A novel class of dual-mode filters with improved high frequency sideband suppression response making use of coplanar waveguide (CPW) square loop resonator is presented. The resonator is placed on the bottom plane and inside the defected area of the ground. The resonant property of the CPW square loop resonator as well as the coupling property between two degenerate modes with different patch perturbation is studied in the paper. Two T-shaped orthogonal feed lines are arranged on the top plane, which not only provide proper excitation to the resonator, but also introduce an additional source-load coupling, so the proposed filter is found to have two transmission zeros at high frequency sideband and takes on asymmetric frequency response. Such a compact dual-mode CPW square loop resonator filter operating at 2.68 GHz is designed and fabricated.
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