Analysis of Dielectric Loaded Circular Waveguide Probes for Time Domain Reflectometry

Time domain reflectometry has been used for fluid level measurement and characterization since the 1960's, however, there is little research present in the literature to improve the measurements by using alternate probe designs. Numerical analysis of a dielectric loaded circular waveguide probe has determined that appropriately chosen dimensions and dielectric loading will allow for control of the reflection coefficient and consequently its permittivity accuracy. Additionally, the resulting reduction in signal velocity allows for an increase in height measurement accuracy. A detailed optimization procedure has been developed and implemented with MATLAB to determine the required probe dimensions and dielectric loading for a particular lossless application. Using Ansoft HFSS the validity of the lossless method was confirmed at low frequencies for lossy ethanol-water and acetic acid solutions. At 1GHz the procedure yields parameters that allow for accuracy increases on the order of 5 to 20 times that of a traditional probe.