[TowerTalk] Re: Metal roof & antenna patterns

[jaw@lion.cc]

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 the supplier.

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.

73s

Juergen - OE5CWL



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