[TowerTalk] Measure / Check phase difference between two antennas?

[Jim Lux]

Having done this for a variety of antennas at a variety of frequencies.

for HF the observation point needs to be some wavelengths away AND equidistant from both antennas you're testing.
So, for 20m, maybe 100 meters away?
( one could use the 2 D^2/lambda rule - where D is the distance between the two antennas you're comparing - so for 20m, and the two antennas 10m apart, 200/20 = 10 meters)
The 2D^2/lambda is to get the phase "flat" within lambda/10 or lambda/14 or something like like that. (I'm too lazy to do the calculation) which is sufficient that a gain measurement on boresight won't be off by more than some amount.  (the rule is for things like dishes)

You might also need an amplifier for the source.   
 Let's say 100 meters away, 20 m wavelength (15 MHz)

free space loss between isotropes is 32.44 + 20log10(0.1) + 20log10(15) = 32.4-20+12  = about 24 dB.
100 meters of RG6 cable TV coax is about 2 dB.

So you've got about 20-30 dB of loss (less the gain of the antenna under test) so that should be ok.



On Wed, 11 Mar 2026 09:37:28 -0700, Gary Johnson via TowerTalk <towertalk at contesting.com> wrote:

I’m working on an article that addresses this problem, among others.

A way to do this is with a VNA and an observation antenna. The observation antenna can be almost anything with the same polarization as the antenna(s) you are testing. It does not need to be resonant. It does need to be in the "far field,” a subject that some may debate but let’s say a couple of wavelengths away should be enough. The temporary feedline can be something cheap, like RG6 or RG58. OR… if you have another antenna of almost any kind that happens to be far enough away, that can be the observation antenna.

Using a VNA in transmission (S21) mode, you can see the _relative_ magnitude and phase of the path between your antennas under test and the observation antenna. Because it’s all relative, you don't have to worry about the observation antenna being calibrated in any way, nor do feedline losses matter. If the observation antenna is located farther from one antenna under test than the other, you can correct the phase by estimating the path lengths and using the speed of light.

-Gary NA6O

>
> 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?

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