Microstrip balanced quad-channel diplexer using dual-open/short-stub loaded resonator

A microstrip balanced quad-channel diplexer is reported for the first time, using dual-open/short-stub loaded resonator (DOSLR/DSSLR) which consists of a uniform impedance end-shorted resonator loaded with two open/short stubs. The resonant frequencies of dual-open-stub loaded resonator (DOSLR) and dual-short-stub loaded resonator (DSSLR) under differential-mode (DM) and common-mode (CM) excitation are fully analyzed. Based on their respective characteristics, two individual balanced dual-band filters are initially designed with a frequency ratio of larger and smaller than 2 respectively. The designed filters are then combined to construct the first balanced quad-channel diplexer adopting distributed coupling technique. In order to demonstrate the efficacy of the proposed design methodology, a compact quad-channel balanced diplexer prototype (2.4/5.2 GHz and 3.2/4.75 GHz) is implemented and measured. Good agreement between the theoretical analyses and the measured data has been achieved.