Velocity of propagation
Encyclopedia
The velocity factor, also called wave propagation speed or velocity of propagation (VoP or 
), of a transmission medium
is 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, relative to the speed of light. 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, while velocity factor is used for the ratio.
s and radio transmission line
s. Plenum data cable
typically has a VF between 0.42 and 0.72 (42% to 72% of the speed of light) 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 are provided in the ARRL Handbook:
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 c is the speed of light
, L is the distributed inductance
(in henries per unit length) and C is the capacitance
between the two conductors (in farads per unit length).
), of a transmission mediumTransmission medium
A transmission medium is a material substance that can propagate energy waves...
is the speed at which a wavefront (of an acoustic signal, for example, or an electromagnetic signal, a radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
signal, a light pulse in a fibre channel
Fibre Channel
Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
or a change of the electrical voltage on a copper wire) passes through the medium, relative to the speed of light. For optical signals, the velocity factor is the reciprocal of the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
.
The speed of radio signals in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, for example, is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, 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, while velocity factor is used for the ratio.
Typical velocity factors
Velocity factor is an important characteristic of communication media such as Category 5 cableCategory 5 cable
Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...
s and radio transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
s. Plenum data cable
Plenum cable
Plenum cable is cable that is laid in the plenum spaces of buildings. The plenum is the space that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows...
typically has a VF between 0.42 and 0.72 (42% to 72% of the speed of light) 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
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
to travel one meter.
Some typical velocity factors for radio communications cables are provided in the ARRL Handbook:
| VF% | Transmission line |
|---|---|
| 95-99 | Open-wire "Ladder" Line |
| 80 | Belden 9085 twin-lead Twin-lead Twin-lead cable is a two-conductor ribbon cable used as a transmission line to carry radio frequency signals.- Characteristics and uses:... |
| 82 | RG-8X Belden 9258 coaxial cable Coaxial cable Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis... (foamed polyethylene dielectric) |
| 66 | RG-213 CXP213 coaxial cable Coaxial cable Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis... (solid polyethylene dielectric) |
Calculating velocity factor
VF equals the reciprocal of the square rootSquare root
In mathematics, a square root of a number x is a number r such that r2 = x, or, in other words, a number r whose square is x...
of the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
(relative permittivity),
, of the material through which the signal passes:The VF of a lossless transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
is given by:
where c is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, L is the distributed inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
(in henries per unit length) and C is the capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
between the two conductors (in farads per unit length).
See also
- Coaxial cableCoaxial cableCoaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
- Propagation delayPropagation delayPropagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics...
- Signal velocitySignal velocityThe signal velocity is the speed at which a wave carries information. It describes how quickly a message can be communicated between two separated parties...
- Speed of electricitySpeed of electricityThe speed of electricity refers to the movement of electrons through a conductor in the presence of potential and an electric field. The finite velocity of the electric field thus requires consideration, and may even become the dominating factor in the electrical phenomena: In the conduction of...
- Speed of soundSpeed of soundThe speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
- Telegraphers equationsTelegraphers equationsThe telegrapher's equations are a pair of linear differential equations which describe the voltage and current on an electrical transmission line with distance and time. The equations come from Oliver Heaviside who in the 1880s developed the transmission line model which is described in this article...