Technically, one can use LTSpice as it features a transmission line model, but obviously it doesn't like dangling nodes (ie. open lines) and isn't really suited for that purpose anyway due to lack of RF-related measurements.
What other software can be used to do a schematic-level simulation of RF circuits with transmission lines and lumped elements? I want to model distributed filters (like made from coax), matching circuits (tuners or baluns) etc. Amplifiers would be nice, but not necessary.
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.
