FDTD validation on an improved virtual transmission-line conversion model of open-ended coaxial probe

A GPR (ground-penetrating radar) system for locating leakage points of a water supply pipe is to be tested using a plastic tank filled with dry sand. To design our wide-band GPR antenna, we must know the electrical properties of dry and wet sand. An open-ended coaxial probe is effective in measuring complex permittivities of materials in a wide frequency range. A model of the open-ended coaxial probe is used to convert the measured reflection coefficient into the corresponding complex permittivity. We find that the deviation between the given and our measured complex permittivities of ethanol, whose electrical properties are similar to those of saturated sand, grows as the measured frequency increases. This discrepancy may be generated from experimental errors or the conversion model. To verify whether our conversion model is erroneous or not, we need the exact reflection coefficient without errors. One suitable tool may be the FDTD method. The FDTD simulation results lead us to conclude that our conversion model is very accurate in a wide range of frequency.