Miniaturized filtering rat‐race coupler based on hybrid circular substrate integrated waveguide and coplanar waveguide resonators.

In this paper, an approach to designing filtering rat‐race coupler (FRRC) with hybrid circular substrate integrated waveguide (CSIW) and coplanar waveguide (CPW) resonators is proposed. The FRRC is developed in a single CSIW cavity embedded with four CPW resonators and four feeding ports, featuring miniaturized size. The desired 0° and 180° phase characteristics of the RRC are realized by the inherent in‐phase and out‐of‐phase electromagnetic fields of the dual‐mode CSIW cavity. By properly arranging the sum, difference and other two output ports, the FRRC is exploited to achieve the desired properties of a RRC with bandpass filtering responses. In addition, transmission zeros are generated in the vicinity of the upper stopband. For validation and demonstration, a miniaturized FRRC utilizing hybrid CSIW and CPW resonators is synthesized, fabricated and experimented in a single‐layered process. Comparisons with previously state‐of‐the‐art research, the proposed FRRC prototype indicates the superiorities of miniaturized footprint, high‐quality‐factor, high‐order filtering response and enhanced out‐of‐band rejection. [ABSTRACT FROM AUTHOR]