[ieee 2011 mediterranean microwave symposium (mms) - yasmine hammamet, tunisia...

New Methodology Of Conceiving Stacked Proximity Feed Antenna

Mehdi ALI* , Abdennacer KACHOURI** and Mounir SAMET* Laboratory LETI, *University of Sfax, National School of Engineers of Sfax, Route Soukra Km 3.5 B.P W 1173, TUNISIA

** University of Gabes, ISSIG Higher Institute Of Industrial Systems Gabes CP 6011 TUNISIA [email protected], [email protected], [email protected]

Abstract— Proximity coupled microstrip antenna is a noncoplanar feed structure using at least two layers. Coupling between the patch and the microstrip feeding line is capacitive in nature. The coupling capacitor is in series with the R-C-L resonant circuit representing the patch. The capacitance is designed for impedance matching. The radiating patch being placed on the double layer gives a larger bandwidth.

Iterative method is commonly used to determine the correct position and dimension of each element. Investigating a methodology to calculate the optimal dimension and position of both radiating patch and feeding line will be a useful tool to conceive proximity-coupled microstrip antennas.

In this paper, an easy methodology to calculate and to conceive a stacked proximity feed antenna was presented. The methodology is confirmed for a compact rectangular microstrip antenna used for wireless local area networks (WLAN) application.

The proposed method suggests fixing the feeding line length at (0.4. λg), the location of the feeding point at 5 Ohms antenna input impedance position and the width of the line is determine to provide the sufficient capacitance to acquire good matching impedance. This method tested for different frequencies led to an optimal antennas characteristics; a gain of 4.65 dB was attained for 2.45 GHz, 3.29 dB for 915MHz and 4.74 dB for 5GHz antenna.

Keywords-WLAN; proximity coupled; stacked patch; purity.

I. INTRODUCTION Many methods were investigated to overcome the narrow

bandwidth involved when the microstrip technology is used. One can use thick substrate Single-layer to enhance the band width, or multi-layer proximity coupled structure reaching higher bandwidth [1–5].

Proximity coupled antenna technique is associated with two main difficulties to solve; the first is that the input impedance of a proximity coupled microstrip antenna is a sensitive function of length and width of the microstrip feed line, the second is the discrepancy of the obtained resonant frequency due to etching and substrate parameters errors.

Proximity coupled microstrip antenna is composed of a radiating patch fabricated on a dielectric substrate and excited by a microstrip line on the same or another substrate. The

dielectric constant and height of the substrates used for microstrip patch and microstrip line may be different.

Iterative method is commonly used to determine the correct position and dimension of each element. Investigating a methodology to calculate the optimal dimension of both radiating patch and line feed will be a useful tool to conceive proximity-coupled microstrip antennas.

A combination of proximity-coupled microstrip antennas and stacked patch suggests a supplementary mean to enhance the bandwidth [6] and offer an accurate device to tune precisely the resonant frequency of the antenna to cover the entire range of frequency required.

In this paper, theoretical and simulated studies of a new type of proximity-coupled microstrip antenna are reported. A comparative study is carried out between three antennas proximity-coupled microstrip stacked patch antennas, thick substrate rectangular fed by an interdigital capacitor and a circular polarised interdigital capacitor fed. Results show that the proposed microstrip patch antennas have small size, wide bandwidth, and high gain suitable for WLAN application and excellent purity.

II. CONCEIVING ANTENNA FOR WLAN APPLICATION. The basic limitation of the microstrip antenna is its narrow

band width. Increasing band width has been studied and a special field of interest. Various techniques have been introduced to enhance the bandwidth of a microstrip antenna. As a solution to the bandwidth problem as discussed below, one can for instance use a lossy substrate involving the problem of inductive shift with increasing the substrate thickness. This problem was overcome by introducing capacitor in the feed circuit or by using multi layer associated to patch proximity coupled to a microstrip line. This method increases the overall height of the antenna but the size in the plane remains the same as the conventional patch antenna. Two different types of multi-layer structure antenna were used. One is the electromagnetically coupled microstrip antenna and the other is the aperture coupled microstrip antenna. The patches can be fabricated on different substrates and air gaps.

A. Planar rectangular antenna. The common method to obtain a wide band antenna is to

increase the substrate thickness. The proposed design which is

shown on Fig.1, is composed of a radiating patch printed on FR4 substrate 3.2 mm thick the loss tangent is taken 0.018.

The dimension of the radiating patch is determined using the following Equations:

W c2f 2є 1 1

L c2f є 2ΔL 2

L L 2ΔL (3)

ΔL 0.412 h є 0.3 W 0.264є 0.258 W 0.813 4

є є 12 є 12 1 12 5

(W=37.8mm, єeff = 4.676, ΔL= 1.32, L=25.66mm)

The antenna bandwidth should set up from 2.4 until 2.484GHz.

The antenna is electro-magnetically coupled with the feed line. The link is provided by an interdigital capacitor which is a planar configuration of a multi layer capacitor the dimension and the number of the fingers is determined using the Eq. 6 using elliptic functions.

є 1 є 1 tan1 tan 6

l, n, and w are respectively the length, the number and the width of the interdigital capacitor fingers, g is the gap between the fingers [7].

A capacitance of about 4.36 pF provided by the interdigital capacitor match the impedance at -50dB.

The impedance at the port is exactly 50Ω using a microstrip transformer, the wildness is fixed w = 6,24 mm determined using LineCal which is one tool of the simulator Advanced Design System (ADS).

Figure 1. Rectangular patch antenna 3.2mm thick FR4 substrate

The reflexion obtained is plotted in Fig.2 the band obtained is suitable for WLAN application.

Figure 2. Reflexion S11 of the thick rectangular antenna.

The TM10 mode is exited as demonstrated by the 3D simulation showing the current distribution Fig.3.

Figure 3. Current distribution TM10 mode

TABLE I. ANTENNA PERFORMANCE

Performances Values

Power radiated (watt) 0.217274

Effective Angle (degrees) 150.52

Directivity(dB) 6.797245

Gain(dB) 4.604970

Maximum Intensity (Watt/Steradian) 0.082703

The purity is plotted in Fig 4. the cross polarisation is about -25dB.

Figure 4. Cross polarisation

The Band width of the designed antenna is 100MHz from 2.4 until 2.5 GHz. A good gain, good impedance and acceptable cross polarisation are obtained.

2.3 2.4 2.5 2.62.2 2.7

-40

-30

-20

-10

-50

0

Frequency

Mag

. [dB

]

S11

-80

-60

-40

-20

0 20 40 60 80-100

100

-50

-40

-30

-20

-10

-60

0

THETA

Mag

. [dB

]

E_co E_cross

This work is supported by LETI and SNDP AGIL

B. Proximity coupled Microstrip antenna. A conventional proximity-coupled microstrip antenna is

made up of two layers, a radiating patch is etched on the top of the higher layer, the feeding line is printed on the top of the lower layer the bottom side is the ground plane.

1) Conventional proximity coupling antenna For proximity coupling antenna using multilayer structure,

the resonant frequency for TM10 mode is expressed by Equation 7. c2 L 2ΔL є 7

є 2є 1 A1 A 8

1 12hW 9

h12 : is the thickness between the patch and the ground plane.

The equivalent circuit of a proximity feed line antenna is shown in Fig.5.

Figure 5. Equivalent circuit of conventional proximity feed line antenna

Figure 6. Conventional proximity coupled antenna

Figure 7. Simulation result of a conventional proximity feed antenna

The band covered is from 2.4 to 2.476 GHz is not enough unless an air gap is introduced between the two layers to widen the band width.

2) Stacked patch proximity coupled antenna In order to enhance the gain and the bandwidth as well as

acquire a supplementary mean to tune the resonance frequency, a third layer is added on which a stacked patch is printed. The upper patch is electromagnetically coupled with the lower one as shown in Fig.8.

Figure 8. Electric field generated in stacked patch proximity feed antenna.

a) Lamped equivalent circuit of a stacked proximity feed antenna. The deduced equivalent circuit is shown in Fig.9

Figure 9. Lamped equivalent circuit of a stacked patch proximity fed

antenna.

b) Design procedure. The design of an efficient and optimal stacked patch

proximity fed antenna according to our experience is as follow:

• Calculate the optimal higher antenna dimension Using Eq. (1..5).

• Calculate the optimal lower antenna width fixed to 25% narrow than the higher patch.

• Calculate the input impedance of the upper antenna.

• Calculate the feed line location.

• Calculate the feed line and the stub length.

• Calculate the feed line and the stub width.

• Calculate the microstrip impedance transformer between the feed line and the 50 Ω port.

c) Antenna dimension calculation. The antenna impedance is determined using transmission

line theory for different feed point locations can be calculated with the network model. In resonance, the input impedance at an arbitrary feed point a distance x from one end of the microstrip element is pure real. By transforming the slot admittances GT+jB =(G+G21) + jB to the common point and adding them together, the input impedance at resonance is found as:

CL

Feed Line C

Patch 1

L

Patch 1R

Patch 1

Feed Line

2.2 2.3 2.4 2.5 2.6 2.72.1 2.8

-30

-20

-10

-40

0

Frequency

Mag

. [dB

]

m1 m2

S11

m1freq=dB(Hantenna_a..S(1,1))=-9.965

2.401GHz

m2freq=dB(Hantenna_a..S(1,1))=-10.017

2.476GHz

CL

Feed Line C

Patch 1

L

Patch 1

R

Patch 1

Feed Line

Mutuel coupling

L

Patch2C

Patch2

R

Patch2

Antenna patch

Upper patch Lower patch

Mutuel coupling

Higher patch

Lower patch

Feeding line

Ground plane

x 12 cos βxGT BY sin βx BY sin 2βx 10

Where Yc is the characteristic admittance and β=2π/λ is the propagation constant of a microstrip line of width W. BY and GTY 1

We can write:

x 12 cos 11

For 2

edge 12 12

The conductance G is determined using the power radiated when the antenna is loaded to a source V0. P|V | 13

|V |2 sin W coscos sin d 14

1120 sin W coscos sin d 15

G21 is the mutual conductance G12 between the radiating ends.

1120 sin W coscos J k L sin sin d 16

Were J0 Bessel coefficients, x=k0L, k0=2π/λ and y=k0W/2.

G21= 0.001055445888 and G = 0.001148798393.

The input impedance at the edge is R in (edge)= 217.6Ω.

The current is low at the ends of a half-wave patch and increases in magnitude toward the center, the input impedance could be reduced if the patch was fed closer to the center.

In order to determine the input impedance at a located point the Eq.14 is used. Z edge cos .YL (17)

The investigation carried out to determine the feed line dimension and the inset length beneath the patches. We note that, first the feeding system length formed out of the feed line and the stub affects the resonant frequency and the antenna performances, second the feed line and the stub width depend widely of the input impedance of the patches and third the length of the feed system must (l) must be at least 7.5 times the width (w) the design procedure is listed as follow:

• The feed system length made up of the feed line and the stub length must not surpass the half wave length in order not to make the feed system resonating at the same frequency of that the antenna. We propose to take the feeding system length as 80% of λg/2.

• The location of the feed point is taken were the impedance is 5Ω (Table III and table IV).

• Calculate the width of the feed system.

d) Feeding line length

The antenna performance will be compared for different feeding system length mentioned in Table III.

TABLE II. ANTENNA PERFORMANCE VARIATION FOR DIFFERENT FEEDING LENGTH

Feeding line length(mm) P rad Direc. Gain Max

int.

Antenna 915MHz

0.4 λg (56.7) 0.638 6.66 3.29 0.23

0.575 λg (59) 0.535 6.576 2.26 0.19

Antenna 2.45GHz

0.4 λg (20) 0.865 6.94 4.65 0.34

0.575 λg (28.75) 0.798 6.88 4.29 0.31

Antenna 5GHz

0.4λg ( 8.43) 0.746 7.66 4.8 0.346

0.575 λg (12.96) 0.739 7.69 4.741 0.345

From table III it is clear to deduce that the optimal feeding line Length is that corresponding to 0.4. λg.

e) Feeding line location

The performance of the antenna will be noticed for different feeding point location after what a comparison will be made.

TABLE III. VARIATION OF THE PERFOMANCE OF THE 2.45 GHZ ANTENNA WHEN THE FEED POINT LOCATION VARY.

ls (mm)

w (mm)

Prad watt

Directivity dB

Gain dB

Max Intensity (Watt/Steradian)

11 5.36 0.865 6.94 4.63 0.35

14 5.36 0.771 6.93 4.62 0.30

10 5.36 0.82 6.92 4.61 0.32

The first line of Table IV corresponding to 5Ω point impedance is the optimal inset feed point.

The Table V below determines the feeding point for different frequencies.

TABLE IV. FEEDING POINT VERSES FRIQ

Frequency 915 MHz 2.45

Antenna dimension (L-W)mm 73.68-134 24.74

Feed point location (mm) 33.11 11.

Feed system length (mm) 56.42 2

f) Feeding line width The feeding line width is determined to generadequate to match the impedance of the antenn . є. . pF m w, h are the width of the line, and the substrate

TABLE V. FEED LINE WIDTH NECESSARY TO MATCIMPEDANCE OF DIFFERENT WIDTH ANTEN

Antenna Width 48 44 37 35

feed line width 9.60 7.50 5.86 4.58

Figure 10. feed line width necessary to match the impe

width 2.45GHz antenna

Fig. 10 show that the capacitance needed to mimpedance has a linear variation when the widvaries.

Experiments was made up confirm the emethod; 915MHz and 5.0GHz antennas will dimensions was calculated using the above me

3) Conceiving 2.45GHz stacked patch antenna.

The antenna is shown in Fig.11. The upproles the first is to enhance the bandwidth, third to adjust the resonant frequency by correcof the upper patch over the radiating patch a fbe achieved.

QUENCY

GHz 5GHz

4-37.8 11.24-8.9

.28 5.07

0 8.37

rate a capacitance na. 18

e thickness.

CH THE MATCH THE NNA

32 28

8 4.00 2.65

edance of different

match the antenna dth of the antenna

fficiency of the be designed, the

ethodology.

proximity feed

per patch has two the gain and the cting the position fine tuning could

Figure 11. designed antenna total di

The antenna uses three layers of

The antenna dimensions are summa

TABLE VI. 2.45GHZ AN

Antenna dimenPart le

Upper patch

Lower patch

Line feed

Impedance transformer

Inset feed line (stub)

The current distribution is showthe lower patches.

Figure 12. Upper view of current distrib

imension (38.26 x 35)mm

FR4 1.6mm thick.

arized in table VII.

NTENNA DIMENSION

nsion ength(mm) width(mm)

24.86 38.26

24.86 30.64

9 5.86

1.4 2.22

11 5.86

wn below for the upper and

bution of the designed antenna.

Figure 13. The lower view the current distribution of the designed antenna.

The reflexion coefficient is plotted in Fig.14, the adjustment of the antenna dimension was done to acquire the best performances we will later explain the methodology followed to determine each value.

Figure 14. Reflexion plot S11 of the designed antenna.

The impedance transformer accurately matches the feeding line 35Ω impedance to fit exactly the port impedance 50Ω.

TABLE VII. DESIGNED 2.45GHZ ANTENNA PERFORMANCE

Performances Values


Effective Angle (degrees) 145.73


Gain(dB) 4.65


The purity of the electric field at almost -50dB of the antenna showed by Momentum simulation which is a one tool of ADS plotted in Fig.15.

Figure 15. Stacked patch proximity feed antenna cross polarisation.

The antenna efficiency is about 55% while the variation of the magnitude of the gain and the directivity is shown on Fig.16.

Figure 16. Variation of the Gain and Directivity.

From a 3D simulation Table IX was filled showing that the gain is constant all along the bandwidth.

TABLE VIII. THE GAIN VARIATION ALONG THE BANDWIDTH

Frequency (GHz) 2.4 2.43 2.45 2.466 2.488

Gain (dB) 4.26 4.286 4.283 4.271 4.213

4) Desining 915MHz stacked patch proximity feed antenna. The antenna will be conceived according to the proposed methodology, the calculated dimensions of the antenna are exposed in Table X.

TABLE IX. 915 MHZ ANTENNA DIMENSION

915 MHz Antenna dimension Part length(mm) width(mm)

Upper patch 73.2 133.65

Lower patch 73.2 114

Line feed 56.7 13.7

Impedance transformer 1.4 2.22

Inset feed line 23.2 13.7

The antenna performance is summarized in Table XI.

TABLE X. 915GHZ ANTENNA PERFORMANCE

Performances Values


Effective Angle (degrees) 155


Gain(dB) 3.29


2.40 2.45 2.502.35 2.55

-40

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-10

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0

Frequency

Mag

. [dB

]

S11

-80

-60

-40

-20

0 20 40 60 80-100

100

-50

-40

-30

-20

-10

0

-60

10

THETA

Mag

. [dB

]

E_co E_cross

-80

-60

-40

-20

0 20 40 60 80-100

100

-40

-30

-20

-10

0

-50

10

THETA

Mag

. [dB

]

Gain Directivity

The simulation of the antenna gives the reflexion plot shown in Fig.17.

Figure 17. S11 plot of 915 MHz antenna

A good current distribution TM10 is shown below for the designed anrenna.

Figure 18. Up view of the 915 MHz antenna showing the currut distribution

for the upper patch

Figure 19. Low view of the 915 MHz antenna showing the currut distribution

for the lower patch

5) Desining 5GHz stacked patch proximity feed antenna. The antenna calculation was done and a reduction of the whole surface was operated in order to study the effect of the reduction and the availability of the procedure used.

TABLE XI. 5 GHZ ANTENNA DIMENSION

5 GHz Antenna dimension Part length(mm) width(mm)

Upper patch 11.28 9

Lower patch 10.28 7.27

Line feed 8.49 2.37

Inset feed line 5.04 2.37

Impedance transformer 2.67 1

We notice that even though the antenna width was reduced from 16.36 mm to 9 mm, the variation of antenna performances was insignificant.

TABLE XII. 5GHZ ANTENNA PERFORMANCE

Performances Values


Effective Angle (degrees) 122


Gain(dB) 4.741


An accurate impedance matching is obtainable using an adjustment of the feed line width.

Figure 20. S11 reflexion plot of 5GHz Antenna

The two patches are exited and a good current distribution is obtained.

Figure 21. Up view of the 5 GHz antenna showing the currut distribution for

the upper patch

860 880 900 920 940840 960

-40

-30

-20

-10

-50

0

Frequency

Mag

. [dB

]

S11

4.6 4.8 5.0 5.2 5.44.4 5.6

-50

-40

-30

-20

-10

-60

0

Frequency

Mag

. [dB

]S11

Figure 22. Low view of the 5 GHz antenna showing the currut distribution for the lower patch

6) Tuning the frequency of the conceived antenna. We find that it is necessary to conceive a tenable system to

attain the bandwidth required and the best impedance matching.

A fine tuning of the resonant frequency is possible by conceiving the fixture of the middle layer with slotted holes that tolerate a precise correction of the position of the stacked patch compared to that of the driven patch as exposed in the Fig.23.

Figure 23. Tuning the resonant frequency by adjuting the position of the

upper patch and the feed line

III. CONCLUSION An easy methodology to calculate and to conceive a stacked

proximity feed antenna was presented. The theory presented was validated for different cases of use including three band 915MHZ, 2.45GHz and 5GHz. A good precision was obtained, furthermore the antenna obtained have good performance compared to the planar structure. A gain of 4.65 dB was attained for 2.45 GHz, 3.29 dB for 915MHz and 4.74GHz for 5GHz antennas. The power radiated and the maximum intensity was increased considerably and the cross polarization purity was about -50dB. Moreover the structure allows a tuning of the resonant frequency by setting the position of the feeding line and the surface facing of the upper and the lower patch.

REFERENCES [1] Zhang Y.P., Wang J.J. (2006) Theory and analysis of differentially-

driven microstrip antennas. IEEE Transactions on Antennas and Propagation. 54(4), 1092-1099.

[2] Matin M.M., Sharif B.S., Tsimenidis C.C. (2007) Probe fed stacked patch antenna for wideband applications. IEEE Transactions on Antennas and Propagation. 55(8), 2385-2388.

[3] M.A. Matin, B.S. Sharif, and C.C. Tsimenidis, “Probe Fed Stacked Patch Antenna for Wideband Applications,” IEEE Trans. Antennas Propagation, vol. 55, no. 8, Aug. 2007, pp. 2385-2388.

[4] Z. Wang, S. Fang, and S. Fu ‘‘Broadband Stacked Patch Antenna with Low VSWR and Low Cross-Polarization,’’ETRI Journal, Volume 32, Number 4, August 2010, pp618-621.

[5] D. Gibbins, M. Klemm, I. J. Craddock, J. A. Leendertz, A. Preece, and R.Benjamin, ‘‘A Comparison of a Wide-Slot and a Stacked Patch Antenna for the Purpose of Breast Cancer Detection,’’ IEEE transactions on antennas and propagation, vol. 58, no. 3, march 2010.

[6] H. Tiwari and M. V. Kartikeyan,’’ A Stacked Microstrip Patch Antenna With Fractal Shaped Defects,’’Progress In Electromagnetics Research C, Vol. 14, 185-195, 2010

[7] A. Mahdi, N. Kachouri and M. Samet, “Novel method for planar microstrip antenna matching impedance,” journal of telecommunications, vol. 2, issue 2, pp. 131-138 may 2010.

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