A Microstrip Resonator With Slotted Ground Plane for Complex Permittivity Measurements of Liquids

A novel microstrip resonator is designed to measure complex permittivities of liquids with medium loss. The enclosed resonator is based on a lambda/2 open-circuit microstrip line with a slot in the ground plane. It is immersed into the liquid under test to measure the S-parameter from 2.2 to 2.6 GHz. The complex permittivities of binary liquid mixtures of methanol and ethanol are reconstructed from the resonant frequency and the unloaded quality factor. The experimental results agree with reference values. The maximum errors are 4.4% and 8.6% for the real and imaginary part of the permittivity, respectively.