Monolithic 3D printed waveguide filters with wide spurious-free stopbands using dimpled spherical resonators

Employing a family of dimpled and slotted spherical resonators, and a set of X-band waveguide bandpass filters (BPFs) is presented in this study. The resonators originated from spherical resonators with depression (hence, dimples are formed) and cutting slots into the resonator shell. With a large number of experiments, the combination of using the twisted coupling topology, depression and slotting on the resonators results in filters with simultaneous low insertion loss (IL), good out-of-band rejection and wide spurious-free stopband. Several BPFs are designed and 3D printed using the selective laser melting (SLM) technique where they are made in one piece. The monolithic implementation of these filters is allowed by the compatibility of the microwave and the mechanical design, which minimises the required supporting structures inside the filters. The measurement results show ILs of 0.2–0.3 dB and are better than 15 dB return loss. There is a small frequency shift of less than 0.9%, and a measured spurious-free stopband covering the entire X–Ku bands (8.2–18 GHz) with rejection over 34 dB. Additionally, for the SLM printed devices, the influences of temperature to the S-parameter responses, as well as polishing, which lowers the losses, are included in this study.