Time domain transmission sensors for soil moisture measurements

Delay time measurements, e.g. time domain reflectometry (TDR), are a well-established method for the measurement of permittivity in various materials, especially in soils. However, the accuracy offered by common TDR measurements is limited due to multiple reflections caused by the material under test. This contribution deals with new sensor designs for the characterization of soil moisture using time domain transmission (TDT) measurements. The basis of the advanced TDT technique is a new waveguide concept, which offers the possibility to perform transmission measurements in soils. Thus multiple reflections along the waveguide do not influence the measurement accuracy. By using a so-called “concentric reversion coupler” TDT measurements can be performed without a measuring port at the end of the sensor.