This paper presents the modified design that can reject harmonic components in the branch-line coupler. After adding open stubs at the center of branch lines of the traditional design, their new network parameters can be found in order to maintain the conventional function at an operating frequency and suppress its harmonic terms chosen. Experimental results show the second and third harmonic suppressions to be -28.3 and -39.6 dBs, while maintaining its traditional performance at the fundamental frequency.
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