Non-unity reflection coefficient at the open end of a coaxial TDD probe

In the last two decades, Time Dornain Reflectometry (TOR) has become a extremely versatile method to measure soil volumetric water content and bulk soil electric conductivity. Recent research has also proved the applicability of TOR for measuring subsoil petroleum contaminant saturation. Next to the traditional time domain approach, many researchers use the frequency domain scatter function to describe the measured signal with a network analyser. Existing models of the scatter function are based on the assumption that the soil in the cross-section of the TOR probe is homogeneous and that the end of the probe can be described as an open ended circuit [1]. The latter assumption implies that all energy is reflected at the end of the probe and hence, all energy loss is the modelIed as due to an intrinsic loss mechanism in the soil. This assumption would work if the probe would be ended in air. As the TOR probe is usually ended in the soil which can have relatively high complex and frequency dependent electric perrnittivities, the reflection at the end of the probe is expected to deviate from unity and to be frequency dependent as weIl. In this paper we show that this is indeed the case and that the unity end reflection coefficient assumption leads to wrong values of the electric perrnittivity