Compact dual-layer substrate-integrated waveguide bandpass filters with controllable transmission zeros

A design method for compact substrate-integrated waveguide (SIW) bandpass filters (BPF) with controllable finite transmission zeros (FTZs) is proposed in this work, which is realised by a dual-layer SIW structure with two single-mode SIW cavities in the upper substrate and one dual-mode SIW cavity in the lower substrate. A basic dual-layer SIW BPF in box-section coupling scheme is proposed and analysed. Two FTZs can be produced for the bypass couplings between the diagonal cross-coupling paths. To enhance the cross-coupling strength, a magnetic coupling window between the two single-mode SIW cavities is designed. To further improve the selectivity of passband, a frequency-dependent coupling is designed between the two single-mode SIW cavities to produce an additional FTZ. Characteristics of the proposed dual-layer SIW filters with controllable FTZs are analysed. For the demonstration, some design examples with the centre frequency of 7.5 GHz and bandwidth of 240 MHz are presented, and two design examples are fabricated to verify the performance of the proposed design method for compact dual-layer SIW filters with controllable FTZs.