Evaluation of a prototype variable frequency mode soil moisture and EC probe for its final development

A prototype dielectric probe developed by the Daiki Rika Kogyo Co., Ltd, Japan, was evaluated by controlled laboratory experiments. The probe simultaneously measures the real and imaginary parts of the dielectric permittivity, which are comparable to the measurements obtained with a vector network analyser (VNA) at frequencies ranging from 10 to 160 MHz. The probe accurately (±2% error) measures the real parts of the dielectric permittivity of oil–ethanol and ethanol–water mixtures over a wide range of real permittivities (3.26 ~ 79) at frequencies ranging from 70 to 90 MHz. The imaginary parts of the dielectric permittivity of aqueous solutions are related to the electrical conductivity (EC) through a unique rational model (±2.5% error) over 80–90 MHz. Dielectric measurements by probes at the desired frequencies can eliminate/reduce the limitations of fixed‐frequency measurements, such as the effects of EC, clay composition, porosity, and organic matter, of currently available probes.