Three Dimensional Printed Vertically-Stacked Single-/Multi-Band Coaxial Filters and RF Diplexers

This article reports on monolithically integrated and low form-factor vertically-stacked coaxial resonator-based single-/multi-band filters and radio frequency (RF) diplexers using stereolithography apparatus (SLA) additive manufacturing (AM). The proposed RF filtering/diplexer concepts exhibit the following unique features: 1) low form-factor due to the use of vertically stacked low-profile capacitively-loaded coaxial cavity resonators; 2) monolithic integration; and 3) versatile and highly selective transfer functions facilitated by the topological arrangements of the resonators within the filter volume that enable cross-couplings and can be exploited to increase selectivity. Theoretical filter design analysis using coupling routing diagrams (CRDs) and eigen-mode electromagnetic (EM) simulations are performed. The proposed concept is validated by the design, fabrication, and testing of four monolithically-integrated prototypes, namely a four-pole two transmission zeros (TZs) bandpass filter (BPF), a three-pole two TZs BPF, a second-order dual-band BPF, and a second-order diplexer operating at $C$ -band and exhibiting an effective quality factors ( $Q_{\mathrm {eff}}s) >$ 700.