PIERS ONLINE, VOL. 4, NO. 1, 2008 1

Design and Implementation of Aperture Coupled Microstrip IFFAntenna

M. N. Jazi1, Z. H. Firouzeh2, H. Mirmohammad-Sadeghi3, and G. Askari3

1Institut National de la Recherche Scientifique, Universite de Quebec, Canada2Department of Electrical Engineering, Amirkabir University of Technology, Iran

3Information and Communication Technology Institute, Isfahan University of Technology, Iran

Abstract— Design and implementation procedure of a wideband aperture coupled microstripantenna in L-band frequency is introduced. To improve radiation performance of the microstripantenna four structures are proposed. The structure with air substrate has the maximum fre-quency bandwidth. Experimental results of frequency bandwidth and radiation patterns of theoptimum structure agree with the simulation results. The antenna has the gain of 8.5 dB, thefrequency bandwidth of greater than 25%, and F/B better than 15 dB. This aperture coupledmicrostrip antenna can be used as an element of microstrip array antennas in IFF systems.

1. INTRODUCTION

Microstrip antennas (MA) are simple planar structures that have advantages such as low profile,conformal availability, simple fabrication using printed circuit technology, low cost and compatibil-ity with integrated circuits. In spite of limitations such as small bandwidth, low gain and low powerhandling capability [1], MA offers over than other type of antennas due to their advantages and in-creasingly used in a variety applications such as military, industry and wireless communication [2].Different techniques have been used to increase bandwidth such as using thick substrate [3]. Thistechnique improves bandwidth less than 5 percent. Aperture coupled stacked patch microstripantenna can improve bandwidth more than 50 percent at the expense of increasing in back loberadiation level. To reduce back radiation, metallic rods or resonance planes are used in the back ofthe antenna or appropriate cavities are put next to the coupling aperture [5–8]. Aperture coupledmicrostrip antenna has been used for IFF (or Secondary Surveillance Radar) systems with 6 percentbandwidth for VSWR < 1.5. However, Front to Back (F/B) ratio has been reported 10 dB whichis not enough for IFF application [9].

In this paper, both techniques have been used to increase the bandwidth and decrease the backradiation. The design process of the proposed structure is organized as followed. First of all, anaperture coupled microstrip antenna will be designed according to the requirements. Then, fourstructures will be introduced and compared to each other. After simulation, the best structure willbe selected and optimized to fabricate. Finally, the measurements and conclusion will be reported.

2. DESIGN AND SIMULATION

The structure of the aperture coupled microstrip antenna has been depicted in Figure 1. Thisantenna has been designed to use as an element in the microstrip array antenna for IFF (or SSR)systems. The central frequency is 1060 MHz and the bandwidth is at least 250 MHz. The thick-ness of patch substrate, h3 is chosen 20 mm due to the desired bandwidth. The feed substrate isRT/Duroid 5880 with thickness of h2 = 0.787mm and relative dielectric constant of 2.2. FR4 isused as cover with thickness of h4 = 1 mm and εr4 = 4.3.

According to [1], the preliminary antenna dimensions are designed and calculated at the centralfrequency of 1060MHz. [4] recommends that aperture width Ws is the half of its length Lsl and Ts

is the one-tenth of Lsl. The length of the stub, Lst is 0.22λg, in which λg is the wavelength in thefeed substrate.

The antenna parameters such as patch dimensions, aperture length, the length of stub, and thedistance between feed line and back plane are optimized by use of full wave simulator IE3D untilfrequency bandwidth greater than 25% and F/B greater than 20 dB will be obtained at the centralfrequency of 1060MHz. Four different structures as shown in Figure 2 are designed, simulated andcompared to recognize the structure with best performance.

Aperture has been designed to resonate near the resonance frequency of the patch. Apertureshape is very important to control the maximum coupling between the feed line and the patch and

PIERS ONLINE, VOL. 4, NO. 1, 2008 2

also the minimum back radiation. A metallic plane is placed behind of the antenna to reduce backradiation power to −20 dB. Actually, this plane functions as a resonator and generates appropriatecurrent distribution to eliminate undesired radiation fields in the rear of the antenna. The distancebetween the back plane and the feed line influences on the amplitude of the current distributionbut the phase of the current distribution is controlled by the size of the back plane.

The four structures have been simulated and optimized by IE3D software. The graph of theirreturn losses has been depicted in Figure 3. According to Figure 2, structure (a) has the maximumbandwidth. The simulation results indicate that the smallest bandwidth will be achieved whenTeflon dielectric is directly beneath the patch. In addition, the frequency center of structure (d) islower than other structures. The simulation results have been indicated in Table 1 in which fu andfl are the low and high frequency corresponding to −10 dB return loss.

Cover

Slot

Feed Line

Back Plane

WpLp

Ws

Ts

Lsl

Lst

h4 εr4,

h3 εr3,

h2 εr2,

h1 εr1,

Figure 1: Aperture coupled microstrip antennastructure.

Air

(a) Air substrate

Foam

(b) Foam substrate

Foam

Foam

(c) Foam-Teflon substrate

(d) Teflon-Foam substrate

h

h

h

h

3

3

3

3

Figure 2: Four different structures of aperture cou-pled microstrip antenna.

Structure

Central Frequency

(MHz)

(fu + fl) /2

Bandwidth

(MHz)

fu − fl

(a) 1038 279

(b) 1030 260

(c) 1018 224

(d) 1000 195

Table 1: Simulation results of four different struc-tures as shown in Figure 2.

Figure 3: Return losses of four structures simulatedby IE3D.

E- and H-plane radiation patterns at 1030 MHz and 1090 MHz have been represented for struc-ture (a) in Figures 4 and 5, respectively. The Front to Back ratio is greater than 20 dB. E-and H-plane Half Power Beam Width (HPBW) at 1030 MHz are 65◦ and 75◦, respectively. At1090MHz, E-plane HPBW is 63◦ and H-plane HPBW is 74◦. Figure 6 shows simulated results forgain of four structures. The gain difference between 1030MHz and 1090 MHz is less than 0.5 dBfor four structures. For example, the gain of structure (a) at 1030 MHz is 8.5 dB but at 1090 MHzis 8.8 dB.

PIERS ONLINE, VOL. 4, NO. 1, 2008 3

After consideration and comparison of the simulation results of four structures show that struc-ture (a) i.e., the antenna with air substrate has the performance to realize the desired antenna.Consequently, this structure is selected to be fabricated and measured its radiation parameters.

Figure 4: E- and H-plane radiation patterns at1030MHz simulated by IE3D.

Figure 5: E- and H-plane radiation patterns at1090MHz simulated by IE3D.

Figure 6: Comparison the gain of four differentstructures simulated by IE3D.

Figure 7: The photo of the fabricated aperture cou-pled microstrip antenna with air substrate.

Figure 8: Simulation and measurement results of return loss of the antenna.

PIERS ONLINE, VOL. 4, NO. 1, 2008 4

Figure 9: Measurement of co- and cross-polarizationradiation patterns at 1030 MHz.

Figure 10: Measurement of co- and cross-polarization radiation patterns at 1090MHz.

3. MEASUREMENT

Figure 7 shows photo of the fabricated aperture coupled microstrip antenna. Return loss of theantenna has been measured and compared with simulation result in Figure 8. A little frequencydifference between two curves is observed because of simulation accuracy, simulation settings, mea-surement accuracy and fabrication tolerance. The return loss of the antenna is less than −10 dBin the frequency range of 875 MHz–1150MHz, i.e., the frequency bandwidth is greater than 25%.Figures 9 and 10 show respectively co- and cross-polarization radiation patterns at 1030 MHz and1090MHz. F/B of E- and H-plane radiation patterns are greater than 15 dB and 13 dB at 1030 MHz,respectively. Figure 10 indicates F/B of E-plane radiation pattern is greater than 20 dB but thatof H-plane radiation pattern is greater than 15 dB. This difference is due to low accurate measure-ment of anechoic chamber for this frequency bandwidth. The gain difference of both planes is lessthan 0.5 dB. This antenna has better performance such as bandwidth and F/B than the antennareported in [9].

4. CONCLUSIONS

In this paper, four different structures of aperture coupled microstrip antenna have been consideredto achieve the maximum bandwidth with good radiation performance in this bandwidth. The airsubstrate antenna was selected and optimized by IE3D simulator. The simulation and measurementresults have the good agreement with each other. The antenna has the gain of 8.5 dB, the bandwidthof greater than 25%, and F/B better than 15 dB. This aperture coupled microstrip antenna can beused as an element of microstrip array antennas in IFF (or SSR) systems.

ACKNOWLEDGMENT

The authors would like to thank the staff of Information and Communication Technology Institute(ICTI), Isfahan University of Technology (IUT), Isfahan, Iran for their co-operator and supportingthis work.

REFERENCES

1. Garg, R., P. Bhartia, I. Bahl, and A. Ittipihoon, Microstrip Antenna Design Handbook, ArtechHouse, 2001.

2. Pozar, D. M. and D. H. Schaubert, Microstrip Antenna, the Analysis, and Design of MicrostripAntenna and Arrays, IEEE Press, New York, 1995.

3. Pozar, D. M., Review of Aperture Coupled Microstrip Antenna: History, Operation, Develop-ment, and Application, Microwave Online System Company world wide web site, July 1996.

4. Targonski, H. S., R. B. Waterhous, and D. M. Pozer, “Design of wideband aperture stackedpatch microstrip antennas,” IEEE Trans. on Antennas and Propagation, Vol. 46, No. 9, 1245–1251, Sept. 1998.

5. Targonski, S. D., R. B. Waterhouse, and D. M. Pozar, “Reflector elements for aperture coupledmicrostrip antennas,” Antennas Propag. Symp. Dig., 1997.

PIERS ONLINE, VOL. 4, NO. 1, 2008 5

6. Blefko, M. J. and W. N. Klimczak, “A secondary surveillance radar (SSR) antenna withintegrated difference and sidelobe suppression (SLS) channel,” APS International Symposium,USA, July 1996.

7. Shin, H. S. and N. Kim, “Wideband and high gain-one patch microstrip antenna coupled withH-shaped aperture,” Electronic Letters, Vol. 38, No. 19, Sept. 2002.

8. Ahmed, Z., M. B. Ihsan, N. Chaudhry, and J. Kiani, “Design of a low sidelobe aperture coupledmicrostrip antenna array,” IEEE, 2004.

9. Kreczkowski, A., T. Rutkowski, A. Buda, and S. Kostka, “The microstrip IFF antenna for theairborne radar,” International Conference on Microwaves, Radar and Wireless Communica-tions, MIKON, 2002.