High Isolation and Compact Microstrip Diplexer for Broadband and WLAN Application

Abstract

A high isolation and compact microstrip diplexer for broadband and wireless local area network (WLAN) application is proposed in this paper. It is achieved by using the combination of a broadband bandpass filter (BPF) composed of one open stub and two short stubs of different size loaded in 50Ω feed lines. The tapped stub can not only generate two zeros to deepen the stopband, but also connect other BPF without deterioration of in-band performance. Hence, the compact diplexer combines two BPFs without extra junction matching network. The mutual loading effect approximately equivalent to a coupled resonator can create one zero at the passband edge to improve the isolation. The proposed microstrip diplexer is designed and fabricated, and it has a 1dB fractional bandwidths (FBW) of 60% at 3.5GHz for broadband channel and 3% at 5.8GHz for WLAN channel . The simulated and measured results are presented and good agreement is obtained.