Measurement of s11 of differential output antenna transmitter

[electronx]

Normally, when we look at the s11 parameters from the feed point of a single-ended antenna, we can easily see the s11 reflection coefficient, yes.

Now, the question I am wondering is this: I have a differential output transmitter. First, it goes to the balun (it converts the differential input to single-ended). Then, the match circuit and a monopole helical antenna. In this case, I want to observe the reflection coefficient in order to adjust the bandwidth and impedance in the same way. In this case, where should I place the ports? Can I create a mathematical equation between the ports and observe the general s11 performance? Or, will connecting the 2nd port to gnd in the balanced part of the balun give me the correct result?