[TowerTalk] cliff in ground for NEC

[jimlux]

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)