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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