Near-Zero-Power Passive Wireless Underground Soil Moisture Sensor

This paper presents the design and laboratory characterization of a near-zero-power passive wireless underground soil moisture sensor. The system employs two inductively coupled LC tank circuits operated under a two-phase operation with a time-domain resonant response-based sensing scheme to achieve a large sensing distance. A 40cm-diameter primary coil and 6cm-diameter secondary coil are designed to obtain high quality factors above 100 at high frequency over a typical soil moisture range. An operating frequency of 6MHz is chosen to maximize system sensitivity achieving a frequency shift of 150kHz for soil moisture ranging from 20% to 45%. The passive sensor can be operated wirelessly under a soil depth of 15cm, limited by the system feed-through signal.