A time domain reflectometry technique to determine effective permittivity in lossless transmission lines

This paper will present a methodology for evaluating effective relative permittivity using a Time Domain Relectometer (TDR). Though the concept of using a TDR to measure permittivity is not new, it has not become widely known and accepted. It will be shown that reasonable results can be obtained with this method and that the values obtained for relative permittivity can be used with excellent results when used in the proper closed form equations or modeling method. This method is particularly useful when processes, as in present day laminated circuit boards, have had an impact on the effective permittivity. In these cases using published values or using conventional methods for determining permittivity can lead to errors in computations for characteristic impedance.