A compact filtering rat-race coupler using dual-mode stub-loaded resonators

A compact microstrip rat-race coupler with fourth-order bandpass response has been proposed in this paper. In this study, the dual-mode stub-loaded resonators are used as the building blocks of the proposed structure. Thus, the proposed rat-race coupler with fourth-order bandpass response can be implemented with only four resonators, resulting in a small circuit size. All measured results are in good agreement with the full-wave simulation results by IE3D. As a consequence, the proposed filtering rat-race coupler not only has a small size, but also has high selectivity and good out-of-band response.