HFSS - how could one check the effectiveness of a balun?

I'm thinking of simulating a 4:1 balun made from a half-wave section of cable. Clearly as the operating frequency departs from the exact frequency the balun is designed for, the balun will become less effective.

I'm wondering how I could measure this in HFSS? I was thinking of something like looking at the near field around the coax feed port, as the coax must radiate. But I don't really know if that's the best way, or even how to use the near-field data. About all I know about the near field, is that if FFT'ed one gets the far-field. I don't really know how to use the near field data in any other sensible way.

Any thoughts how best to check a balun?