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In this paper, a planar microstrip branch-line coupler is designed to have dual-band operation. A Pi-typed structure is used in place of the conventional quarter wavelength transmission line for dual-band application. This structure consists of a pair of coupled line in which one has two ends while the other has open stubs attached to its two ends, and its circuit parameters are determined by the transmission line theory. Explicit design equations are derived using ABCD-matrix. For verification, a 3-dB branch line coupler with operating frequencies of 900 MHz and 3.5 GHz is fabricated and measured on an FR-4 printed circuit board (PCB). The simulated and measured results are in good agreement with each other.
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Cao et al. (2015) studied this question.
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