Velocity factor

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 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:^[8][9]

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:

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

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 inductance (in henries per unit length), ${\displaystyle C'}$  is the capacitance between the two conductors (in farads per unit length), and ${\displaystyle c_{\mathrm {0} }}$  is the speed of light in vacuum.

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

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