Dual-band branch-line couplers with three parallel lines for arbitrary power-split ratios

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Abstract
This work demonstrates the design procedure for a dual-band branch-line coupler that is capable of having an arbitrary coupling level. Simply with an additional parallel line that connect the centers of the two branch lines, the coupler can be designed to have an arbitrary power-split ratio between the through and coupled ports, while its structural simplicity is preserved. Moreover, the practical range of power-split ratio is relatively wider than previous dual-band branch-line couplers. A detailed design procedure is presented. Experimental results for dual-band couplers centered at 1 and 2.5 GHz with ±3-dB power-split ratios show great agreement with theoretical results. The power-split ratio remains within 0.5 dB from the specified, in as much as 230 MHz bandwidths at both 1 and 2.5 GHz.

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