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This paper presents a novel branch-line coupler which has a wide stopband characteristic with attenuation 30 dB from 4 GHz to 8 GHz using defected ground structure (DGS). The conventional branch-line coupler suffers from the presence of spurious response at odd harmonics of the operating frequency. The proposed coupler adopts the two symmetrical dumbbell DGS under the input and output feed lines to suppress the harmonic components. To verify the proposed structure, both simulated results of the conventional and proposed branch-line coupler are presented. It demonstrates that the proposed structure has a high harmonic suppression. Moreover, it produces a much wider passband bandwidth compared with the conventional coupler design. The measured results show good agreement with the simulated results.