Array Solutions AIM 4170
I don't recall what I paid for my AIM 4170. It is an older version and has been replaced with the AIM 4300 ($519). The AIM 4170 exceeded my expectations. It is well worth the price if you are doing any kind of antenna work or network characterization.
The software runs under Windows 7 and works very well. I have not tried to port it to Ubuntu.
The following plot is from a balun I build back in '07.
This plot is from an antenna feed balun I built. Here is a photo:
