Editing Velocity factor (section)

{{Short description|Ratio of the speed at which a wavefront passes through the medium to the speed of light in vacuum}}
The '''velocity factor''' ('''VF'''),<ref>Gottlieb, I.M., ''Practical RF power design techniques'', TAB Books, 1993, {{ISBN|0-8306-4129-7}}, p.251 ('velocity factor')</ref> also called '''wave propagation (relative) speed''' or '''(relative) velocity of propagation''' ('''VoP''' or {{nobreak|'''<math>v_\mathrm{P}</math>'''),}}<ref>''[https://web.archive.org/web/20240712045911/www.generalcable.co.nz/getattachment/f81f14ee-7bec-4841-82d5-84b2df58b4f5/Velocity-of-Propagation.aspx Velocity of Propagation]'', General Cable Australia Pty Ltd, retrieved 2010-02-13</ref> 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 [[List of refractive indices|~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,<ref>"velocity of propagation" in Walker, P.M.B., ''Chambers Science and Technology Dictionary, Edinburgh, 1991, {{ISBN|1-85296-150-3}}</ref> while ''velocity factor'' is used for the ratio.