Compact and Wide Stopband Bandpass Filters Based on Dual-Mode Folded Circular Substrate Integrated Waveguide Cavities

This article presents a novel class of compact and wide stopband bandpass filters based on dual-mode folded circular substrate integrated waveguide cavities (FCSIWCs). By employing both TM ₀₂₀ and TM ₁₁₀ modes of the FCSIWC as resonant modes and TM ₀₁₀ mode as nonresonating mode, a doublet with two poles and two transmission zeros (TZs) is implemented. The TZs can be located either both below or both above the passband, or one below and one above. By combining the dual-mode FCSIWC and single-mode folded rectangular substrate integrated waveguide cavities (FRSIWCs), a class of compact and wide stopband four-pole box-section filters is proposed. In addition, a more selective five-pole extended box-section filter is implemented by loading a complementary split-ring resonator (CSRR) on the four-pole filter structure, without occupying any additional area. Two prototypes are fabricated and measured to validate the proposed filters.