Substrate Integrated Waveguide Filtering Rat-Race Coupler Based on Orthogonal Degenerate Modes

In this paper, a method for designing substrate integrated waveguide (SIW) filtering rat-race couplers based on TE102 and TE201 orthogonal degenerate modes is proposed for the first time. Without additional circuits, the desired 0° and 180° phase differences of the rat-race coupler are realized by the inherent in-phase and out-of-phase characteristics of E-fields at TE102 and TE201 modes. E-field distributions of the SIW cavities at TE102 and TE201 modes are studied for guiding the circuit realization. By properly arranging the locations of input-output ports and coupling regions in the SIW cavities, magnitude and phase properties of the rat-race coupler topology are realized. And then the filtering responses can be integrated with the rat-race coupler. A detailed design method based on coupling matrix is presented. A filtering rat-race coupler is implemented using square SIW cavities to verify the proposed method, showing good performance of filtering responses, amplitude balance, as well as 0° and 180° phase differences. For further verification, another filtering rat-race coupler is fabricated using two circular SIW cavities to simplify the structure. Comparison with other filtering couplers indicates that the proposed designs exhibit the advantages of good filtering responses, relatively high Q factor of the entire filtering circuits, and simple structure.