Velocity factor

The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or 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 vacuum. For optical signals, the velocity factor is the reciprocal of the refractive index. The speed of radio signals in vacuum, for example, is the speed of light, and so the velocity factor of a radio wave in vacuum is 1.0 (unity). 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,"velocity of propagation" in Walker, P.M.B., Chambers Science and Technology Dictionary, Edinburgh, 1991, while velocity factor'' is used for the ratio. Velocity factor is an important characteristic of communication media such as s 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 vacuum) and riser cable around 0.70 (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), 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 distributed inductance (in henries per unit length), is the capacitance between the two conductors (in farads per unit length), and is the speed of light in vacuum.