What is Propagation Velocity in a Cable?

The Velocity of propagation (Vp) of a cable is the speed at which an electrical signal can propagate through the cable in comparison to the speed of light. For example, in a vacuum, the velocity of propagation would be 100% or 1 (depending on how it is represented). So, if the Propagation Velocity is 70%, it means that the signal in a cable will propagate at 0.7 times the speed of a signal if the dielectric of the cable was vacuum.

The Velocity factor or Propagation Velocity of a cable can be calculated by the formula below (where ϵ is the dielectric constant of the dielectric material used in the cable).

The velocity of propagation or velocity factor is a coaxial cable is determined by the dielectric used. There are a number of materials that are used as the dielectric in a cable. Each of these dielectrics has a different dielectric constant and thus a different velocity factor. Here is a table of the most common dielectrics used in coax cables with their dielectric constant and velocity factor.

Velocity of the speed at which an electrical signal travels through a wire or a cable in comparison to the speed of light.  The velocity of an electrical cable i.e., that an electrical signal passes is 85 % is 85 % of the speed of light.