During the July Program Meeting, John Frederick (N8GOU) demonstrated the antenna modelling software EZNEC. This is Windows software that can be used to model all kinds of antennas and then plot the calculated SWR and elevation or azimuth gain for any frequency. There is a free demo available on their website: www.eznec.com.
John modeled a dipole for us and showed the effects of changing the height above ground and the wire diameter, which was very interesting.
Another software that is very similar and compatible is 4NEC2, which is free without the limitations of EZNEC. It can be downloaded here: www.qsl.net/4nec2.