I have been trying to find a free (or cheap) software package that can be used to simulate microwave systems (i.e. NOT an EM simulation package) but so far I haven't had much luck. Essentially I would like to be able to "build" systems with mixers, circulators, amplifiers, waveguides etc and then be able to plot parameters like S11 and S21. It would be great if it could also handle time-domain data (e.g. pulse propagation). It would also be a plus if it can import s2p data. I know there are commercial software packages that can do this, e.g. Microwave Office and the RF Toolbox for Matlab but they are VERY expensive so -at least at the moment- they are not really an option. Any suggestions?