High power and frequency stability of electromagnetic wave input are required in the DNP-NMR system. It is necessary to monitor the electromagnetic wave energy in the transmission line in real-time to judge the magnitude and stability of the energy transmitted to the NMR spectrometer. However, the energy in the transmission system is too large to be directly measured in the transmission line. A part of the energy must be extracted to facilitate the measurement. We design a directional coupler by placing a wire array in the middle of the three-port circular corrugated waveguide for real-time extraction of transmitted energy. In the frequency range of 290 – 360 GHz, 0.05% energy can be stably coupled to port 3, so it is convenient for the energy detection. The S 11 parameter of the designed directional coupler can below −20dB from 290 to 360 GHz, and even −41dB at the operating frequency of 330 GHz.
