Terry Conboy wrote:
> At 10:41 AM 2006-07-03, firstname.lastname@example.org wrote:
>> 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.
> As Ben indicated, multiple antennas with any significant spacing will
> generate a multi-lobed pattern with lots of nulls. This will be the
> case with or without reflections from the water tank.
> Even though the individual Cushcraft units should perform
> satisfactorily, when phased together, the pattern will be a mess.
> 73, Terry N6RY
Yes, I sort of figured that the 'around the catwalk' idea was a problem
and didn't consider it too much because I figured you'd need a lot of
antennas to get good coverage without significant nulls. I'd love to
hear different, however.
Going back to the top of the tank, I figured that a good lightning rod
that is significantly taller than the antenna would provide protection,
provided the antenna system was properly grounded and protected, of
course. But I can only go about three feet from the antenna, so I worry
somewhat about affecting the radiation pattern. This may be a bit silly,
since antennas are mounted near big metallic things all the time (like
side mounts on towers). I can stand 3db loss in any given direction, but
if I got too much downtilt, that might create a problem.
The other option that I mentioned looking at was the Folded Dipole with
a lightning rod on the top of the mast. I worry about the currents from
the strikes in this instance since the wiring harness is a good path for
the current in addition to the mast. So I looked at the Folded Dipoles
with internal harness'. I figured that with the harness internal to the
mast, the currents would predominately want to go through the mast (skin
effect). But even with this, there would still be large currents going
through the harness, and being internal, it would be hard to replace if
it got damaged (assuming the rest of the antenna survived.)
I've noticed on some towers, a system of three dipoles side mounted. If
a single dipole with a lightning arrestor were a problem, I was thinking
that I could try something like this on top. Basically use three dipoles
spaced around a lightning arrestor to get omni coverage. The dipoles
would be close enough to minimize the nodal structure. The lightning
arrestor would be something like the tower and provide enough height to
protect the dipoles. This is somewhat similar to what Gene suggested
except on top of the tank.
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