Ultracompact 3-D Bandpass Filters Using Mixed Subcylindrical and Subspherical Resonators

New types of ultracompact 3-D bandpass filters (BPFs) are presented. They are based on a new class of a half-cylindrical (HC) resonator concept and a mixed filter integration scheme with half-spherical (HS) and quarter-spherical (QS) resonators, aiming to optimally occupy an available cubical 3-D volume. By integrating HC resonators with subspherical cavities, 3-D BPFs with a very low axial ratio (AR) can be obtained. Size compactness and ultralow weight are achieved using monolithic integration enabled by stereolithography apparatus 3-D printing. The concept is validated via four different BPF prototypes at the Ku band, namely a two-pole/one-transmission zero (TZ) HC, a two-pole HC–HS, a three-pole HC–HS–HC, and a four-pole HC–QS–QS–HC BPFs achieving effective quality factors of up to 2550, ARs between 0.7 and 1.3, and a wide spur-free response up to 1.56 times the center frequency.