Microstrip Quad‐channel Diplexer Using Quad‐mode Square Ring Resonators

A new compact microstrip quad-channel diplexer using quad-mode square ring resonators (QMSRRs) is proposed. The quad-channel diplexer is composed of two QMSRRs. A structure consists of two QMSRRs together with one common input and two output coupling lines is proposed, which can remove the matching network and therefore reduce the absolute size of the diplexer. The proposed QMSRR has two transmission zeros at the frequencies between the four resonant frequencies, which can be controlled by changing the length and impedance ratio of the QMSRR. Simulation and measurement results show that the resultant diplexer is of a small circuit size and a high isolation.