Velocity of propagation

Velocity of Propagation (VoP) or velocity factor is a parameter that characterizes the speed at which an electrical signal (e.g. radio) passes through a medium. Expressed as a percentage, it is the ratio of a signal's transmission speed compared to the speed of light in vacuum. Thus, transmission in a vacuum would have a VoP of 1 (100%). VoP equals the reciprocal of the square root of the dielectric constant of the material through which that signal passes.

This parameter is used for communication media such as data cables (e.g., Category 5 cable cables). Plenum data cable typically has a VoP ranging from 0.42 to 0.72 (42% to 72% of the speed of light) and riser cable around 70%. A VoP of 70% would correspond to a speed of approximately 210,000,000 m/s or taking 4.76 ns to travel one meter. In some technologies, such as Ethernet/Fast Ethernet the maximum length is dependent on the propagation delay of the medium.

Typical velocity factors
From table 19.1 in the ARRL Radio Amateur's Handbook, some typical velocity factors: