A New Class of Filtering Coupler Networks in Both Co-Directional and Contra-Directional Configurations Based on Cross-Coupled Resonators

This article presents a comprehensive treatment and a systematic design methodology of filtering couplers based on canonical coupled resonator networks. A theoretical framework for the network design process is first presented, paving the way to a straightforward design procedure for couplers with three types of directional characteristics: quadrature forward couplers, quadrature backward couplers, and backward 180° rat-race couplers. The design aspects of each type are thoroughly discussed, and four design examples are provided to detail the design process. Two different realizations in ridge waveguides are provided as proofs of concepts. However, any of the available technologies can be utilized to realize the coupler networks. The two prototypes were designed, simulated, fabricated, and tested demonstrating measured results that agree with theoretical predictions. These results show the potential of the proposed approach for synthesizing filtering couplers with a canonical topology suitable for higher order filtering couplers.