THE SAMPLE AREAS OF CONVENTIONAL AND ALTERNATIVE TIME-DOMAIN REFLECTOMETRY PROBES

We define the sample area in the plane perpendicular to the long axis of conventional and alternative time domain reflectometry (TDR) probes based on the finite element numerical analysis of Knight et al. [1997 ] and the definition of spatial sensitivity of Knight [1992]. The samp le area of conventional two- and three-rod probes is controlled by the rod separation. Two-rod probes have a much larger sample area than th ree-rod designs. Low dielectric permittivity coatings on TDR rods grea tly decrease the sample area. The sample area of coated rod probes dec reases as the relative dielectric permittivity of the surrounding medi um increases. Two alternative profiling probes were analyzed. The sepa ration of the metal rods of Hook et al. [1992] probes controls the siz e of the sample area. Reducing the height or width of the rods improve s the distribution of sensitivity within the sample area. The relative dielectric permittivity of the probe body does not affect the sample size. The sample size of the Redman and DeRyck [1994] probe is also co ntrolled by the rod separation. Two alternative surface probe designs [White and Zegelin, 1992; Selker et al., 1993] are shown to have simil ar sample areas.