A Compact Size, High Isolation and Low Insertion Loss Microstrip Diplexer

Abstract

A compact microstrip diplexer with low insertion loss and high isolation is proposed by combination of two coupled line bandpass filters (BPFs). An additional zero in the lower BPF transmission is created by a coupling capacitance which can be tuned at the higher channel and improve the isolation, consequently. Moreover, defected ground structures (DGSs) are used for more design flexibility and fine tuning of the structure. A diplexer prototype with two passbands at 1.24[Formula: see text]GHz and 1.5[Formula: see text]GHz is fabricated and tested. The measured insertion losses in the passbands are less than 1.2[Formula: see text]dB while the isolation between two channels is greater than 25[Formula: see text]dB and the input return losses of the three ports are about 20[Formula: see text]dB. These results in comparison with the references verify the good performance and ability of the designed duplexer.