Thermal characterization of a Fabry-Perot open resonator

A Fabry-Perot open resonator has become a versatile tool for the broadband room-temperature microwave characterization of various kind of materials, including dielectric films, low-loss liquids and metals. An emerging need for the measurements at elevated temperatures has prompted the study of its thermal properties presented in this paper. It is shown that the Fabry-Perot open resonator as an ultra-sensitive fixture is capable of quantitatively detecting such barely visible effects as changes of the air refractivity with temperature and humidity. In addition to that, it is shown how the effective electric conductivity of silver-plated spherical mirrors changes with frequency and temperature. The study is concentrated on a double-concave geometry of the resonator operating in the 1450 GHz range.