Coupled evanescent mode resonator filter using a defective ground structure to increase coupling

A bandpass filter design based on a modified type of coupled evanescent mode resonators is presented. The resonators are implemented as a substrate integrated waveguide (SIW) structure using a multilayer stack of organic microwave laminates. To realize a high input and output coupling coefficient required for many wideband filter specifications, the use of a U-shaped defective groundplane structure is proposed. Without this structure, the maximum achievable coupling coefficient is limited by manufacturing constraints. The use of the proposed defective groundplane structure allows to increase the coupling coefficient beyond these limitations. The effect on the external couplings is analyzed by simulations and experiments using a single resonator. Based on the obtained results, a layout for the full bandpass filter with a center frequency of 6GHz and a bandwidth of 500MHz is derived. This filter design is verified by simulation and measurement.