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On Friday, January 1, 2021, 02:02:35 PM PST, John Gord via groups.io <johngord@...> wrote:

Bob,
I don't think you are remembering quite right.

For the first method:
Set the frequency range for Start 50kHz, Stop 500MHz.
Calibrate (open, short, load) the VNA at the point where the unknown cable will be connected.
Turn on Smith Chart display if it is not already on, and set the Smith marker for R+jX mode.
Connect the unknown cable, open circuited at the far end.
Adjust Stop frequency until the arc on the Smith Chart goes from 3 o'clock (open) to 9 o'clock (short).? This is the frequency at which the cable is one-quarter wavelength long.
Set the marker to half the Stop frequency.? At this frequency the cable is one-eighth wavelength long.
Read -jX from the marker.? The X value is the characteristic impedance of the cable.? (The impedance of an eighth-wavelength open cable is -j times the characteristic Z of the cable.)

For the second method:
Set the frequency range for Start 50kHz, Stop 500MHz.
Calibrate (open, short, load) the VNA at the point where the unknown cable will be connected.
Turn on Smith Chart display if it is not already on, and set the Smith marker for R+jX mode.
Connect the unknown cable with an adjustable (not wirewound) resistor at the far end,
Adjust the resistor to give the closest approximation of a single dot on the Smith Chart.? The R at that point is the Z0 of the cable.
This works because a cable terminated in its characteristic impedance looks like that impedance at all frequencies.

There are, of course, other methods, including using the TDR (transform) function.? See message 12494.

--John Gord


On Fri, Jan? 1, 2021 at 12:15 PM, Bob Albert wrote:

? Use Smith chart display.? Sweep the frequency to find the place where the
curve passes the first quarter of the circle, which is one eighth wave.? Move
the marker to that spot and read the reactance,
At least that's how I remember doing it.
Another method is to terminate the line in a variable resistor and adjust for
SWR = 1.00.? Then measure the resistor or read the impedance.
There are other ways also.
Bob
? ? On Friday, January 1, 2021, 12:02:26 PM PST, Noji Ratzlaff
<nojiratz@...> wrote:

? How can I use a NanoVNA to measure the characteristic impedance of an
unmarked length of coaxial cable? I have a NanoVNA-H4.

I saw another thread here titled "characteristic impedance," but the
conversation quickly got off-topic, and remained off-topic, so I bring it up
again.

Thanks,
Noji