Wireless Battery-Free Sub-mW Underground Soil Moisture Sensing System

This paper presents the design and laboratory demonstration of a wireless battery-free underground soil moisture sensing system across a soil depth of at least 30cm. The system is designed by employing a tunnel diode oscillator-based capacitive sensing architecture and dissipates 0.32mW. The low power consumption allows a battery-free operation through RF powering by using a small underground power receiving coil with a diameter of 6cm, which is inductively coupled at 2MHz to an above-ground power transmitting coil exhibiting a diameter of 40cm. The tunnel-diode oscillator operates around 40MHz. Its frequency can be varied by 580kHz for a soil moisture level ranging from 19% to 45%. The frequency-modulated oscillator signal can be wirelessly received above the soil surface.