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The coupler described in this paper is designed to separate the hot electron beam and the high-power microwave signal in a free-electron-maser amplifier. This coupler has been designed, built, and tested and is intended for operation across the 8-18 GHz frequency band. It consists of an arc of quadridged waveguide with the electron beam entering between ridges. The RF performance was found to be critically dependent on the radius and length of the arc. The measured return loss is better than -30 dB across the band with an insertion loss of less than 0.1 dB.