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

Abstract

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 TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2m1</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">101</sub> modes in the hemispherical resonators are effectively suppressed by interrupting the surface current with slots, without significantly degrading the unloaded quality factor of the fundamental TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">101</sub> mode. 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.