In this paper, a highly efficient microstrip diplexer with low insertion loss, high selectivity, and high isolation is introduced. The proposed diplexer employed two compact size coupled squared open-loop resonator (SOLR) based band pass filters (BPFs). Firstly, a matching network is utilized to ensure that the two BPFs and the antenna load are properly matched. This is accomplished by connecting the two BPFs and the antenna with a conventional T-junction that acts as a combining circuit, resulting in good isolation between the up-link and down-link BPFs. As a second step, a defected ground structure (DGS) is used to improve the overall filter response in terms of insertion loss and isolation without affecting the diplexer selectivity. Based on this structure, the proposed diplexer has two resonance frequencies of 2.5 GHz and 2.8 GHz. The structure provides good insertion losses of about 1.6 and 1.3 dB for the two channels, respectively with fractional bandwidth of 2.8% at 2.5 GHz and 3.2% at 2.8 GHz. The measured isolation levels are 70 dB and 50 dB for 2.5 GHz and 2.8 GHz, respectively. The proposed diplexer is useful for several wireless communication applications such as WiMAX. The good agreements between simulated and measured results verified the practical validation of the proposed diplexer.
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