In this paper, a planar simplified dual composite right/left-handed (SD-CRLH) transmission line (TL) structure is proposed and applied to the design of branch-line coupler. The SD-CRLH TL is obtained by a microstrip line with an open-ended stub in spiral form. Since this structure has unusual phase shift characteristics with a transmission zero out of the passband, the branch-line coupler with the planar SD-CRLH TL can achieve both size reduction and harmonic suppression. Such a branch-line coupler operating at 0.915 GHz is investigated and fabricated. The equivalent circuit simulation, full-wave simulation and measurement results agree well with each other. From the results, it is shown that the area of the proposed branch line coupler is reduced by 74% compared to the conventional one while maintaining similar performance, and the second harmonic suppression can be lower than -45 dB.
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