How does a ferrite bead or core reduce common-mode RF current on the shield of a coaxial cable?
By creating an impedance in the current’s path✓Bonne réponse
It converts common-mode current to differential mode current
By creating an out-of-phase current to cancel the common-mode current
Ferrites expel magnetic fields
Explication
The equal and opposite currents on the inner conductor and inside of the shield produce fields that cancel, so the ferrite only sees the uncancelled common-mode current and presents a series impedance to it, reducing it. The ferrite does not generate a cancelling current or convert one mode into another.