A Compact Quad-Band SIW Bandpass Filter with Two Closely Spaced Passband Pairs Using Interdigital-Loaded CSRRs

This paper presents a compact quad-band substrate integrated waveguide (SIW) bandpass filter employing two interdigital-loaded co-directional complementary split-ring resonators (ILCD-CSRRs). Owing to the interdigital loading, each ILCD-CSRR exhibits dual-resonant behavior, enabling two independently controllable resonant frequencies from a single resonator. By tuning the geometrical parameters of the upper and lower ILCD-CSRRs, four resonant frequencies are obtained and organized into two pairs of closely spaced passbands, with simulated frequency ratios of 1.10 and 1.038, respectively. A coupling matrix representation is used to provide theoretical insight into the resonant interactions and quad-band filtering response. The proposed filter achieves quad-band operation using only two dual-resonant resonators without additional resonators or complex physical coupling networks, resulting in a compact normalized footprint of 0.29 λg². A prototype is fabricated and experimentally characterized. The measured center frequencies are 7.08, 7.84, 9.60, and 10.22 GHz, with insertion losses ranging from 1.47 to 3.71 dB and return losses better than 16 dB for all passbands. The measured results agree well with the simulations, confirming the feasibility of the proposed design for compact multiband SIW filtering applications requiring closely spaced spectral allocations.