Design and air–water calibration of a waveguide connector for TDR measurements of soil electric permittivity in stony soils

Time Domain Reflectometry (TDR) is one of the most widely used methods for measuring soil water content. TDR determines water content by measuring the apparent relative electric permittivity of the soil or relative permittivity (ɛra), which is directly related to soil water. For TDR waveguides used in stony soils, the connection between the Trase TDR probe head (Model Trase System 6051X1, Soilmoisture Equipment Co., USA) and the waveguides can be difficult because of the lack of parallelism between the waveguides. Such a lack of parallelism is caused by the occurrence of a large amount of rock fragments in the soil profile, which produces air gaps and compression zones around the waveguides causing errors in the estimation of ɛra when the Trase TDR probe head is connected to the waveguides. For this reason, a new type of flexible connector that minimises the existing problems has been designed. The signal travel time correction factor t0, which accounts for signal travel in the designed flexible waveguide connector, and the electrical path length of the waveguide Le have been calibrated. In addition, a software for analysing the waveform obtained using the Trase TDR system has been developed. The calibration results were considered to be good and the sensitivity of soil water content values to the estimates of t0 was low. Higher values of soil apparent electric permittivity were found when the waveform was measured with the flexible connector than when the Trase connector was used; mainly with the waveguides 150 mm long and when the soil water content in the superficial layer was high.