Failure estimating transmission line Zo – λ/8 method – nanoVNA discussed the potential for failure using this ‘no-brainer’ method of estimating differential mode characteristic impedance Zo.
Well, as the article showed, it is not quite the no-brainer but with care, it can give good results. This article documents such a measurement of a 0.314mm cable.
The nanoVNA was carefully SOLT calibrated from 1 to 201MHz. Care includes that connectors are torqued to specification torque… no room here for hand tight, whether or not with some kind of handwheel adapter or surgical rubber tube etc.
Above is the Smith chart view over the frequency range from a little under λ/8 to a little over λ/8. It is as expected, a quite circular arc with no anomalies. Since the DUT is coax, and the connector is tightened to specification torque, we would expected nothing less. The situation may be different with two wire lines if great care is not taken to minimise common mode excitation. The sotware does not show Marker 2 properly, it should be between ‘c’ and ‘i’ of the word Capacitive.
Above is the R,X scan, Marker 2 is at the λ/4 resonance (X≈0) and Marker 1 is exactly half of that frequency, so λ/8 electrical length at which point |X|=|Zo|.
From the λ/8 measurement, we can calculate VF=0.314/(1.767384/4)=0.710 which reconciles with the datasheet.