High isolation, compact microstrip diplexers using quarter-wave stepped-impedance resonators

A novel microstrip diplexer with a compact size and high isolation is proposed in this letter. It is formed by two four-pole cross-coupled bandpass filters placed side by side and combined through a T-junction. The filters are designed with quarter-wave stepped-impedance resonators (SIRs) to achieve the optimum size reduction. Though the filters are closely placed, higher isolation can still be obtained by properly designing the T-junction. Moreover, the possibility for magnetic-coupled SIRs in the proposed diplexer to use a common shorting via is also discussed, and a modified design is presented. The performances of the proposed diplexers are verified through both simulation and measurement, and they agree well.