Battery-Less Wireless Chipless Sensor Tag for Subsoil Moisture Monitoring

Inefficient soil management is a hindrance to food productivity and a contributing factor to the degradation of soil quality. Despite decades of research confirming the benefits of using sensors in effective soil management and crop yield, they are still not widely used in farms for several reasons including high cost, difficulty in large area deployment, and requirement of labor-intensive wiring. In this paper, we report a low-cost battery-less chipless wireless sensor tag that can be batch-manufactured for large-scale agricultural surveillance. The sensor tag works based on backscattering techniques for the wireless measurements of the volumetric water content (VWC) of the soil. The sensor tag consists of shorted dipole resonators whose resonance frequencies are dependent on the effective permittivity of the VWC of its surrounding soil. The resonant frequency of the sensor tag is measured by an orthogonally polarized reader antenna that observes the peak in the backscattered spectrum. As a proof-of-concept, a sensor tag with two resonators of length 10 cm and 9 cm, is buried at a depth of 10 cm in the soil and is wirelessly read from a free-space distance of 50 cm, to demonstrate its capability to measure the changes in VWC. The resonators of length 10 cm and 9 cm have demonstrated a linear change of -3.92 MHz/VWC and -4.85 MHz/VWC, respectively, within the VWC range 4-27%. By changing the functional passivation layers on the demonstrated sensor tag, this concept can be applied to a wide range of low-cost subsoil sensors for precision agriculture applications.