EZNEC is an excellent tool for designing and/or analyzing antennas, written by W7EL. It used to be a free version for hams, and a more thorough professional one (for $500) for engineers. They are both now available for FREE!
I downloaded it and I like it. I have been using the free version (basic) of MMANA-GAL antenna software at: http://gal-ana.de/promm/index.htm
They compliment each other very well.
Sometime this winter I want to work on a multiband vertical HF antenna to backup my 80m loop sky antenna. This will help.
Thanks for the info from firstname.lastname@example.org sent by Glenn.