Vertically Integrated Coaxial Resonator-Based Multiband Bandpass Filters Using SLA 3-D Printing

This letter reports on a novel multiband RF filtering concept and a compact low-loss integration scheme for coaxial resonator-based multiband bandpass filters (BPFs). Size compactness is achieved by: 1) vertically stacking its constituent resonators, so that the available 3-D volume is optimally occupied; 2) the use of subwavelength capacitively loaded coaxial resonators; and 3) monolithic integration enabled by stereolithography apparatus (SLA) additive manufacturing (AM). A unique coupling routing diagram (CRD) is proposed and implemented with capacitively loaded coaxial resonators that allow to independently control the passband and stopband frequencies and facilitate both symmetric and asymmetric responses. A dual-band and triple-band BPFs operating at sub-6 GHz were designed, manufactured, and tested demonstrating highly miniaturized form factors and passbands with in-band insertion loss (IL) < 0.2–0.3 dB, i.e., $Q_{\mathrm {eff}}$ : 640–1030.