A Compact Substrate Integrated Waveguide Diplexer Using Fundamental Dual-Mode Resonances

A design methodology for developing a compact substrate integrated waveguide (SIW) diplexer based on fundamental dual-mode resonances is proposed in this paper. The diplexer is composed of two TE101/TE201-based dual-mode rectangular cavities and a TE101-based single-mode rectangular cavity. By placing one output port at the location of the weakest electric field intensity of TE201 mode in the dual-mode cavity, the isolation near the second channel passband can be generated. Meanwhile, the isolation near the first channel passband can be enhanced based on the evanescent characteristics of the coupled single-mode cavity. Compared to previous studies, this design employs the fundamental resonant mode of TE101, thus further reducing the circuit size. A 1^st/2^nd-order SIW diplexer is demonstrated as a proof of concept whereas a 2^nd/3^rd-order diplexer is synthesized, designed, fabricated, and tested for a validation. The experimental results closely match the synthesized and simulated responses, demonstrating its potential in the development of miniaturized diplexers for frequency division duplexing wireless communication systems.