# Can ferrite beads lower coax cable velocity factor?

For VHF and UHF, it seems reasonable to slip ferrite beads over a coaxial cable to act as a transmission line common mode choke. It also seems reasonable to use beads with the smallest diameter possible. This can lead to ferrite-over-cable lengths on the order of 250-400mm or more with a slip-fit ferrite cover. Can this affect the cable's velocity factor or impedance?

No, adding ferrite beads to choke currents on the outside of the shield of a coaxial cable does not affect its impedance or velocity factor. Impedance and velocity factor are determined by the inside construction of the cable: the outside diameter of the inner conductor, d, the inside diameter of the outer conductor (shield), D, and the magnetic permeability, $$\mu$$, and dielectric constant, $$\epsilon$$, of the material that separates them: $$Z_0=\frac{1}{2\pi}\sqrt{\frac{\mu}{\epsilon}}ln\frac{D}{d}$$