This is where tools like the NanoVNA are really nice - you can calibrate at the
end of the coax. Or, if you've measured the coax once, you can save it, and
post process your data in one of the apps that takes VNA data (I use
NanoVNA-Saver, but there's lots of others).
A lot of the newer antenna analyzers are basically a 1 port VNA, and so, they
do the Short, Open, Load calibration (SOL) on an "as needed basis" as opposed
to, say, a MFJ 259 where the calibration is done at the factory.
And, there's even a trickier technique with a VNA - you can make a measurement
over a wide frequency range, have the VNA do a transform to time domain, figure
out where the end of the transmission line is. This is done by looking for the
impedance discontinuity at the feedpoint - even if it's perfectly matched in
band, it's probably not perfectly matched at lots of other frequencies, while
the coax is usually pretty "smooth". Once you know that, you can then back out
the feedline effect. Computationally more complex, but if someone has done the
algorithm and embedded it in the software tool, it's pretty sweet.
On Tue, 10 Feb 2026 07:37:48 -0700, Tom Hellem <tom.hellem@gmail.com> wrote:
In order to make measurements of impedance, etc on an antenna with any of
the various analyzers on the market, one must connect the analyzer to the
antenna
with some length of coax. And unless the antenna being measured has a
feedpoint
impedance of 50 +/- j0, the impedance as seen by the analyzer is going to
vary
continuously all along the length of the coax, a phenomenon easily seen
with the use
of a program like TLW, etc.
Question is, unless I can attach the analyzer directly to the feedpoint,
how can I
obtain a reliably accurate measurement?
I just looked at the user manual for one of the popular analyzers on the
market.
It makes no mention of this whatsoever. And I don't remember it being in
any other manual I have seen in over 50 years of being in ham radio except
for one:
I have a manual for a simple and inexpensive VNA built from a kit several
years ago, sold by a German company, that makes a big deal out of doing
what it calls an SOL compensation for
the coax and any other connectors used between the VNA and the antenna.
This
supposedly removes any of the impedance transforming effect of the test
setup from the
equation and allows for a precise measurement of the antenna under test.
So what gives? Has anybody here ever wondered about this?
Am I overthinking it or is there something to it?
Tom
K0SN
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