Design of Access-Tube TDR Sensor for Soil Water Content: Testing

Soil water measurement is important in water management for irrigation and in hydrologic sciences. The purpose of this paper is to develop and test the design of a cylindrical access-tube mounted waveguide for use in time-domain reflectometry (TDR) for in situ soil water content sensing. Several prototypes with varying geometrical parameters were constructed. The sensors were compared by evaluating the characteristics of reflected waveforms from a (200-ps) step pulse in different media, including air, triethylene glycol, deionized water, and over a range of water contents in sand and a clay loam soil. Sensors with greater separation between electrodes, achieved by means different tube diameters or the separation angles, tend to have greater field penetration in both sand and clay. In addition, sensors with the shortest electrode separation show greater sensitivity to soil electrical conductivity. Together, these trends demonstrate that the propagating electromagnetic fields above 0 Hz do not take the transverse electromagnetic form commonly assumed in the analysis of TDR probes.