Time domain reflectometry (TDR) is an increasingly popular method for measuring the water content θ. As opposed to most or all previous studies, in this study we investigated a TDR probe that employs only one single metallic rod for measuring θ. The probe is based on the concept of a Sommerfeld wire. The electromagnetic behavior of the probe is analyzed and experimental results are presented that provide evidence of the probe's applicability and its working principles. Dielectric properties, and hence water contents, obtained with the single rod probe were compared with those of a standard two-wire TDR probe, and with gravimetrically determined water contents. Additionally, we calculated the probe's region of influence (RoI), which was considerably larger than that of standard two-wire probes. We also tested the RoI experimentally. The new probe is easier to install, robust, and has a larger sphere of influence over which the dielectric permittivity distribution is averaged.