In this paper, a planar microstrip diplexer consisting of two resonators integrated by three coupled lines is presented. The proposed resonators are composed of coupled lines and microstrip spiral cells. The realized diplexer operates at 2.88 GHz for wireless and 3.29 GHz for WiMAX applications. To obtain a compact size, low insertion losses and tune the resonance frequencies a designing method is presented based on the analysis of scattering and Z parameters of the proposed resonator. The designed diplexer has a miniaturized size of 0.028 kg 2 . Another advantage of the proposed structure is a significant improvement in the insertion losses at the both passbands so that they are 0.36 and 0.44 dB at 2.88 and 3.29 GHz respectively where the return losses at the both passbands are better than 23.7 dB. The designed diplexer is fabricated and measured. There is a good agreement between simulated and measured results, which validates the introduced methodology.
