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A 390 GHz harmonic gyrotron based on a cusp electron gun was designed to operate on a TE71 mode and the 7th cyclotron harmonic of a large orbit electron beam. This beam is produced by a cusp electron gun with a voltage of 40 keV, a current of 1.5 A and an adjustable velocity ratio from 1 to 3. Smooth cylindrical waveguide was studied and built as the interaction cavity. The relationship between the cavity dimensions and cavity Q was optimized for output at the design mode. A simple linear output taper with moderate mode conversion was used in the gyrotron output. Both theoretical analysis and numerical simulation results (using the 3D PIC code MAGIC) of this gyrotron will be presented. When the beam thickness and beam spread were introduced into the simulation, competitive modes were observed in the simulation resulting in a drop in the output power. A high frequency (325-500 GHz) VNA was used to measure the millimeter wave properties of the gyrotron cavity and the results are presented in this paper.