Velocity factor

The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or v P {displaystyle v_{mathrm {P} }} ), of a transmission medium is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, a light pulse in an optical fibre or a change of the electrical voltage on a copper wire) passes through the medium, to the speed of light in a vacuum. For optical signals, the velocity factor is the reciprocal of the refractive index. The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or v P {displaystyle v_{mathrm {P} }} ), of a transmission medium is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, a light pulse in an optical fibre or a change of the electrical voltage on a copper wire) passes through the medium, to the speed of light in a vacuum. For optical signals, the velocity factor is the reciprocal of the refractive index. The speed of radio signals in a vacuum, for example, is the speed of light, and so the velocity factor of a radio wave in a vacuum is unity, or 100%. In electrical cables, the velocity factor mainly depends on the insulating material (see table below). The use of the terms velocity of propagation and wave propagation speed to mean a ratio of speeds is confined to the computer networking and cable industries. In a general science and engineering context, these terms would be understood to mean a true speed or velocity in units of distance per time, while velocity factor is used for the ratio. 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 (42% to 72% of the speed of light in a vacuum) and riser cable around 0.70. A VF of 0.70 corresponds to a speed of approximately 210,000,000 m/s or 4.76 ns per metre. Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table: VF equals the reciprocal of the square root of the dielectric constant (relative permittivity), κ {displaystyle kappa } or ϵ r {displaystyle epsilon _{mathrm {r} }} , of the material through which the signal passes: in the usual case where the relative permeability, μ r {displaystyle mu _{mathrm {r} }} , 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: