Velocity factor
The velocity factor of a transmission medium is the ratio of the speed at which a wavefront passes through the medium, to the speed of light in vacuum. For optical signals, the velocity factor is the reciprocal of the refractive index.
The speed of radio waves in vacuum, for example, is the speed of light, and so the velocity factor of a radio wave in vacuum is 1.0. In air, the velocity factor is ~0.9997. In electrical cables, the velocity factor mainly depends on the insulating material.
In the computer networking and cable industries, the terms velocity of propagation and wave propagation speed is also used to mean a ratio of speeds. In a general science and engineering context, these terms would be understood to mean an actual speed or velocity, with dimension of distance per time, while velocity factor is used for the ratio.
Typical velocity factors
Velocity factor is an important characteristic of communication media such as category 5 cables and radio transmission lines. Plenum data cable typically has a VF between 0.42 and 0.72 and riser cable around 0.70.Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table:
Calculating velocity factor
Electric wave
VF equals the reciprocal of the square root of the dielectric constant, or, of the material through which the signal passes:in the usual case where the relative permeability,, is 1. In the most general case:
which includes unusual magnetic conducting materials, such as ferrite.
The velocity factor for a lossless transmission line is given by:
where is the lineic inductance, is the lineic capacitance between the two conductors, and is the speed of light in vacuum.