How does a ferrite bead or core reduce common-mode RF current on the shield of a coaxial cable?
Why A ferrite bead slipped over the coax acts as a common-mode choke: the ferrite's high permeability raises the inductance, and its lossy nature adds resistance, so the outside of the shield sees a large series impedance at RF. Since the current on the outer surface of the shield has to push through that impedance, it drops (I = V/Z). The desired signal inside the coax is differential (equal and opposite currents on center conductor and shield interior), its fields cancel inside the cable, so the ferrite has no effect on it.
Watch out The cancellation idea is tempting because chokes and cancellation both suppress noise, but a ferrite is a passive lossy inductor, not a device that generates an out-of-phase current; and nothing converts common mode into differential mode. Expelling magnetic fields describes a superconductor, not ferrite, which concentrates flux.
Ferrite = choke, not canceller. It adds impedance in series with the shield's outside surface.