Mark,
Your metal barn will make a great ground-plane, but I'd like to bring up a
few issues that you ought to consider.
In total area, the barn roof is acceptable for the higher HF bands, but
would be a bit short for 40 and 80 meters. If you mount the vertical
antenna at the center of the roof, the effective 'radial' length would be
about 15 by 24 feet, depending upon the slope of the roof. A typical 80
meter radial is over 60 feet long. I have a question for you... What is
the distance from ground level to the beginning of the roof..? Since the
barn is all metal, the vertical walls will become part of the ground-plane,
so you can include the walls in your calculations.
>From my personal experience, metal buildings, in general, are well made in
structural terms, but not too well electrically. Several years ago, I used
a metal building as a ground-plane on 15 through 6 meters. When the
building was new, everything worked very well, but after a few years, I
experienced variations in SWR in windy weather. The problem was that there
was little or no bonding between the metal panels. I tried to install
bonding straps between panels to cure the problem, but I ran into mechanical
problems. In my case, the roof was made of very long interlocking panels
which were difficult to bond using strapping, so I emptied the building to
avoid a fire, and used an arc welder to tack the panels together. Ever
since I welded the panels, the SWR problems went away, and a side benefit is
that the building became much quieter in windy weather <grin>...
73, Larry - W1GOR
----- Original Message -----
From: "Mark Erbaugh" <mark@microenh.com>
To: <tentec@contesting.com>
Sent: Tuesday, January 21, 2003 12:23 PM
Subject: [TenTec] [OT] Metal Roof as Ground Plane
> 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.
>
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