Higher order modes in dielectric resonators

In order to utilize dielectric resonators as reliable frequency-selective elements at millimeter frequencies, one has to increase their size. One possible way of increasing the size of the dielectric resonator at millimeter frequencies is by operating the resonator at substantially higher resonant modes. Therefore, it is appropriate to study the suitability of higher-order modes, in particular whispering-gallery modes in dielectric resonators for the use in an integrated circuit environment. The results of such a study should be a valuable guide for the selection and dimensioning of oversized dielectric resonators at millimeter wave frequencies. The diagrams of normalized resonant frequencies for various modes are an important guidance in designing such resonators. The Q factors of various resonant modes in various application environments are also of considerable interest. The field plots for various modes provide assistance to the user in selecting the appropriate method of coupling (or suppressing) a particular mode. Analytical as well as numerical methods are limited in their validity, because they are necessarily based on various assumptions and approximations. One of the goals of this paper is to provide a critical assessment of these methods.