Realization of compact broadband rat-race coupler

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Abstract
In this article, a method to miniaturize the broadband rat-race couplers using microstrip nonuniform transmission lines (MNTLs) is presented. The proposed coupler features a compact size on a single circuit layer structure without reactive components and via-holes. The design formulas are developed using ABCD matrix and even–odd mode analysis. The theoretical analysis has been verified by measurement of the manufactured broadband MNTL rat-race coupler with center frequency of 1.35 GHz and 89% fractional bandwidth. The isolation, magnitude, imbalance and phase imbalance are less than –20 dB, 0.5 dB, and 4 degrees, respectively, within 0.75–1.95 GHz frequency bandwidth.

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