Velocity factor

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The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or ${\displaystyle v_{\mathrm {P} }}$), 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 a vacuum. For optical signals, the velocity factor is the reciprocal of the refractive index.

The speed of radio signals in a vacuum, for example, is the speed of light, and so the velocity factor of a radio wave in a vacuum is unity, or 100%. 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, while velocity factor is used for the ratio.

Video Velocity factor

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 (42% to 72% of the speed of light in a vacuum) 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 to travel one meter.

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

Maps Velocity factor

Calculating velocity factor

VF equals the reciprocal of the square root of the dielectric constant (relative permittivity), ${\displaystyle \kappa }$ 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:

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

which includes unusual magnetic conducting materials, such as ferrite.

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

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

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

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See also

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References

Source of article : Wikipedia