Prototype of Circular Split Ring Resonator based Sensor for Estimating Soil Moisture as a Function of Soil Particle Distribution

Soil particle size significantly influences the water-holding capacity of the soil. Smaller particles have a greater ability to retain water compared to larger particles. As a result, the dielectric behaviour of soil with varying particle sizes changes as the moisture content fluctuates. Therefore, it’s important to consider both the moisture content and the particle size when assessing the dielectric characteristics of soil. This article presents the design and experimental testing of a C-SRR based sensor for determining the dielectric properties of different soil textures as a function of moisture content. The analysis is conducted on three distinct soil types: sandy soil, clayey soil, and loam soil. It operates in both transmissive (S
₁₂
) and reflective (S
₁₁
) modes but is more sensitive in the S
₁₁
mode. The estimation of soil dielectric properties with varying moisture levels is achieved through the analysis of the displacement in resonance frequency, specifically the S
₁₁
notch. It is worth noting that sandy soil exhibits a higher relative permittivity compared to clay and loam soil. Due to its larger particle sizes, the relative permittivity of sandy soil is positively correlated with its M.C. and this correlation is evidenced by the gradual increase in the relative permittivity from 2.6461 to 7.038 with a rise in the M.C. from 0% to 13%. In contrast, clayey soil exhibits a comparatively lower relative permittivity, ranging between 2.7366 and 3.9818 as the M.C. changes from 0% to 13%, attributed to a higher bound water fraction. The sensor proposed in this work offers a higher sensitivity, approximately 76.44 MHz, and can estimate M.C. (%) in different soil textures by analyzing the frequency shift. A second-order polynomial equation (R
²
= 0.99994) is also formulated to calculate the relative permittivity based on the observed frequency shift. Moreover, three equations have been derived to estimate M.C. (%) in different soil textures as a function of relative permittivity, resulting in impressive R2 values of 0.9968, 0.9962, and 0.9926 for sandy, loam, and clayey soils, respectively.