Non-Contact Moisture Content Measurement by Horn-Antenna Reflectometric Differencing

Abstract

This study introduces a novel non-contact method for measuring moisture content, employing a horn- antenna reflectometric differencing technique. By leveraging water’s pronounced attenuation across microwave and RF bands, our approach involves taking measurements of reflected signals at two distinct stand-off distances from the same specimen. From the resulting phase and magnitude shifts, the wavenumber 𝑘 = 𝛽 − 𝑗𝛼is accurately derived. This estimated 𝑘is then transformed into the complex permittivity 𝜀'and 𝜀'', which is subsequently mapped to moisture content using either a physics-based mixture model or direct regression models developed from 𝛽and 𝛼. Validation of this technique is performed by directly comparing in-situ measurements (MC_meter) against oven-dry reference values (MC_ref) obtained from meticulously paired specimens. Our comprehensive performance assessment incorporates several key metrics: the consistent co-variation between the two value sets, their average disparity (diff), the repeatability quantified by the coefficient of variation (CV%) from repeated measurements, and the proportion of data falling within predefined control limits. Notably, this proposed methodology is applied to both horizontally arranged paper sheets and vertically loaded bulk wood chips, thereby establishing concrete quantitative criteria essential for effective on-site quality control.