A novel microstrip dual-layer rat-race coupler with compact size and enhanced bandwidth

Microwave hybrid couplers are crucial components of mixers, phase shifters, amplifiers and other high-frequency systems. Conventional couplers are characterized by large size which limits their usefulness in modern applications. In this work, a novel compact rat-race coupler with enhanced bandwidth has been proposed. The structure consists of six compact microstrip resonant cells. It is implemented on two separate layers which permits further size reduction as compared to single-layer topologies. The size of the optimized structure is only 15.3 mm × 19.5 mm (footprint of 297 mm2). The coupler is characterized by 300 MHz bandwidth (symmetrical w.r.t. 1 GHz operating frequency) and nearly perfect power split at the center frequency. The coupler has been favorably compared to state-of-the-art compact structures reported in the literature.