Moisture content estimation of wet sand from free-space microwave techniques

Abstract

The book chapter presents theory and best-practice measurement techniques to extract complex permittivity and hence volumetric moisture content from moist sand. The proposed technique can also be applied to other well-mixed particulate materials. In the scenario discussed a wideband horn antenna is positioned above the sample which is backed by a metal reflector plate and S11 measurements taken in the range 1 – 6GHz. Necessary EM theory derived from transmission line techniques is reviewed in the context of use in reflection measurements. Practical considerations such as antenna-to-sample separation distance, sample size and thickness constraints are discussed. Antenna calibration issues are investigated along with the use and impact of background subtraction for removal of system responses. Residual system responses require the use of more sophisticated techniques for removal and in the proposed technique we demonstrate the use of time-domain gating, and present the effects of zero-padding and windowing prior to transforming into the time domain and post time-gating operations. Next numerical solutions are used on the processed S11 to extract the complex permittivity of the sand. By using range information and time-of-flight calculations we present a method to provide accurate initial values for the numerical solution to ensure rapid convergence and reliable solutions. Using integration of the time domain reflection pulse from the free-space to sample interface we propose a relationship between the energy contained in the reflection pulse and permittivity of the sample. This information is used to position the time-gate window in time and for the generation of initial values for the numerical solution. Measured results on moist sand with thicknesses of 100-300mm and with moisture contents of ~5 - 25% show that the approach presented can extract moisture content values to within 1% accuracy.