Nondestructive sensing of physical properties of granular materials by microwave permittivity measurement

The dielectric properties of wheat, corn, and soybeans were determined by measuring the scattering transmission coefficient S₂₁ in free space at frequencies between 2 and 13 GHz. For better accuracy, a pair of horn-lens antennas was used and time-domain gating was applied to the main response. Variations of the dielectric properties with frequency and physical properties such as bulk density, moisture content, and temperature were investigated. Both the dielectric constant and the loss factor decreased with frequency and increased linearly with bulk density, moisture content, and temperature. Three different approaches are used to correlate the measured dielectric properties and physical properties. Explicit relationships between the dielectric properties and the different physical properties are given at a midrange frequency of 7.0 GHz as an example. The potential use of these relationships is shown for the development of indirect methods for the nondestructive and instantaneous determination of the physical properties of cereal grain and seed from measurements of their dielectric properties. Different sources of error in attenuation and phase shift measurement are discussed, and their effects on the accuracy for the determination of the relative complex permittivity and physical properties are investigated