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I'm wondering if the correct way to measure the length of window line
with a NanoVNA is fundamentally different than that of measuring coax.

I'm following the method described by Arie
(/g/nanovna-users/message/26832). I have 5.175m of "JSC
1320 300 Ohm Ladder Line 300 Ohm 20 AWG / 7 Strands Bare Copper". That's
a quarter wavelength of 20.7m, or a frequency of 14.5MHz.

The first step is to sweep it to determine the velocity factor. Yet,
when I sweep from 12-17MHz, I get the Smith chart attached. There's no
point when the impedance is close to zero.

Am I doing something wrong (most probable), or is measuring the length
of window line with an NanoVNA fundamentally different than measuring coax?

Thanks for your advice and suggestions.

-Kevin
KC3KZ