Dielectric measurements using a coaxial resonator opened to a waveguide below cut-off

A model of a quarter-wavelength coaxial resonator opened to a semi-infinite circular waveguide below cut-off is presented in the paper. 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 with a dielectric material. As a consequence, the parameters of the resonator, resonant frequency, quality factor and resonant transmission, change in relation to the complex permittivity of the material. A fully automated L-band microwave system with resonator sensor was fabricated and used for measurements of the permittivity of low-loss and high-loss powdered materials. Results obtained using the system are compared with those of other method.