Compact Dual-Mode SIW Bandpass Filters with Ultrawide Stopband Performance

This letter presents novel dual-mode bandpass filters (BPFs) based on a substrate-integrated waveguide (SIW) cavity loaded with slots, demonstrating small sizes and wide stopband performance. By etching C-shaped split-ring slots on the top surface, the resonant frequency (f0) of the fundamental mode transverse magnetic (TM)₁₀ is remarkably reduced, thereby increasing the frequency ratios between the higher order modes and TM₁₀ mode. The introduction of a metallized via enables the dual-mode operation by shifting the TM₀₀ mode closer to TM₁₀ mode while simultaneously moving TM₂₀ mode to approximately 5 f0. CPW feeding with coupling slots is utilized to further enhance the stopband suppression. The operating principles are demonstrated through eigenmode solvers, facilitating the analysis and design of the filters. Two prototypes were fabricated and measured, achieving upper stopband rejections of 9.84 f0 and 13.6 f0 , both at a rejection level of 30 dB, thus effectively contributing to the suppression of out-of-band interference from adjacent wireless systems or internal nonlinear front-end modules.