Cavity Resonator for Permittivity Measurements of Liquids at Millimeter-wave Frequencies

Permittivity data of liquids is necessary for applications such as dielectric heating, remote sensing, and moisture detection, and is also used for molecular characterization. Dispersive molecular mechanisms occur for field excitations of frequencies mainly above 10~GHz and extending into terahertz and optical frequencies. Around 100~GHz there is less data, due to the frequency limits of microwave and quasi-optical techniques. This work presents an over-moded cavity resonator for liquid permittivity measurements. Novel full-wave modeling of a four-port inhomogeneous waveguide junction removes the limits imposed by previous methods. A cavity with environmental control was designed and tested. The parameters estimated from the modeling and measurement inputs are plausible and comparable to literature. Based on repeatability measurements and a sensitivity analysis, recommendations are made for future cavity designs that will enable permittivity measurements at frequencies previously little measured.