Analysis of the Compact Graded Dumbbell Coaxial Resonator With Its Filter Design Applications

This article presented a modified coaxial resonator, i.e., the graded dumbbell coaxial resonator (GDCR), which can be constructed by concaving the lateral surface of the inner metal cylinder from both ends to the center. The modification will not increase the design complexity for the classical topology coaxial bandpass filters (BPFs). Compared with the conventional coaxial resonator (CCR), the GDCR can provide a volume saving of 35%–38% while maintaining comparable unloaded quality values. The analytical expression was given in detail. Three fourth-order filters with simple inline cascading were designed to prove the merits of the GDCR. On the other hand, the GDCR showed some interesting features for the inline mixed-coupling topology. Then, a simple novel spout-shaped structure was introduced and analyzed for constructing electrical magnetic (EM) mixed coupling. For demonstration, a fourth-order cascaded quarter (CQ) and a sixth-order inline mixed-coupling topology filter were designed. Both filters achieved controllable transmission zeros (TZs) that highly improved the stopband performance. The complete design methodology of all filters is also discussed in detail. For rapid prototyping, the filters with TZs were fabricated by direct metal 3-D printing technology. The printing tolerance is acceptable, and the tested results agreed well with the simulated ones.