Direct-Coupled TE–TM Dual-Mode Waveguide Cavities

This letter introduces a novel quadruplet structure in waveguide technology, comprising a dual-mode TM cavity within two $\lambda $ /4 TE cavities. This structure demonstrates the capability of generating responses with four poles and two transmission zeros (TZs). Notably compact, it accommodates both wideband and narrowband responses effectively. The TZs are obtained by exploiting the nonresonating modes within the TM section. The coupling between the resonant mode of the TE section and both resonating and nonresonating modes is achieved by precise positioning of the TE section relative to the TM section. This inherent flexibility in TZ positioning renders the structure highly adaptable. Moreover, this structure can be exploited as building block for the design of higher order filters. Experimental results validate the viability and effectiveness of the proposed structure.