A Wideband Quintuple-mode Cavity Bandpass Filter: Design and Miniaturization

This paper presents a novel five-mode cavity filter based on a composite structure of stepped impedance resonator (SIR) and L-shaped feeding lines. The filter adopts a cylindrical metal cavity as its basic structure, where three conductive columns are arranged at the bottom to excite and regulate three TM modes ( \textTM_01 , \textTM_11 , and \textTM_02 ). A pair of L-shaped feeding structures is employed to achieve the excitation and separation of degenerate \textTE_11 modes while providing sufficient port coupling, ultimately realizing a 40% fractional bandwidth centered at 2.8 GHz. Innovatively applying the SIR structure to multimode filter design, the proposed configuration maintains five-mode resonant characteristics while successfully reducing the filter volume by 45%, significantly enhancing its miniaturization performance. Furthermore, by controlling the spatial arrangement of the conductive columns, cross-coupling between resonant modes is achieved, generating a pair of controllable transmission zeros. To validate the effectiveness of the design methodology, a prototype was fabricated and tested. The measured results show good agreement with simulations, confirming the feasibility of the proposed approach.