New type of microwave resonator sensors with waveguide below cut-off

A model of a quarter-wavelength coaxial resonator opened to a semi-infinite circular waveguide below cut-off is presented. A full-wave analysis of the transition between coaxial-line and circular waveguide was used. The analysis was based on a mode-matching formulation. The circular waveguide is filled by a dielectric material. As a consequence, the parameters of resonator: resonance frequency, quality factor, resonance transmission change in relation to the permittivity of the material. The fully automated microwave system was developed and used for measurements of low-loss and high-loss dielectric materials permittivity. Good agreement between theoretical calculations and results of measurements were obtained.