Helmut Wabnig wrote:

How do I simulate a sheet metal or other metal structure in NEC,
when the software only knows (infinitesimally thin) "wires"

w.

You simulate it as a wire grid, like a screen. Download the free EZNEC
demo program, or just the manual, from http://eznec.com, and look in the
index under "Wire Grid Modeling" for more information.

This technique is widely used and generally gives very good results.

Roy Lewallen, W7EL