When I was living in Upperaustria we had a house and antenna similar to your
description. Our building had two stories with a flat metal roof and the
antenna was mainly a 5 element tribander. The size of the roof was about 6
meters times 15 meters. So it was a rather big metallic surface beyond the
antenna. The Tribander itself was on a 17 meters high mast, about 5 meters away
from the building.
We changend the antenna several times (building some homemade quad-antennas and
later the commercial tribander) but we never noticed any influence of the
metallic roof. The commercial tribander worked exactly with the dimensions of
We also measured the radiation diagramm of one quad at zero elevation angle,
which seemed okay. Of course a metallic surface beyond the antenna can change
your elevation angle and we never had the possibility to test the antennas
without the roof (or without the building, hi). But from my point of view I
would not worry to much about a metallic roof, which is almost a wavelength
away from your antennas.
By the way, we even were very happy to have the (flat) metallic roof, as we
installed a quarterwave vertical for 40 meters on the roof. Because of the good
conductivity of the metallic surface, this antenna worked very well without
radials. From your antenna-description I see, you don´t have an antenna for 30
meters. Perhaps a vertical on a metall roof would make you as happy on 30 as we
were on 40.
For lightning protection the roof must be grounded. We used several ground-rods
for this purpose and also connected it to the groundsystem of the mast (which
was a steel-wire of about 1 cm diameter and about 20 meters long, buried about
60 cm deep in the garden). In almost all cases a lightning strike will hit your
tower and not the roof.
Juergen - OE5CWL
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