Velocity factor

The velocity factor (VF),^[1] also called wave propagation speed or velocity of propagation (VoP or ${\displaystyle v_{\mathrm {P} }}$),

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

Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table:[8]^[9]

Calculating velocity factor

Electric wave

VF equals the

), ${\displaystyle \kappa }$ or ${\displaystyle \epsilon _{\mathrm {r} }}$, of the material through which the signal passes:

${\displaystyle \mathrm {VF} ={\frac {1}{\sqrt {\kappa }}}\ }$

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

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

which includes unusual magnetic conducting materials, such as ferrite.

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

${\displaystyle \mathrm {VF} ={\frac {1}{c_{\mathrm {0} }{\sqrt {L'C'}}}}\ }$

where ${\displaystyle L'}$ is the distributed

${\displaystyle C'}$

${\displaystyle c_{\mathrm {0} }}$

Optical wave

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

${\displaystyle \mathrm {VF} ={\frac {1}{n}}\ }$

See also

References