Velocity factor
The velocity factor, also called wave propagation speed or velocity of propagation 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 unity, or 100%. In electrical cables, the velocity factor mainly depends on the insulating material.
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.
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. 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 |
| 86 | RG-8 Belden 7810A coaxial cable |
| 83 | RG-6 Belden 1189A coaxial cable, RG-11 Belden 1523A coaxial cable |
| 82 | RG-8X Belden 9258 coaxial cable |
| 80 | Belden 9085 twin-lead |
| 77 | RG-8/U generic |
| 66 | Belden 8723 twin shielded twisted pair stranded |
| 66 | RG-213 CXP213 |
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 distributed inductance, is the capacitance between the two conductors, and is the speed of light in vacuum.