Single-Layer Substrate-Integrated Waveguide Evanescent-Mode Filter

Single-layer capacitively coupled structure integrated with substrate-integrated waveguide (SIW) is developed to design evanescent-mode bandpass filters. The planar gap-coupled structure is proposed as an alternative for the conventional inductive aperture-coupled one for filter excitation below the cutoff frequency of the SIW. The proposed planar capacitive excitation enables considerable miniaturization and resonates at a small fraction of a 3-D structure's cutoff frequency. The proposed integration is analytically discussed using lumped-element equivalent circuit accompanied with experimental verification. Measured scattering parameters are compared to those of simulations, which demonstrate good agreement.