4NEC2 is a freeware antenna simulation software program. The program can be used to model an antenna, and then determine through simulation properties such as the radiation pattern and SWR of the modelled antenna. It is very useful for those designing home made antennas for their RTL-SDR or other radio, or for those just trying to understand how antennas work.
4NEC2 can be a complicated piece of software and to get the most out of it a guide is needed. We’ve recently been informed that Mark Schoonover KA6WKE, is writing an eBook guide on this topic. His guide is currently only 20% complete, but a free sample of his book in its current state can be downloaded from leanpub. The guide already shows what the main windows of 4NEC2 do, and shows you how to model and simulate a simple dipole antenna.