Microwave dielectric properties of low-loss materials at low temperature

The design and development of high-Q solid state microwave systems are generally limited by dielectric or conductor losses. One of the most accurate techniques for the evaluation of both permittivity and dielectric loss tangent of low-loss materials is to employ the specimen under test as a dielectric resonator. Resonant frequencies and Q factors of whispering gallery modes may then be measured on a single specimen to determine both permittivity and dielectric loss tangent over a broad frequency range. With the use of both quasi-TE and quasi-TM whispering gallery mode resonances, dielectric measurements of uniaxially anisotropic materials may be performed. The microwave losses of high quality single-crystal sapphire axe measured as a function of temperature from 20-297 K and at frequencies from 14-23 GHz. Loss tangents on the order of 10(-8) are observed at temperatures close to 20 K and at a frequency of 23 GHz. Measured frequency and temperature dependence of microwave absorption in the crystal are compared with theoretical predictions. They are also compared with those observed for high-purity polycrystalline alumina.