A 20–400 MHz microstrip-line measurement system for soil permittivity and electrical conductivity determination

Electromagnetic (EM) sensors are widely used to determine soil water content in agricultural, geotechnical, and ecological applications. However, devices operating at lower frequencies, typically in the tens to one hundred MHz range, are more susceptible to interference from various soil properties and mechanisms, which can impact measurement accuracy. In this study, we introduced a microstrip-line-based measurement system designed to operate at variable frequencies for soil property determination. The system leveraged a direct digital synthesizer (DDS) to generate sinusoidal signals within the 20–400 MHz range, enabling the determination of soil dielectric permittivity ( ε ) and electrical conductivity ( σ ) through phase difference ( PD ) and magnitude ratio ( MR ) measurements between test and reference signals. Both EM simulations and laboratory experiments validated the correlation between ε and PD , as well as σ and MR , for the materials under test (MUT). Results showed that higher MUT ε produced larger PD values, while increased MUT σ led to lower MR values. Quadratic polynomial fits achieved high R 2 , with root mean square errors below 3 units for ε and 0.6 dS m −1 for σ across the examined frequency range. The effect of σ on ε determination accuracy was significantly reduced at higher frequencies (200–400 MHz), with relative error below 10 % and optimal performance of 8 % observed at 250 MHz. The development of this microstrip-line-based measurement system provides a flexible and practical approach for estimating soil ε and σ across a range of frequencies within the VHF/UHF bands, with potential for adaptation to in situ applications.