On 4/8/22 8:58 AM, Grant Saviers wrote:
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.
the rule of thumb for "mesh to simulate solid surface" is that the
circumference of the wire should be approximately the spacing. And
around 1/10th wavelength is good, both for spacing and segment length.
If you're doing it in raw NEC, the GM card and clever wire numbering is
your friend.
The tricky parts are joining the meshes at the edges, so that segments
line up.
These days, it runs pretty fast: I was running some models of two
crossed fan dipoles supported by a steel pipe over a steel mesh laying
on the ground, with the full SOMNEC soil model, and fill times for 2100
segments were around 30 seconds and factor time about 2 seconds. Fill
time goes as the square of number of segments, factor goes as the cube.
(This was on a 2 year old Macbook pro, running NEC4.2)
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