Development of a Circularly Polarized Hmsiw Antenna

Abstract

A circularly polarized (CP) half-mode substrate integrated waveguide (HMSIW) cavitybacked antenna is developed by a novel design method in this paper. The single-fed antenna contains two coupled HMSIW cavities with orthogonal polarizations. Its design method evolves from filter synthesis procedure, which takes the advantage of equivalent circuit model to accelerate the optimization. The antenna radiates high-purity right handed CP waves with measured axial ratio (AR) of 0.33 dB at 3.55 GHz, while its gain and AR bandwidth are comparable to other CP SIW antennas. With the robust design method, the proposed antenna is competitive in practical applications. INTRODUCTIONAntenna with circular polarization (CP) plays an important role in modern wireless communications. It solves the problem of the polarization mismatch caused by the multipath interference and the wave propagation in ionosphere. CP antenna with microstrip technology has been well studied, and the design procedure is mature [1-3]. However, microstrip antennas suffer from high conduction loss, which makes them less competitive in the high-frequency domain.The substrate integrated waveguide (SIW) cavity-based CP antennas have low conduction loss and high gain, while maintaining the advantages of low profile, low cost, and easy integration with planar circuits [4][5][6][7][8]. However, generating pure CP waves in SIW cavities is not as straightforward as that in microstrip patches. The general solutions are either to design two standalone radiating elements [8], or to cut slots on top (or bottom) of the SIW cavity [4][5][6][7]. The first solution requires external feeding to provide the phase difference. For the second one, though widely used, the performance of the antennas is highly dependent on the shape, size, and location of the slots. Therefore, the design procedure typically involves heavy optimization in full-wave simulators, which takes a lot of time and computation.In this paper, a CP antenna with SIW technique is developed by a fast and robust design methodology. The antenna contains two coupled half-mode SIW (HMSIW) cavities. The even and odd eigen-modes are excited inside the structure, similar to the coupled-mode patch antenna [9][10][11]. The design method evolves from filtering theory, where the equivalent circuit model is optimized to realize the target operating frequency and bandwidth. The final structure is directly synthesized from the circuit model, avoiding the heavy optimization in full-wave simulator. The proposed antenna has very low axial ratio (AR) at target frequency while still maintaining decent gain and AR bandwidth. The design procedures, along with the results and discussions, are presented in the following sections. DESIGN METHOD AND THEORYThe schematic layout of the proposed antenna is shown in Fig. 1. Two identical HMSIW cavities have orthogonal polarization to each other. A gap on the center wall brings magnetic coupling between the

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