Radiation losses of sapphire WGM resonators: Effects of dielectric disk shape

Radiation losses of Ka-band sapphire whispering gallery mode (WGM) resonators are studied using COMSOL program. The resonators with two and one conducting endplates (CEP) are analyzed. Such resonators are promising for monitoring of microwave surface impedance of (super)conductors. A hemi-cylindrical resonator as a specific kind of the cut disk resonator with two CEPs has a high radiation Q-factor. The numerical studies performed also for different shape resonators with one CEP, show high values of radiation Q-factor for the resonators, among which the hemispherical resonator has the highest radiation Q-factor. Experimental measurements are performed for the resonators with one CEP, where the shape of dielectric disk differs from cylindrical one. The experiment shows smaller values of Q-factor compared with the numerical results. This can be due to additional radiation losses at the coupling elements of quasi-optical dielectric resonators.