A time domain transmission sensor with TDR performance characteristics

Time domain reflectometry (TDR) has become a standard method for determining water content in soils. However, the cost of instrumentation and the level of user-ability required often limit its application, especially for agricultural and urban soil water management. The objective of this paper is to report on a new, lower cost time domain transmission (TDT) sensor and to demonstrate that it has the performance characteristics of commonly used TDRs. We approximate the sampling volume of the TDT sensor and compare it to that of TDR. We demonstrate that the sensor accurately determines permittivity from travel time measurements by comparing the TDT sensor with two TDR instruments, and a network analyzer providing measurements of frequency-dependent permittivity. Both TDRs and the TDT operated within ±3 permittivity units of each other across the permittivity range of 9-80. The reported rise time of the TDT sensor is 180 ps, which suggests a frequency bandwidth and upper frequency limit similar to TDR. We determined the maximum passable frequencies of all three instruments in non-relaxing media and demonstrate that they are in fact quite similar with the average maximum passable frequencies of the two TDR systems being 1.64 and 1.45 GHz, and the average maximum passable frequency of the TDT being 1.23 GHz. In addition to having accuracy and frequency characteristics similar to TDR, the TDT sensor offers the advantage of having the pulse generating and sampling electronics mounted in the head of the probe which reduces attenuation and overcomes the constraint that prevents TDR being used with long coaxial cables and multiplexers connecting the instrument to TDR probes.