Millimetre-wave reflectometer

This paper describes a reflectometer, which can operate at either 35 or 94GHz. A broadband signal is produced by a modulated noise source in either vertical or horizontal polarisation. After reflection by the sample under test a superheterodyne receiver detects this signal. The noise source and the receiver are mounted on two opposed 0.5m parabolic antennas. These antennas are supported on arms, which can be rotated by stepper motors under computer control. The computer also controls the data logging system, which consists of a lock-in amplifier and analogue to digital converter. This instrument allows reflectivity to be measured for incidence angles from 20 to 70degrees down to a minimum of 0.05%, which is sufficient to measure diffuse reflection.