Wireless Battery-Free Active and Passive Underground Soil Moisture Sensing System Design and Field Characterization for Future Agricultural Applications

This paper presents the design, lab characterization, and field test of two wireless battery-free underground soil moisture sensing systems for future sustainable agricultural application. To overcome the limitations of cable connection and battery maintenance associated with the conventional technologies, we propose a wireless battery-free active sensing system employing a capacitive sensing scheme coupled with an ultra-low-power tunnel diode oscillator to provide frequency modulated data telemetry with a total power dissipation of 0.32mW, and a wireless passive sensing system utilizing a time-domain resonant response-based sensing technique that consumes zero DC power. Both systems are operated wirelessly via inductive coupling to enable the active sensing system and the passive sensing system capable of operating at 30cm and 15cm below the ground surface, respectively. Robust 3D-printed sensor packaging ensures long-term field deployment. Field tests over 40 days demonstrate that the wireless active sensing system achieves an accurate soil moisture measurement with an error less than 3.5% after temperature compensation, while the passive sensing system attains an error less than 1.5%. The proposed systems can provide reliable and low-maintenance solutions for large-scalein-situsoil moisture monitoring, critical for enabling precision irrigation and efficient water resource usage.