Wireless Power Transfer Through Soil Over a Range of Moisture Levels for In-Situ Soil Health Monitoring

This paper presents the design and measurement results of wireless power transfer through soil over a range of soil moisture levels for future low-cost, wireless, battery-less, and in-situ soil health monitoring technologies. A pair of packaged 6 cm-diameter coils can wirelessly transfer 0.56 mW from an external RF power source of 37 mW through a soil depth of 15 cm over a typical moisture level ranging from low 30% to 40%. An optimal frequency of 1.4 MHz and 1 MHz was chosen for an efficient operation in loamy soil and sandy soil, respectively. The demonstrated power transfer is sufficient to energize underground soil health monitoring devices. In addition, a pair of packaged 12 cm-diameter coils, taking the loss contributed by the surrounding soil into design consideration, can achieve a 130 µW power delivery with an efficiency of approximately 8% over a 30 cm air gap. It is expected that a similar performance can be achieved through a 30 cm soil depth.