High-Q cryogenic dielectric resonator filters for C-band and Ku-band frequencies

Dielectric resonator filters for C-band and Ku-band were designed and built. The C-band filter was based on the HEM(11^δ) dual-mode of hemispherical resonators machined from BaMgTaO ceramics, which has a dielectric constant of around 24. The Ku-band filter was based on the HEM(11^δ) dual-mode of cylindrical resonators machined from sapphire single crystals. For both filters with cavity made from copper, unloaded quality factors of 30,000 and more than 60,000 were registered at room temperature and 77 K, respectively. For a four-pole quasi-elliptic filter with center frequency of around 3.72 GHz and bandwidth of 36 MHz, the measured insertion loss was 0.17 dB at room temperature and 0.08 dB at 77 K. For a four-pole quasi-elliptic filter with center frequency of 10.76 GHz and bandwidth of 24 MHz, the measured insertion loss was 0.9 dB at room temperature. We also estimated the unloaded quality factor of the C-band filter when the top copper endplate was replaced by a YBa₂Cu₃O ₇ superconducting film. In this case the unloaded quality factor was 100,000 at 77 K