Sample area of two‐ and three‐rod time domain reflectometry probes

Recent advances in the application of numerical analyses to the spatial sensitivity of conventional two‐ and three‐rod time domain reflectometry (TDR) probes allow for investigation of the response of TDR in spatially heterogeneous media. In this study, we present numerical analyses and laboratory measurements of the effects of steep gradients in relative dielectric permittivity (K) on the spatial sensitivity of TDR probes. Two‐ and three‐rod probes were placed horizontally through the walls of an experimental box. These horizontal probes were placed either within the same horizontal plane (horizontal probes) or within the same vertical plane (vertical probes). Then, an air/liquid interface was raised upward from below and past the probe. Three liquids (sunflower oil, ethanol, and water) with varying K values were used. The same system was analyzed using a numerical model of the spatial sensitivity of TDR probes to lateral variations in K. There was good agreement between measured and modeled K, demonstrating that the spatial sensitivity of TDR probes is biased toward the area of lowest K. The sample areas of all of the probe designs depend strongly on the relative dielectric permittivity distribution in the plane transverse to the direction of wave propagation. Two‐rod instead of three‐rod probes should be used if sharp changes in K are expected in the direction transverse to the plane containing the probe rods, owing to separation of the traveling electromagnetic (EM) waves in the three‐rod case. Generally, a horizontal probe orientation is more appropriate for monitoring across sharp vertical boundaries such as wetting fronts.