Design of high-isolation diplexer based on microstrip coupled lines

The miniaturization of the microwave diplexers becomes more important and a challenging task in the development of the modern communication system. Many researches have been conducted to achieve a more compact size and high isolations. Although the size of the diplexer becomes more and more compact, how to reduce the coupling between two signal channels and improve the isolation between two out-put ports are still problems. In this work, a kind of high-isolation diplexers are designed based on the microstrip parallel-coupled line structures and open-ended stubs. By controlling multiple transmission zeros, the filter of each channel has more than one adjustable transmission zeros that could be utilized to restrict the coupling between two channels and improve the isolation. Finally, a diplexer which consists of these two channel filters is designed and fabricated. The measured results show that low insertion losses can be achieved at 1.8 GHz and 2.4 GHz respectively, while a remarkable isolation using microstrip line resonator achieves more than 50 dB over the whole concerned frequency band.