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This article presents a Ka-band fourth-order slotted spherical resonator waveguide bandpass filter (BPF) with a wide spurious suppression stopband. It uses the first higher-order TM211 mode rather than the fundamental TM101 mode of the spherical resonator in order to obtain a higher unload quality factor (Qu) for smaller in-band insertion loss, as well as a larger filter volume that gives better tolerance to fabrication errors in high frequency applications. By introducing slots that interrupt surface currents, the fundamental TM101 and two higher spurious modes (TE101 and TM311) can be suppressed without compromising the unloaded quality factor of the TM211 mode. In addition, the filter topology is optimized to further enhance the suppression level of the spurious passbands. AZ-shaped topology has been found effective in decreasing the coupling strength of the spurious TM311 mode. An analysis is performed to demonstrate the better tolerance of the Z-shaped filter over the conventional TM101-mode spherical resonator filter. For verification, a fourth-order slotted spherical resonator waveguide BPF with a center frequency of 31 GHz and bandwidth of 880 MHz is designed and manufactured using a selective laser melting (SLM) 3-D printing process. The measured results show an average in-band insertion loss of 1.53 dB, and a passband return loss better than 13.6 dB. The stopband of the filter is extended up to 40.9 GHz with a rejection level greater than 20 dB.

In this paper, a third-order waveguide bandpass filter (BPF) based on slotted spherical resonators with a wide spurious-free stopband is presented. The resonator consists of a spherical cavity with slots opened at the top and bottom. Compared with a non-slotted spherical resonator, the slotted resonator suppresses the two spurious modes (TM211 and TE101) whilst maintaining the fundamental TM101 mode. The unloaded quality factor of the TM101 mode is not significantly degraded. This is achieved by interrupting surface current and radiating the unwanted spurious modes with the slots. The BPF is designed at a center frequency of 10 GHz with a fractional bandwidth of 1%. Two filter prototypes are fabricated, one using metal-based selective laser melting (SLM) and the other by polymer-based stereolithography apparatus (SLA) techniques. The slots also facilitate the copper electroplating process for the SLA-printed filter. The measured results show that the average passband insertion losses of the SLM- and SLA-printed filters are 0.33 and 0.2 dB, respectively. The corresponding passband return losses are better than 22 and 20 dB. The filters demonstrate excellent passband performance and wide spurious-free stopbands up to 16 GHz with stopband rejections of over 20 dB.

A new class of slotted spherical antenna is proposed in this letter. The antenna is composed of a single air-filled high-quality-factor spherical resonator that is properly slotted on the cavity shell, and is fed by a rectangular waveguide on the other side of the slots, radiating the TM101 and TM211 modes. A three-slot antenna prototype operating at X- and Ku-bands is designed and implemented as a proof of concept. Good directional radiation patterns are achieved at both bands by optimizing dimensions of the three circumferential slots. The prototype is additively and monolithically manufactured by incorporating stereolithography apparatus and electroless copper plating techniques, showing enhanced flexibility and improved efficiency in the fabrication. The antenna demonstrates a light weight (11.2 g) and a reflection coefficient below –10 dB at the lower band from 9.88 to 10.68 GHz and the upper band from 13.05 to 15.05 GHz, with the maximum realized gains of 8.8 and 10.1 dBi, respectively. [ABSTRACT FROM AUTHOR]