Printed Circuit Board Time Domain Reflectometry Sensors for Near-Surface Soil Moisture Measurement.

Time domain reflectometry (TDR) is a well-established method for measurement of volumetric soil moisture content. Traditional TDR sensors are two- or three-rod waveguides exhibiting cylindrical sampling volume around the probe rods and along their length. We present a non-invasive TDR probe with surface-mounted electrodes on a printed circuit board (PCBs) for sensing near-surface (<5 cm) soil moisture. The electrode PCBs were designed with single-sided three-wire spiral traces as waveguides for the TDR sensor. When placing the PCB onto the soil surface, the electromagnetic (EM) field of the traces fringes into the soil, which alters the signal propagation time, depending on the soil water content (i.e., permittivity). The waveform acquisition and interpretation were implemented using the recently developed Acclima Inc. TDR-310S circuitry, which was directly soldered to the electrode PCB to omit signal bandwidth constraints due to the coaxial cable. The apparent relative dielectric permittivity determined by the PCB TDR was calibrated in air and water and further verified in dielectric liquids. High-frequency numerical simulations of the EM field were performed to examine the sensing volume of the PCB waveguide. A potential application of the PCB TDR sensor is for on-the-go mapping of near-surface soil moisture when mounted on the tire of an autonomous vehicle. The sensor is also a viable option for ground-truth calibration of soil moisture remote sensing platforms (e.g., SMAP, SMOS).