Optimum Design of a Miniaturized Multilayer SIW Diplexer using Orthogonal-Mode Resonator

The optimum design of a miniaturized multilayer SIW diplexer in the X-band is presented using the orthogonal mode square resonator, which replaces the conventional T-junctions connecting the lower and upper frequency channels. The placement of metallic posts in the SIW excites two degenerate orthogonal modes. The two mode channel frequencies are adjusted by the displacement of positions of posts. The magnetic couplings between the middle SIW layer and the upper and lower ones are achieved by perpendicular rectangular apertures, where each one excites one of the orthogonal modes. Consequently, the isolation between the output ports is maximized. The ports are made by tapered microstrip lines. The proposed structure leads to substantial miniaturization of the SIW diplexer. A prototype model is fabricated and measured to verify its proposed structure and design procedure.