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New Method for Back Lobe Suppression of

Microstrip Patch Antenna for GPS

Won-Gyu Lim #1, Hyeong-seok Jang 2, Jong-Won Yu 3

#Korea Aerospace Research Institute (KARI)

Daejeon, 305-333, [email protected]

Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology

Daejeon, [email protected]

[email protected]

Abstract A new method for back lobe suppression of mi-crostrip patch antenna with a small ground is proposed andinvestigated for GPS receiver at 1.575 GHz. To reduce back loberadiation, we use the slotted ground choke by etching slots on

the corner of the ground plane. Experiment results show thatthe back lobe radiation is reduced lower than -35 dBi and thefront-to-back ratio more than 40 dB.

I. INTRODUCTION

Global Positioning System (GPS) is a global navigation

satellite system (GNSS) to determine the instantaneous loca-

tion, the time and the velocity information almost anywhere

on or above the surface of the earth at any time. Despite

continuing improvements in GPS receivers, multipath signal

propagation has remained a dominant cause of error in differ-

ential positioning. Multipath refers to the existence of signals

reflected from objects in the vicinity of a receivers antenna

that corrupt the direct line-of-sight signals from the GPSsatellites, thus degrading the accuracy of both code-based and

carrier phase-based measurements. The multipath signals can

be mitigated by using a low back lobe antenna. A conventional

approach to mitigate the multipath interference is to use a

choke ring ground plane [1]. Choke ring ground plane consists

of multiple concentric ring shorted at the bottom and open

at the top. The choke ring ground plane effectively reduces

the multipath signals; however it significantly increases the

antenna size, making its adoption difficult in applications that

need small form factor antennas. Alternative technologies have

been pursued using the Electromagnetic Band Gap (EBG)

structures [2], artificial magnetic conductor [3] and vertical

choke ring [4].

In this paper, we present a new compact structure, the

slotted ground choke, to reduce the back lobe of the microstrip

patch antenna. The slotted ground choke is as effective as a

conventional choke ring ground plane and the increase of the

antenna size is kept minimal.

II . ANTENNA DESIGN

Circular polarization is required for GPS applications. Cir-

cularly polarized patch antennas can be constructed using var-

ious structures and configurations such as slot-loaded patches,

Fig. 1. The geometry of the proposed microstrip patch antenna with slottedground choke in (a), side view in (b) and top view in (c)

near-square patches, square patches with orthogonal feeding,

etc. In this study, in order to get circular polarization, a double

feed is used which allows to excite two orthogonal TM01mode on the square patch by feeding the two inputs

90o out of phase. Fig.1 shows the geometry and dimensions

28-30 September 2010, Paris, France

Proceedings of the 40th European Microwave Conference

978-2-87487-016-3 2010 EuMA 679


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Fig. 2. The simulated radiation patterns of microstrip patch antenna on thefinite ground plane with/without slots (Wg=Lg=90mm, Wa=La=82.4mm,Ha

=5mm, Ls

=30mm,Ws

=5.8mm)

s

of the proposed patch antenna with slotted ground choke. A

rectangular patch is designed to operate at frequency f =

1.575GHz and is fabricated with copper plate ( t=1mm and

Wa=La=82.4mm). It is fed directly by using copper wire with

1mm diameter and height Ha=5mm above the

FR4 substrate (Hd=0.6mm,Wg=Lg=90mm and relative per-

mittivity 4.6). The slotted ground choke is created by etching

four slots at the corner of the ground plane of the FR4

substrate and the quadrature hybrid coupler is designed on

the other side of the ground plane. Backward radiation isgenerated at microstrip patch antenna on the finite ground

plane due to the ground plane edge diffraction. To reduce

backward radiation, we insert four slots in the corner of the

ground plane with Ls=30mm and Ws=5.8mm. Fig. 2 shows

the simulated radiation patterns of the microstrip patch antenna

on the finite ground with slots and without slots. It shows that

the backward radiation from the microstrip patch antenna with

slotted ground choke can be dramatically reduced about 45dB

more than without slots. The simulations are performed using

the CST Microwave Studio Package which utilizes the finite

integration technique for electromagnetic computation. Fig. 3

shows the simulated radiation patterns for different length (Ls)and different width (Ws) of the slotted ground choke. IfLs is

30mm and Ws is 5.8mm, the backward radiation is decreased

considerably about -45dBi at =180o.

Fig. 4 shows the simulated surface current distribution on

the slotted ground plane at 1.575GHz. We can see more

and stronger surface current is concentrated around the edge

of slots and is oppositely directed between the interior and

exterior of the slot. This causes the slotted ground to operate

in a transmission-line-like mode, which transforms the nearly

zero impedance (short circuit) at the edge of the slot to nearly

high impedance(open circuit) at the edge of the ground plane.

Fig. 3. The simulated radiation patterns for different length (Ls) andWs=5.8mm in (a) and for different width (Ws) and Ls=30mm in (b)

Fig. 4. The simulated surface current distribution on the slotted ground planeat 1.575GHz

The high impedance leads to the desired high attenuation of

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Fig. 5. A photograph of the fabricated antenna

Fig. 6. The measured return loss of the proposed antenna

edge diffraction and cut off backward radiation.

III. MEASUREMENTS

Fig.5 shows a photograph of the fabricated microstrip patch

antenna with slotted ground choke. Fig. 6 shows the measured

return loss of proposed antenna. The impedance bandwidth

defined by -10 dB return loss about 400 MHz centered at about

1.575 GHz is observed, which is due to the wide bandwidth

characteristic of the feed network . The measured peak gain

and axial ratio are shown in Fig. 7. The proposed antenna has

a good axial ratio under 0.5 dB and peak gain about 6dBi at

1.575 GHz. Fig. 8 shows the measured radiation pattern. We

can see that forward and backward gain are 6dBi and -35dBi,

respectively. By using the slotted ground choke, we can get

the high front-to-back ratio about 41dB.

i

Fig. 7. The measured peak gain and axial ratio of the proposed antenna

Fig. 8. The measured radiation pattern of the proposed antenna at 1.575GHz

IV. CONCLUSION

The slotted ground choke is proposed to reduce the back-

ward radiation of microstrip patch antenna with finite and

small ground plane. The simulated and experimental results

show that the back lobe can be successfully reduced below

-45dBi and -35dBi, respectively. The proposed antenna offersgood circular polarization and uniform hemispherical gain

pattern for the operating band, and provides an excellent

multipath rejection characteristic. The new structure effectively

suppresses back lobe without an increase in the size of the

ground plane.

REFERENCES

[1] Tranquilla, J.M., Carr, J.P. and Al-Rizzo, H.M. Analysis of a chokering groundplane for multipath control in Global Positioning System(GPS) applications IEEE Transcations on Antennas and Propaga-tion,vol.42,no.7,pp. 905 - 911, July, 1994.

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[2] Baggen, R., Martinez-Vazquez, M., Leiss, J., Holzwarth, S., Drioli,L.S. and de Maagt, P., Low Profile GALILEO Antenna Using EBGTechnology,IEEE Transactions on Antennas and Propagation, vol. 56,no.3, pp. 667-674, March, 2008. Page(s):667 - 674

[3] McKinzie, W.E., III, Hurtado, R.B., Klimczak, B.K. and Dutton, J.D.,Mitigation of multipath through the use of an artificial magneticconductor for precision GPS surveying antennas, IEEE InternationalSymposium on Antennas and Propagation, vol.4, pp. 16-21, June, 2002.

[4] Yoonjae Lee, Ganguly, S. and Mittra, R., Multi-band L5-capable GPSantenna with reduced backlobes, IEEE International Symposium on

Antennas and Propagation, vol. 1A, pp. 3-8, July, 2005.

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2010 new method for back lobe suppression of

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