Effects of temperature on soil dielectric permittivity measured in time and frequency domains

Measuring the electrical permittivity of soil is currently the basic method of determining its water content using commercially available sensors and meters. However, the relationship between permittivity and soil water content is also determined by other factors including soil texture, salinity, and temperature. The paper presents the electrical permittivity of various soil samples at different temperatures measured by using two techniques: Time Domain Reflectometry (TDR) and Frequency Domain Reflectometry (FDR). In the first one, the apparent electrical permittivity is determined from the propagation time of an electric pulse along a metallic and parallel transmission line placed in the soil. The applied frequencies are not exactly defined as the measured material acts as a filter. Therefore, the TDR-measured soil dielectric permittivity is named as bulk soil dielectric permittivity $\varepsilon_{b}$. The FDR technique measures the soil complex electrical permittivity for each frequency from the range $20 \mathrm{MHz}-3 \mathrm{GHz}$ using a setup with a coaxial cell filled with soil. The results showed complicated temperature behavior of soil dielectric spectrum, depending on water content, salinity, frequency, and texture. Also, the parameters of a permittivity/moisture conversion function were established at various temperatures and frequencies.