A case study of novel slow-wave resonant structures dedicated to efficient microstrip branch-line coupler size reduction has been presented. A compact 3-dB branch-line coupler has been designed to mimic the performance of its conventional counterpart, simultaneously demonstrating a substantial 88% surface area reduction. The vital usefulness of the approach in the process of microstrip line abbreviation has been experimentally validated. The measured in-band results are in excellent agreement with theoretical characteristics, while the out-of-band performance depicts an efficient suppression of unwanted signals up to the 6th harmonic.
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