Failure estimating transmission line Zo – λ/8 method – nanoVNA discussed traps in using the λ/8 method to estimate Zo… it is not the no-brainer that is often suggested.
This article shows the use of the Smith chart to look for departures from pure transmission line behavior in that test, or any other that depends on measuring purely Zin of a length of line in purely differential mode with short circuit or open circuit termination.
Above is a Smith chart plot of what we should see looking into a line of similar characteristic swept from 1 to 20MHz. There is no magic there, this is basic transmission lines and Smith chart.
Above is the Smith chart plot of a scan of the NEC model with the USB cable included. A significant anomaly can be seen and the measurement frequency is in the seriously disturbed area, little wonder the test gave wrong results. The whole curve is wrong, and will give erroneous results.
Any significant common mode current gives rise to power lost to radiation, and the measured impedance is not that of the transmission line alone. The Smith chart might not have strong hints as in the above, but it is a first place to look for departure from expected pure transmission line behavior.
You would not see this huge hint of an invalid experimental setup if you focus on the narrow band of frequencies where the electrical length is λ/8.
Above is the NEC Smith chart plot with the USB cable removed, quite as expected.