Measuring dielectric constant from group delay

Updated August 16, 2008

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New for August 2008! Our S-parameter Utilities spreadsheet now has the capability of calculating effective dielectric constant over frequency for measurements of transmission line structures (microstrip, CPW, coax, etc). The basic idea is from the group delay calculation, if you know the length a a measured transmission line, you can calculate the phase velocity (meters/second); here you can see the dispersion of the media up close and personal! From VP you can calculate effective dielectric constant (Keff). Using transmission line equations for a given geometry you can determine the filling factor, and back out the true dielectric constant.

This measurement technique can have improved accuracy if you measure two different lengths of transmission lines, and then subtract them. This reduce the reference plane errors associated with the measurement, you won't even have to de-embed the data.

Can someone supply us with two sets of S-parameters for two lengths of transmission lines and we'll show how this calculation works?