A Compact Bandpass Filter Based on a Dual-Mode Folded Substrate Integrated Rectangular Cavity

A compact substrate integrated waveguide (SIW) bandpass filter (BPF) using the first two resonances in a folded substrate integrated rectangular cavity (FSIRC) is proposed. The usage of a single dual-mode FSIRC brings a miniaturized circuit size. The first two modes of TE0.5,0,1 and TE1,0,1 in the FSIRC can be flexibly controlled and arranged to be closely spaced by reasonably setting the width and length of the FSIRC so as to construct a second-order filtering passband. The two feeding ports are set at specific positions to control the finite transmission zeros (TZs), the interference between different modes can also be used to produce TZs. A second-order BPF centered at 9.2 GHz is synthesized, designed, fabricated, and tested with a compact circuit size of $0.28{\lambda _g}^2$, realizing a good agreement between the measured and simulated results.