Compact high-Q hemispherical resonators for 3-D printed bandpass filter applications

A new class of hemispherical resonators featuring a high unloaded quality factor (Qu) and a compact geometrical configuration is proposed for the first time for 3-D printed bandpass filter (BPF) applications. The hemispherical resonator exhibits a volume only half that of a spherical one at a same dominant-mode resonant frequency, without losing its intrinsic characteristic of a high Qu. Electromagnetic field analysis of the hemi-spherical resonator is expounded, and second-order BPFs based on such resonators are designed at X and Ka bands. The Ka-band second-order BPF is manufactured with a high-temperature-resistant ceramic-filled resin using a fast and low-cost stereo-lithography-based 3-D printing technique for validation purpose. The filter's surface metallization is achieved by employing electroless copper/silver plating, which contributes to an improved fabrication accuracy in thickness and uniformity of the conductive layer. The RF-measured results demonstrate the Ka-band filter an insertion loss of 0.56–0.7 dB at 31.95–32.13 GHz, a passband return loss of better than 17 dB, and a small frequency shift of 0.04%.