Whispering gallery modes in hollow spherical dielectric resonators

Whispering gallery modes have been used for precise measurements of dielectric losses in hollow spherical samples of single crystal quartz versus single crystal YAG at frequency range from 10 to 40 GHz. It has been shown that for single crystal YAG the product of frequency times Q-factor due to dielectric losses is almost constants, while for single crystal quartz dielectric loss tangent increases much slower than linearly.