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 inKu-band, one 3rdorder triple-band filter with a fundamental TM101mode and two 5thorder dual-band filters with TM101and TE101modes, are designed to validate the model. Compared to the previous multiband waveguide filter model using cylindrical resonators, the presented model can realise all expectedNtransmission zeros between the bands, whereNis the filter order, and has a 112% higher simulated Quvalue. Meanwhile, the designed very narrowband dual-band filter using spherical resonators with TE101mode has about 287% higher simulated Quvalue than the same filter would have if using rectangular resonator with TE101mode. For experimental validation, the 3rdorder 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.