EZNEC has a feature to make a XYZ grid of wires, the manual advises less
than 1/10wl between them, although I use a finer mesh. So some insight
can be gained with the roof insulated from ground (no walls connecting
it to ground).
The hard part is how the metal roof is grounded. With NEC4.2 a buried
ground can be set up, not so in NEC2. Connecting the roof via something
(mesh, separate wires) can take a lot of time to set up, then the high
accuracy compute time can be significant. I've also run some models
that run into numerical computation problems, I think because of the
large number of segments and junctions. The current distributions made
no sense and some tweaking of the model yielded sensible patterns.
From my modeling, I think (and one experience) the grounding of the
roof may have a significant effect. Elevated radials should not be
grounded.
Grant KZ1W
On 4/7/2022 20:09, Jim Brown wrote:
On 4/7/2022 6:55 PM, Stan Stockton wrote:
I am interested in knowing what the detriment might be if you had a 80
foot tower with a large Yagi for top loading within four feet of a 20
foot tall concrete building with lots of rebar and a metal roof.
Hi Stan,
Intuitively, I'd expect the pattern to be down a bit in the direction of
the building.
73, Jim K9YC
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