BWO-circuit was designed to generate hundreds MW electromagnetic wave on X-band in this study. A RF coupler is designed and fabricated to measure the power of an electromagnetic wave which is generated in the slow wave structure of a relativistic backward wave oscillator (RBWO) before this wave is radiated through an antenna. To pickup about -63dB of total output power and avoid a RF breakdown in this coupler, a 2mm X 5mm slot is made in a cylindrical waveguide. This coupler is installed before the radiation antenna and the measured value is compared with the power which is measured after the radiation. In the end, the radiation pattern was identified by using of fluorescent lamps.