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A novel dual-band coupled-line coupler with an arbitrary coupling coefficient is proposed and investigated in this paper. The coupler has a simple structure and can realize a wide range of the same coupling coefficient over two designated frequencies. The mathematical design formula is also developed, which not only provides a straightforward design procedure, but also reveals the attractive features of the proposed coupler, including: 1) the same coupling coefficient in the dual frequency bands; 2) the sole dependence of a coupling coefficient on the ratio of the impedances of even and odd modes; and 3) a large range of the ratio of the dual frequencies. Two prototypes of the proposed couplers, one operating at 2.4/5.8 GHz with a -10-dB coupling coefficient and the other working at 0.9/1.8 GHz with a -15-dB coupling coefficient, have been designed, fabricated, and measured. The measured results demonstrate that the bandwidths for the 2.4- and 5.8-GHz bands are 22% and 19.8%, respectively, and that for the 0.9- and 1.8-GHz bands are 28.4% and 15.8%, respectively. Good correlation between the measured results and those of the theoretically designed justifies the proposed coupler circuit and the design theory.