tables ease horn design
I think you can use many different programs to analyze horn antennas.
HFSS is probably tempting with its interface, but it is very time and memory consuming, at least for the projects I've done with it.
There are several free programs, but the only problem is that you don't get technical support and quality manuals, so you would spend a lot of time to learn how to use those programs.
I recommend WIPL-D, I am using it regularly. It is a general 3D solver, and it analyzes horns with ease. There is a professional version called WIPL-D Pro, but there is also a low-cost version that costs couple of hundred $.
Maybe I sound emotional, but WIPL-D has pulled me out of some serious design trouble in the past. By the way, I am not working for those guys, if you are wondering.
Regards,
Axel