An interferometer with compensation for vibration and large scale mechanical movements has been designed and built to measure the line integral electron density along three different lines of sight through the JET divertor plasma. Overcoming the effects of a long transmission path, having an estimated 65 dB loss, requires oversized waveguide transmission lines, sensitive heterodyne detection, low loss quasioptical circuits, and highly stable sources. The sources are frequency doubled, phase-locked, Gunn oscillators producing 15 mW at 130 GHz and 10 mW at 200 GHz. Waveguide Schottky mixer diodes generate reference and output signals at an IF of 10.7 MHz and the LO Gunn diodes are phase locked to the reference IF. Corrugated feedhorns and ellipsoidal mirrors are used for beam control and polarizing wire grids for beam splitting and recombination. To minimize unwanted, direct coupling of source power info the signal detectors, Brewster angle beam dumps and Faraday rotation isolators are used in the transmit and receive QO circuits, which in turn are separated, on opposite faces of a vertical plate. Martin-Pupplet polarizing interferometers are used to multiplex the two colors into a single coahgned, copolar output beam and to demultiplex the return beam. Constant fraction discriminators are used to optimize the accuracy of the phase detectors, which have sampling and recording rates of 1 MHz and a resolution of ~7degrees (0.02 fringe).