Compact Quad-Mode Bandpass Filter Using Modified Coaxial Cavity Resonator With Improved $Q$ -Factor

This paper presents a novel quadruple-mode coaxial cavity resonator and its application to bandpass filters with compact size and improved quality factor (Q-factor). Unlike the conventional coaxial cavity resonator, the proposed resonator is made up of four inner conductive posts within a single cavity, which provides new quadruple resonant modes to realize bandpass filter. No metallic walls inside the cavity are required, and thus the utilization efficiency of the cavity space is improved. As a result, the unloaded Q can be approximately 15% higher in comparison to the conventional coaxial resonator, or 30%-35% volume saving can be achieved while maintaining the similar Q-factor value with the conventional designs. In addition, due to the multiple cross-coupling occurring within the cavity, including the source-to-load direct coupling, four flexible transmission zeros can be created to realize different filtering functions. The complete design methodology of filters based on the proposed resonator is discussed, starting with the resonator characteristics and the coupling scheme. Simulations as well as experimental results of four- and eight-pole filters are presented to validate this attractive design concept. Good agreement between measured and computed results is obtained.