Wireless Power Transfer Through Soil for Energizing an Underground Soil Moisture Sensor

This paper presents the design and measurement result of an inductively coupled power transfer system, which can wirelessly transmit 20mW through a 15cm-deep soil to energize a soil moisture sensor. The power transfer system consists of a tuned primary coil exhibiting 6cm-diameter and 9-turn, which is inductively coupled to a secondary coil, tuned to the same frequency of 2.5MHz, exhibiting 6cm-diameter and 1-turn while being sandwiched between two ferrite sheets. Ferrite materials are employed to enhance the secondary coil's inductance value and quality factor, which are critical for wireless power transfer. A soil moisture level varied between 31% and 46% under a controlled manner was detected by the soil moisture sensor under wireless powering.