I've had excellent results with a metal roof as the ground plane for a
two different single band verticals on two different structures. You
have a real asset if it is of decent size - 1000 sq ft +. Check out the
recent QEX articles re elevated ground planes. It is possible to
extrapolate a bit from that data to see that an elevated metal roof will
work very well as a "radial". I've tried several EZNEC alternative
representations and it seems that the NEC2 engine does not handle
elevated ground planes well.
So a traditional trapped vertical (several makes, I have no opinion)
that needs radials should work well and fit your length constraint 10 to
40m. Perhaps there is a top loaded one that also works on 80m but I
haven't come across one. Otherwise a 80m base loading coil will get you
on 80m but without much bandwidth.
When asking tower talkians a few years ago about metal roofs as ground
planes a lot of concerns were expressed about PIM - passive
intermodulation. It is caused by partially conducting metal
connections, possible in metal roofs, metal gutters, tower bolts, coax
braid, etc. etc. PIM is a big concern in the cellular industry where
there are many full duplex radios on a single tower. So, unless you are
a multi-multi (unlikely from your question) I think it is highly
unlikely that PIM will be a concern. I had no problems with an old barn
(50 yoa) somewhat rusty galvanized roof, an extreme test case, but there
were no multi-multi, SO2R, or full duplex operations at my QTH. On a
second structure I had to connect two electrically isolated roof
sections together and did that with #16 wire jumpers and would recommend
those about every 18" if you have such a situation. I could see the
difference on an AIM4170 as jumpers were added.
A hip or mansard roof also helps raise the feedpoint impedance a bit,
the same affect as drooping radials on a VHF ground plane vertical. You
want your antenna to be on a ridge line, of course.
On 8/23/2012 9:01 AM, email@example.com wrote:
> 1. I'm in a situation where I need to try to use a different kind of
> antenna. I would like ot try a vertical, and see what changes it makes in
> my transmitting and receiving ability.
> I would like an antenna (commercial or homebuilt) which:
> * Is somewhere around 30' in vertical length
> * Base to be mounted 25' above ground level
> * Uses few radials (4 or so) <the radials will be insulated wire, and also
> be guys>
> * does not need a base tuner (one in the shack is fine)
> Would greatly appreciate links to an antenna similar to this.
> 2. We have a metal roof. Is there any particular advantage mounting this
> centered on my roof? Or, are radials the trick?
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