Antenna spacing for coherent diversity

VK3ICM
Posts: 34
Joined: Tue Nov 12, 2019 1:44 pm

Antenna spacing for coherent diversity

Postby VK3ICM » Sat Jul 03, 2021 10:25 pm

I'm muddling my way through trying to use Diversity to null out a local interference source (power pole in my street). Read all the posts here, but can't get my head around whether significant spacing between my main and 'interferer' antenna is needed, and haven't found a suitable beam-forming 101 reference.

For coherent diversity on my 7000, my head says adjusting the phase in the phasing control panel is essentially mathematically moving one antenna vs the other. Am I off track? I don't have the real estate for any significant fraction of a wavelength separation unfortunately.

Chris - VK3ICM
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: Antenna spacing for coherent diversity

Postby w-u-2-o » Sun Jul 04, 2021 12:32 pm

Chris,

The answer to your questions is non-trivial and involves a bunch of math. I've just updated some mathematical links in the tacked topic in the Diversity Operations sub-forum, including a link to a video because the old links had gone stale on us.

In general one would want the two antennas to be a minimum of a quarter wavelength apart, and probably no more than a half wavelength apart, in order to allow both lobes and nulls to easily be formed and steered. More than a half wave apart just puts you on the backside of the unit circle.

Remember that if you steer the array to point a null in the direction of the interfering signal, the lobes will also move. But the lobes may not move to point in the direction of the desired signals. Thus the angle the baseline (the line between the two elements) of your two element array makes towards both the interferer and the desired signal is also very important.

Also remember that the intrinsic pattern of your antenna comes into play. Two verticials are going to be a lot easier to calculate for than two dipoles.

What a lot of folks do is get a rotating loop antenna of some sort to pair with their main antenna and spin both the loop around and the phasing controls in the hope of getting lucky rather than do the math and design the antennas. If they don't get lucky the first time then they move the loop to another place to be on a different side of the main antenna and try again.

If it's really a bad power pole, at the end of the day it's probably better and easier to call the power company and get them to fix the pole.

73,

Scott
VK3ICM
Posts: 34
Joined: Tue Nov 12, 2019 1:44 pm

Re: Antenna spacing for coherent diversity

Postby VK3ICM » Sun Jul 04, 2021 8:49 pm

Brilliant Scott, thanks. I will go through the links.

I'm using a magnetic receive loop for my 2nd antenna. Moved it as far as physically possible (probably 1/5 of a wavelength on 80m) from my main EFHW and now am seeing some results. If I'm understanding correctly, I have a challenge in that the noise source roughly lies in the desired orientation of my main antenna, and it still hears the interferer better (I assume because the mag loop is inefficient, or responds primarily to the h-field or both..). So I must use Rx2 as the reference source which is generally weaker for desired signals. Even so, it's made 80m bearable again.

I've raised the power pole issue with the local utility.. I've no idea what kind of response to expect..

Thanks again.

Chris - VK3ICM

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