Monolithic 3D-printed slotted hemisphere resonator bandpass filter with extended spurious-free stopband

A method to effectively suppress the spurious modes in a hemispherical resonator is proposed and the suppression contributes to extension of the spurious-free stopband for the hemispherical-resonator-based bandpass filters. The TM _2m1 and TE ₁₀₁ modes in a hemispherical resonator are suppressed by interrupting the corresponding surface current via properly slotting the cavity shell, without significantly interfering with the fundamental TM ₁₀₁ mode. Resonant modes in a slotted hemispherical (SH) resonator are analysed, and design and implementation of a proof-of-concept X-band fourth-order SH-resonator waveguide filter are presented. The filter is monolithically prototyped by employing metallic additive manufacturing technology. The filter demonstrates passband insertion and returns losses of averagely 0.9 dB and >20 dB, respectively, a small frequency shift of <0.9%, and a spurious-free stopband frequency ratio of >1.8:1 with a stopband rejection over 34 dB at 10.5-16.4 GHz.