Abstract
This letter presents planar microstrip branch-line (BL) couplers with improved phase and amplitude stability bandwidth along with significant size reduction. The prototypes are designed to operate at 433 MHz frequency band suitable for ISM 433 MHz applications. The prototypes with symmetric polygonal modification (SPM) and asymmetric polygonal modification (ASPM) offer a wide quadrature phase bandwidth of 37.8% and 35.5% respectively, over which the phase imbalance is within 90°±5°. In SPM all four transmission lines of the BL coupler have been symmetrically modified to bring in 75.99% size reduction compared to the conventional design. Further asymmetric modification has been performed to bring an additional 12.34% size reduction, compared to symmetric, and providing an overall size reduction of 78.95% compared to conventional BL coupler. The prototypes are able to offer compactness without compromising much on the matching, transmission and isolation parameters. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1369–1374, 2016
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