Are you looking for the "electrical length" of the two feedlines?
A sub $100 NanoVNA can do it - esp with some software on a PC to drive it.
Do the "Time domain" calculation over a fairly wide bandwidth. Sure, your
antenna is a "good match" at some frequencies, but it's terrible at others. So
you'll see a discontinuity where the feedpoint is in the time domain
measurement.
You could probably do the measurement at a single frequency, chosen to be
something where you know the antenna is a mismatch, and then look at the phase
of the S11. Pick a frequency close to your operating frequency (but still off
resonance) so if there's any frequency dependent change in velocity it's
minimized. And you'll need to resolve the ambiguities (is the round trip 270
degrees, or is it 630 degrees.
This won't find the phase difference in the antennas themselves, which is
affected by things like the surroundings.
On Tue, 10 Mar 2026 11:22:53 -0500, George Fremin III <geoiii@kkn.net> wrote:
I have posed this question to a number of people over the last few years - and
I have gotten some interesting ideas but I am not sure any of them have
provided a simple or repeatable way of making such a measurement. I have not
tried any of they yet but I think I am getting to that point.
So, I thought I would cast a larger net.
On the simple side of things - if I have two antennas - lets say verticals at
some distance from each other with coax coming from them is there some way to
measure the phase difference? Could I get some value out so that I know how
much delay I need to add to one to get them in phase for a given signal?
Or lets say I have two yagis on a tower fed with coax and I would like to
verify they are in phase or at least close - can I measure this in some way?
What if I have say a 48 foot boom 10m yagi and a 24 ft boom 10m yagi on a tower
at 60ft over 30ft. If there a way for me to measure the phase difference?
The last question is a real use case - and on the air use of this antenna seems
to show that I might have lucked into these antennas being in phase - at least
they work better together for EU and the USA. But it would be nice to repeat
this luck or measure it.
It would be nice if this did not require $100k in test equipment.
George Fremin III
K5TR
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