Miniaturized Half-Mode Substrate Integrated Waveguide Diplexer based on FCSRR Unit-Cell

In this paper a miniaturized diplexer for WLAN and WiMAX applications is presented, based on half-mode substrate integrated waveguide (HMSIW) technology by loading a novel metamaterial unit-cell. The proposed metamaterial unit-cells are called fractal open complementary split-ring resonators (FCSRRs). The proposed FCSRRs behave as electric dipoles if appropriately stimulated, and are able to generate a forward-wave passband region below the cutoff frequency of the waveguide structure. The electrical size of the proposed FCSRRs unit-cell is smaller than the conventional CSRRs unit-cell. Therefore, the FCSRR unit-cell is a good candidate to miniaturize the SIW structure. The proposed diplexer has been designed by cascading two bandpass filters with different center frequencies. The HMSIW bandpass filters are implemented by etching two FCSRR unit-cells with different sizes. The design procedure is based on the theory of evanescent mode propagation in which the FCSRR unit-cells behave as electric dipoles. A forward-wave passband below the intrinsic cutoff frequency of the HMSIW structure has been achieved by loading the FCSRR unit-cells on the metal surface of the HMSIW structure. This proposed diplexer displays high selectivity and compact size by using sub-wavelength resonators. The designed diplexer has been fabricated and experimental verifications have been provided. The measured results are in a good agreement with the simulated ones. The total size of the proposed diplexer is about 0.30 λg × 0.09 λg. The proposed diplexer shows significant advantages in terms of size reduction, low loss, high selectivity, high Q-factor, easy bandpass frequency shifting, easy fabrication and easy integration with other planar microwave circuits.