An Open-ended Coaxial Probe for Broad-band Permittivity Measurement of Agricultural Products

Abstract

The moisture content of most agricultural materials is closely related to their dielectric properties, and to characterize their response, measurement over a broad frequency range is desirable. A coaxial probe and calibration system has been designed for measurement of the permittivity of semi-solid or liquid samples over the frequency range 1 MHz–6 GHz. The reflection coefficient of the probe in the sample was measured using a vector network analyser. Calibration was achieved by using three references: an open circuit, short circuit (indium foil) and a reference liquid which was chosen so its permittivity was similar to that of the sample. The sample permittivity was calculated by an accurate numerical model which used as inputs, the true reflection coefficient obtained from the VNA, and physical measurements of the geometry of the system.The measurement system was tested using reference liquids, and the experimental configuration was optimized for accuracy over the whole frequency range. Numerical instability generated errors at low frequencies, and resonances limited accuracy at the top of the measurement frequency range. Over the frequency range 20 MHz–6 GHz and using an ethanol reference, the error in the measured relative permittivity of ethanol was less than 0·3. (or 1·3%).