High-Q dielectric resonator devices at cryogenic temperatures

Dielectric resonators machined from various single crystalline materials ex hibit a strong increase of quality factor upon cooling to cryogenic tempera tures. As an example, single crystalline lanthanum aluminate prepared by th e Verneuil crystal growing technique exhibits a variation of the loss tange nt with temperature at 4 GHz from 8.10(-6) at 300K to about 2.10(-6) betwee n 77K and 120K. As an application, we have developed a dielectric dual-mode filter for satellite communication based on two degenerated modes in a hem isphere machined from lanthanum aluminate. Employing aperture coupling betw een adjacent hemispheres, we have built a C-band quasielliptic four-pole fi lter for use in output multiplexers of future cryogenic satellite transpond ers. Our filter with a centre frequency of 3.72GHz and a bandwidth of 32MHz exhibits an in-band insertion loss of -0.04dB below T=120K corresponding t o an unloaded quality factor of 87,000. No degradation of the filter perfor mance was observed up to power levels of 180 watts. Our results indicate th e great potential of cryogenic dielectric resonator devices for future comm unication satellites up to Kaband frequencies.