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A planar dual-band quadrature branch-line coupler with coupled lines is presented in this letter. The quarter-wavelength transformers in a conventional branch-line coupler are replaced by coupled lines. With derived explicit design formula, the proposed branch-line coupler can not only be operated at two arbitrary frequencies, but it can also exhibit the required output power ratio at the designed dual frequencies. The measured return losses and isolation are all better than 16 dB at the operating frequencies. Phase imbalance less than 1° is obtained. The measured data validate the proposed design methodology, and show good agreement with the simulation results.