[Antennas] Vertical coaxial endfed dipole...

[Harvey&Bessie]

Several turns of the co-ax feed line right at the antenna acts as a
current balun. It has no effect on the current in the inner conductor
because the field for the inner conductor is completely confined within
the shield (formed by the outer conductor). But it acts as a choke
(creating a higher impedance to any currents that would otherwise flow
down the outside of the outer conductor.
Harvey/W4TG