I have a tin roof on my house. I asked a lot of questions like yours and got
darn few answers. I think no one really knows. It definitely isn't "ground"
and you can't consider the roof a monolith because there are seams. The
seams are not electrically bonded and with a little corrosion may act like
diodes and spin off all sorts of RFI. Mine don't but it is possible in
theory.
I think the roof is just a mess -- hard to model and hard to figure. A roof
slopes and the antenna may not be in the middle in all four directions. How
do you figure all that?
I mounted a Cushcraft R7000 about five feet above the tin and it worked
great. A GAP Titan mounted the same way gave me fits with RFI until I ran
the feedline straight down to the ground instead of across the roof. Neither
antenna has radials and neither was "grounded" to the tin roof. A ladder
line fed multiband dipole mounted away from the roof seems to do better than
either of the verticals ever did. Can you blame the roof?
I now have a TGM mini beam (quagi) about 15 feet above the roof. It is
probably 40 feet in the air and the roof slopes away from the base pretty
steeply. It works on 20 and above but I have can't compare it to the same
antenna mounted in the clear.
I suspect this is like a lot of other antenna questions. Just put the dang
thing up and see what happens. The K4IA antenna axiom is "Any antenna in
the air will get more QSOs and work more DX than every antenna on the drawing
board."
Radio K4IA
Craig Buck
Fredericksburg, Virginia USA
QRP ARCI #2550 FISTS #6702 CC 788
K1 #470
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