Gap-Coupled Excitation for Evanescent-Mode Substrate Integrated Waveguide Filters

This paper reports on a study of substrate integrated waveguides integrated with gap-coupled (capacitive) excitation to design multipole multiband bandpass filters (BPFs). Waveguide structures are conventionally coupled using inductive aperture to realize a BPF. The capacitive gap-coupled structure is proposed as an alternative for the conventional inductive aperture-coupled ones. It is shown that the proposed structure could operate at the frequencies considerably below the cutoff frequency of the waveguide. Therefore, the filters designed by this technique can be considered as evanescent-mode filters, resulting in a significant size reduction. Moreover, the dual-pole and dual-resonance evanescent-mode BPFs are demonstrated by integrating the new and the conventional evanescent-mode resonators. It is shown that the proposed capacitive excitation enables significant miniaturization and resonates at a small fraction of a 3-D structure cutoff frequency. The proposed integration is analytically discussed using lumped element equivalent circuit accompanied by experimental verification.