I've had many EZNEC users tell me they thought the course was very
worthwhile.

As others have mentioned, there's no upper frequency limit to NEC or its
derivative programs, but some of the geometries used at microwave
frequencies don't lend themselves well to NEC modeling. I've done
considerable modeling with EZNEC at 6 GHz and found the results to be
satisfactory.

Roy Lewallen, W7EL