balun designs
To do measurements you will need another balun to bring the impedance back dowo to your other port. You have two choices. The first is to use a laboratory quality one that is available from the anazyer manufacturer. The other is to use two. Put an attenuator between the balanced prorts. When you do the S12 and S21 measurements, subtract the attenuation and divide theresult by two to get the parameters of just one balun. The S11 and S22 measurements are correct as is. Of course, you need a good attenuator which you will have to be careful in designing and constructing and be at the impedance level of the balanced side.