Single-rod probes for time domain reflectometry; sensitivity and calibration

Time domain reflectometry (TDR) probes consisting of one conducting rod and a wave mode converter are an alternative configuration that overcomes some of the disadvantages of conventional probes. We examined four different single-rod probes (SRPs) and a two-rod probe for sensitivity to a small and a large conductive scatterer in their vicinity. The SRPs were assembled combining a small and large wave mode converter with an uncoated and coated rod. We found that the volume sampled by SRPs is larger and more symmetric than in the case of a two-rod probe of equal size. A comparison of the mode converters showed a higher loss for the smaller converter but only a small difference concerning the spatial sensitivity. Coating the conducting rod with a high dielectric constant material reduces the spatial sensitivity. One of the SRPs and the two-rod probe were calibrated in a sand tank (particle size 0.08-0.2 mm) with volumetric water content up to 0.35 m3 m-3. The calibration showed only small differences in the measured bulk dielectric constant between the single-rod and the two-rod probe. Based on this study the SRP is a promising new tool for improved TDR measurement of soil moisture.