Development of a Sensitive Automatic Reflecto-Interferometer for Measurements in the Q and E Bands (35 and 70 GHz) Applications

The determination of the complex permittivity for 35 and 70 GHz (8 and 4 milimetre wavelengths) presents several difficulties as soon as it becomes necessary to study materials with large permittivities and having large loss tangents (tg ¿¿1). Therefore, for the chosen method, it is necessary to measure the displacements of the interferometric fringes of the order of 10¿4 ¿, which represent the limit of resolution of mechanical millimetre phase shifters. Furthermore, manual measurements - necessitate long and tedious double readings - cannot be separated from the inherent slowness of the temperature stabilization - are, by their slowness, at the mercy of source drift - do not lend themselves easily to the necessary repetition. For these reasons, it was decided to develop an automation of the existing interferometers, with the following aims: 1) increased sensitivity 2) rapidity of measurement. To illustrate the performance of the apparatus the authors give the permittivity curves for water and ethylene glycol over a large temperature range.