the GN card allows you to simulate an electromagnetic cliff
If you say 0 for the number of radials (the default), the first two
floating point parameters (F1, F2) specify the epsilon and sigma of the
"ground" around the antenna.
If you specify 4 more numbers (F3,F4,F5, F6), you can specify a
different epsilon and sigma, and a distance where the change occurs, and
what's cool, the distance by which the medium is below the surface of
the central medium.
So basically, you could do
GN 2,0,0,0,5.e-3,13.0,5.e-3,13.0,100.0,20.0
to make your antenna on a 200 meter diameter "mesa" that is 20 meters high.
If your antenna is at a place like Dunnottar Castle
(https://www.dunnottarcastle.co.uk/)
your GN card might be more like
GN 2,0,0,0,5.e-3,13,1,80,100,100
and you'll have a booming signal into EU (which actually isn't that far
away)
If you have a radial screen, and you want to use the GN card to do it
(or you want to have your antenna over a perfect ground, GN 1)
or you want a linear cliff, you use the GD card
http://www.nec2.org/part_3/cards/gd.html
which lets you specify a linear cliff at a particular X coordinate (the
cliff is parallel to the Y axis)
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