Design of Single- and Dual-Band Waveguide Filters Using Quadruple-Mode Cavity Resonators

In this letter, the designs of narrow-band filters using two doublets of orthogonal modes in a quadruple-mode cavity resonator are presented. By properly arranging the frequency locations of these quadruple modes, namely TE₀₁₁/TE₁₀₁/TM₁₂₀/TM₂₁₀, single-band or dual-band performance can be realized. Since a pair of degenerate modes can be regarded as a doublet producing one transmission zero (TZ), two TZs are generated beside the passband because of two pairs of doublets (fundamental and high-order modes) in a single cavity resonator. A high-order filtering response is also explored by mounting a quarter-wavelength waveguide section nonresonating nodes (NRNs) between the cascaded cavity resonators. To verify the design methodology, an eighth-order single-band filter (at 11.0 GHz) and a fourth-order dual-band filter (at 11.1 and 12.6 GHz) have been simulated, fabricated, and tested. A good agreement between measurement and simulation is achieved.