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 HE $_{11}$ 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.