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High power millimeter wave and terahertz sources are used in DNP/NMR spectroscopy to greatly enhance the NMR signal. A key issue is the efficient coupling of the source power to the sample. We present HFSS calculations showing that the MIT 250 GHz DNP/NMR system provides a B1 value of 13 μT/W1/2. The simulation shows the importance of the diffraction of radiation passing through the RF coil, leading to non-uniform illumination of the sample. A simple single pass model yields insight into the origin of the observed B1 value. Methods of optimizing the B1 value are described.