A Filtering Rat-Race Coupler Using Low-Profile Tri-Mode Dielectric Resonator

This brief proposes a low-profile tri-mode dielectric resonator (DR) for implementing a rat-race coupler. The proposed DR has a hexagonal shape with a hybrid boundary, where three of its lateral surfaces are plated with silver. The hybrid boundary enables three operation modes, whose resonant frequencies can be gathered by subtly adjusting the DR dimension. By exploiting the inherent phase relationship among the three modes, filtering in-phase power divider or filtering balun can be implemented by selectively exciting specific modes. Furthermore, by combining the feeding schemes of the aforementioned two circuits, a filtering rat-race coupler can be achieved. Additionally, the source-load coupling provides two transmission zeros beside the passband for enhancing selectivity. For demonstration, a filtering rat-race coupler centered at 5.8 GHz is designed, fabricated, and measured. The simulated and measured results show good agreement.