Microstrip Patch Antenna Fed by Substrate Integrated Waveguide

T. Mikulasek1, J. Lacik

1 Department of Radio Electronics, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic, e-mail: mikulasek.t@phd.feec.vutbr.cz, tel.: +420 541 14 91 23, fax: +420 541 14 92 44.

Abstract − A microstrip patch antenna fed by Substrate Integrated Waveguide (SIW) is presented in this paper. The antenna operates in centimeter-waveband, at 9 GHz frequency. In the first step, the antenna is modeled in Ansoft HFSS. Influence of the most important parameters on the return loss of the antenna is investigated. In the second step, the final model of the antenna is simulated, fabricated and measured. At the operating frequency, the antenna reaches 8.8% impedance bandwidth (for s11 better than –10 dB) and 8.2 dBi gain.

1 INTRODUCTION

The Substrate Integrated Waveguide (SIW) structures have gained considerable attention in recent years [1]. This is mainly the fact that the substrate integrated waveguide is electrically similar to a conventional metallic waveguide. What’s more, it can be easily fabricated by a low cost Printed Circuit Board (PCB) process. The SIW structure consists of two rows of metal vias created in a dielectric substrate. The substrate is on its top and bottom side covered by metal sheets which are connected by vias.

If a narrow slot is etched in a metal sheet of a SIW wall, a low profile slot antenna is obtained. The slot antennas based on the SIW technology have usually a low gain and a narrow impedance bandwidth (for s11 is less than –10 dB typically about 2 %). However, if the slot antenna is extended by a secondary radiator, a wider impedance bandwidth and a higher gain can be obtained. This modification was introduced in [2] where a dielectric radiator was used.

In this paper, we propose to use as a radiator a microstrip rectangular patch instead of the dielectric radiator. This antenna configuration is designed (with help of Ansoft HFSS) for the frequency 9 GHz.

2 ANTENNA CONFIGURATION

The proposed antenna consists of a rectangular patch fed by a SIW through a slot located in the top wall of the SIW. For the analysis, the SIW is replaced by its equivalent waveguide of the width WWG.

A model of the proposed antenna consists of two dielectric layers (Figure 1). The rectangular patch is placed on the top of the first layer (a patch layer). The rectangular waveguide is created in the second

layer (a waveguide layer). The waveguide operates in the fundamental mode TE10. One end of the waveguide is shorted.

The rectangular patch dimensions are the length Lp and the width Wp. The patch is located in the centre of the patch layer. The slot in the top wall of the waveguide (a ground plane) provides coupling between the waveguide and the patch. The centre of the slot is the same with the centre of the patch. The slot dimensions are the length Ls and the width Ws. The maximum coupling of the fundamental mode is at the distance ys = λg/4 from the waveguide short. The offset xs, and the patch length Lp set primarily the resonance frequency. The antenna parameters (Figure 1) are summarized in Table 1.

The patch is etched on the FoamClad with the dielectric permittivity εr1 and the thickness hsub1. The CuClad217 with the dielectric permittivity εr2 and the thickness hsub2 was chosen as a dielectric substrate for the waveguide layer.

(a) Top view

(b) Side view

Figure 1: The proposed antenna configuration withsolid walls.

978-1-61284-978-2/11/$26.00 ©2011 IEEE

1209

Parameter Value Lp 10.20 mm Wp 15.70 mm Ls 12.51 mm Ws 0.30 mm xs 3.74 mm ys 8.05 mm

WWG 15.91 mm hsub1 1.88 mm hsub2 1.52 mm εr1 1.25 [-] εr2 2.17 [-]

Table 1: Parameters of the proposed antenna.

3 PARAMETRIC STUDY

The most important parameters for the design of the antenna are the the length of the slot Ls and the offset of the slot xs. These parameters control an amount of the coupling energy between the waveguide and the patch. Obviously, the dimensions of the patch influence the coupling too, but their size is given by the calculation [3]. The influence of these parameters on the behavior of the return loss is shown in Figure 2.

The length of the slot Ls controls mainly the amount of the coupling and shifts slightly the resonance frequency (Figure 2a). The offset of the slot (and of the patch too) in the x-direction has primarily an effect on the resonance frequency (Figure 2b). The width of the slot Ws has not a significant effect on the impedance matching of the antenna (Figure 2c).

4 SIMULATION AND EXPERIMENTAL RESULTS

The equivalent waveguide in Figure 1 can be converted into the SIW [1]. The structure of the SIW is depicted in Figure 3, where WSIW is the center-to- -center distance between two opposite rows of vias, pvia is the pitch of the consecutive vias and dvia is the diameter of a via. The SIW is about one guide wavelength long (it is not dimensioned in Figure 3).

There are a few approaches which can be used for the excitation of the SIW fundamental mode [4], [5]. The SMA-to-microstrip-to-SIW transition is not appropriate due to the radiation of the microstrip line. In this paper, a direct non-radiated SMA-to-SIW transition is used. The design is carried out by the optimization procedure. The final position of the SMA connector and the supplementary parameters of the SIW antenna are given in Table 2.

To verify the obtained results, the antenna was modeled by the transient solver of the CST MICROWAVE STUDIO (CST MWS), and then

(a) Length of the slot

(b) Offset of the slot in x-direction

(c) Width of the slot

Figure 2: The influence of the slot parameters on thereturn loss of the model in Figure 1.

Parameter Value WSIW 17.00 mm dvia 1.50 mm pvia 2.56 mm

xSMA 1.81 mm ySMA 10.71 mm

Ls 12.08 mm xs 4.33 mm

Table 2: Supplementary parameters of the SIWantenna.

1210

fabricated and measured. The SIW antenna was fabricated by a low cost PCB process. The both layers were fixed using an epoxy resin (this was considered in the simulation too). The photographs of the fabricated antenna prototype are shown in Figure 4.

The simulated and measured return loss of the SIW antenna is shown in Figure 5. The obtained results from Ansoft HFSS fit very well with the ones from CST MWS. The resonance frequency of the fabricated SIW antenna is about 100 MHz lower than the designed frequency. It is shifted primarily due to fabrication tolerances and the fixing process. The describe antenna reaches up to 8.8 % impedance bandwidth (for s11 is less than –10 dB) in comparison with the simulated impedance bandwidth 8.6 %.

The radiation pattern of the proposed antenna was simulated and measured in E-plane (xz) and H-plane (yz). The simulated and measured radiation patterns are shown in Figure 6. There is a good agreement between the simulation and the measurement. The radiation pattern is symmetrical in the both planes. The SIW antenna has a very low level cross- -polarized radiation –30 dB. The simulated and measured gain of the antenna is 8.8 dBi and 8.2 dBi.

(a) Top view

(b) Bottom view

Figure 4: Photographs of the fabricated SIW antenna.

Figure 5: Return loss of the SIW antenna.

(a) E-plane

(a) Top view

(b) Side view

Figure 3: Schematics of the final model of theproposed antenna.

1211

(b) H-plane

Figure 6: Radiation patterns of the SIW antenna.

5 CONCLUSION

In the paper, the low cost low profile patch antenna fed by SIW has been introduced for centimeter wave applications. The antenna was simulated, fabricated and measured. In comparison with the mentioned dielectric radiator antenna, the proposed antenna configuration achieves wider impedance bandwidth 8.8 % and higher gain 8.2 dBi.

Acknowledgments

The presented research was financially supported by the Czech Grant Agency project no. 102/08/H027 “Advanced Methods, Structures and Components of Electronic Wireless Communication,” by the project CZ.1.07/2.3.00/20.0007 WICOMT of the operational program Education for competitiveness, and by the research program MSM 0021630513. The research is the part of the COST Action IC 0603 “Antenna Systems & Sensors for Information Society Technologies (ASSIST).”

References

[1] L. Yan, W. Hong, K. Wu, T. J. Cui, “Investigations on the propagation characteristics of the substrate integrated waveguide based on the method of lines,” in IEE Proceedings – Microwaves, Antennas and Propagation, 2005, vol. 152, no. 1, pp. 35-42.

[2] Z. C. Hao, W. Hong, A. Chen, J. Chen, K. Wu, “SIW fed dielectric resonator antennas (SIW- -DRA),” in Proceedings of the IEEE MTT-S International Microwave Symposium, 2006, pp. 202-205.

[3] E. O. Hammerstad, “Equations for microstrip circuit design,” In Proceedings of the 5th European Micro-strip Conference, 1975, pp. 268--272.

[4] D. Deslandes, “Design equations for tapered microstrip-to-Substrate Integrated Waveguide transitions,” in Proceedings of the IEEE MTT-S International Microwave Symposium, 2010, pp. 704-707.

[5] A. Morini, M. Farina, C. Cellini, T. Rozzi, G. Venanzoni, “Design of Low-Cost non-radiative SMA-SIW Launchers,” in Proceedings of the 36th European Microwave Conference, 2006, pp. 526-529.

1212