Velocity factor: Difference between revisions
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The '''velocity factor (VF)''',<ref>Gottlieb, I.M., ''Practical RF power design techniques'', TAB Books, 1993, ISBN |
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 speed''' or '''velocity of propagation (VoP or <math>v_\mathrm{P}</math>)''',<ref>''[http://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 acoustic signal, for example, or an electromagnetic signal, a [[radio]] signal, a light pulse in a fibre channel 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]]. |
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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 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). |
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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 |
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. |
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== Typical velocity factors == |
== Typical velocity factors == |
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Revision as of 22:26, 2 July 2017
The velocity factor (VF),[1] also called wave propagation speed or velocity of propagation (VoP or ),[2] of a transmission medium is the ratio of the speed at which a wavefront (of an acoustic signal, for example, or an electromagnetic signal, a radio signal, a light pulse in a fibre channel 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,[3] 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 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:[4][5]
| VF% | Transmission line |
|---|---|
| 95–99 | Open-wire "Ladder" Line |
| 83 | RG-6 Belden 1189A coaxial cable |
| 82 | RG-8X Belden 9258 coaxial cable (foamed polyethylene dielectric) |
| 80 | Belden 9085 twin-lead |
| 66 | Belden 8723 twin shielded twisted pair stranded (polypropylene insulator)[6] |
| 66 | RG-213 CXP213 coaxial cable (solid polyethylene dielectric) |
Calculating velocity factor
VF equals the reciprocal of the square root of the dielectric constant (relative permittivity), , of the material through which the signal passes:
The VF of a lossless transmission line is given by:
where L is the distributed inductance (in henries per unit length), C is the capacitance between the two conductors (in farads per unit length), and c is the speed of light in vacuum.
See also
References
- ^ Gottlieb, I.M., Practical RF power design techniques, TAB Books, 1993, ISBN 0-8306-4129-7, p.251 ('velocity factor')
- ^ Velocity of Propagation, General Cable Australia Pty Ltd, retrieved 2010-02-13
- ^ "velocity of propagation" in Walker, P.M.B., Chambers Science and Technology Dictionary, Edinburgh, 1991, ISBN 1-85296-150-3
- ^ H. Ward Silver, N0AX, ed. (2011). "Chapter 22: Component Data and References". The ARRL Handbook For Radio Communications (88th ed.). ARRL. p. 22.48. ISBN 978-0-87259-096-0.
{{cite book}}: CS1 maint: numeric names: editors list (link) - ^ Kaiser, Kenneth L. (2005). Transmission Lines, Matching, and Crosstalk. CRC Press. p. 2-24. ISBN 9780849363627.
- ^ http://www.belden.com/techdatas/english/8723.pdf