Velocity factor
Template:Short description The velocity factor (VF),[1] also called wave propagation (relative) speed or (relative) velocity of propagation (VoP or Template:Nobreak[2] 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 ~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,[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 vacuum) and riser cable around 0.70 (approximately 210,000,000 m/s or 4.76 ns per metre).
Minimum velocity factors allowed for network cable standards VF
(%)Cable type Ethernet physical layer 74~79%Script error: No such module "Check for unknown parameters".Cat-7 twisted pair 77%Script error: No such module "Check for unknown parameters".RG-8/U Minimum for 10BASE5[4] 67%Script error: No such module "Check for unknown parameters".Optical fiber (silica glass) Minimum for 10BASE-FL,[5]
100BASE-FX, ...67%Script error: No such module "Check for unknown parameters".Plastic optical fiber (PMMA) 1000BASE-RHx 63%Script error: No such module "Check for unknown parameters".Plastic optical fiber (polystyrene) 65%Script error: No such module "Check for unknown parameters".RG-58A/U Minimum for 10BASE2[6] 65%Script error: No such module "Check for unknown parameters".Cat-6A twisted pair 10GBASE-T 64%Script error: No such module "Check for unknown parameters".Cat-5e twisted pair 100BASE-TX, 1000BASE-T 58.5%Script error: No such module "Check for unknown parameters".Cat-3 twisted pair Minimum for 10BASE-T[7]
Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table:[8][9]
VF
(%)Transmission line Center insulation 95~99%Script error: No such module "Check for unknown parameters".open-wire line ("ladder line") air insulated 93%Script error: No such module "Check for unknown parameters".HJ8-50B 3 inch Heliax coaxial cable air dielectric[10] 86%Script error: No such module "Check for unknown parameters".RG-8 Belden 7810A coaxial cable gas-injected foamed
high-density polyethylene[11]83%Script error: No such module "Check for unknown parameters".RG-6 Belden 1189A coaxial cable,
RG-11 Belden 1523A coaxial cable82%Script error: No such module "Check for unknown parameters".RG-8X Belden 9258 coaxial cable foamed polyethylene dielectric 80%Script error: No such module "Check for unknown parameters".Belden 9085 twin-lead 77%Script error: No such module "Check for unknown parameters".RG-8/U generic foamed polyethylene 66%Script error: No such module "Check for unknown parameters".Belden 8723 twin shielded
stranded twisted pairpolypropylene insulator[12] 66%Script error: No such module "Check for unknown parameters".RG-213 CXP213 solid polyethylene dielectric
Calculating velocity factor
Electric wave
VF equals the reciprocal of the square root of the dielectric constant (relative permittivity), or , of the material through which the signal passes:
in the usual case where the relative permeability, , is 1. In the most general case:
which includes unusual magnetic conducting materials, such as ferrite.
The velocity factor for a lossless transmission line is given by:
where is the distributed inductance (in henries per unit length), is the capacitance between the two conductors (in farads per unit length), and is the speed of light in vacuum.
Optical wave
VF equals the reciprocal of the refractive index of the medium, usually optical fiber.
See also
- Coaxial cable
- Propagation delay
- Signal velocity
- Speed of electricity
- Speed of sound
- Telegrapher's equations
References
- ↑ Gottlieb, I.M., Practical RF power design techniques, TAB Books, 1993, Template:ISBN, 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, Template:ISBN
- ↑ IEEE 802.3 Clause 8.4.1.3 The minimum required velocity of propagation is 0.77 c.
- ↑ IEEE 802.3 clause 15.3.1.3 The propagation delay shall be ≤5 μs/km. (This is equivalent to a velocity of propagation of 0.67c.)
- ↑ IEEE 802.3 Clause 10.5.1.3 The minimum required velocity of propagation is 0.65 c.
- ↑ IEEE 802.3 Clause 14.4.2.4 The maximum propagation delay of twisted pair shall be 5.7 ns/m (minimum velocity of 0.585 × c).
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- ↑ Template:Cite report
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