Adding to Duane's warning note, even if you use radials, etc.some
return current is still going to flow in the sheets of metal. And,
even if you use something like the R-7000 that doesn't require a
"ground" its presence will still induce currents in the metal roof.
Every "joint" where one piece touches another, unless thoroughly
bonded electrically for r-f, can become a very efficient diode
(semiconductor) capable of generating a wide range of interfering and
unwanted signals. Also, anything within the barn when "illuminated" by
r-f could be a potential harmonic generator. The metal roof will not
be a perfect shield if it has currents flowing in itself from the
antenna. They will just radiate into the space below, less what they
dissipate in resistive loss.
Just something to be alert for - it may work perfectly or it may also
take out every TV in the house.
How's that for "negative!" <:}
I also tend to agree with Duane in that in my experience
ground-mounted radials have always provided better performance than
any elevated (roof) radials that I have used.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489 IC-765 #2349 IC-756 PRO #2121 IC-756 PRO2 #3235
----- Original Message -----
From: "Duane A Calvin" <ac5aa@juno.com>
To: <tentec@contesting.com>
Sent: Tuesday, January 21, 2003 12:49 PM
Subject: Re: [TenTec] [OT] Metal Roof as Ground Plane
> Mark,
> Quite often, metal roofing is not electrically bonded well
from
> one section to the next. So, your results may be good or very bad.
I
> tried it once on an old farm house with a flat metal roof on one
newer
> addition. It was horrible. I had to run radials before it became
> useful. So, if it's not too difficult, try it and hope it works,
but if
> it doesn't then you'll have to decide whether you want to run
radials, or
> relocate it to where radials will work better (ground-mounted.)
>
> 73, Duane
>
>
> On Tue, 21 Jan 2003 12:23:39 -0500 "Mark Erbaugh"
<mark@microenh.com>
> writes:
> > Since most of my HF equipment is TenTec, there is some tangential
> > relationship to this list <g>.
> >
> > I have a 24 by 48 foot metal barn with a metal roof. The peak of
the
> > roof is
> > about 20 feet high. Is there a wait to exploit this structure as a
> > ground
> > plane for an HF vertical antenna? What kind of performance
> > improvement (if
> > any) could I expect over the same antenna mounted on the ground.
> > Would I
> > still need radials?
> >
> > Currently, I have a Butternut HF6V and a Cushcraft AV5 antenna
> > available. I
> > also have a Gap Challenger, but I understand that it doesn't use a
> > ground
> > plane (elevated feed point). Would it be worth trying any of these
> > antennas
> > on the barn roof?
> >
> > The ham shack is not in the barn and the barn would not normally
be
> > occupied
> > (it's just storage) when I'm on the air, but would RF exposure
> > inside the
> > barn be an issue.
> >
> > 73,
> > Mark
> >
> > P.S. There's 2 inches of snow on the ground and temps in the 20s
and
> > I have
> > good antennas up now, so this is just a though experiment for
later
> > in the
> > spring.
> >
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
> >
>
>
> Duane Calvin, AC5AA
> Austin, Texas
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
|