SIW bandpass filters in modified box‐section scheme with bypass/constant/frequency‐dependent coupling in diagonal cross‐coupling path

Abstract

Here, three fourth-order bandpass filters (BPFs) in modified box-section schemes using substrate integrated waveguide (SIW) are proposed and researched. The proposed filters are based on a conventional box-section structure with considering bypass/constant/frequency-dependent coupling (FDC) in the diagonal cross-coupling path to construct new filtering responses. Based on the different type of the cross-coupling path, the frequency response of each proposed structure is analysed and presented, which indicates that the proposed filters have flexible response in terms of finite transmission zeros (FTZs). The proposed filters can realise generalised Chebyshev responses with one/two/three FTZs at arbitrary positions. Moreover, one FTZ may be produced by using bypass coupling and direct inductive coupling window to improve lower stopband performance, which is verified by a coupling matrix. For the demonstration, three quasi-elliptic response filters were designed, fabricated, and measured. Good agreement is achieved between simulated and measured results.