Common mode choke

If a symmetrical antenna (e.g. half wave dipole) is fed with coaxial cable, a common mode choke (or current BALUN) is required to suppress unwanted currents on the outer side of the braid.
The mechanism is described nicely in the antenna book of HB9ACC (Chapter 3.7.4).
It is also recommended to read the ‚Transmission Line Transformers Handbook‚ of W2FMI.