Ben:
My recommendation would be to use four DC-grounded folded vertical dipoles
spaced around the perimeter of the water tank, and fed with the appropriate
phasing harness. Check out Cushcraft's commercial antennas such as 4504P at
http://www.cushcraft.com/ (click on land mobile/paging and follow through to
their VHF and UHF arrays.) You might be able to get some technical assistance
from their engineers on phasing harness design for your specific app.
73 de
Gene Smar AD3F
-------------- Original message ----------------------
From: Ben Thorson <bthorson@4smps.com>
>
> Hello,
>
> I've read a lot of the submissions to this list with interest. One
> question that I haven't seen addressed that is near and dear to me is
> designing antenna structures for lightning prone environments.
> Specifically, if you've got an installation with a nice ground system,
> the cable grounded to the tower at proper intervals, and you've done all
> the good surge protection/ground stuff, you obviously still don't want a
> strike hitting your antenna. So, if you want an omnidirectional pattern,
> you typically would like an antenna on top of your tower (Specifically,
> I'm talking about vertically polarized dipole style antennas typically,
> anywhere from 150-450MHz) But putting a dipole on top of a tower is just
> asking for trouble. So, I've looked at Folded Dipoles which have a nice
> center mast above the elements that you could even put a lightning rod
> on. As long as the mast was a heavy enough gauge, it should take a
> strike ok. However, the mast is still fairly intimate with the antenna.
>
> So, my question is, what are some good ways to set up an omnidirectional
> pattern on a tower in a way that you don't expose the antenna or
> antennas unduly. For example, are there good side mount strategies with
> multiple antennas? Is there a nice way to mount a lightning rod on top
> with a dipole or something similar that won't screw up the pattern too
> much? Other ideas?
>
> The reason this has come up is I have an antenna installation that
> appears to be the favored Anvil of God, and his hammer is coming down
> regularly. I have plenty of installations that survive lightning storms
> well, but this one in particular is the problem child. We get big
> strikes with antennas literally blown off or vaporized (at least we
> can't find the pieces.) In this case, it is mounted on the top of an
> elevated water tank, not a tower, and there is about a three foot
> diameter landing to mount antennas and such. My current plans are either to:
>
> 1) Mount a lightning rod on a separate mast on top with enough to height
> to protect the antenna. Mounting this on the far side of the landing and
> try to tune it to minimize the interaction and shadowing.
>
> 2) Figure out how to set up a system of antennas on the catwalk around
> the circumference of the tank to generate an omnidirectional pattern.
> This would probably be the safest, but I'm not sure how to generate a
> good omni signal without nodes with a reasonable number of antennas
> around the tank.
>
> My favorite solution, moving the antenna off of the tank, has been nixed.
>
> Any thoughts, comments, and help on this subject are greatly appreciated.
>
> BenT
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>
>
>
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