Freeware program for system simulation

In my final project i have to construct circuit for measuring s-parameters like feedback's microstrip trainer
(http://www.fbk.com/telecommunications/MST532.asp)
and i need a program for system simulation. I have thought already SystemVew or ADS but i would like also to know if there are any freeware programs to do system simulation.
Please help...

Sorry for my poor english.