a novel metamaterial mimo antenna with high isolation for wlan applications

10
Research Article A Novel Metamaterial MIMO Antenna with High Isolation for WLAN Applications Nguyen Khac Kiem, 1 Huynh Nguyen Bao Phuong, 2 Quang Ngoc Hieu, 3 and Dao Ngoc Chien 4 1 Hanoi University of Science and Technology, Hanoi 100000, Vietnam 2 Quy Nhon University, Binh Dinh 820000, Vietnam 3 Chuo University, Tokyo 1920393, Japan 4 Ministry of Science and Technology, Hanoi 100000, Vietnam Correspondence should be addressed to Nguyen Khac Kiem; [email protected] Received 11 September 2014; Revised 24 December 2014; Accepted 8 January 2015 Academic Editor: Xinyi Tang Copyright © 2015 Nguyen Khac Kiem et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A compact 2×2 metamaterial-MIMO antenna for WLAN applications is presented in this paper. e MIMO antenna is designed by placing side by side two single metamaterial antennas which are constructed based on the modified composite right/leſt-handed (CRLH) model. By adding another leſt-handed inductor, the total leſt-handed inductor of the modified CRLH model is increased remarkably in comparison with that of conventional CRLH model. As a result, the proposed metamaterial antenna achieves 60% size reduction in comparison with the unloaded antenna. e MIMO antenna is electrically small (30 mm × 44 mm) with an edge- to-edge separation between two antennas of 0.06 0 at 2.4 GHz. In order to reduce the mutual coupling of the antenna, a defected ground structure (DGS) is inserted to suppress the effect of surface current between elements of the proposed antenna. e final design of the MIMO antenna satisfies the return loss requirement of less than 10 dB in a bandwidth ranging from 2.38 GHz to 2.5 GHz, which entirely covers WLAN frequency band allocated from 2.4 GHz to 2.48 GHz. e antenna also shows a high isolation coefficient which is less than 35 dB over the operating frequency band. A good agreement between simulation and measurement is shown in this context. 1. Introduction Recently, social demand on multimedia communication has been rapidly increasing resulting in development of modern wireless communication systems such as Wi-Fi, WiMAX, and 3G/4G. Along with these applications, modern antennas are required to have small size and light weight. However, the typical antennas are usually large in size due to the operating wavelength, so they are difficult to meet the requirements of modern antennas. ere are several techniques used to decrease the size of antenna, such as incorporating a shorting pin in a microstrip patch [1], using short circuit [2], and cutting slots in radiating patch [3, 4], by partially filled high permittivity substrate [5] or by Fractal microstrip patch configuration [6]. Besides, transmission line metamaterial (TL-MM) [7] is one of the methods that provides a conceptual way for implementing small resonant antenna [815]. e first proposals of using TL-MM structures at resonance to implement small sprinted antennas have been documented in [9, 10]. Wireless LANs have experienced phenomenal growth during the past several years. e new WLANs standard (IEEE 802.11n) promises both higher data rates and increases reliability. is standard is based on MIMO communication technology which has received much attention as a practical method to substantially increase wireless channel capacity without additional power and spectrum. A multiple antenna system is needed for MIMO system. However, it is difficult to integrate two or more antennas in a mobile device. ere are two critical factors for MIMO antenna system. One is total size of antenna system with a limited space of mobile device. In such a way the antenna elements must be compact and Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2015, Article ID 851904, 9 pages http://dx.doi.org/10.1155/2015/851904

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Research ArticleA Novel Metamaterial MIMO Antenna withHigh Isolation for WLAN Applications

Nguyen Khac Kiem1 Huynh Nguyen Bao Phuong2

Quang Ngoc Hieu3 and Dao Ngoc Chien4

1Hanoi University of Science and Technology Hanoi 100000 Vietnam2Quy Nhon University Binh Dinh 820000 Vietnam3Chuo University Tokyo 1920393 Japan4Ministry of Science and Technology Hanoi 100000 Vietnam

Correspondence should be addressed to Nguyen Khac Kiem kiemnguyenkhachusteduvn

Received 11 September 2014 Revised 24 December 2014 Accepted 8 January 2015

Academic Editor Xinyi Tang

Copyright copy 2015 Nguyen Khac Kiem et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

A compact 2 times 2metamaterial-MIMO antenna for WLAN applications is presented in this paper The MIMO antenna is designedby placing side by side two single metamaterial antennas which are constructed based on the modified composite rightleft-handed(CRLH) model By adding another left-handed inductor the total left-handed inductor of the modified CRLH model is increasedremarkably in comparison with that of conventional CRLH model As a result the proposed metamaterial antenna achieves 60size reduction in comparison with the unloaded antennaTheMIMO antenna is electrically small (30mm times 44mm) with an edge-to-edge separation between two antennas of 006120582

0at 24GHz In order to reduce the mutual coupling of the antenna a defected

ground structure (DGS) is inserted to suppress the effect of surface current between elements of the proposed antenna The finaldesign of the MIMO antenna satisfies the return loss requirement of less than minus10 dB in a bandwidth ranging from 238GHz to25GHz which entirely coversWLAN frequency band allocated from 24GHz to 248GHzThe antenna also shows a high isolationcoefficient which is less than minus35 dB over the operating frequency band A good agreement between simulation and measurementis shown in this context

1 Introduction

Recently social demand on multimedia communication hasbeen rapidly increasing resulting in development of modernwireless communication systems such asWi-FiWiMAX and3G4G Along with these applications modern antennas arerequired to have small size and light weight However thetypical antennas are usually large in size due to the operatingwavelength so they are difficult to meet the requirementsof modern antennas There are several techniques used todecrease the size of antenna such as incorporating a shortingpin in a microstrip patch [1] using short circuit [2] andcutting slots in radiating patch [3 4] by partially filledhigh permittivity substrate [5] or by Fractal microstrip patchconfiguration [6] Besides transmission line metamaterial(TL-MM) [7] is one of themethods that provides a conceptual

way for implementing small resonant antenna [8ndash15] Thefirst proposals of using TL-MM structures at resonance toimplement small sprinted antennas have been documentedin [9 10]

Wireless LANs have experienced phenomenal growthduring the past several years The new WLANs standard(IEEE 80211n) promises both higher data rates and increasesreliability This standard is based on MIMO communicationtechnology which has received much attention as a practicalmethod to substantially increase wireless channel capacitywithout additional power and spectrum A multiple antennasystem is needed for MIMO system However it is difficult tointegrate two or more antennas in a mobile device There aretwo critical factors for MIMO antenna system One is totalsize of antenna system with a limited space of mobile deviceIn such a way the antenna elements must be compact and

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2015 Article ID 851904 9 pageshttpdxdoiorg1011552015851904

2 International Journal of Antennas and Propagation

Metallic ground

Metallic via

Metallic patch

LR

CR LL

CL

(a)

LR

CRLL

CL

(b)

CL1 LR12 LR12

LL1 CR1

LLA

(c)

Figure 1 CRLH transmission line (a) mushroom-like EBG model (b) equivalent circuit of unit cell and (c) equivalent circuit of proposedmetamaterial antenna

be put very close The other factor is the isolation betweenantenna elements Due to the close space between antennaelements the coupling coefficient among radiating elementsis very high This will degrade the performance of MIMOsystem Therefore it will be a real challenging task to designa MIMO antenna with small size while obtaining a very highisolation coefficient

In this paper a very compact metamaterial MIMOantenna is proposed The MIMO antenna consists of twoantennas which are based on composite leftright handed(CLRH) transmission lines for reducing the antenna dimen-sion In the proposed configuration a defected ground struc-ture (DGS) is employed to increase the isolation betweentwo antenna elements Thus a novel metamaterial MIMOantenna is proposed which has a high isolation with only75mm (006120582

0) distance between antenna elements This

antenna is built on a FR4 substrate with total volume of 30 times44 times 16mm3 and has very compact radiating elements withtotal size of 892 times 326mm2 and operates at the frequencyband of 238ndash25GHzwhile the values of isolation coefficientsare below minus35 dB over operating frequency band

The rest of this paper is organized as follow In Section 2detailed designs of the single metamaterial antenna arepresented The proposed MIMO antenna is then introducedin both cases of initial and final design The simulatedand measured results are shown in Section 3 while someconclusions are provided in Section 4

2 Design of Metamaterial MIMO Antenna

In this work the design of the antenna is divided into twoparts In the first one a metamaterial antenna is designed forWLAN frequency ranging from 24GHz to 248GHz In thesecond part the two identical single metamaterial antennasare utilized as elements to form a 2 times 2 MIMO antenna

Finally the defected ground structure is implemented todiminish the mutual coupling of the antennas

21 Design of Single Metamaterial Antenna The configu-ration of metamaterial antenna is shown in Figure 2(a)The antenna is printed on a low-cost FR4 substrate withthe thickness of 16mm dielectric constant 120576 of 44 andloss tangent tan 120575 of 002 As a reference comparison anunloaded microstrip fed rectangular strip with the lengthof 1198971is chosen as the monopole radiating element In order

to maintain compact electrical length while decreasing theoperating frequencies the monopole antenna is constructedby a modified CRLH single-cell

The model of conventional CRLH transmission line isshown in Figure 1(a)This is amushroom-like EBGwhich canbe interpreted by equivalent circuit depicted in Figure 1(b)From this figure the serial left-handed (LH) capacitor (119862L)is created by two adjacent metallic patches placed on thetop surface of the structure while the shunt LH inductor(119871L) created by the current flows from the metallic patch toground plane throughmetallic viaMoreover the serial right-handed (RH) inductor (119871R) is formed by the metallic patchand the shunt RH capacitor (119862R) is created due to the parallelarrangement of metallic patch and ground plane

The equivalent circuit of proposed metamaterial antennais shown in Figure 1(c) In this design the metamaterial-loading is carried out in an asymmetric fashion where serialLH capacitor (119862L1) is formed between two strips separatedby a distance 119904

1(as shown in Figure 2(a)) while the shunt

LH inductor (119871L1) is formed similarly to the shunt LH oneshown in Figure 1(b) Moreover the additional LH inductor(119871LA) is built up by meandered strips which connect thestructure and the ground plane Regarding RH componentsthe serial RH inductor (119871R1) is formed by themain patchwithlength of 119897

1and shunt RH capacitor (119862R1) is formed similarly

International Journal of Antennas and Propagation 3

TopBottom

Metallic viaw1

l3

l2

l1

s1

s2

l4

l5

X

YZ

(a)

TopBottom

L3

L2 L1

L4

L6

L

S

(b)

Figure 2 Configuration of the proposed antennas (a) single metamaterial antenna and (b) metamaterial MIMO antenna

Table 1 Dimensions of single metamaterial antenna

1198971

8mm 1198974

105mm 1199041

042mm1198972

05mm 1198975

67mm 1199042

03mm1198973

28mm 1199081

6mm

to the RH components of conventional CRLH model As aresult a singlemetamaterial antenna is proposedwith the sizeof radiating element of 892 times 126mm2 (007120582

0times 01120582

0at

24GHz) and printed in a substratewith two dimensions of 27times 30mm2 Finally the center resonant frequency of proposedmetamaterial antenna is defined as follows

119891119862=

1

2120587radic(119871L1 + 119871LA) 119862R1

(1)

22 Design ofMetamaterial MIMOAntenna In this design aMIMO model is constructed by placing two single antennasside by side at the distance of 20mm (016120582

0at 24GHz)

from center-to-center or 75mm (0061205820at 24GHz) from

edge-to-edgemaking the overall the dimension of this designvery compact The layout of the MIMO antenna is shownin Figure 2(b) In order to increase the isolation betweenelements of MIMO antenna a defected ground structure isetched in a part of ground between two elements Firstly twoparallel slots are etched on the ground plane As a result theMIMO antenna satisfies the isolation requirement while theoperating frequencies were shifted compared to the WLANfrequency bandTherefore two I-shaped slots which are usedas an impedance matching circuit are etched on the metallicground (as shown in Figure 2(b)) The final MIMO antennasystem is proposedwith total size of radiating elements of 892times 326mm2 and satisfies all requirements of MIMO systemwith very high isolation between antenna elements All thedimensions of the proposed single metamaterial antenna andMIMO antenna are given in Tables 1 and 2 respectively

Table 2 Dimensions of metamaterial MIMO antenna

119871 5mm 1198713

12mm 119878 25mm1198711

25mm 1198714

1mm1198712

6mm 1198716

05mm

3 Results and Discussions

The performance of the proposed antennas is discussed indetail in terms of simulation and measurement results

31 SingleMetamaterial Antenna Asmentioned in Section 2the resonant frequency of proposed metamaterial antennadepends on themeandered strip lengthwhich is controlled bytuning the length 119897

3as well as the gap between strip steps 119904

2

The simulated 11987811results of the single metamaterial antenna

with different values of 1198973and 1199042are shown in Figure 3 In

Figure 3 the resonant frequency reduces with the increasingthe value of 119897

3and 119904

2 Actually the increase of 119897

3and 119904

2

will lead to the increase of the additional LH inductor 119871LAand therefore making the decrease of the resonant frequencyThis is entirely consistent with formula (1) The optimizedbandwidth is obtained when the 119897

3and 1199042are set at 28mm

and 03mm respectively It can be seen from Figure 7 that thebandwidth of the antenna defined by the 119878

11less than minus10 dB

entirely covers theWLAN frequency range which is allocatedfrom 24 to 248GHz

The size reduction of the proposed antenna is carried outby taking the simulated 119878

11of antennas in case of loaded

(proposed antenna) and unloaded (conventional antenna)The two antennas are given the same dimensions of substratelayer and radiation elements As can be seen from Figure 4the resonant frequency of the unloaded antenna centers at6GHz while the resonant one of the proposed antenna ismaintained at 244GHz It is clear that the proposed antennaexhibits smaller resonant frequency than the conventionalone In this case the proposed antenna achieves 60 sizereduction in comparison with the conventional one

4 International Journal of Antennas and Propagation

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

235mm255mm

275mm295mm

(a)

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

020mm025mm

030mm035mm

(b)

Figure 3 Simulated 11987811of single metamaterial antenna for different values of (a) 119897

3and (b) 119904

2

1 2 3 4 5 6 7 8Frequency (GHz)

Unloaded antennaProposed antenna

Proposedantenna

Unloadedantenna

minus40

minus30

minus20

minus10

0

S 11

(dB)

Figure 4 Simulated 11987811of unloaded and proposed antennas

Current distributions of the metamaterial antenna at thecenter frequency of WLAN are exhibited in Figure 5 Asobserved in Figure 5 the current distribution on antenna at244GHz mainly focuses on the meandered strips instead ofon the radiating patch as the principle of microstrip antenna

The radiation pattern of single antenna at the centerfrequency of 244GHz is plotted in Figure 6 The solid linesdisplay the 119864-plane and the dotted lines represent119867-plane Itcan be observed that the single antenna possesses an isotropicradiation pattern confirming its operation in the fundamental

Jsurf (Am)

10000e + 002

92911e + 001

85822e + 001

78732e + 001

71643e + 001

64554e + 001

57465e + 001

50376e + 001

43286e + 001

36197e + 001

29108e + 001

22019e + 001

14930e + 001

78405e + 000

75131e minus 001

Figure 5 Surface current distribution on single antenna at244GHz

resonantmodeTherefore its gain is small with themaximumtotal gain of 14 dB

Finally the fabricated single metamaterial antenna ispresented in Figure 13The simulated andmeasured results of11987811of singlemetamaterial antenna is shown in Figure 7 From

this figure it can be observed that the antenna can operateover the range spreading from 24GHz to 248GHz and from2405GHz to 2495GHz in simulation and measurementrespectively

32 Metamaterial MIMO Antenna The simulated results ofreflection coefficients of the initial MIMO antenna (withoutDGS) are shown in Figure 8 From this figure it is observedthat the initial antenna does not satisfy the impedance

International Journal of Antennas and Propagation 5

minus20

minus10

0

030

60

90

120

150180

210

240

270

300

330

minus20

minus10

0

E-planeH-plane

Figure 6 Simulated radiation pattern of single metamaterial ante-nna at center frequency of 244GHz

22 23 24 25 26 27minus35

minus30

minus25

minus20

minus15

minus10

minus5

0

Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 7 Simulated and measured results of single metamaterialantenna

matching condition due to the effect of mutual coupling The119878-parameters of antenna are changed and could not meetthe requirements of MIMO antenna from which 119878

11and 11987822

are not below minus10 dB and 11987821

and 11987812

are not below minus15 dBin WLAN band This fact is clearly demonstrated by thesurface current distribution on the initial MIMO antenna inFigure 10(a) As can be observed from Figure 10(a) when thefirst element (Port 1) is excited the surface current is stronglyinduced on the second element (Port 2) resulting in a riseof the mutual coupling (119878

21and 119878

12) Actually the mutual

coupling can be reduced by increasing the distance between

22 23 24 25 26 27minus50

minus40

minus30

minus20

minus10

0

With full DGSWith dual slotsWithout DGS

Frequency (GHz)

S-pa

ram

eter

s (dB

)

S11

S21

Figure 8 Simulated 119878-parameters in three cases without DGS withdual slots and with full DGS

the elements However this will lead to the larger size ofthe proposed MIMO antenna These drawbacks of the initialMIMO antenna can be solved thanks to the use of defectedground structure etched on the common ground of MIMOantenna by the following two steps

At first two parallel slots are added to central groundplane between two ports (as shown in Figure 2(b)) Thelength slot 119871

3is varied to find out the value from which

the impedance matching and mutual coupling have the bestsolution The optimized length of 119871

3is chosen as 12mm

Simulated 119878-parameters ofMIMO antennawith dual slots areshown in Figure 8 From this figure it can be seen that theisolation coefficient 119878

21is lower than minus18 dB for all frequency

in WLAN band However the impedance is not matchedenough in this band so that the 119878

11is below minus10 dB over the

frequency ranging from 242GHz to 25 GHz and thereforethe antenna could not cover the WLAN frequencies

The current distribution of MIMO antenna with theimplementation of dual slots at 244GHz is shown in Fig-ure 10(b) It can be seen from Figure 10(b) that the surfacecurrent partly focuses on the slots and somewhat coupling tothe radiation strips of the adjacent antenna element

In order to solve this problem in the second step two I-shaped slots are etched on the ground plane and used as animpedance matching circuit The effect of I-shaped slots toimpedance matching of the MIMO antenna is investigatedvia the length 119871 Figure 9 shows the simulated 119878-parametersof the MIMO antenna for the different values of 119871 It can beobserved that the isolation coefficient is below minus15 dB for allcases and the return loss is changed with the various sizesof slots When the length 119871 of I-slots increases the inputimpedance decreases make the impedance highly matchedThe final MIMO antenna with full DGS is formed as thevalue of 119871 fixed at 5mm As a result the full DGS MIMO

6 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

1mm

5mm3mm

Figure 9 Simulated 119878-parameters of full DGS MIMO antenna for different values of 119871

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(a)

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(b)

Jsurf (Am)

50000e + 00146429e + 00142857e + 00139286e + 00135714e + 00132143e + 00128571e + 00125000e + 00121429e + 00117857e + 00114286e + 00110714e + 00171429e + 00035714e + 00000000e + 000

(c)

Figure 10 Surface current distribution at 244GHz on metamaterial MIMO antenna (a) without DGS and (b) with the implementation ofdual slots and (c) with full DGS

antenna achieves high isolation coefficient which is lessthan minus35 dB over all frequency of WLAN band while theoperating bandwidth covers from 238GHz to 252GHzThe current distribution of the final MIMO antenna at244GHz is focused on the defected ground structure shown

in Figure 10(c) Therefore the effect of the surface current tothe second element is significant reduced

Simulated radiation patterns of final MIMO antenna inthe119909119910119910119911 and119909119911planes at 244GHzwhen the antenna is fedeach port in turn is shown in Figure 11 The antenna displays

International Journal of Antennas and Propagation 7

minus20

00

30

60

90

120

150180

210

240

270

300

330

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

xy plane yz plane xz plane

(a)

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

Gain Φ

Gain 120579

Gain Φ

Gain 120579

Gain Φ

Gain 120579

xy plane yz plane xz plane

(b)

Figure 11 Radiation patterns of proposed metamaterial MIMO antenna at central frequency of 244GHz when (a) excited Port 1 and (b)excited Port 2

22 23 24 25 26 2720

30

40

50

60

70

80

90

100

Ant

enna

effici

ency

()

Frequency (GHz)

Figure 12 Simulated radiation efficiency of proposed antenna

good omnidirectional radiation patterns in the 119910119911 and 119909119911planes (119867-plane) while the separate far field patterns areproduced in the 119909119910 plane (119864-plane) Therefore the diversity

(a)

(b)

Figure 13 Fabrication of the single metamaterial antenna initialand final MIMO antenna (a) front view and (b) back view

for the antenna is achieved Thanks to this characteristic theantenna is a promising candidate for MIMO system

Figure 12 gives the simulated radiation efficiency of theproposed antenna This figure indicates that the proposedantenna shows good radiation efficiency which has theaverage value of 85 in over the operating bandwidth ofWLAN system

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

2 International Journal of Antennas and Propagation

Metallic ground

Metallic via

Metallic patch

LR

CR LL

CL

(a)

LR

CRLL

CL

(b)

CL1 LR12 LR12

LL1 CR1

LLA

(c)

Figure 1 CRLH transmission line (a) mushroom-like EBG model (b) equivalent circuit of unit cell and (c) equivalent circuit of proposedmetamaterial antenna

be put very close The other factor is the isolation betweenantenna elements Due to the close space between antennaelements the coupling coefficient among radiating elementsis very high This will degrade the performance of MIMOsystem Therefore it will be a real challenging task to designa MIMO antenna with small size while obtaining a very highisolation coefficient

In this paper a very compact metamaterial MIMOantenna is proposed The MIMO antenna consists of twoantennas which are based on composite leftright handed(CLRH) transmission lines for reducing the antenna dimen-sion In the proposed configuration a defected ground struc-ture (DGS) is employed to increase the isolation betweentwo antenna elements Thus a novel metamaterial MIMOantenna is proposed which has a high isolation with only75mm (006120582

0) distance between antenna elements This

antenna is built on a FR4 substrate with total volume of 30 times44 times 16mm3 and has very compact radiating elements withtotal size of 892 times 326mm2 and operates at the frequencyband of 238ndash25GHzwhile the values of isolation coefficientsare below minus35 dB over operating frequency band

The rest of this paper is organized as follow In Section 2detailed designs of the single metamaterial antenna arepresented The proposed MIMO antenna is then introducedin both cases of initial and final design The simulatedand measured results are shown in Section 3 while someconclusions are provided in Section 4

2 Design of Metamaterial MIMO Antenna

In this work the design of the antenna is divided into twoparts In the first one a metamaterial antenna is designed forWLAN frequency ranging from 24GHz to 248GHz In thesecond part the two identical single metamaterial antennasare utilized as elements to form a 2 times 2 MIMO antenna

Finally the defected ground structure is implemented todiminish the mutual coupling of the antennas

21 Design of Single Metamaterial Antenna The configu-ration of metamaterial antenna is shown in Figure 2(a)The antenna is printed on a low-cost FR4 substrate withthe thickness of 16mm dielectric constant 120576 of 44 andloss tangent tan 120575 of 002 As a reference comparison anunloaded microstrip fed rectangular strip with the lengthof 1198971is chosen as the monopole radiating element In order

to maintain compact electrical length while decreasing theoperating frequencies the monopole antenna is constructedby a modified CRLH single-cell

The model of conventional CRLH transmission line isshown in Figure 1(a)This is amushroom-like EBGwhich canbe interpreted by equivalent circuit depicted in Figure 1(b)From this figure the serial left-handed (LH) capacitor (119862L)is created by two adjacent metallic patches placed on thetop surface of the structure while the shunt LH inductor(119871L) created by the current flows from the metallic patch toground plane throughmetallic viaMoreover the serial right-handed (RH) inductor (119871R) is formed by the metallic patchand the shunt RH capacitor (119862R) is created due to the parallelarrangement of metallic patch and ground plane

The equivalent circuit of proposed metamaterial antennais shown in Figure 1(c) In this design the metamaterial-loading is carried out in an asymmetric fashion where serialLH capacitor (119862L1) is formed between two strips separatedby a distance 119904

1(as shown in Figure 2(a)) while the shunt

LH inductor (119871L1) is formed similarly to the shunt LH oneshown in Figure 1(b) Moreover the additional LH inductor(119871LA) is built up by meandered strips which connect thestructure and the ground plane Regarding RH componentsthe serial RH inductor (119871R1) is formed by themain patchwithlength of 119897

1and shunt RH capacitor (119862R1) is formed similarly

International Journal of Antennas and Propagation 3

TopBottom

Metallic viaw1

l3

l2

l1

s1

s2

l4

l5

X

YZ

(a)

TopBottom

L3

L2 L1

L4

L6

L

S

(b)

Figure 2 Configuration of the proposed antennas (a) single metamaterial antenna and (b) metamaterial MIMO antenna

Table 1 Dimensions of single metamaterial antenna

1198971

8mm 1198974

105mm 1199041

042mm1198972

05mm 1198975

67mm 1199042

03mm1198973

28mm 1199081

6mm

to the RH components of conventional CRLH model As aresult a singlemetamaterial antenna is proposedwith the sizeof radiating element of 892 times 126mm2 (007120582

0times 01120582

0at

24GHz) and printed in a substratewith two dimensions of 27times 30mm2 Finally the center resonant frequency of proposedmetamaterial antenna is defined as follows

119891119862=

1

2120587radic(119871L1 + 119871LA) 119862R1

(1)

22 Design ofMetamaterial MIMOAntenna In this design aMIMO model is constructed by placing two single antennasside by side at the distance of 20mm (016120582

0at 24GHz)

from center-to-center or 75mm (0061205820at 24GHz) from

edge-to-edgemaking the overall the dimension of this designvery compact The layout of the MIMO antenna is shownin Figure 2(b) In order to increase the isolation betweenelements of MIMO antenna a defected ground structure isetched in a part of ground between two elements Firstly twoparallel slots are etched on the ground plane As a result theMIMO antenna satisfies the isolation requirement while theoperating frequencies were shifted compared to the WLANfrequency bandTherefore two I-shaped slots which are usedas an impedance matching circuit are etched on the metallicground (as shown in Figure 2(b)) The final MIMO antennasystem is proposedwith total size of radiating elements of 892times 326mm2 and satisfies all requirements of MIMO systemwith very high isolation between antenna elements All thedimensions of the proposed single metamaterial antenna andMIMO antenna are given in Tables 1 and 2 respectively

Table 2 Dimensions of metamaterial MIMO antenna

119871 5mm 1198713

12mm 119878 25mm1198711

25mm 1198714

1mm1198712

6mm 1198716

05mm

3 Results and Discussions

The performance of the proposed antennas is discussed indetail in terms of simulation and measurement results

31 SingleMetamaterial Antenna Asmentioned in Section 2the resonant frequency of proposed metamaterial antennadepends on themeandered strip lengthwhich is controlled bytuning the length 119897

3as well as the gap between strip steps 119904

2

The simulated 11987811results of the single metamaterial antenna

with different values of 1198973and 1199042are shown in Figure 3 In

Figure 3 the resonant frequency reduces with the increasingthe value of 119897

3and 119904

2 Actually the increase of 119897

3and 119904

2

will lead to the increase of the additional LH inductor 119871LAand therefore making the decrease of the resonant frequencyThis is entirely consistent with formula (1) The optimizedbandwidth is obtained when the 119897

3and 1199042are set at 28mm

and 03mm respectively It can be seen from Figure 7 that thebandwidth of the antenna defined by the 119878

11less than minus10 dB

entirely covers theWLAN frequency range which is allocatedfrom 24 to 248GHz

The size reduction of the proposed antenna is carried outby taking the simulated 119878

11of antennas in case of loaded

(proposed antenna) and unloaded (conventional antenna)The two antennas are given the same dimensions of substratelayer and radiation elements As can be seen from Figure 4the resonant frequency of the unloaded antenna centers at6GHz while the resonant one of the proposed antenna ismaintained at 244GHz It is clear that the proposed antennaexhibits smaller resonant frequency than the conventionalone In this case the proposed antenna achieves 60 sizereduction in comparison with the conventional one

4 International Journal of Antennas and Propagation

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

235mm255mm

275mm295mm

(a)

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

020mm025mm

030mm035mm

(b)

Figure 3 Simulated 11987811of single metamaterial antenna for different values of (a) 119897

3and (b) 119904

2

1 2 3 4 5 6 7 8Frequency (GHz)

Unloaded antennaProposed antenna

Proposedantenna

Unloadedantenna

minus40

minus30

minus20

minus10

0

S 11

(dB)

Figure 4 Simulated 11987811of unloaded and proposed antennas

Current distributions of the metamaterial antenna at thecenter frequency of WLAN are exhibited in Figure 5 Asobserved in Figure 5 the current distribution on antenna at244GHz mainly focuses on the meandered strips instead ofon the radiating patch as the principle of microstrip antenna

The radiation pattern of single antenna at the centerfrequency of 244GHz is plotted in Figure 6 The solid linesdisplay the 119864-plane and the dotted lines represent119867-plane Itcan be observed that the single antenna possesses an isotropicradiation pattern confirming its operation in the fundamental

Jsurf (Am)

10000e + 002

92911e + 001

85822e + 001

78732e + 001

71643e + 001

64554e + 001

57465e + 001

50376e + 001

43286e + 001

36197e + 001

29108e + 001

22019e + 001

14930e + 001

78405e + 000

75131e minus 001

Figure 5 Surface current distribution on single antenna at244GHz

resonantmodeTherefore its gain is small with themaximumtotal gain of 14 dB

Finally the fabricated single metamaterial antenna ispresented in Figure 13The simulated andmeasured results of11987811of singlemetamaterial antenna is shown in Figure 7 From

this figure it can be observed that the antenna can operateover the range spreading from 24GHz to 248GHz and from2405GHz to 2495GHz in simulation and measurementrespectively

32 Metamaterial MIMO Antenna The simulated results ofreflection coefficients of the initial MIMO antenna (withoutDGS) are shown in Figure 8 From this figure it is observedthat the initial antenna does not satisfy the impedance

International Journal of Antennas and Propagation 5

minus20

minus10

0

030

60

90

120

150180

210

240

270

300

330

minus20

minus10

0

E-planeH-plane

Figure 6 Simulated radiation pattern of single metamaterial ante-nna at center frequency of 244GHz

22 23 24 25 26 27minus35

minus30

minus25

minus20

minus15

minus10

minus5

0

Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 7 Simulated and measured results of single metamaterialantenna

matching condition due to the effect of mutual coupling The119878-parameters of antenna are changed and could not meetthe requirements of MIMO antenna from which 119878

11and 11987822

are not below minus10 dB and 11987821

and 11987812

are not below minus15 dBin WLAN band This fact is clearly demonstrated by thesurface current distribution on the initial MIMO antenna inFigure 10(a) As can be observed from Figure 10(a) when thefirst element (Port 1) is excited the surface current is stronglyinduced on the second element (Port 2) resulting in a riseof the mutual coupling (119878

21and 119878

12) Actually the mutual

coupling can be reduced by increasing the distance between

22 23 24 25 26 27minus50

minus40

minus30

minus20

minus10

0

With full DGSWith dual slotsWithout DGS

Frequency (GHz)

S-pa

ram

eter

s (dB

)

S11

S21

Figure 8 Simulated 119878-parameters in three cases without DGS withdual slots and with full DGS

the elements However this will lead to the larger size ofthe proposed MIMO antenna These drawbacks of the initialMIMO antenna can be solved thanks to the use of defectedground structure etched on the common ground of MIMOantenna by the following two steps

At first two parallel slots are added to central groundplane between two ports (as shown in Figure 2(b)) Thelength slot 119871

3is varied to find out the value from which

the impedance matching and mutual coupling have the bestsolution The optimized length of 119871

3is chosen as 12mm

Simulated 119878-parameters ofMIMO antennawith dual slots areshown in Figure 8 From this figure it can be seen that theisolation coefficient 119878

21is lower than minus18 dB for all frequency

in WLAN band However the impedance is not matchedenough in this band so that the 119878

11is below minus10 dB over the

frequency ranging from 242GHz to 25 GHz and thereforethe antenna could not cover the WLAN frequencies

The current distribution of MIMO antenna with theimplementation of dual slots at 244GHz is shown in Fig-ure 10(b) It can be seen from Figure 10(b) that the surfacecurrent partly focuses on the slots and somewhat coupling tothe radiation strips of the adjacent antenna element

In order to solve this problem in the second step two I-shaped slots are etched on the ground plane and used as animpedance matching circuit The effect of I-shaped slots toimpedance matching of the MIMO antenna is investigatedvia the length 119871 Figure 9 shows the simulated 119878-parametersof the MIMO antenna for the different values of 119871 It can beobserved that the isolation coefficient is below minus15 dB for allcases and the return loss is changed with the various sizesof slots When the length 119871 of I-slots increases the inputimpedance decreases make the impedance highly matchedThe final MIMO antenna with full DGS is formed as thevalue of 119871 fixed at 5mm As a result the full DGS MIMO

6 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

1mm

5mm3mm

Figure 9 Simulated 119878-parameters of full DGS MIMO antenna for different values of 119871

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(a)

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(b)

Jsurf (Am)

50000e + 00146429e + 00142857e + 00139286e + 00135714e + 00132143e + 00128571e + 00125000e + 00121429e + 00117857e + 00114286e + 00110714e + 00171429e + 00035714e + 00000000e + 000

(c)

Figure 10 Surface current distribution at 244GHz on metamaterial MIMO antenna (a) without DGS and (b) with the implementation ofdual slots and (c) with full DGS

antenna achieves high isolation coefficient which is lessthan minus35 dB over all frequency of WLAN band while theoperating bandwidth covers from 238GHz to 252GHzThe current distribution of the final MIMO antenna at244GHz is focused on the defected ground structure shown

in Figure 10(c) Therefore the effect of the surface current tothe second element is significant reduced

Simulated radiation patterns of final MIMO antenna inthe119909119910119910119911 and119909119911planes at 244GHzwhen the antenna is fedeach port in turn is shown in Figure 11 The antenna displays

International Journal of Antennas and Propagation 7

minus20

00

30

60

90

120

150180

210

240

270

300

330

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

xy plane yz plane xz plane

(a)

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

Gain Φ

Gain 120579

Gain Φ

Gain 120579

Gain Φ

Gain 120579

xy plane yz plane xz plane

(b)

Figure 11 Radiation patterns of proposed metamaterial MIMO antenna at central frequency of 244GHz when (a) excited Port 1 and (b)excited Port 2

22 23 24 25 26 2720

30

40

50

60

70

80

90

100

Ant

enna

effici

ency

()

Frequency (GHz)

Figure 12 Simulated radiation efficiency of proposed antenna

good omnidirectional radiation patterns in the 119910119911 and 119909119911planes (119867-plane) while the separate far field patterns areproduced in the 119909119910 plane (119864-plane) Therefore the diversity

(a)

(b)

Figure 13 Fabrication of the single metamaterial antenna initialand final MIMO antenna (a) front view and (b) back view

for the antenna is achieved Thanks to this characteristic theantenna is a promising candidate for MIMO system

Figure 12 gives the simulated radiation efficiency of theproposed antenna This figure indicates that the proposedantenna shows good radiation efficiency which has theaverage value of 85 in over the operating bandwidth ofWLAN system

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 3

TopBottom

Metallic viaw1

l3

l2

l1

s1

s2

l4

l5

X

YZ

(a)

TopBottom

L3

L2 L1

L4

L6

L

S

(b)

Figure 2 Configuration of the proposed antennas (a) single metamaterial antenna and (b) metamaterial MIMO antenna

Table 1 Dimensions of single metamaterial antenna

1198971

8mm 1198974

105mm 1199041

042mm1198972

05mm 1198975

67mm 1199042

03mm1198973

28mm 1199081

6mm

to the RH components of conventional CRLH model As aresult a singlemetamaterial antenna is proposedwith the sizeof radiating element of 892 times 126mm2 (007120582

0times 01120582

0at

24GHz) and printed in a substratewith two dimensions of 27times 30mm2 Finally the center resonant frequency of proposedmetamaterial antenna is defined as follows

119891119862=

1

2120587radic(119871L1 + 119871LA) 119862R1

(1)

22 Design ofMetamaterial MIMOAntenna In this design aMIMO model is constructed by placing two single antennasside by side at the distance of 20mm (016120582

0at 24GHz)

from center-to-center or 75mm (0061205820at 24GHz) from

edge-to-edgemaking the overall the dimension of this designvery compact The layout of the MIMO antenna is shownin Figure 2(b) In order to increase the isolation betweenelements of MIMO antenna a defected ground structure isetched in a part of ground between two elements Firstly twoparallel slots are etched on the ground plane As a result theMIMO antenna satisfies the isolation requirement while theoperating frequencies were shifted compared to the WLANfrequency bandTherefore two I-shaped slots which are usedas an impedance matching circuit are etched on the metallicground (as shown in Figure 2(b)) The final MIMO antennasystem is proposedwith total size of radiating elements of 892times 326mm2 and satisfies all requirements of MIMO systemwith very high isolation between antenna elements All thedimensions of the proposed single metamaterial antenna andMIMO antenna are given in Tables 1 and 2 respectively

Table 2 Dimensions of metamaterial MIMO antenna

119871 5mm 1198713

12mm 119878 25mm1198711

25mm 1198714

1mm1198712

6mm 1198716

05mm

3 Results and Discussions

The performance of the proposed antennas is discussed indetail in terms of simulation and measurement results

31 SingleMetamaterial Antenna Asmentioned in Section 2the resonant frequency of proposed metamaterial antennadepends on themeandered strip lengthwhich is controlled bytuning the length 119897

3as well as the gap between strip steps 119904

2

The simulated 11987811results of the single metamaterial antenna

with different values of 1198973and 1199042are shown in Figure 3 In

Figure 3 the resonant frequency reduces with the increasingthe value of 119897

3and 119904

2 Actually the increase of 119897

3and 119904

2

will lead to the increase of the additional LH inductor 119871LAand therefore making the decrease of the resonant frequencyThis is entirely consistent with formula (1) The optimizedbandwidth is obtained when the 119897

3and 1199042are set at 28mm

and 03mm respectively It can be seen from Figure 7 that thebandwidth of the antenna defined by the 119878

11less than minus10 dB

entirely covers theWLAN frequency range which is allocatedfrom 24 to 248GHz

The size reduction of the proposed antenna is carried outby taking the simulated 119878

11of antennas in case of loaded

(proposed antenna) and unloaded (conventional antenna)The two antennas are given the same dimensions of substratelayer and radiation elements As can be seen from Figure 4the resonant frequency of the unloaded antenna centers at6GHz while the resonant one of the proposed antenna ismaintained at 244GHz It is clear that the proposed antennaexhibits smaller resonant frequency than the conventionalone In this case the proposed antenna achieves 60 sizereduction in comparison with the conventional one

4 International Journal of Antennas and Propagation

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

235mm255mm

275mm295mm

(a)

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

020mm025mm

030mm035mm

(b)

Figure 3 Simulated 11987811of single metamaterial antenna for different values of (a) 119897

3and (b) 119904

2

1 2 3 4 5 6 7 8Frequency (GHz)

Unloaded antennaProposed antenna

Proposedantenna

Unloadedantenna

minus40

minus30

minus20

minus10

0

S 11

(dB)

Figure 4 Simulated 11987811of unloaded and proposed antennas

Current distributions of the metamaterial antenna at thecenter frequency of WLAN are exhibited in Figure 5 Asobserved in Figure 5 the current distribution on antenna at244GHz mainly focuses on the meandered strips instead ofon the radiating patch as the principle of microstrip antenna

The radiation pattern of single antenna at the centerfrequency of 244GHz is plotted in Figure 6 The solid linesdisplay the 119864-plane and the dotted lines represent119867-plane Itcan be observed that the single antenna possesses an isotropicradiation pattern confirming its operation in the fundamental

Jsurf (Am)

10000e + 002

92911e + 001

85822e + 001

78732e + 001

71643e + 001

64554e + 001

57465e + 001

50376e + 001

43286e + 001

36197e + 001

29108e + 001

22019e + 001

14930e + 001

78405e + 000

75131e minus 001

Figure 5 Surface current distribution on single antenna at244GHz

resonantmodeTherefore its gain is small with themaximumtotal gain of 14 dB

Finally the fabricated single metamaterial antenna ispresented in Figure 13The simulated andmeasured results of11987811of singlemetamaterial antenna is shown in Figure 7 From

this figure it can be observed that the antenna can operateover the range spreading from 24GHz to 248GHz and from2405GHz to 2495GHz in simulation and measurementrespectively

32 Metamaterial MIMO Antenna The simulated results ofreflection coefficients of the initial MIMO antenna (withoutDGS) are shown in Figure 8 From this figure it is observedthat the initial antenna does not satisfy the impedance

International Journal of Antennas and Propagation 5

minus20

minus10

0

030

60

90

120

150180

210

240

270

300

330

minus20

minus10

0

E-planeH-plane

Figure 6 Simulated radiation pattern of single metamaterial ante-nna at center frequency of 244GHz

22 23 24 25 26 27minus35

minus30

minus25

minus20

minus15

minus10

minus5

0

Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 7 Simulated and measured results of single metamaterialantenna

matching condition due to the effect of mutual coupling The119878-parameters of antenna are changed and could not meetthe requirements of MIMO antenna from which 119878

11and 11987822

are not below minus10 dB and 11987821

and 11987812

are not below minus15 dBin WLAN band This fact is clearly demonstrated by thesurface current distribution on the initial MIMO antenna inFigure 10(a) As can be observed from Figure 10(a) when thefirst element (Port 1) is excited the surface current is stronglyinduced on the second element (Port 2) resulting in a riseof the mutual coupling (119878

21and 119878

12) Actually the mutual

coupling can be reduced by increasing the distance between

22 23 24 25 26 27minus50

minus40

minus30

minus20

minus10

0

With full DGSWith dual slotsWithout DGS

Frequency (GHz)

S-pa

ram

eter

s (dB

)

S11

S21

Figure 8 Simulated 119878-parameters in three cases without DGS withdual slots and with full DGS

the elements However this will lead to the larger size ofthe proposed MIMO antenna These drawbacks of the initialMIMO antenna can be solved thanks to the use of defectedground structure etched on the common ground of MIMOantenna by the following two steps

At first two parallel slots are added to central groundplane between two ports (as shown in Figure 2(b)) Thelength slot 119871

3is varied to find out the value from which

the impedance matching and mutual coupling have the bestsolution The optimized length of 119871

3is chosen as 12mm

Simulated 119878-parameters ofMIMO antennawith dual slots areshown in Figure 8 From this figure it can be seen that theisolation coefficient 119878

21is lower than minus18 dB for all frequency

in WLAN band However the impedance is not matchedenough in this band so that the 119878

11is below minus10 dB over the

frequency ranging from 242GHz to 25 GHz and thereforethe antenna could not cover the WLAN frequencies

The current distribution of MIMO antenna with theimplementation of dual slots at 244GHz is shown in Fig-ure 10(b) It can be seen from Figure 10(b) that the surfacecurrent partly focuses on the slots and somewhat coupling tothe radiation strips of the adjacent antenna element

In order to solve this problem in the second step two I-shaped slots are etched on the ground plane and used as animpedance matching circuit The effect of I-shaped slots toimpedance matching of the MIMO antenna is investigatedvia the length 119871 Figure 9 shows the simulated 119878-parametersof the MIMO antenna for the different values of 119871 It can beobserved that the isolation coefficient is below minus15 dB for allcases and the return loss is changed with the various sizesof slots When the length 119871 of I-slots increases the inputimpedance decreases make the impedance highly matchedThe final MIMO antenna with full DGS is formed as thevalue of 119871 fixed at 5mm As a result the full DGS MIMO

6 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

1mm

5mm3mm

Figure 9 Simulated 119878-parameters of full DGS MIMO antenna for different values of 119871

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(a)

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(b)

Jsurf (Am)

50000e + 00146429e + 00142857e + 00139286e + 00135714e + 00132143e + 00128571e + 00125000e + 00121429e + 00117857e + 00114286e + 00110714e + 00171429e + 00035714e + 00000000e + 000

(c)

Figure 10 Surface current distribution at 244GHz on metamaterial MIMO antenna (a) without DGS and (b) with the implementation ofdual slots and (c) with full DGS

antenna achieves high isolation coefficient which is lessthan minus35 dB over all frequency of WLAN band while theoperating bandwidth covers from 238GHz to 252GHzThe current distribution of the final MIMO antenna at244GHz is focused on the defected ground structure shown

in Figure 10(c) Therefore the effect of the surface current tothe second element is significant reduced

Simulated radiation patterns of final MIMO antenna inthe119909119910119910119911 and119909119911planes at 244GHzwhen the antenna is fedeach port in turn is shown in Figure 11 The antenna displays

International Journal of Antennas and Propagation 7

minus20

00

30

60

90

120

150180

210

240

270

300

330

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

xy plane yz plane xz plane

(a)

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

Gain Φ

Gain 120579

Gain Φ

Gain 120579

Gain Φ

Gain 120579

xy plane yz plane xz plane

(b)

Figure 11 Radiation patterns of proposed metamaterial MIMO antenna at central frequency of 244GHz when (a) excited Port 1 and (b)excited Port 2

22 23 24 25 26 2720

30

40

50

60

70

80

90

100

Ant

enna

effici

ency

()

Frequency (GHz)

Figure 12 Simulated radiation efficiency of proposed antenna

good omnidirectional radiation patterns in the 119910119911 and 119909119911planes (119867-plane) while the separate far field patterns areproduced in the 119909119910 plane (119864-plane) Therefore the diversity

(a)

(b)

Figure 13 Fabrication of the single metamaterial antenna initialand final MIMO antenna (a) front view and (b) back view

for the antenna is achieved Thanks to this characteristic theantenna is a promising candidate for MIMO system

Figure 12 gives the simulated radiation efficiency of theproposed antenna This figure indicates that the proposedantenna shows good radiation efficiency which has theaverage value of 85 in over the operating bandwidth ofWLAN system

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

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Control Scienceand Engineering

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International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

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Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

4 International Journal of Antennas and Propagation

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

235mm255mm

275mm295mm

(a)

22 23 24 25 26 27minus40

minus30

minus20

minus10

0

Frequency (GHz)

S 11

(dB)

020mm025mm

030mm035mm

(b)

Figure 3 Simulated 11987811of single metamaterial antenna for different values of (a) 119897

3and (b) 119904

2

1 2 3 4 5 6 7 8Frequency (GHz)

Unloaded antennaProposed antenna

Proposedantenna

Unloadedantenna

minus40

minus30

minus20

minus10

0

S 11

(dB)

Figure 4 Simulated 11987811of unloaded and proposed antennas

Current distributions of the metamaterial antenna at thecenter frequency of WLAN are exhibited in Figure 5 Asobserved in Figure 5 the current distribution on antenna at244GHz mainly focuses on the meandered strips instead ofon the radiating patch as the principle of microstrip antenna

The radiation pattern of single antenna at the centerfrequency of 244GHz is plotted in Figure 6 The solid linesdisplay the 119864-plane and the dotted lines represent119867-plane Itcan be observed that the single antenna possesses an isotropicradiation pattern confirming its operation in the fundamental

Jsurf (Am)

10000e + 002

92911e + 001

85822e + 001

78732e + 001

71643e + 001

64554e + 001

57465e + 001

50376e + 001

43286e + 001

36197e + 001

29108e + 001

22019e + 001

14930e + 001

78405e + 000

75131e minus 001

Figure 5 Surface current distribution on single antenna at244GHz

resonantmodeTherefore its gain is small with themaximumtotal gain of 14 dB

Finally the fabricated single metamaterial antenna ispresented in Figure 13The simulated andmeasured results of11987811of singlemetamaterial antenna is shown in Figure 7 From

this figure it can be observed that the antenna can operateover the range spreading from 24GHz to 248GHz and from2405GHz to 2495GHz in simulation and measurementrespectively

32 Metamaterial MIMO Antenna The simulated results ofreflection coefficients of the initial MIMO antenna (withoutDGS) are shown in Figure 8 From this figure it is observedthat the initial antenna does not satisfy the impedance

International Journal of Antennas and Propagation 5

minus20

minus10

0

030

60

90

120

150180

210

240

270

300

330

minus20

minus10

0

E-planeH-plane

Figure 6 Simulated radiation pattern of single metamaterial ante-nna at center frequency of 244GHz

22 23 24 25 26 27minus35

minus30

minus25

minus20

minus15

minus10

minus5

0

Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 7 Simulated and measured results of single metamaterialantenna

matching condition due to the effect of mutual coupling The119878-parameters of antenna are changed and could not meetthe requirements of MIMO antenna from which 119878

11and 11987822

are not below minus10 dB and 11987821

and 11987812

are not below minus15 dBin WLAN band This fact is clearly demonstrated by thesurface current distribution on the initial MIMO antenna inFigure 10(a) As can be observed from Figure 10(a) when thefirst element (Port 1) is excited the surface current is stronglyinduced on the second element (Port 2) resulting in a riseof the mutual coupling (119878

21and 119878

12) Actually the mutual

coupling can be reduced by increasing the distance between

22 23 24 25 26 27minus50

minus40

minus30

minus20

minus10

0

With full DGSWith dual slotsWithout DGS

Frequency (GHz)

S-pa

ram

eter

s (dB

)

S11

S21

Figure 8 Simulated 119878-parameters in three cases without DGS withdual slots and with full DGS

the elements However this will lead to the larger size ofthe proposed MIMO antenna These drawbacks of the initialMIMO antenna can be solved thanks to the use of defectedground structure etched on the common ground of MIMOantenna by the following two steps

At first two parallel slots are added to central groundplane between two ports (as shown in Figure 2(b)) Thelength slot 119871

3is varied to find out the value from which

the impedance matching and mutual coupling have the bestsolution The optimized length of 119871

3is chosen as 12mm

Simulated 119878-parameters ofMIMO antennawith dual slots areshown in Figure 8 From this figure it can be seen that theisolation coefficient 119878

21is lower than minus18 dB for all frequency

in WLAN band However the impedance is not matchedenough in this band so that the 119878

11is below minus10 dB over the

frequency ranging from 242GHz to 25 GHz and thereforethe antenna could not cover the WLAN frequencies

The current distribution of MIMO antenna with theimplementation of dual slots at 244GHz is shown in Fig-ure 10(b) It can be seen from Figure 10(b) that the surfacecurrent partly focuses on the slots and somewhat coupling tothe radiation strips of the adjacent antenna element

In order to solve this problem in the second step two I-shaped slots are etched on the ground plane and used as animpedance matching circuit The effect of I-shaped slots toimpedance matching of the MIMO antenna is investigatedvia the length 119871 Figure 9 shows the simulated 119878-parametersof the MIMO antenna for the different values of 119871 It can beobserved that the isolation coefficient is below minus15 dB for allcases and the return loss is changed with the various sizesof slots When the length 119871 of I-slots increases the inputimpedance decreases make the impedance highly matchedThe final MIMO antenna with full DGS is formed as thevalue of 119871 fixed at 5mm As a result the full DGS MIMO

6 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

1mm

5mm3mm

Figure 9 Simulated 119878-parameters of full DGS MIMO antenna for different values of 119871

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(a)

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(b)

Jsurf (Am)

50000e + 00146429e + 00142857e + 00139286e + 00135714e + 00132143e + 00128571e + 00125000e + 00121429e + 00117857e + 00114286e + 00110714e + 00171429e + 00035714e + 00000000e + 000

(c)

Figure 10 Surface current distribution at 244GHz on metamaterial MIMO antenna (a) without DGS and (b) with the implementation ofdual slots and (c) with full DGS

antenna achieves high isolation coefficient which is lessthan minus35 dB over all frequency of WLAN band while theoperating bandwidth covers from 238GHz to 252GHzThe current distribution of the final MIMO antenna at244GHz is focused on the defected ground structure shown

in Figure 10(c) Therefore the effect of the surface current tothe second element is significant reduced

Simulated radiation patterns of final MIMO antenna inthe119909119910119910119911 and119909119911planes at 244GHzwhen the antenna is fedeach port in turn is shown in Figure 11 The antenna displays

International Journal of Antennas and Propagation 7

minus20

00

30

60

90

120

150180

210

240

270

300

330

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

xy plane yz plane xz plane

(a)

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

Gain Φ

Gain 120579

Gain Φ

Gain 120579

Gain Φ

Gain 120579

xy plane yz plane xz plane

(b)

Figure 11 Radiation patterns of proposed metamaterial MIMO antenna at central frequency of 244GHz when (a) excited Port 1 and (b)excited Port 2

22 23 24 25 26 2720

30

40

50

60

70

80

90

100

Ant

enna

effici

ency

()

Frequency (GHz)

Figure 12 Simulated radiation efficiency of proposed antenna

good omnidirectional radiation patterns in the 119910119911 and 119909119911planes (119867-plane) while the separate far field patterns areproduced in the 119909119910 plane (119864-plane) Therefore the diversity

(a)

(b)

Figure 13 Fabrication of the single metamaterial antenna initialand final MIMO antenna (a) front view and (b) back view

for the antenna is achieved Thanks to this characteristic theantenna is a promising candidate for MIMO system

Figure 12 gives the simulated radiation efficiency of theproposed antenna This figure indicates that the proposedantenna shows good radiation efficiency which has theaverage value of 85 in over the operating bandwidth ofWLAN system

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 5

minus20

minus10

0

030

60

90

120

150180

210

240

270

300

330

minus20

minus10

0

E-planeH-plane

Figure 6 Simulated radiation pattern of single metamaterial ante-nna at center frequency of 244GHz

22 23 24 25 26 27minus35

minus30

minus25

minus20

minus15

minus10

minus5

0

Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 7 Simulated and measured results of single metamaterialantenna

matching condition due to the effect of mutual coupling The119878-parameters of antenna are changed and could not meetthe requirements of MIMO antenna from which 119878

11and 11987822

are not below minus10 dB and 11987821

and 11987812

are not below minus15 dBin WLAN band This fact is clearly demonstrated by thesurface current distribution on the initial MIMO antenna inFigure 10(a) As can be observed from Figure 10(a) when thefirst element (Port 1) is excited the surface current is stronglyinduced on the second element (Port 2) resulting in a riseof the mutual coupling (119878

21and 119878

12) Actually the mutual

coupling can be reduced by increasing the distance between

22 23 24 25 26 27minus50

minus40

minus30

minus20

minus10

0

With full DGSWith dual slotsWithout DGS

Frequency (GHz)

S-pa

ram

eter

s (dB

)

S11

S21

Figure 8 Simulated 119878-parameters in three cases without DGS withdual slots and with full DGS

the elements However this will lead to the larger size ofthe proposed MIMO antenna These drawbacks of the initialMIMO antenna can be solved thanks to the use of defectedground structure etched on the common ground of MIMOantenna by the following two steps

At first two parallel slots are added to central groundplane between two ports (as shown in Figure 2(b)) Thelength slot 119871

3is varied to find out the value from which

the impedance matching and mutual coupling have the bestsolution The optimized length of 119871

3is chosen as 12mm

Simulated 119878-parameters ofMIMO antennawith dual slots areshown in Figure 8 From this figure it can be seen that theisolation coefficient 119878

21is lower than minus18 dB for all frequency

in WLAN band However the impedance is not matchedenough in this band so that the 119878

11is below minus10 dB over the

frequency ranging from 242GHz to 25 GHz and thereforethe antenna could not cover the WLAN frequencies

The current distribution of MIMO antenna with theimplementation of dual slots at 244GHz is shown in Fig-ure 10(b) It can be seen from Figure 10(b) that the surfacecurrent partly focuses on the slots and somewhat coupling tothe radiation strips of the adjacent antenna element

In order to solve this problem in the second step two I-shaped slots are etched on the ground plane and used as animpedance matching circuit The effect of I-shaped slots toimpedance matching of the MIMO antenna is investigatedvia the length 119871 Figure 9 shows the simulated 119878-parametersof the MIMO antenna for the different values of 119871 It can beobserved that the isolation coefficient is below minus15 dB for allcases and the return loss is changed with the various sizesof slots When the length 119871 of I-slots increases the inputimpedance decreases make the impedance highly matchedThe final MIMO antenna with full DGS is formed as thevalue of 119871 fixed at 5mm As a result the full DGS MIMO

6 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

1mm

5mm3mm

Figure 9 Simulated 119878-parameters of full DGS MIMO antenna for different values of 119871

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(a)

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(b)

Jsurf (Am)

50000e + 00146429e + 00142857e + 00139286e + 00135714e + 00132143e + 00128571e + 00125000e + 00121429e + 00117857e + 00114286e + 00110714e + 00171429e + 00035714e + 00000000e + 000

(c)

Figure 10 Surface current distribution at 244GHz on metamaterial MIMO antenna (a) without DGS and (b) with the implementation ofdual slots and (c) with full DGS

antenna achieves high isolation coefficient which is lessthan minus35 dB over all frequency of WLAN band while theoperating bandwidth covers from 238GHz to 252GHzThe current distribution of the final MIMO antenna at244GHz is focused on the defected ground structure shown

in Figure 10(c) Therefore the effect of the surface current tothe second element is significant reduced

Simulated radiation patterns of final MIMO antenna inthe119909119910119910119911 and119909119911planes at 244GHzwhen the antenna is fedeach port in turn is shown in Figure 11 The antenna displays

International Journal of Antennas and Propagation 7

minus20

00

30

60

90

120

150180

210

240

270

300

330

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

xy plane yz plane xz plane

(a)

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

Gain Φ

Gain 120579

Gain Φ

Gain 120579

Gain Φ

Gain 120579

xy plane yz plane xz plane

(b)

Figure 11 Radiation patterns of proposed metamaterial MIMO antenna at central frequency of 244GHz when (a) excited Port 1 and (b)excited Port 2

22 23 24 25 26 2720

30

40

50

60

70

80

90

100

Ant

enna

effici

ency

()

Frequency (GHz)

Figure 12 Simulated radiation efficiency of proposed antenna

good omnidirectional radiation patterns in the 119910119911 and 119909119911planes (119867-plane) while the separate far field patterns areproduced in the 119909119910 plane (119864-plane) Therefore the diversity

(a)

(b)

Figure 13 Fabrication of the single metamaterial antenna initialand final MIMO antenna (a) front view and (b) back view

for the antenna is achieved Thanks to this characteristic theantenna is a promising candidate for MIMO system

Figure 12 gives the simulated radiation efficiency of theproposed antenna This figure indicates that the proposedantenna shows good radiation efficiency which has theaverage value of 85 in over the operating bandwidth ofWLAN system

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

6 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

1mm

5mm3mm

Figure 9 Simulated 119878-parameters of full DGS MIMO antenna for different values of 119871

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(a)

Jsurf (Am)

50000e + 00146439e + 00142879e + 00139318e + 00135758e + 00132197e + 00128637e + 00125076e + 00121515e + 00117955e + 00114394e + 00110834e + 00172730e + 00037124e + 00015185e minus 001

(b)

Jsurf (Am)

50000e + 00146429e + 00142857e + 00139286e + 00135714e + 00132143e + 00128571e + 00125000e + 00121429e + 00117857e + 00114286e + 00110714e + 00171429e + 00035714e + 00000000e + 000

(c)

Figure 10 Surface current distribution at 244GHz on metamaterial MIMO antenna (a) without DGS and (b) with the implementation ofdual slots and (c) with full DGS

antenna achieves high isolation coefficient which is lessthan minus35 dB over all frequency of WLAN band while theoperating bandwidth covers from 238GHz to 252GHzThe current distribution of the final MIMO antenna at244GHz is focused on the defected ground structure shown

in Figure 10(c) Therefore the effect of the surface current tothe second element is significant reduced

Simulated radiation patterns of final MIMO antenna inthe119909119910119910119911 and119909119911planes at 244GHzwhen the antenna is fedeach port in turn is shown in Figure 11 The antenna displays

International Journal of Antennas and Propagation 7

minus20

00

30

60

90

120

150180

210

240

270

300

330

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

xy plane yz plane xz plane

(a)

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

Gain Φ

Gain 120579

Gain Φ

Gain 120579

Gain Φ

Gain 120579

xy plane yz plane xz plane

(b)

Figure 11 Radiation patterns of proposed metamaterial MIMO antenna at central frequency of 244GHz when (a) excited Port 1 and (b)excited Port 2

22 23 24 25 26 2720

30

40

50

60

70

80

90

100

Ant

enna

effici

ency

()

Frequency (GHz)

Figure 12 Simulated radiation efficiency of proposed antenna

good omnidirectional radiation patterns in the 119910119911 and 119909119911planes (119867-plane) while the separate far field patterns areproduced in the 119909119910 plane (119864-plane) Therefore the diversity

(a)

(b)

Figure 13 Fabrication of the single metamaterial antenna initialand final MIMO antenna (a) front view and (b) back view

for the antenna is achieved Thanks to this characteristic theantenna is a promising candidate for MIMO system

Figure 12 gives the simulated radiation efficiency of theproposed antenna This figure indicates that the proposedantenna shows good radiation efficiency which has theaverage value of 85 in over the operating bandwidth ofWLAN system

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 7

minus20

00

30

60

90

120

150180

210

240

270

300

330

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

xy plane yz plane xz plane

(a)

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

minus20

0

minus40

minus20

0

030

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

330

Gain Φ

Gain 120579

Gain Φ

Gain 120579

Gain Φ

Gain 120579

xy plane yz plane xz plane

(b)

Figure 11 Radiation patterns of proposed metamaterial MIMO antenna at central frequency of 244GHz when (a) excited Port 1 and (b)excited Port 2

22 23 24 25 26 2720

30

40

50

60

70

80

90

100

Ant

enna

effici

ency

()

Frequency (GHz)

Figure 12 Simulated radiation efficiency of proposed antenna

good omnidirectional radiation patterns in the 119910119911 and 119909119911planes (119867-plane) while the separate far field patterns areproduced in the 119909119910 plane (119864-plane) Therefore the diversity

(a)

(b)

Figure 13 Fabrication of the single metamaterial antenna initialand final MIMO antenna (a) front view and (b) back view

for the antenna is achieved Thanks to this characteristic theantenna is a promising candidate for MIMO system

Figure 12 gives the simulated radiation efficiency of theproposed antenna This figure indicates that the proposedantenna shows good radiation efficiency which has theaverage value of 85 in over the operating bandwidth ofWLAN system

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

8 International Journal of Antennas and Propagation

22 23 24 25 26 27Frequency (GHz)

SimulatedMeasured

minus40

minus30

minus20

minus10

0S-

para

met

ers (

dB)

S11

S21

(a)

SimulatedMeasured

22 23 24 25 26 27Frequency (GHz)

minus50

minus40

minus30

minus20

minus10

0

S-pa

ram

eter

s (dB

)

S11

S21

(b)

Figure 14 Simulated and measured 119878-parameters of (a) initial MIMO antenna and (b) final MIMO antenna

Figure 14 presents the measured 11987811

and 11987821

of thefabricated initial and finalMIMOantenna shown in Figure 13From this figure it is observed that the final MIMO antennacan operate over the range spreading from 238 to 25 GHzwhich is covering the WLAN band Meanwhile the mutualcoupling between two elements (119878

21) is less than minus35 dB over

WLAN range It should be noted that the measured resultsare in good agreement with the simulated results

33 MIMO Characteristics MIMO antennas are required tobe characterized for their diversity performance In eachsystem the signals can be usually correlated by the distancebetween the antenna elements [16] The parameter used toassess the correlation between radiation patterns is so-calledenveloped correlation coefficient (ECC) Normally the valueof ECC at a certain frequency is small in case of the radiationpattern of each single antenna differently from each otherOtherwise the same patterns of these antennas will exhibitthe larger value of enveloped correlation coefficient Thefactor can be calculated from radiation patterns or scatter-ing parameters For a simple two-port network assuminguniform multipath environment the enveloped correlation(120588119890) simply square of the correlation coefficient (120588) can be

calculated conveniently and quickly from 119878-parameters [17]as follows

120588119890=

1003816100381610038161003816119878lowast

1111987812+ 119878lowast

2111987822

1003816100381610038161003816

2

(1 minus1003816100381610038161003816119878111003816100381610038161003816

2minus1003816100381610038161003816119878211003816100381610038161003816

2) (1 minus1003816100381610038161003816119878221003816100381610038161003816

2minus1003816100381610038161003816119878121003816100381610038161003816

2)

(2)

The calculated ECC of proposed antenna by using thesimulated and measured 119878-parameters is shown in Figure 15From this figure the proposed MIMO antenna has thesimulated ECC lower than 001 while the measured one haslower than 002 over the operating frequenciesTherefore the

22 23 24 25 26 27000

001

002

003

004

ECC

Frequency (GHz)

SimulatedMeasured

Figure 15 Simulated and measured proposed MIMO antennarsquosenvelope correlation coefficient

proposed antenna is suitable for mobile communication witha minimum acceptable correlation coefficient of 05 [18]

4 Conclusions

The compact 2 times 2 metamaterial MIMO antenna is designedto operate in WLAN frequency band By using the mod-ified CRLH model the proposed metamaterial antennaachieves 60 size reduction in comparisonwith the unloadedantenna The defected ground structures are inserted to

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 9

suppress the effect of surface current on the elements ofthe proposed antenna for reducing the mutual coupling Theantenna offers the compact size with the diversity radiationpatterns The fabricated MIMO antenna shows isolation lessthan minus35 dB over its operating frequency band spreadingfrom238 to 25GHzTheproposedMIMOantenna has also aminimum correlation coefficient which is less than 002 overtheWLAN frequency range Summing up the result it can beconcluded that the proposed antenna is a good candidate forWLAN applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] J P Gianvittorio and Y Rahmat-Samii ldquoFractal antennas anovel antenna miniaturization technique and applicationsrdquoIEEEAntennas and PropagationMagazine vol 44 no 1 pp 20ndash36 2002

[2] G Monti L Catarinucci and L Tarricone ldquoCompact micro-strip antenna for RFID applicationsrdquo Progress in Electromagnet-ics Research Letters vol 8 pp 191ndash199 2009

[3] W J Krzysztofik ldquoModified Sierpinski fractal monopole forISM-bands handset applicationsrdquo IEEE Transactions on Anten-nas and Propagation vol 57 no 3 pp 606ndash615 2009

[4] A Mehdipour I D Rosca A-R Sebak C W Truemanand S V Hoa ldquoFull-composite fractal antenna using carbonnanotubes for multiband wireless applicationsrdquo IEEE Antennasand Wireless Propagation Letters vol 9 pp 891ndash894 2010

[5] M N Jahromi A Falahati and R M Edwards ldquoBandwidthand impedance-matching enhancement of fractal monopoleantennas using compact grounded coplanar waveguiderdquo IEEETransactions on Antennas and Propagation vol 59 no 7 pp2480ndash2487 2011

[6] A Arazi ldquoUltra wideband fractal microstrip antenna designrdquoProgress in Electromagnetic Research C vol 2 pp 7ndash12 2008

[7] GV Eleftheriades A K Iyer andP C Kremer ldquoPlanar negativerefractive index media using periodically L-C loaded trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 50 no 12 pp 2702ndash2712 2002

[8] G V Eleftheriades ldquoEnabling RFmicrowave devices usingnegative-refractive-index transmission-line (NRI-TL)metama-terialsrdquo IEEE Antennas and Propagation Magazine vol 49 no2 pp 34ndash51 2007

[9] G V Eleftheriades A Grbic and M Antoniades ldquoNegative-refractive-index transmission-line metamaterials and enablingelectromagnetic applicationsrdquo in Proceedings of the IEEE Anten-nas and Propagation Society Symposium vol 2 pp 1399ndash1402IEEE June 2004

[10] A Sanada M Kimura I Awai C Caloz and T Itoh ldquoA planarzeroth-order resonator antenna using a left-handed transmis-sion linerdquo in Proceedings of the 34th European MicrowaveConference pp 1341ndash1344 October 2004

[11] M A Antoniades and G V Eleftheriades ldquoA folded-monopolemodel for electrically small NRI-TL metamaterial antennasrdquoIEEE Antennas andWireless Propagation Letters vol 7 pp 425ndash428 2008

[12] A Lai K M K H Leong and T Itoh ldquoInfinite wavelengthresonant antennas with monopolar radiation pattern basedon periodic structuresrdquo IEEE Transactions on Antennas andPropagation vol 55 no 3 pp 868ndash876 2007

[13] J-G Lee and J-H Lee ldquoZeroth order resonance loop antennardquoIEEE Transactions on Antennas and Propagation vol 55 no 3pp 994ndash997 2007

[14] J Zhu and G V Eleftheriades ldquoDual-band metamaterial-inspired small monopole antenna for WiFi applicationsrdquo Elec-tronics Letters vol 45 no 22 pp 1104ndash1106 2009

[15] J Zhu and G V Eleftheriades ldquoA compact transmission-linemetamaterial antenna with extended bandwidthrdquo IEEE Ante-nnas andWireless Propagation Letters vol 8 pp 295ndash298 2009

[16] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress in Electromagnetics Research Cvol 19 pp 245ndash257 2011

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (119873119873)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of