Compact and High Isolation Microstrip Diplexer for Broadband and WLAN Application

In this paper, a compact and high isolation microstrip diplexer is designed for broadband and wireless local area network (WLAN) application, simultaneously. The bandpass filter (BPF) for broadband channel is formed by three-coupled-line structure and two short stubs with different size loaded in 50 feed lines, and the BPF for WLAN channel consists of two coupling quarter-wavelength resonators (QWR) and one open stub loaded in short parallel-coupling feed structure. Multiple transmission zeros can be generated due to their intrinsic characteristics, so the broadband BPF with sharp skirt and wide upper-stopband performance and the WLAN BPF with sharp roll-off and lower-stopband characteristic can be realized. The tapped stub not only can generate transmission zeros to deepen the stopband, but also can connect other BPF as an its part without deterioration of in-band performance. Hence, a compact microstrip diplexer combines of two BPFs without the extra junction matching network. The mutual loading effect approximately equivalent to a coupled short QWR can also generate new transmission zero at the passband edge to improve the isolation. A microstrip diplexer with the 3 dB fractional bandwidth (FBW) of 80% for broadband channel and 5% for WLAN channel is designed and fabricated. Good agreement between the simulated and measured results is observed.