A new microstrip diplexer using open-loop resonators

In this paper, a new microstrip diplexer using open loop resonators with compact size and high isolation is designed and fabricated for the WLANs (IEEE 802.11b/g at 2.4 GHz and IEEE 802.11a at 5.2 GHz) applications. The diplexer is formed by two dual-mode band pass filters (BPFs) using an asymmetric fork-form feed line and two open loop resonators. The diplexer has less than 3 dB insertion loss and the isolation between the two channels is more than 25 dB. Furthermore, the proposed diplexer shows several transmission zeros beside the pass bands which improves the selectivity of the BPFs and thus improves the isolation between lower and upper channel filters and achieves a significant attenuation of the undesired harmonics. The electrical performances of the diplexer are investigated numerically by using Momentum integrated in ADS Agilent and CST microwave software. Good agreement between the simulation and measurement results is achieved.