The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or of a

transmission medium A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulat ...

is the ratio of the speed at which a wavefront (of an electromagnetic signal, a

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...

signal, a light pulse in an optical fibre or a change of the electrical voltage on a

copper wire Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor. Copper is the electri ...

) passes through the medium, to the speed of light in vacuum. For optical signals, the velocity factor is the reciprocal of the

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

. The speed of radio signals in

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

, for example, is the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

, 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 A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

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 line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

s. 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:

Calculating velocity factor

Electric wave

VF equals the

reciprocal Reciprocal may refer to: In mathematics * Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal'' * Reciprocal polynomial, a polynomial obtained from another pol ...

of the

square root In mathematics, a square root of a number is a number such that ; in other words, a number whose ''square'' (the result of multiplying the number by itself, or  ⋅ ) is . For example, 4 and −4 are square roots of 16, because . ...

of the dielectric constant (

relative permittivity The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insul ...

\kappa

\epsilon_\mathrm

, of the material through which the signal passes: :

\mathrm =  \

in the usual case where the

relative permeability In multiphase flow in porous media, the relative permeability of a phase is a dimensionless measure of the effective permeability of that phase. It is the ratio of the effective permeability of that phase to the absolute permeability. It can be ...

\mu_\mathrm

, is 1. In the most general case: :

\mathrm =  \

which includes unusual magnetic conducting materials, such as ferrite. The velocity factor for a lossless

is given by: :

\mathrm =  \

where

L'

is the distributed inductance (in henries per unit length),

C'

is the

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...

between the two conductors (in

farads The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units (SI). It is named after the English physicist Michael Faraday (1791–1867). In SI base units ...

per unit length), and

c_\mathrm

is the speed of light in vacuum.

Optical wave

VF equals the reciprocal of the

of the medium, usually

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...

. :

\mathrm =  \

References

{{Reflist Electromagnetic radiation

Typical velocity factors

Calculating velocity factor

Electric wave

Optical wave

See also

References