What free software can be used to simulate transmission lines and other RF circuits?
Speaking of free software, you can give Qucs (webpage, wikipedia info) a try, which will probably fill your requirements. 

From its web page:


  The Qucs GUI is well advanced and allows setting up schematics and
  presenting simulation results in various types of diagrams. DC, AC,
  S-parameter, noise and transient analysis is possible, mathematical
  equations and use of a subcircuit hierarchy (with parameterised
  subcircuits) are available. Qucs can also import existing SPICE models
  for use in your simulations.


However, keep in mind that Qucs is an academic project, aimed at demonsrtating various circuit simulation algorithms, so its simulation core is not specially optimized for speed. Also, is developed under the GNU/Linux OS using the standard autotools with no special effort to support other operating systems. However Qucs is said to be successfully compiled and run on Windows, Solaris, NetBSD, FreeBSD, MacOS, and Cygwin.