Multilayer microwave diplexers based on dual-mode resonators for ISM/WiFi bands

In this paper, novel multilayer microstrip diplexer is presented for ISM band at 2.45 GHz and WiFi at 3.6 GHz. The receiver and transmitter filters of the diplexer are implemented using square loop dual-mode resonators. First, we propose the planar diplexer realized in one single PCB layer. In that way, the characteristic in both bands can be independently controlled by changing the resonator dimensions. Moreover, several transmission zeros are located at the passband edges resulting in improved passband selectivity. The multilayer modification is proposed as transition from coplanar to microstrip architecture in order to obtain high isolation between filters in different layers. By using the multilayer design the size reduction and wide passbands are achieved with isolation better than 20 dB.