Theoretical model of a multisection time domain reflectometry measurement system

Abstract

A multisection model is presented to simulate electromagnetic wave propagation in an unmatched time domain reflectometry (TDR) probe and layered soil system. The model uses a linear‐time‐invariant feedback system to model each 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 equivalent sections and matching the simulated waveform with the actual waveform. The excellent match between the simulated and the recorded waveforms verifies the model. This model eliminates the requirement of a matched 50‐Ω probe handler in probe design for dielectric measurement. It can also be used to assist probe design and TDR data interpretation. The reflected waveforms in layered soil specimens are modeled, and the simulated waveforms are compared to those obtained from experiments. The method of using apparent dielectric constant to obtain average volumetric water content is examined for the layered soil system. The results show that apparent dielectric constant is applicable for the measurement of average volumetric water content for a layered soil, but caution has to be used when interpreting the waveforms.