Theoretical model of a multisection time domain reflectometry measurement system

A multisection model is presented to simulate electromagnetic wave propagat ion in an unmatched time domain reflectometry (TDR) probe and layered soil system. The model uses a linear-time-invariant feedback system to model eac h section and links each section in a bottom-up fashion. Multiple sections can be incorporated in this model by a simple extension of a single-section system. An unmatched TDR probe system is modeled by dividing it into equiv alent sections and matching the simulated waveform with the actual waveform . The excellent match between the simulated and the recorded waveforms veri fies the model. This model eliminates the requirement of a matched 50-Ohm p robe handler in probe design for dielectric measurement. It can also be use d to assist probe design and TDR data interpretation. The reflected wavefor ms in layered soil specimens are modeled, and the simulated waveforms are c ompared to those obtained from experiments. The method of using apparent di electric constant to obtain average volumetric water content is examined fo r the layered soil system. The results show that apparent dielectric consta nt is applicable for the measurement of average volumetric water content fo r a layered soil, but caution has to be used when interpreting the waveform s.