Analysis of wave reflection from open-ended coaxial lines and application to the measurement of soil moisture and salinity

Open-ended coaxial lines among others have been used to measure permittivity and soil moisture because of their simplicity. The preciseness of the permittivity measurement, however, depends on the calibration of the dielectric probe. An exact analysis of the open-ended coaxial line is also required for development of a good dielectric probe. In this paper, at first, the wave reflections from various types of open-ended coaxial lines contacted on the various materials have been analyzed exactly using the finite-difference time domain (FDTD) technique. Due to the coordinate transformation from three-dimensional rectangular structure to two dimensional cylindrical structure, the characteristics of wave reflection were computed very efficiently. Then, an open-ended coaxial line was selected based on numerical analysis, and calibrated precisely for permittivity measurement. Many samples of soil and saline water were used for calibration of the probe. It was found that the soil moisture and the salinity estimated by the dielectric probe agreed very well with the measurements.