Kevin Schmidt, W9CF, has written about measuring an antenna system's differential and common-mode impedances -- he describes it in the appendix of this article:
I've tried using this method, but with so-so results. The issue, for me, with measuring an antenna system's common-mode impedance is that I want to know it at the point where I plan to insert the choke. For example, at a dipole's feedpoint (or my loop's feedpoint), where the two wire arms leave the coax, rather than back at the transmitter.
I haven't figured out a way to do this apart from simulations with EZNEC. So for the moment I simply shoot for a choke that has a large amount of common-mode resistance, hoping that it will do the job.
- Jeff, k6jca
P.S. Regarding the measurement of a choke's common-mode impedance -- those who have a two-port VNA that can make all four S-parameter measurements (S11, S12, S21, S22) might want to look at this technique that I just put up on my blog:
It offers some improvement over the S21 technique described by G3TXQ. But the S21 technique is perfectly fine, too, in many cases (for those who have, for example, a nanoVNA).
