Velocity factor

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 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,[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

  1. ↑ Gottlieb, I.M., Practical RF power design techniques, TAB Books, 1993, ISBN 0-8306-4129-7, p.251 ('velocity factor')
  2. ↑ Velocity of Propagation, General Cable Australia Pty Ltd, retrieved 2010-02-13
  3. ↑ "velocity of propagation" in Walker, P.M.B., Chambers Science and Technology Dictionary, Edinburgh, 1991, ISBN 1-85296-150-3
  4. ↑ 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.
  5. ↑ Kaiser, Kenneth L. (2005). Transmission Lines, Matching, and Crosstalk. CRC Press. pp. 2–24. ISBN 9780849363627.
  6. ↑ "8723 Multi-Conductor - Shielded Twisted Pair Cable" (PDF). Belden.com. Retrieved 2017-07-06.
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