Prototype of Circular Split Ring Resonator-Based Sensor for Estimating Soil Moisture as a Function of Soil Particle Distribution
Swaranpreet Kaur, Surinder Singh, M. M. Sinha
Abstract
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 behavior of soil with varying particle sizes changes as the moisture content (MC) fluctuates. Therefore, it is important to consider both the MC 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 MC The analysis is conducted on three distinct soil types: sandy soil, clayey soil, and loam soil. It operates in both transmissive (S<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{12}}\text {)}$ </tex-math></inline-formula> and reflective (S<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{11}}\text {)}$ </tex-math></inline-formula> modes but is more sensitive in the S<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> 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<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> 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 MC, and this correlation is evidenced by the gradual increase in the relative permittivity from 2.6461 to 7.038 with a rise in the MC from 0% to 13%. In contrast, clayey soil exhibits a comparatively lower relative permittivity, ranging between 2.7366 and 3.9818 as the MC 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 MC (%) in different soil textures by analyzing the frequency shift. A second-order polynomial equation (R<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}} =0.99994$ </tex-math></inline-formula>) is also formulated to calculate the relative permittivity based on the observed frequency shift. Moreover, three equations have been derived to estimate MC (%) in different soil textures as a function of relative permittivity, resulting in impressive R<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> values of 0.9968, 0.9962, and 0.9926 for sandy, loam, and clayey soils, respectively.