Considerations on measurement frequency of electromagnetic sensors for soil water content determination

• Comprehensive examination of operating frequency impacts on EM sensor performance. • Both sensor variables and soil properties affect water content determination accuracy. • Standards simulating relaxation mechanisms are needed to improve EM sensor evaluation. Electromagnetic (EM) sensors are widely used to measure soil water content for different applications. The dielectric response of soil over the operational frequency of EM sensors in the megahertz to gigahertz range can be affected by a number of factors other than soil water content. It is therefore beneficial to examine the measurement frequency of sensors, for better understanding the sensor output (i.e., permittivity or circuit response), as well as its impact on water content determination. Previous investigations differed in measurement equipment, tested frequencies, and soil variables and hence found inconsistent conclusions regarding various EM sensors. In this paper, we try to provide comprehensive considerations on measurement frequency for EM sensing of soil water content, which could clarify sensor performance, selecting appropriate sensors, and designing new sensors.