I started RF antenna modelling using various versions of NEC, like EZNEC. You can play around with simple wire structures and see what happens as you change sizes, spacings and frequency. There are many free sources of this software. It grew out of a DOS version to which a number of people added Windows front-ends.
For circuit simulation, again free I believe, there is APPCAD, which can be downloaded from http://hp.woodshot.com
If you can get hold of a copy of Mathcad, then the following site has a number of Mathcad routines for RF design, including filter synthesiswww.circuitsage.com
Hope that helps.