Abstract: A fault interior to an overmoded waveguide transmission line is located by pneumatically propelling an electrically semi-transparent reflecting piston within the waveguide line. The piston is designed to give low forward insertion loss yet at the same time a sufficient reflection in response to a transmitted millimeter wave test pulse stream. By measuring the magnitude of the signal reflected from the piston and returned to the transmitting terminal as the piston is moved along the waveguide length, the transmission loss of the waveguide is determined as a function of distance from the transmitting terminal. By examining the transmission loss measurements for any abnormal step or slope change or ripple which would be indicative of a waveguide fault, the location of a fault can be precisely determined.