Time domain reflectometry developments in soil science: I. Unbalanced two-rod probe spatial sensitivity and sampling volume

The use of parallel two-rod time domain reflectometry (TDR) probes is widespread because the two-rod probe is the least destructive of the conventional TDR probes and has a larger sample volume than a three-rod probe of equal dimensions. However, in order to transform the electromagnetic signal from unbalanced to balanced at the point of connection with the cable, two-rod probes have been thought to require a balun, making the probe construction both more expensive and more complicated. In this study, a two-rod probe without a balun (unbalanced) was exposed to a rising air-water interface, creating a sharp dielectric permittivity boundary within the sample volume of the probe. The probes were horizontal, but they were located within a vertical plane, i.e., one rod was placed above the other. A shorting diode technique was used to improve the location of the end reflection on the TDR traces. Two experiments were carried out differing only in the connection of the coaxial cable to the probe rods. In one experiment, the conductor was connected to the lower rod and the shield was connected to the upper rod. In the second experiment these connections were reversed. Using a numerical model, the relative dielectric permittivity (K) responses of two-rod balanced and unbalanced TDR probes were predicted as a function of the fluid interface height. The measured and modeled responses of the unbalanced two-rod probe matched perfectly, and there was no observed increase in the spatial sensitivity of the probe adjacent to either rod. Furthermore, the modeled probe responses as well as the sample areas for the balanced and unbalanced probe configurations were identical. Based on these results, we suggest that baluns be omitted from two-rod TDR probe designs.