| Summary: | In this paper, two miniaturized planar diplexers based on the substrate integrated waveguide (SIW) structure loaded by open complementary split-ring resonators (OCSRRs) are proposed. The working principle is based on the theory of evanescent mode propagation. The proposed SIW diplexers operate below the cutoff frequency of the waveguide. Both the complementary split-ring resonators (CSRRs) and the OCSRRs behave as electric dipoles however, the resonance frequency of the OCSRRs is approximately half of the resonance frequency of the CSRRs. Therefore, the electrical size of the OCSRRs is larger than the CSRRs. Accordingly, the OCSRRs are more appropriate for the SIW miniaturization. At first, the filtering response of the SIW structure loaded by OCSRR unit cells is investigated. Then, two miniaturized SIW diplexers which consist of two cascaded OCSRR unit cells with different orientations are designed. For the first diplexer (Type I), the fractional bandwidths of operation for the up and down channels are 9.52% and 2.59% at 4.2 GHz and 5.8 GHz, respectively. For the second diplexer (Type II), the fractional bandwidths of operation for the up and down channels are 5.95% and 2.51% at 4.7 GHz and 5.6 GHz, respectively. Finally, in order to validate the ability of the proposed OCSRR unit cells in the size reduction, two designed diplexers are fabricated and experimental verification are provided. A good agreement between the results of measurement and simulation is achieved. The proposed diplexers show significant advantages in terms of size reduction, low loss, high isolation, and integration with other planar circuits.
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