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Re: [TowerTalk] Vertical with no radial wires

To: WA3GIN <wa3gin@comcast.net>
Subject: Re: [TowerTalk] Vertical with no radial wires
From: jimlux <jimlux@earthlink.net>
Date: Sat, 18 Oct 2008 17:58:41 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
WA3GIN wrote:
> Hi Folks,
> 
> Looking for a commercial 80-10m for a building roof top install.  The 
> building roof is rubber so we're using non-penetrating roof mounts for the 
> base of the vertical. The building owner prefers no wires on the roof top.
> 
> Vendor and Model recommendations are welcome. Please reply direct.
> 

What sort of mounting base is possible? How much load can it take? 
What's your wind/ice/snow requirement?

Any vertical and a SGC tuner at the base, if you can lay some radials on 
the roof or tie to some form of counterpoise (surely, there's some sort 
of lightning ground?)

An aluminum flagpole might be a good choice.  They're sturdy, come with 
engineering drawings, etc. Mount it on an insulated base (Structural 
fiberglass, like Extren(tm)).

If no radials allowed, then an elevated vertical dipole, with SGC tuner 
at the feedpoint, coax fed down through the center of the lower half of 
the dipole, bunch of ferrite toroids as a choke.  I wouldn't contemplate 
an elevated vertical dipole with the tuner at the base, though.. the 
problems from the wires to the feedpoint will be substantial.  At least 
if it's going to look good.

For "commercial" application, you need to think how you're going to meet 
NEC and NFPA 780 rules for grounding, bonding, etc.

Some lengths of vertical will work better than others (i.e. there's some 
particularly "bad" lengths that are hard to tune), but in general, 
longer would be better. The SGC mobile whip is actually two whips in one 
of different lengths.

I've used a similar technique on my mobile installation, wrapping about 
25 feet of wire helically around a fairly stock fiberglass/SS 10m whip. 
The bare antenna on my car shows resonances at around 8 MHz and around 
29 MHz.

In general, though, the loss in the tuner won't be all that high, and it 
makes you basically immune to issues of trying to get resonance on the 
antenna.  You can make it what ever length is needed structurally, and 
not worry about exact dimensions.  80m is going to be a compromise, 
regardless, eh? (unless you're putting up a 60 foot mast)

Jim, W6RMK.
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