What you are looking for is the "port extension" feature, which is "only
software" but nevertheless is missing from many implementations. (Back
in the stone age, port extension was a hardware feature implemented with
a mechanical line stretcher, but now is pure mathematics). It's not as
good as actually doing SOL at the end of the coax of coarse and depends
on knowing the physical length of the coax and its velocity factor.
Some implementations have a TDR function that allows you to determine
these numbers for coax that is already up on the tower. Then you can
enter them into the port extension feature.
In a prior life I designed network analyzers for Agilent.
---
Rick Karlquist
N6RK
On 2026-02-11 12:32, Jim Lux wrote:
Almost all of the current inexpensive VNAs can do this.
It's the de-embedding of the coax that might not be automated (unless
you cal at the end of the coax, which is inconvenient if the end of the
coax is at the top of the tower). Not all the VNAs have the ability to
"subtract out" the transmission line. Most of the programs that run on
a computer that talk to the NanoVNA and its ilk can do this.
On Tue, 10 Feb 2026 11:40:20 -0800, Jim Brown
<jim@audiosystemsgroup.com> wrote:
On 2/10/2026 11:03 AM, Jim Lux wrote:
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 common use of analyzers is to simply use the SWR as a rough indicator
or resonance. The best can export a measurement as a data file (plain
text) that can be imported by freeware like SimSmith, which can then
"tune out" the coax by inserting a length of coax and varying its
length
until we see the normal Smith Chart view of resonance. I've used that
model to design switchable stub matching networks for my 80 and 40M
dipoles on the CW and SSB portions of the band. I've also used the
model
to optimally place along the line stubs to suppress power amp
harmonics,
and to eliminate interaction between harmonically related antennas in
close proximity. See k9yc.com/7QP.pdf for an example.
For more than ten years, I've used the VNWA, built by hams in the UK
based on a design by German EE prof DG8SAQ, and using his control
software. That software can do that transformation and a lot of other
math, like transforming a measurement of an antenna fed by 75 ohm line
to show its true impedance.
73, Jim K9YC
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