Velocity factor

 ( Electromagnetic Radiation ) 

The velocity factor ( VF),Gottlieb, I.M., Practical RF power design techniques, TAB Books, 1993, , p.251 ('velocity factor') also called wave propagation (relative) speed or (relative) velocity of propagation ( VoP or Velocity of Propagation, General Cable Australia Pty Ltd, retrieved 2010-02-13 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,"velocity of propagation" in Walker, P.M.B., Chambers Science and Technology Dictionary, Edinburgh, 1991, while velocity factor'' is used for the ratio.

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Typical velocity factors Velocity factor is an important characteristic of communication media such as category 5 cables and radio transmission lines. Plenum 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 nanosecond per metre).

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+ Minimum velocity factors allowed for network cable standards
Minimum for 10BASE5IEEE 802.3 Clause 8.4.1.3 The minimum required velocity of propagation is 0.77 c.
Minimum for 10BASE-FL,IEEE 802.3 clause 15.3.1.3 The propagation delay shall be ≤5 μs/km. (This is equivalent to a velocity of propagation of 0.67c.) 100BASE-FX, ...
1000BASE-RH x
	Plastic optical fiber (polystyrene)
Minimum for 10BASE2IEEE 802.3 Clause 10.5.1.3 The minimum required velocity of propagation is 0.65 c.
10GBASE-T
100BASE-TX, 1000BASE-T
Minimum for 10BASE-TIEEE 802.3 Clause 14.4.2.4 The maximum propagation delay of twisted pair shall be 5.7 ns/m (minimum velocity of 0.585 × c).

Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table:

(2025). 9780872590960, ARRL. ISBN 9780872590960

Kenneth L. Kaiser (2025). 9780849363627, CRC Press. . ISBN 9780849363627

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air insulated

air dielectric

gas-injected foamed high-density polyethylene

foamed polyethylene dielectric

foamed polyethylene

polypropylene insulator

solid polyethylene dielectric

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Calculating velocity factor

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Electric wave

VF equals the reciprocal of the square root of the dielectric constant (relative permittivity), $\backslash kappa$ or $\backslash epsilon\_\backslash mathrm\{r\}$, of the material through which the signal passes:

$\backslash mathrm\{VF\}\; =\; \{\; \backslash frac\{1\}\{\backslash sqrt\{\backslash kappa\}\}\; \}\; \backslash$

in the usual case where the relative permeability, $\backslash mu\_\backslash mathrm\{r\}$, is 1. In the most general case:

$\backslash mathrm\{VF\}\; =\; \{\; \backslash frac\{1\}\{\backslash sqrt\{\backslash mu\_\backslash mathrm\{r\}\backslash epsilon\_\backslash mathrm\{r\}\}\}\; \}\; \backslash$

which includes unusual magnetic conducting materials, such as ferrite.

The velocity factor for a lossless transmission line is given by:

$\backslash mathrm\{VF\}\; =\; \{\; \backslash frac\{1\}\{c\_\backslash mathrm\{0\}\backslash sqrt\{L\text{'}C\text{'}\}\}\; \}\; \backslash$

where $L\text{'}$ is the distributed inductance (in henries per unit length), $C\text{'}$ is the capacitance between the two conductors (in farads per unit length), and $c\_\backslash mathrm\{0\}$ is the speed of light in vacuum.

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Optical wave

VF equals the reciprocal of the refractive index $\{n\}$ of the medium, usually optical fiber.

$\backslash mathrm\{VF\}\; =\; \{\; \backslash frac\{1\}\{n\}\; \}\; \backslash$

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See also

• Coaxial cable
• Propagation delay
• Signal velocity
• Speed of electricity
• Speed of sound
• Telegrapher's equations

Categories: Electromagnetic Radiation, Velocity

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