Time domain transmission sensor for soil moisture profile probe, selected technical aspects

Time Domain Transmissometry (TDT) is one of the methods of electromagnetic metrology. This method, similarly to TDR, is mainly used to measure parameters of transmission lines and connectors. It is also suitable for determining the dielectric properties of materials from which the transmission line is built. It is known that water has high dielectric permittivity of about 80. This means that the volumetric water content of the soil has a decisive influence on its dielectric properties. A transmission line placed in the soil can be a detector of dielectric properties and then soil moisture. For these reasons, the TDR and TDT methods are commonly used to measure and monitor soil moisture. The selection of materials shape and construction of the transmission line are a constant challenge for the sensor designers. The probe should be watertight and materials used for construction should not absorb water. The paper presents selected research results concerning the design of soil moisture sensor and signal analysis to determine bulk dielectric permittivity.