Metal-Dielectric Coaxial Dual-Mode Resonator Bandpass Filters

A metal-dielectric coaxial (MDC) dual-mode resonator for bandpass filters is proposed in this work and is applied to various practical filter realizations in a compact in-line layout. Since the MDC resonator supports the TEM mode of the metal coaxial resonator and one of HE11 modes of a metal-loaded dielectric resonator (DR), both of which are dissimilar fundamental modes, the dual-mode resonator occupies a comparable volume that a conventional coaxial resonator can achieve. In addition to the basic properties of the dual mode resonator, the mechanisms for controlling the couplings between various modes are also discussed in detail. The in-line realization of the cascaded quadruplet (CQ) and cascaded triplet (CT) sections using the MDC dual-mode resonator are particularly investigated for facilitating the design of practical filters with different filtering characteristics. To demonstrate the applicability of the proposed MDC dual-mode resonator, three six-pole bandpass filter prototypes in the in-line configuration are designed, fabricated, and tuned in the 3.5-GHz band with different transmission zero (TZ) arrangements. All experimental results affirm the unique features of the MDC dual-mode resonator and demonstrate its applicability to compact in-line bandpass filters with a wide range of symmetric/asymmetric filtering responses for future wireless base stations.