Compact In-Line Multiband Bandpass Filters With Independently Controllable Passbands Using Quarter-Mode SIW Cavities

This paper presents a compact multi-band bandpass filter using a novel multimode quarter-mode substrate integrated waveguide cavity (QMSIWC) for sub-6 GHz frequencies. The first four resonating modes (TM₀₁₀, TM₂₁₀, TM₀₂₀ and TM₄₁₀) are utilized for realizing multiple passbands. The slot perturbations are introduced in the cavity for the flexible and independent control of center frequencies and bandwidths of passbands. The design incorporates transmission zeros for passband isolation in an in-line topology using modal bypass coupling. Five experimental prototypes, including three second order designs of dual/tri/quad-band responses, two fourth order designs of dual/tri-band responses with high selectivity using novel coupling structure are synthesized, designed, fabricated and tested to validate the proposed design procedure. The measured insertion loss and return loss at the center frequency of these are better than 2 dB and 18 dB, respectively.