Studying the Additional Mode of a Profiled Sapphire Disk Resonator

Abstract

Sapphire dielectric resonators used in low-noise microwave self-oscillators for improving the oscillatory system Q-factor usually have the form of a smooth cylinder or disk inside a coaxial metal screen. Since only high azimuthal modes of the resonator possess a high Q-factor, a multitude of additional resonances able to upset normal operation of the generator exist in the vicinity of the chosen mode. To rarefy the spectrum of the resonator’ interfering oscillations near the prescribed frequency, it was proposed to replace the resonator’s smooth cylindrical wall by a certain periodically profiled surface. During the investigation, a previously unknown mode was revealed, which had a Q-factor commensurable with that of the planned modes. Its antinodes were found to be adjacent (from the inside) to the cut out circles and not in the sapphire element "petals" as might be expected. It is shown that by using the new disk structure it becomes possible to improve the resonator’s Q-factor in modes with a lower azimuthal number, and it should be noted that the Q-factor gain decreases with increasing the azimuth number when a shift is made from the plain disk to a profiled one. It has been found that with using a profiled sapphire element it becomes possible to substantially reduce the resonator volume owing to a smaller required screen radius. It can be expected that a more rarefied spectrum of resonant frequencies will simultaneously be obtained.