[TowerTalk] sheet metal roof influencing antenna - any hints / simulation...?

[corneliuspaul at gmx.net]

Hello,


a friend of mine is having problems with mounting a 3ele  20m Beam  6m
(18ft) above a sheet metal roof.

I was quite sure that would be enough separation, but the roof seems to
greatly detune the antenna, as the point of lowest SWR moves down to 13.8
MHz and his calls on the band remain un-answered...

The interaction seems to be way less pronounced when he moves the antenna
to only 2m (6ft) above the metal roof.

I was trying run a 4NEC2 simulation of the antenna and roof but it seems I
can model the roof only as a very coarse wire mesh to avoid running out of
computation power (inspite 2.4GHz PC and 528MB RAM) and the results seem
useless.

Am I doing something wrong here?
Any other hints, how to make the antenna play...?

Your help is very much appreciated

73s Con DF4SA