Slotted Hemispherical Resonators for 3-D Printed Waveguide Filters With Extended Spurious-Free Stopbands

This paper presents a comprehensive study on a new family of slotted hemispherical resonators and their applications in waveguide bandpass filters (BPFs) to extend the spurious-free stopbands. The proposed scheme focuses on suppressing unwanted spurious modes in the air-filled hemispherical resonators by adding functional slots to the metallic cavity shells. The spurious TM2m1and TE101modes in the hemispherical resonators are effectively suppressed by interrupting the surface current with slots, without significantly degrading the unloaded quality factor of the fundamental TM101mode. Resonant modes in the slotted hemispherical resonators are analyzed. Design, fabrication, and measurement of X-band proof-of-concept waveguide BPFs based on slotted hemispherical resonators are expounded. Each filter is monolithically prototyped by employing metallic additive manufacturing technology. The filters demonstrate good passband performance-average insertion loss: 0.6-1.1 dB, frequency shift: 0.9%-1%, and return loss: >20 dB, and more significantly extended stopbands with spurious-free frequency ratios of >1.8:1 and stopband rejections over 36 dB.