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Re: [TowerTalk] Omnidirectional Antenna Installations(Specifically) and

To: Terry Conboy <>
Subject: Re: [TowerTalk] Omnidirectional Antenna Installations(Specifically) and Lightning
From: Ben Thorson <>
Date: Mon, 03 Jul 2006 20:34:36 -0400
List-post: <>
Terry Conboy wrote:
> At 10:41 AM 2006-07-03, 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 (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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