Dielectric resonators machined from various single crystalline materials exhibit a strong increase of quality factor upon cooling to cryogenic temperatures. As an example, single crystalline lanthanum aluminate prepared by the Verneuil crystal growing technique exhibits a variation of the loss tangent with temperature at 4 GHz from 8/spl middot/10/sup -6/ at 300 K to about 2/spl middot/10/sup -6/ between 77 K and 120 K. As an application, we have developed a dielectric dual-mode filter for satellite communication based on two degenerated modes in a hemisphere machined from lanthanum aluminate. Employing aperture coupling between adjacent hemispheres, we have built a C-band quasielliptic four-pole filter for use in output multiplexers of future cryogenic satellite transponders. Our filter with a centre frequency of 3.72 GHz and a bandwidth of 32 MHz exhibits an in-band insertion loss of -0.04 dB below T=120 K corresponding to an unloaded quality factor of 87,000. No degradation of the filter performance was observed up to power levels of 180 watts. Our results indicate the great potential of cryogenic dielectric resonator devices for future communication satellites up to Ka-band frequencies.
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