Miniaturized evanescent-mode cavity SIW bandpass filter with spurious suppression

A compact evanescent-mode cavity SIW band-pass filter with a wide stopband is presented. To achieve a smaller profile, resonators are required to be highly loaded. However, the spurious responses become closer to the main passband when the loading is accomplished radially. Lumped element capacitive loading along the post axis is used to suppress the spurious responses. Axial loading, compared to radial loading, changes the configuration of the higher resonance modes and pushes them to higher frequencies. As a proof of concept, two second-order prototype bandpass filters with axial and radial capacitive loading are fabricated and measured. Measurements are in good agreement with simulations and demonstrate the validity of the proposed technique.