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Velocity of Propagation (VoP) or velocity factor (VF) is a parameter that characterizes the speed at which an electrical signal (e.g. radio) passes through a medium. Expressed as a number between 0 and 1 (or a percentage), it is the ratio of a signal's transmission speed 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 the signal passes.
This parameter is used to describe communication media such as data cables (e.g., Category 5 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 corresponds to a speed of approximately 210,000,000 m/s or 4.76 ns to travel one meter.
Typical velocity factors
From table 19.1 in the ARRL Radio Amateur's Handbook, some typical velocity factors:
| VF% | Transmission line |
|---|---|
| 95 | ladder line |
| 82 | twin-lead |
| 79 | coaxial cable (foamed polyethylene dielectric) |
| 66 | coaxial cable (solid polyethylene dielectric) |
