The velocity factor (VF),[1] also called wave propagation speed or velocity of propagation (VoP or [2] 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 air, the velocity factor is ~0.9997. 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,[3] 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 vacuum) and riser cable around 0.70 (approximately 210,000,000 m/s or 4.76 ns per metre).
| VF (%) | Cable | Ethernet physical layer | |
|---|---|---|---|
| 74–79 | |||
| 77 | RG-8/U | Minimum for 10BASE5[4] | |
| 67 | Optical fiber (silica glass) | Minimum for 10BASE-FL,[5] 100BASE-FX, ... | |
| 67 | Plastic optical fiber (PMMA) | 1000BASE-RHx | |
| 63 | Plastic optical fiber (polystyrene) | ||
| 65 | RG-58A/U | Minimum for 10BASE2[6] | |
| 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[7] |
Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table:[8] [9]
| VF (%) | Transmission line | |
|---|---|---|
| 95–99 | Open-wire "Ladder" Line | |
| 93 | HJ8-50B 3 inch Heliax coaxial cable (air dielectric)[10] | |
| 86 | RG-8 Belden 7810A coaxial cable (gas-injected foam high-density polyethylene)[11] | |
| 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)[12] | |
| 66 | RG-213 CXP213 (solid polyethylene dielectric) |
VF equals the reciprocal of the square root of the dielectric constant (relative permittivity),
\kappa
\epsilonr
VF={
| 1 | |
| \sqrt{\kappa |
in the usual case where the relative permeability,
\mur
VF={
| 1 | |
| \sqrt{\mur\epsilonr |
which includes unusual magnetic conducting materials, such as ferrite.
The velocity factor for a lossless transmission line is given by:
VF={
| 1 | |
| c0\sqrt{L'C' |
where
L'
C'
c0
{n}
VF={
| 1 | |
| n |
}