Measurement of permittivity and dielectric loss with a millimetre-wave Fabry-Perÿt interferometer

The application of a Fabry-Perÿt interferometer for the measurement of the permittivities and loss tangents of materials at millimetric and sub-millimetric wavelengths is considered. Theoretical expressions for the fields in the dielectric and air regions of the resonant interferometer are derived for a general position of the sheet between the reflectors. Measurements were made on an interferometer at wavelengths of about 6mm, and the results for the permittivities and loss tangents of Teflon, polystyrene and Plexiglas are given. Values of permittivity are believed to be accurate to well within 1%. The accuracy of the tan ¿ determinations is dependent on diffraction losses in the interferometer, and on the reading precision. Results obtained show the correct gradation for these dielectric materials, and are in reasonable agreement with other determinations. The method is potentially applicable to many other similar types of measurement, and becomes easier and more accurate at shorter wavelengths.