Design of a Printed Circuit Board TDR/Tdt Near-Surface Soil Moisture Sensor.

SSSA 2016 Meeting Advances in Soil Sensing and Model Integration with Instrumentation Poster Design of a Printed Circuit Board TDR/TDT Near-Surface Soil Moisture Sensor Wenyi Sheng, Rong Zhou, Scott B. Jones, Scott Anderson and Markus Tuller Department of Plants, Soils and Climate, Utah State University Abstract: Surface soil moisture governs heat and mass exchange processes between soil and atmosphere. We present a novel design of a time-domain reflectometry/transmissometry (TDR/TDT) sensor with printed circuit boards (PCBs) as electrodes for surface soil moisture sensing. The electrode PCBs were designed with single-sided spiral traces as waveguides for the TDR/TDT 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. Because of the challenges in examining the highly nonlinear fringing field analytically, the influence of the PCB-based electrode geometry (e.g., trace width, length, and spacing) on sensor performance was evaluated primarily by means of EM field simulations Three prototypes of electrode PCB with varying electrode spacing (i.e., 1.0, 2.5, and 5.0 mm) were simulated and experimentally evaluated to obtain specific penetration depths. To verify sensor permittivity measurement accuracy, electrodes were immersed in different dielectric materials of known permittivity. A comparison between TDR- and TDT-based sensors suggests that TDT may be advantageous as it involves a larger sensing volume for a given PCB area, due to its two-way waveguide compared with the one-way reflection traces of the TDR.