Properties of calcium fluoride (CaF 2 ) have been well researched at UV, visible and IR range of frequencies but not at microwave frequencies. In this work we report the loss tangent and the real part of relative permittivity " r of CaF 2 measured in the temperature range 15-81 K and at frequency 29.25 GHz. The tan and " r were determined by measurements of the resonant frequency and the Q 0 -factor of a TE 011 mode cylindrical copper cavity with superconducting plates containing the sample under test. The measured " r of CaF 2 was found to change from 6.484 to 6.505, and the tan from 3.1Â10 À6 to 22.7Â10 À6 when temperature was varied from 15 to 81 K. Due to the low losses CaF 2 can be useful in construction of high Q-factor microwave circuits and devices operating at cryogenic temperatures.

Accurate data of complex permittivity of dielectric substrates are needed for efficient design of HTS microwave planar circuits. We have tested MgO substrates from three different manufacturing batches using a dielectric resonator with superconducting parts recently developed for precise microwave characterization of laminar dielectrics at cryogenic temperatures. The measurement fixture has been fabricated using a SrLaAlO 3 post dielectric resonator with DyBa 2 Cu 3 O 7 end plates and silver-plated copper sidewalls to achieve the resolution of loss tangent measurements of 2 × 10 −6 . The tested MgO substrates exhibited the average relative permittivity of 9.63 and tan δ from 3.7 × 10 −7 to 2 × 10 −5 at frequency of 10.5 GHz in the temperature range from 14 to 80 K.

A split post dielectric resonator in a copper enclosure and a single post dielectric resonator in a cavity with superconducting cnd-plates have been constructed and used for the complex permittivity measurements of single crystal substrates. (La,Sr)(AI,Ta)03, LaAJ0 3 , MgO and quartz substrates have been measured at temperatures from 20 K to 300 K in the split post resonator and from 15 K to 80 K in the single post resonator. The TE oio mode resonant frequencies and unloaded Qu-factors of the empty resonators at temperature of 20K were: 9.952GHz and 25,000 for the split post resonator and 10.808GHz and 240,000 for the single post resonator respectively.

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