DESIGN OF TRIPLE-WIRE TIME-DOMAIN REFLECTOMETRY PROBES IN PRACTICE AND THEORY

Time domain reflectometry (TDR) is accepted as a valuable tool for mea suring soil water content and bulk soil electrical conductivity. The a ccuracy of TDR measurements depends on the quality and type of probes, as well as on the length of cable used. The objective of this study w as to test the effects of different triple-wire TDR probe dimensions a nd cable lengths on the measurements. Additional measurements were don e in order to test the performance of small triple-wire probes in soil s with a wide range of water contents. Measurements in air and in wate r showed that the position of the first reflection from the connection between cable and probe is influenced by the dielectric medium. This problem was solved by using the zero of the cable tester as a time ref erence, and calibrating the TDR probes before measurements. The major effect of increasing cable lengths is that the rise time of the TDR vo ltage pulse increases, spreading each reflection across a larger time interval, which influences the accuracy of the wave form analysis and causes a possible underestimation of the apparent dielectric permittiv ity in dry soils. As a result, it is not possible to use short probes with long cable lengths. Smaller spacing of the wires results in steep er reflections from the end of the probe. Small triple-wire probes are convenient in laboratory applications. Large triple-wire probes are c onvenient for automated field applications in which long lengths of ca ble are required.