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

[Jim Lux]

----- Original Message -----
From: <corneliuspaul at gmx.net>
To: <towertalk at contesting.com>
Sent: Wednesday, September 24, 2003 4:33 AM
Subject: [TowerTalk] sheet metal roof influencing antenna - any hints /
simulation...?


> 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.

What's the diameter of the wires in the mesh?  One rule of thumb for
surfaces is to use wires with circumference equal to the spacing, and make
them, say, 0.1 wavelength apart, segmented appropriately.  How big is that
roof?

One technique, useful for a qualitative analysis, is just to put a few wires
where the roof is, and look at the currents induced in the wires. If you've
got significant current, then the roof is interacting, and you need to do
more...

Another approach, which I haven't tried, but might work, is to model the
house in one shot, saving it as a Numerical Green's Function, then load it
in a second sim where you add the antenna.  They use this approach when
simulating, e.g., antennas on a ship.