Investigation of Metal 3-D Printed High-Q Multiband Waveguide Filters Using Spherical Resonators

This paper proposes a high-Q multiband waveguide filter implementation using spherical resonators and an in-band transmission zeros approach. Three prototype filters in Ku-band, one 3 ^{\mathrm{ rd}} order triple-band filter with a fundamental TM101 mode and two 5 ^{\mathrm{ th}} order dual-band filters with TM101 and TE101 modes, are designed to validate the model. Compared to the previous multiband waveguide filter model using cylindrical resonators, the presented model can realise all expected N transmission zeros between the bands, where N is the filter order, and has a 112% higher simulated Qu value. Meanwhile, the designed very narrowband dual-band filter using spherical resonators with TE101 mode has about 287% higher simulated Qu value than the same filter would have if using rectangular resonator with TE101 mode. For experimental validation, the 3 ^{\mathrm{ rd}} order triple-band filter was fabricated using Selective Laser Melting since using traditional milling techniques is not feasible for spherical resonator filters. Monolithic near-net shape fabrication eliminated assembly, improved reliability, and reduced weight. The measured results had a relatively good agreement with the simulations. The measured insertion loss was about 0.54-0.72 dB at the centre frequency of each band, which is a noticeable improvement over the 1.1 dB insertion loss of the previous cylindrical resonator triple-band filter prototype. As there were some deviations between simulated and measured results, the proposed design was inspected in detail, and several improvements for optimising the design for Additive Manufacturing were suggested.