In-situ characterization of soil moisture content using a monopole probe

In this paper, a microwave non-destructive experimental tool based on a monopole antenna is used for the determination of the complex permittivity of soils. The monopole mounted on a ground plane is buried in the soil, and its reflection coefficient measured at the feed point by a vector network analyzer (VNA) depends on the dielectric properties of the surrounding medium at a given frequency. In particular, this study is focused on the evaluation of the change of dielectric properties of different soils with the moisture content. In general, the dependence of the dielectric properties of a soil with the antenna reflection coefficient is nonlinear in nature, and the solution of the inverse problem requires a sophisticated algorithm. Thus, we have used two types of modeling, analytical (Wu's approach) and numerical (FDTD), to further understand the influence of fundamental parameters (antenna geometry and soil properties) on the frequency response of the antenna reflection coefficient. These models have allowed us to develop a novel inverse algorithm for the determination of the mean real permittivity based on the position of the antenna resonant frequencies in a wide frequency range. Moreover, a high resolution algorithm based on the Prony's approach has been developed in order to fit the complex reflection coefficient and determine afterwards the complex permittivity and the volumetric moisture content of a soil. Experimental results from different types of sands have been analyzed.