The velocity factor (VF), ), 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.also called wave propagation speed or velocity of propagation (VoP or
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.
|VF (%)||Cable||Ethernet physical layer|
|74–79||Cat-7 twisted pair|
|77||RG-8/U||Minimum for 10BASE5|
|67||Optical fiber||Minimum for 10BASE-FL, 100BASE-FX, ...|
|65||RG-58A/U||Minimum for 10BASE2|
|65||Cat-6A twisted pair||10GBASE-T|
|64||Cat-5e twisted pair||100BASE-TX, 1000BASE-T|
|58.5||Cat-3 twisted pair||Minimum for 10BASE-T|
Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table:
|VF (%)||Transmission line|
|95–99||Open-wire "Ladder" Line|
|93||HJ8-50B 3 inch Heliax coaxial cable (air dielectric)|
|86||RG-8 Belden 7810A coaxial cable (gas-injected foam high-density polyethylene)|
|83||RG-6 Belden 1189A coaxial cable, RG-11 Belden 1523A coaxial cable|
|82||RG-8X Belden 9258 coaxial cable (foamed polyethylene dielectric)|
|80||Belden 9085 twin-lead|
|77||RG-8/U generic (foamed polyethylene)|
|66||Belden 8723 twin shielded twisted pair stranded (polypropylene insulator)|
|66||RG-213 CXP213 (solid polyethylene dielectric)|
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.
VF equals the reciprocal of the refractive index of the medium, usually optical fiber.