Compact Narrowband Filtering Rat-Race Coupler Using Quad-Mode Dielectric Resonator

In this paper, we propose a method for designing the narrowband filtering rat-race couplers using the quad-mode dielectric resonator (DR) for the first time. The filtering ratrace couplers are designed in single-cavity configuration with one quad-mode DR and four feeding probes, featuring compact size. Based on the investigation on resonant frequencies of a rectangular DR, a quad-mode DR is first constructed. Electromagnetic (EM) field distributions at two resonant mode pairs are studied for guiding the filtering rat-race DR coupler realization. The desired 0° and 180° phase differences of the rat-race coupler are realized by the inherent in-phase and outof-phase EM-fields of the DR. By properly arranging the four feeding probes, the single-cavity configuration is designed to fulfill the transmission characteristics of a rat-race coupler topology, such as the magnitude and phase properties. The detailed design method is presented. For demonstration, a filtering rat-race coupler based on the rectangular DR is fabricated and measured, showing excellent performance of filtering responses, amplitude balance, as well as the 0° and 180° phase differences. Moreover, the other filtering rat-race coupler is implemented by using a cylindrical DR with an improved Q-factor. Comparison with other reported filtering couplers indicates that proposed designs exhibit the advantages of compact size, high Q-factor, and good filtering responses.