I. Introduction
Dielectric resonators (DRs) have been widely employed in integrated circuits at microwave frequencies. However, DR excited on conventional modes (TE, TM and hybrid) has too small a quality factor at higher microwave frequencies. This problem can be solved if the DR is operated on its higher order modes. In particular, DR working in their Whispering Gallery Modes (WGM) is interesting for a variety of reasons [1]. They are endowed with larger dimensions, higher ease of integration with MICs or LTCC and very high Q value as it is limited mainly by the loss tangent of the resonator material. WGMs also offer good suppression of spurious modes, which axially leak out of the resonator and can be absorbed without perturbing the desired modes as well as the quasi insensibility with respect to the absorbing and conducting materials, so the modal energy is well confined within the resonator itself. Dielectric resonators, including Planar Dielectric Resonator (PDR), require a cavity wall to reduce radiation loss and high dielectric constant disk or sphere to confine a mode, and an air gap to form an evanescent field region. Within this complicated structure, the dielectric disk or sphere should be placed accurately for proper operation. The cavity provides good performance, but the technique is very expensive because of the complicated structure.