June 17, 2004 19:46 Research Publishing: Trim Size: 8.50in x 11.00in (IEEE proceedings) ieee-emc08:P54

A Circularly Polarized Microstrip Antenna Array with Butler MatrixMohamed Elhefnawy#1, Widad Ismail#2

School of Electrical and Electronic Engineering, Universiti Sains Malaysia14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia

1elhefnawy.ld06@student.usm.my 2eewidad@eng.usm.my

Abstract— We have implemented a planar microstrip antennaarray with a Butler matrix to form a switched beam smartantenna array that ensures real time mobile tracking in dynamicenvironments. If the transmitting and receiving antennas are both linearly polarized the misalignment between antennasexpected to occurs frequently and affects strongly on theperformance of the mobile tracking system; in order to improvethe system performance, we have generated a circular polarizedmicrostrip array that consists of four identical linearly polarizedpatches. A 2 2 circular polarized patch array and a 4 4Butler matrix have been designed using ADS (Advanced Design System) and matlab software. The simulated results show that a combination of a circularly polarized patch array and a Butlermatrix creates four narrower beams; each of these beams has abetter gain, directivity, and radiated power than a single patchbeam.

I. INTRODUCTION

The effect of multi-path and interference on theperformance of a radio communications link continues torepresent one of the major challenges to wireless systemsespecially in indoor environments. The implementation of a switched beam or an adaptive beam smart antenna helps tocombat impairments such as multi-path fading andinterference, thereby improving the performance of thewireless system. The switched beam smart antenna has beenselected because it is relatively simpler to implement [1]. Themicrostrip antenna has been implemented due to its small size,low profile and cheap manufacturing costs. Four linearlypolarized rectangular microstrip patch antenna are orthogonally oriented to form a planar microstrip antennaarray. The radiation pattern upon the microstrip antenna arrayhas been processed in an analog beamforming network calledButler matrix; this results in several array radiation patterns or fixed beams in different angular directions. The polarizationmismatch can degrade the signal more than 20 dB in a linearlypolarized system [3]. The polarization mismatch will beeliminated if both the transmitter and the receiver implement a circularly polarized smart antenna system; but the polarizationmismatch will cause up to 3 dB loss in the signal strength if a circularly polarized smart antenna is worked with a linearlypolarized antenna [4]. Nhi T. Pham and Franco De Flaviispresented a linearly polarized switched beam smart antennathat implemented a linear microstrip antenna array with a butler matrix [2]. In this paper, A circularly polarizedswitched beam smart antenna has been developed by using a

Butler matrix as a feeding network for a planar microstripantenna array; because a circular polarization which notrequires an alignment between the transmitting and receivingantennas is more suitable for indoor dynamic environmentsand applications like a real time mobile tracking.

II. DESIGN OF A RECTANGULAR MICROSTRIP PATCH

ANTENNA

We have designed the rectangular microstrip patch antennabased on the transmission line model; in this model therectangular microstrip patch antenna is considered as a verywide transmission line terminated by radiation impedance.Fig. 1 shows a rectangular microstrip patch antenna of lengthL and width W. Fig. 2 shows the transmission line model ofthe antenna where GR and C represent the radiation losses andfringing effects, respectively. The input impedance of an insetfed rectangular microstrip patch antenna is given by theEquation (1) [5].

4 oin

R 12

x1Z cos

2 G G L. (1)

G12 is the coupled conductance between the radiating slots ofthe antenna [6].

Fig. 1 An inset fed rectangular microstrip patch antenna

Fig. 2 A transmission line model of the rectangular microstrip patch antenna

WFeed

MsMs

ySlot #2

s s

xo

x

Patch

Slot #1

L

Slot #1 Slot #21 1Y G jB

GR C

\ 2L

GRCZo

2008 Asia-Pacific Sympsoium on Electromagnetic Compatibility &554 19th International Zurich Symposium on Electromagnetic Compatibility, 19–22 May 2008, Singapore

June 17, 2004 19:46 Research Publishing: Trim Size: 8.50in x 11.00in (IEEE proceedings) ieee-emc08:P54

2008 Asia-Pacific Sympsoium on Electromagnetic Compatibility, 19–22 May 2008, Singapore

III. RADIATION PATTERN OF A RECTANGULAR MICROSTRIP

PATCH ANTENNA

The far field radiation pattern from one of the two radiatingslots is obtained based on finding the magnetic current (Ms)which used for obtaining the electric vector potential (F); thenthe magnetic field associated with the electric vector potentialis obtained (HF); finally HF is used to get the electric field (EF).Also an equivalent Ms is created due to the ground plane, sothe total electric field will equal to 2EF [7]. The total electric field can be obtained from Equation (2):

j r

o

Wsin( sin sin )e 2E j E Ws

W2 r sin sin2

. (cos cos sin ). (2)The normalized E-plane radiation pattern for one slot ( ) is

obtained by putting :Eg

0

(3)Eg 1The normalized H-plane radiation pattern for one slot ( ) is

obtained by putting :Hg

9 0

H

Ws in ( s in )

2g cW

s in2

o s . (4)

The far field radiation pattern for the two slots can be obtainedby replacing the two slots by the two isotropic point sourceslocated at the center of the slots, as shown in Fig. 3. Thenormalized array factor for the two slots can be obtained asthe following:

j Lsin cosEAF 1 e . (5)

The total radiation pattern for the E-plane ( E ) and the H-

plane ( H ) for the two slots can be obtained by multiplying

with , g respectively:EAFEg H

j LsinE 1 e . (6)

Ws in ( s in )

2H cW

s in2

o s . (7)

Fig. 3 Array factor for the two radiated slots

IV. DESIGN OF A BUTLER MATRIX

The Butler matrix is used as a feeding network for theantenna array; it works equally well in receive and transmit

mode. The geometry of the Butler matrix is shown in Fig. 4;the 4 4 Butler matrix consists of 4 inputs, 4 outputs, 4 hybrids, 1 crossover to isolate the cross lines in the planarlayout and some phases shifters [8]. Each input of the 4 4Butler matrix inputs produces a different set of 4 orthogonalphases; each set which used as an input for the four elementsantenna array creates a beam with different direction. Theswitching between the four Butler inputs changes the directionof the microstrip antenna array beam. The ADS has been usedfor simulating the 44 Butler matrix; Table I shows a summary of the obtained phases that are associated with theselected input of the 44 Butler matrix.

Fig. 4 Butler matrix geometry

TABLE IPHASES ASSOCIATED WITH THE SELECTED INPUT OF THE 4 4 BUTLER

MATRIX

Phase B(Ant #1)

Phase D (Ant #2)

Phase A (Ant #3)

Phase C (Ant #4)

Port 1 (set 1)

0 267.018 313.183 223.587

Port 2 (set 2)

0 -85.436 -222.974 49.256

Port 3 (set 3)

0 -272.064 -135.594 -49.651

Port 4 (set 4)

0 88.326 41.742 -224.771

V. DESIGN OF A MICROSTRIP ANTENNA ARRAY

The microstrip antenna array consists of 4 identical linearlypolarized microstrip elements; each element is an inset fedrectangular patch shape. The circular polarization can begenerated with linearly polarized elements when all the adjacent elements are orthogonally oriented and are excited bya Butler matrix to form two orthogonally polarized E-fieldsfrom the four linearly polarized E-fields of the planar arrayelements as shown in Fig. 5, where the arrows are the E-fieldpolarization directions [9] [10]. The total normalized radiationpattern of the planar microstrip array for E-plane is obtainedby the following Equation:

T _ T _ EE E (8)AF

where_T EAF is the normalized E-plane array factor for the

planar microstrip array and can be obtained from thefollowing Equation [11]:

yx xjj( d sin )

T _ EAF 1 e 1 e . (9)

The total normalized radiation pattern and the normalizedarray factor for the H-plane are obtained from Equation (10) and Equation (11), respectively:

x

cossinL

L

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June 17, 2004 19:46 Research Publishing: Trim Size: 8.50in x 11.00in (IEEE proceedings) ieee-emc08:P54

19th International Zurich Symposium on Electromagnetic Compatibility, 19–22 May 2008, Singapore

T _ T _ HH H AF (10)

y y xj( d sin ) j

T _ HAF 1 e 1 e (11)

Where ,x yare the feeding phases for the antenna number

4 and the antenna number 2 respectively. ,x yare the

spacing between patches in x direction and y directionrespectively.

Fig. 5 A circularly polarized microstrip antenna array generated with linearlypolarized patches

VI. RESULTS

The inset fed rectangular microstrip patch antenna has been designed using matlab software based on the expression forthe input impedance which given by Equation (1). The inputresistance depends on the micostrip line feed position asshown in Fig. 6; the distance into the patch is found to be 7.674 mm versus patch input impedance equals to 5 0 .

The length (L) and the width (W) of the inset fed rectangularmicrostrip patch are 26.1909 mm, 34.4081 respectively. Thevalues of L and W have been obtained by matlab using FR4PCB substrate ( h=1.6 mm, and

t=0.0356 mm) [12]. The return loss of the patch antennaversus the frequency is shown in Fig. 7. The radiation patternor beam of the patch antenna is omni-directional as shown inFig. 8. The directivity, the gain and the transmitted power ofthe patch antenna are 6.19 dB, -0.0835 dB and 9.37 mWrespectively. In order to design the microstrip antenna array,

the spacing distance between the patches in x direction ( )

has been determined based on the simulation of Equations (6),(8) and (9) by matlab, Fig. 9 shows the total normalizedradiation pattern of the planar microstrip array (

5 .4 ,r tan 0 .035,

xd

T _E ) versus

the elevation angle at different values of . The fixed

beams are narrower, and the switching between them is more

stable at . Also has been determined based on

the simulation of Equations (7), (10) and (11) by matlab. Thetotal normalized radiation pattern ( ) versus the

elevation angle is shown in Fig. 10. All beams have a higher magnitude at . The microstrip array

radiation pattern associated with the selected input of theButler matrix is shown in Fig. 11. The gain, the

directivity, the transmitted power and the polarization type related to each Butler matrix input port are listed in Table II.

xd

xd 0.5 yd

T _H

yd 0.5

4 4

Fig. 6 Dependence of the input impedance on the distance into the patch

Fig. 7 Return loss of patch antenna versus frequency

Fig. 8 Co and cross-polarized components for the patch antenna

Fig. 9 Total normalized radiation pattern of the planar microstrip array

(T _E ) versus the elevation angle

Antenna # 1

Antenna # 4

y

Antenna # 2

Antenna # 3

x

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June 17, 2004 19:46 Research Publishing: Trim Size: 8.50in x 11.00in (IEEE proceedings) ieee-emc08:P54

2008 Asia-Pacific Sympsoium on Electromagnetic Compatibility, 19–22 May 2008, Singapore

Fig.10 Total normalized radiation pattern of the planar microstrip array

( ) versus the elevation angleT _H

Fig. 11 Array radiation pattern associated with the selected input of the Butler matrix4 4

Fig. 12 Return losses versus frequency for a circularly polarized microstripantenna array with a Butler matrix

TABLE IIMICROSTRIP ANTENNA ARRAY PARAMETERS ASSOCIATED WITH THE

SELECTED INPUT OF THE 4 4 BUTLER MATRIX

Gain(dB)

Directivity(dB)

TransmittedPower (mW)

Polarization

Port 1 3.15 9.41 32.5 RHCPPort 2 1.48 7.32 36.4 RHCPPort 3 1.42 7.21 36.9 LHCPPort 4 3.33 9.58 32.6 LHCP

If the loss tangent of the PCB substrate has a smaller value,this will increase the efficiency factor and enhance the gain.The return loss at each input port versus the frequency band of 1 to 3 GHZ is shown in Fig. 12. The results shown in Figures7, 8, 11, 12 and Table II have been obtained using ADS.

VII. CONCLUSIONS

The radiation pattern of the inset fed rectangular microstrippatch antenna is omni-directional and linearly polarized. Acircularly polarized microstrip antenna array can be generatedwith linearly polarized patches. The separation distancebetween patches in x direction has a strong effect on the E-plane of the array pattern only. The separation distancebetween patches in y direction affects only on the H-plane ofthe array pattern. Four narrow beams at four distinct directionsare obtained due to the excitation of a planar microstripantenna array by a Butler matrix; these four beams have a directivity, gain and transmitted power higher than the beamof the single patch antenna. A circularly polarized microstriparray with Butler matrix is suitable for indoor wirelessdynamic environments.

REFERENCES

[1] Chryssomallis, M, “Smart Antennas,’’ IEEE Antennas and Propagation Magazine, Vol. 42, No.3, 2002.

[2] Pham, N.T, Gye-An Lee, De Flaviis, F, “Microstrip antenna array withbeamforming network for WLAN applications,” Antennas and Propagation Society International Symposium, vol. 3A Page(s):267 – 270, 2005.

[3] Air-Stream Wireless Incorporated. [Online]. Available: http://www.air–stream.org.au/Polarization.

[4] David M. Pozar, Microwave and RF Design of Wireless Systems, JohnWiley & Sons, Inc, 123-124, 2001.

[5] Basilio, L.I, Khayat, M.A, Williams, J.T, Long, S.A, “The Dependence of the Input Impedance on Feed Position of Probe and Microstrip Line-Fed Patch Antennas,” IEEE Transactions on Antennas and Propagation,Volume 49, Issue 1 Page(s):45 – 47,2001.

[6] M. Mathian, E. Korolkewicz, P. Gale and E.G. Lim, “Design of A Circularly Polarized 2x2 Patch Array Operating in the 2.45 GHZ ISMBand,” Microwave Journal, 2002.

[7] Johan Lagerqvist, “Design and Analysis of an Electrically SteerableMicrostrip Antenna for Ground to Air use,” Master’s thesis,Department of Computer Science and Electrical Engineering, LuleåUniversity of Technology, 2002.

[8] Tayeb. A. Denidni and Taro Eric Libar, “Wide Band Four-Port Butler Matrix for Switched Multibeam Antenna Array,” The 14th IEEE 2003International Symposium, Indoor and Mobile Radio CommunicationProceedings. 2461-2464, 2003.

[9] John Huang, “A Technique for an Array to Generate CircularPolarization with Linearly Polarized Elements,” IEEE Transactions onAntennas and Propagation, Volume 34, Issue 9, Page(s):1113 – 1124,1986.

[10] Huang, J, “C.P. microstrip array with wide axial ratio bandwidth and single feed L.P. elements,” Antennas and Propagation Society International Symposium, Volume 23, Page(s):705 – 708, 1985.

[11] KAI CHANG, RF and Microwave Wireless Systems, Wiley & Sons, Inc, 2000.

[12] Widad Ismail. 2003, “Active Integrated Antenna (AIA) With ImageRejection. PHD Thesis 2, Department of Electronic," Electrical andComputer, University of Birmingham, UK, 2003.

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