Single-Rod Probes for Time Domain Reflectometry: Sensitivity and Calibration

Time domain reflectometry probes consisting of one conducting rod and a wave mode converter are an alternative configuration which overcomes some of the disadvantages of conventional probes. We examined four different single-rod probes and a two-rod probe regarding their sensitivity to a small and a large conductive scatterer in their vicinity. The single-rod probes were assembled combining a small/large wave mode converter with an uncoated/coated rod. We found that the volume sampled by single-rod probes 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 single-rod probes 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 m³m^-3. The calibration showed only small differences between the single-rod and the two-rod probe regarding the measured bulk dielectric constant. Based on this study the single-rod probe is a promising new tool for improved time domain reflectometry measurement of soil moisture.