The transmission properties of a compact circular corrugated waveguide system are investigated experimentally in the WM-380 band from 500 to 750 GHz. Up to 700 GHz, the average transmission losses range from below the detection limit to 3.4 dB/m, i.e. more than four orders of magnitude lower than rectangular waveguides. The propagation of the lowest-loss HE11 mode is found to be dominant, and long-term measurements of phase and amplitude demonstrate high transmission stability