Dipole measurement without a balun dedicated to single-to-differential conversion

Hello everybody,

Here is one question about differential antenna, e.g. dipole, folded dipole, etc. In order to measure properly such an antenna using a network analyzer, one has to design a balun dedicated to single to differential signal conversion. Then a TRL based calibration is used to get the s-parameter at the correct reference plane, i.e. at the differential antenna input.

What happens if we do this measurement without the balun? I guess the signal at the input is not perfectly differential but what are the quantitative/qualitative impacts?

Thanks for your comment.

The radiation pattern will distort due to the current distribution outside the coax cable. Also, your s11 might be lower than it is since some of reflected current are not going back into the network analyzer. you might use a network analyzer which has a differential port. You can then do the measurement without the balun

Thanks for your answer.
It is not clear to me in what you say in which case is S11 going to be lower? In the differential or single-ended measurement setup?

If it is in unbalance mode, the current will flow inside the outter conductor. Hence, all return current will go back in the network analyzer. If there are both current inside and outside outter conductor, the only current in the inside return to the network analyzer. Hence, S11 will lower than it is. This will occur when you do the measurement in single-end set up.