Compact Tapered Fed Dual-Band Monopole Antenna for WLAN and WiMAX Application

Abstract— In this paper a compact tapered fed dual band monopole antenna based on complimentary split ring resonators(CSRR) used for WLAN and WiMAX application. Two CSRR are placed side by side symmetrically, which produces a narrow stop band in a single band for which dual band observed. As over all bandwidth reduces the gain is improved. The composite metamaterial CSRR can provide dual band operation at 1.9-2.6 and 2.92-4.3 GHz with reflection coefficient less than -10 dB by the two resonant modes due to CSRR and length of the patch. The gain at lower resonant band -2-0.2 dB and at higher resonant band 0.5-2 dB. A 50 Ω microstrip line, followed by parabolic feed and rectangle step feed is adopted for impedance matching. The uniqueness of this design is that the CSRR generates a narrow stop band in a wideband produced by an element without having any CSRR, which makes the dual band antenna very compact. Antenna parameters like reflection coefficient, radiation pattern, gain are analyzed with numerical simulation.

Index Terms— Dual band antenna, parabolic tapered ,metamaterial, CSRR, SRR, HFSS.

I. INTRODUCTION

The rapid growth of WLAN and WiMAX application forced researchers to design dual band antennas such that WLAN & WiMAX band do not interfere with each other. WLAN most popular band is 2.4 GHz ISM band(2.4-2.48 GHz) and WiMAX band 3.5(3.3-3.8) GHz. The recent progress in the design of dual band antennas has attracted comprehensive research interest in wireless communication, radio frequency identification, microwave energy harvesting wireless sensor network as well as MIMO(Multiple Input and Multiple Output) system because it can reduce the numbers of antennas. Compared to conventional antennas the microstrip antennas [1-4] are efficient, low profile & electrically small to be integrated in modern wireless terminal. They are very promising in dual band operation. One of the best way to realize dual band antennas is combining two resonant elements to derive two different radiation modes. For example, dual band printed dipole antennas were designed using resonators with different arm length [5],[6].By etching slots like L-shaped, or U-shaped on the surface of a planar antenna or shortening pins, the planar antenna can be operated in two frequency bands[7]-[11] As microstrip antenna corresponds to narrow band width, it is necessary to cut either the ground structure or the antenna patch or both to achieve an electrically small wide band or multiband antenna. CSRR is the dual of the Split Ring Resonator(SRR) [12] first proposed by J.B. Pendry[13].They are the kind of artificial material that

have novel properties. They do not depend upon crystal, lattice, or composition of the material. They only depends upon size, shape and structure leading to many exciting capabilities for electromagnetic application[14],[15]. The most attractive feature of these elements is their ability to exhibit quasi static resonant frequency at wave length that are much larger than their own size. The application of CSRR received much attraction in the design of small antennas in recent years[16].A CSRR is excited by means of a dynamic electric field with a non negligible components in the axial direction these particles make the artificial line where they are inserted to behave as a negative permittivity medium[17,18].The negative value permittivity in the material medium forced antenna to produce stop-band. As bandwidth gain product is constant due to stop band we get two different band with low return loss. The proposed antenna fed by 50Ω microstrip line followed by parabolic tapered feed and rectangular step feed to get impedance bandwidth. The parabolic tapered feed corresponds to the bandwidth of WiMAX band. Rectangle step feed corresponds to the bandwidth of WLAN band. The proposed antenna has ominidirectional radiation pattern in H-plane which is suitable for ISM and WiMAX application. The proposed antenna has good impedance matching at centre frequencies .The fundamental characteristics of the proposed design simulated return loss, gain, radiation pattern extensively discussed here. In this paper authors designed two antennas. The author proposed how to design antenna 1 and from antenna 1which provides single band how to design antenna 2 which corresponds to two bands. The simulation result is carried out with the Finite Element Method (FEM) based Ansoft High Frequency Structure Simulator (HFSS) software.

(a) (b)

Shashanka Sekhar Behera1, Ambika Singh2, Dr.Sudhakar Sahu3, Pravanjana Behera4 M.Tech1,2,4 , Associate Professor3

School of Electronics Engineering, KIIT University shashanka1988@gmail.com1,ambikasingh1991@gmail.com2, pravanjanabehera@gmail.com4

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(c) (d)

(e)

Fig 1.Geometryof proposed antennas a. Antenna 1 (Top view) b. Ground (Bottom view) c. Geometry of parabolic feed d. Geometry of parabolic and step feed e. Antenna 2 (top view)

Parameters Antenna 1,(mm) Antenna 2, (mm)

W 44.6 44.6

L 35.6 35.6

l1 22.1 22.1

l3 14 14

w1 26 26

w5 7 7

w3 9.4 9.4

a 1.89 1.89

b 1.18 1.18

w6 3 3

h 1.6 1.6

l4 21.5 21.5

w8 0.8227 0.8227

r 3.2 3.2

p1 1.58097 1.58097

R 2.23 2.23

l5 22.555 22.555

l6 1.35 1.35

l2 - 10

w4 - 6.5

w2 - 7

II. ANTENNA DESIGN

The geometrical structure and the dimension of the proposed printed monopole antennas depicted in fig 1 and in table 1 and 2.The substrate is FR4 epoxy with dielectric constant 4.4 and loss tangent .02,The frequency of operation 3.2 GHz. The antenna dimensions are calculated from following equation:-

Antenna 1 dimensions: from [19]

w =

W=w1+6h

=

Ɛ

In fig 1a shows the top view of the proposed antenna 1.in monopole antenna ground length and ground shape has significant effect on resonant frequency. Fig 1b shows the bottom view of the proposed antenna 2. patch width w1 calculated as 28.53mm.. From equation(3) l1 is calculated as 20.43mm.An antenna has three region of operation called feed line, feed region and patch. The feed region consists of parabolic tapered feed followed by a rectangle step of 1.35x7 mm2 .parabolic tapered feed is assigned by taking a lower half of a circle of radius 2.7mm from the end of the feed line followed by a rectangle step of 1.35x7 mm2. The main purpose to inset feed region to smooth the current's path thus providing wider impedance bandwidth. The fig 1(c) indicates the exponential tapered profile of tapered feed region which is defined by the R(radius of parabolic feed) region and the two points p and q. Fig 1(d) shows a parabolic tapered feed followed by a rectangle step feed. All dimensions of antenna 1 is given in table 1a.Due to insertion of feed region the patch width and length reduced and assumed to be 26mm and 14 mm respectively.

The R value is optimized by a series of parametric study. From where a and b value is calculated.

The impedance function for a parabolic tapered feed is given by[20]

Z(z)=f(x)

Z(z)=Z0

Where Z0 the characteristic impedance of the microstrip

feed line. k is any constant can be found out from equation

a=k

As patch radiate EM(Electro Magnetic Wave) from the

point of discontinuity, so patch radiate from the feed region. From where we can understand the main patch length further reduced due to insertion of feed region. Also the Ground tailored also affect to the bandwidth and the resonant frequency .So the modified resonant frequency is approximated as

(1)

(2)

(3)

(4)

(5)

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=

2(( + + + + ( − ))Ɛ

In antenna 1 design the above equation is proved by compared with simulation result.

Antenna 2 dimensions: Two CSRR is placed side by side above the patch in

antenna2 in fig 1e. Insertion of CSRR result high return loss in a specific band so it provides a stop band region .which results a dual band in the single band of antenna 1.All dimensions of antenna 2 is given in table 1b.The ground dimension of antenna 2 is same as antenna1.

III. SIMULATION AND MEASUREMENT RESULTS

The desired frequency band to cover WLAN 4.2 and WiMAX 3.5 GHz band. Antenna 1 covers both WLAN and WiMAX band 2.214-4.0817 GHz with bandwidth 1.8737 with good return loss. A series of parametric study result is compared with the theoretical resonant frequency in below table 3a. In table 3a we kept R value constant. Then effect of ground length l4 is studied.

Table 3a. Parametric study of l4.

S.No l4

(mm) simulated fr (GHz)

Band- Width (GHz)

fr

(GHz) (Theo.)

%error

1 21.5 3.1545 1.8737 3.17524 0.65 2 22 3.2686 1.7560 3.2473 0.66 3 21.7 3.1656 1.8197 3.20369 1.19 4 22.2 3.2489 1.7658 3.2771 0.86

From the above table when l4 is 21.5mm the antenna has

better bandwidth with low return loss. So the ground is selected with 26mmx21.5mm2.The return loss curve for above parametric study is given in fig 3a .

0 1 2 3 4 5-30

-25

-20

-15

-10

-5

0

s11 (

dB

)

f requency (G H z)

l4=22.2 m m

l4=22 m m

l4=21.7 m m

l4=21.5 m m

Fig 3a. Return loss of the proposed antenna 1a.

From the above return loss curve we observed l4=21.5 mm is having good return loss properties with improved impedance bandwidth. The parametric study of the ground notch having radius r is given in the below table 3b.

Table 3b. parametric study of r. S. No

r mm

simulated fr ( GHz)

BW (GHz)

fr (GHz) (Theo)

%error

1 3.2 3.1545 1.8969 3.17524 0.65 2 3.4 3.1410 1.898 3.13763 0.11 3 3 3.1803 1.873 3.1936 0.42

From the table 3b. Although r=3.2mm and r=3mm have

equal bandwidth and r=3mm has low return loss but when transition from antenna 1 to antenna 2 r=3.2 mm have good bandwidth and low return loss that is compared latter on this paper. The return loss and frequency curve is given below.

0 1 2 3 4 5

-30

-25

-20

-15

-10

-5

0

s11

(d

B)

F re q u e n c y(G H z )

r= 3 .4 m m r= 3 .2 m m r= 3 m m

Fig. 3b.Return loss of the proposed antenna 1 with ground notch parametric study.

The parametric study of p1 and R is given in the below table 3c. Table 3c. parametric study of p1 and R. S.No p1

( mm) R (mm)

Simulated fr (GHz)

bandwidth (GHz)

fr GHz (Theo)

%error

1 1.58097

2.23 3.1545 1.8967 3.175 0.65

2 1.49 2.32 3.1214 1.89 3.175 1.6 3 1.770

96 2.04 3.1361 1.92 3.174 1.21

From the above table we can conclude the p1 value and R

value at 1.58097 and R value 2.23 respectively is having low return loss and we observed the calculated resonating frequency and simulated resonating frequency leads error percentage maximum up to 1.5 percentage. The return loss curve with respect to frequency is shown in fig 3c.

From the figure we observe all different values of p1 and R are having almost same bandwidth and return loss .So all the curves are overlap with each other.

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0 1 2 3 4 5-30

-25

-20

-15

-10

-5

0

S1

1 (

dB

)

fre q u e n cy (G H z)

p1=1 .7 709 6m m R = 2 .0 44 6

p1=1 .4 9m m R =2 .3 22

p1=1 .5 809 7m m R = 2 .23 m m

Fig 3c. Return loss of the proposed antenna with parabolic tapered feed and step function feed.

Antenna 2 followed by antenna 1 where a resonating structure CSRR inserted in main patch side by side in order to get a stop band in wide band of antenna 1.AS gain and bandwidth product is constant. Due to decrease in bandwidth the gain of the antenna increases. And due to a narrow stop band two different resonating mode we can observe in two different bands. The first resonating frequency due to the CSRR and the second resonating frequency due to the radiating patch and feed region. A parametric study has done regarding the distance w2 between two CSRR and shown in table 3d.

Table 3d.Parametric study of w2

S. No

w2 (mm) fr (GHz)

fr1 (GHz)

1 7 2.46 3.4 2 4 2.3 3.3 3 3 2.26 3.2823 4 2 2.2 3.2

1 2 3 4 5 6

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

S1

1 (

dB

)

frequency (GHz)

w2 2m m

w2 3m m

w2 4m m

w2 7m m

Fig 3d.Return loss of the proposed antenna 2 with parametric study of w2.

From the above table and plot we realized decreasing the width w2 the resonant frequency decreases. But the proposed antenna application to design a dual band which covers entire 2.4 ISM band and 3.5 WiMAX band. So finally W2 value is optimized as 7mm.

Now finally the reflection coefficient and frequency curve of two antennas is shown in fig 3e.

1 2 3 4 5 6

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

S11(d

B)

F re q u e n cy(G H z)

a n te n n a 1 a n te n n a 2

Fig 3e. Return loss of the proposed antenna 1 and antenna 2.

From the above figure we realized that insertion of CSRR generates a stop band in the single wideband of antenna 1, for which antenna 2 generates dual band with good return loss but over all band width reduces as bandwidth gain product remains constant. For antenna 1 resonating frequency 3.1545, bandwidth 1.8969 GHz corresponds to band 2.2-4.1 GHz. For antenna 2 a dual band observed ,with first resonance at 2.46 GHz corresponds to band 1.98-2.6 GHz with band width 0.62 GHz and second resonance at 3.4 GHz corresponds to band 2.92-4.3 GHz with band width 1.38 GHz. The lower resonant frequency is due to CSRR and length of the patch corresponds to the higher resonance.

1 2 3 4 5

-300

-200

-100

0

100

impend

ence

(ohm

)

Frequency(GHz)

resistance reactance

Fig 4.simulated resistance and reactance results.

Fig 4 shows the simulated input impedance(Zin) graph against frequency. The real part input impedance of antenna at the frequency where the reactance is zero also shown in Fig 4 is equal to the antenna impedance at resonating frequency. The antenna is fed with a line 50 Ω characteristic impedance. When the microstrip line characteristic impedance and antenna impedance both 50 Ω the load impedance matched with the line impedance. It can be observed from the simulated result at resonating frequency 2.46 GHz and 4 GHz where low return loss the input impedance equal to 50 Ω. The

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simulated results are obtained using full wave electromagnetic field simulation software HFSS version 14[21].

1 2 3 4 5 6

-8

-6

-4

-2

0

2

4

6

Ga

in d

B

F requency(GHz)

antenna 2 gain

Fig 5. Simulated gain of antenna 2.

Fig 2 shows simulated gain of antenna 2.The gain at

lower resonant band -2-0.2 dB and at higher resonant band .5-2 dB .The gain at centre frequency -1 dB and 1 dB respectively. The gain increases with increase in frequency from 1 GHz to 5.4 GHz then gain decreases.

(a)

(b)

Fig 6. Current distribution

a. Magnitude of surface current density and surface current

density vector at 2.46 GHz

b. Magnitude of surface current density and vector of

surface current density vector at 3.4 GHz

From the current distribution the current is mainly radiating at the edge of the patch as the current guided towards the edge by tapering both at 2.46 GHz and 4 GHz. Radiation Pattern:

The radiation pattern in fig 7 of the antenna is shown for both E-plane and H-plane at 2.46GHz and 3.4GHz. These pattern shows antenna has nearly ominidirectional pattern in H-plane and figure of eight pattern in E-plane shows the

bidirectional pattern. Here in the radiation pattern we observed the cross polarization level increases at higher frequencies.

-60

-50

-40

-30

-20

-10

0

0

30

60

90

120

150

180

210

240

270

300

330

-60

-50

-40

-30

-20

-10

0

C o Pol C x Pol

(a)

-35

-30

-25

-20

-15

-10

-5

0

0

30

60

90

120

150

180

210

240

270

300

330

-35

-30

-25

-20

-15

-10

-5

0

(b)

- 7 0

- 6 0

- 5 0

- 4 0

- 3 0

- 2 0

- 1 0

0

0

3 0

6 0

9 0

1 2 0

1 5 0

1 8 0

2 1 0

2 4 0

2 7 0

3 0 0

3 3 0

- 7 0

- 6 0

- 5 0

- 4 0

- 3 0

- 2 0

- 1 0

0

(c)

-60

-50

-40

-30

-20

-10

0

0

30

60

90

120

150

180

210

240

270

300

330

-60

-50

-40

-30

-20

-10

0

(d)

Fig 7.Radiation pattern a. Simulated H-plane at 2.46 GHz. b. Simulated E-plane at 2.46 GHz. c. Simulated H-plane at 3.4 GHz. d. Simulated E-plane at 3.4 GHz.

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IV. CONCLUSION

A compact tapered feed dual band monopole antenna inspired by composite metamaterial CSRR has been simulated. The two frequency band of operation is originated from the composite material CSRR, which provides a stop-band to antenna 1 and provides dual band for WLAN and WIMAX application. Moreover a parabolic tapered feed followed by a rectangle step feed was used for impedance matching. It was observed that the proposed antenna was dual impedance bandwidth ranging from 1.92-2.6 GHz in the lower frequency band and 2.92-4.3 GHz at higher frequency band. In both the band the antenna generate a good ominidirectional monopole like radiation pattern. The proposed antenna generating narrow lower band and broad upper band which has wide application in modern wireless communication.

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