Coaxial cable

From Academic Kids

Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, and usually surrounded by a final insulating layer.

Radio-grade flexible coaxial cable.A:outer plastic sheathB:copper screenC:inner dielectric insulatorD:copper core
Enlarge
Radio-grade flexible coaxial cable.
A:outer plastic sheath
B:copper screen
C:inner dielectric insulator
D:copper core

The cable is designed to carry a high-frequency or broadband signal, as in a high-frequency transmission line. Sometimes DC power (called bias) is added to the signal to supply the equipment at the other end, as in direct broadcast satellite receivers. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.

Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, both usually of thin copper wire. The inner insulator, also called the dielectric, has a significant effect on the cable's properties, such as its characteristic impedance and its attenuation. The dielectric may be solid or perforated with air spaces. Connections to the ends of coaxial cables are usually made with RF connectors.

Contents

Electromagnetics

Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, twisted or otherwise shaped without changing their characteristic impedance. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line.

Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The line itself forms a coaxial waveguide, and the transmission of energy in the line occurs totally through the wave that propagates inside the cable between the conductors. Coaxial lines can therefore be bent and twisted without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them.

Coaxial lines are filled with a dielectric material that maintains the spacing between the center conductor and shield. Unfortunately, all dielectrics have loss associated with them, which causes most coaxial lines to be lossier than open wire lines.

Important parameters

  • The characteristic impedance in ohms (Ω) is calculated from the ratio of the inner and outer diameters and the dielectric constant. Assuming the dielectric properties of the material inside the cable does not vary appreciably over the operating range of the cable, this impedance is frequency independent.
  • Capacitance, in farads per metre.
  • Resistance, in ohms per metre.
  • Attenuation or loss, in decibels per metre. This is dependent on the loss in the dielectric material filling the cable, and resistive losses in the center conductor and shield. These losses are frequency dependent, the losses becoming higher as the frequency increases. In designing a system, engineers must consider not only the loss in the actual cable itself, but also the insertion loss in the connectors.
  • Outside diameter, which dictates which connectors must be used to terminate the cable.
  • Velocity of propagation, which depends on the type of dielectric.

Standard cable types

Most coaxial cables have a characteristic impedance of either 50 or 75 ohms. The RF industry uses standard type-names for coaxial cables.

A series of standard types of coaxial cable were specified for U.S. military uses, in the form "RG-#" or "RG-#/U" (from Radio Guide). These were listed in MIL-HDBK-216 published in 1962. These desigations are now obsolete. The current military standard is MIL-SPEC MIL-C-17. MIL-C-17 numbers, such as M17/75-RG214.are given for military cables and manufacturer's catalog numbers for civilian applications. However, the RG-series designations were so common for generations that they are still used, although critical users should be aware that since the handbook is withdrawn there is no standard to guarantee the electrical and physical characteristics of a cable described as "RG=# type". The RG designators are mostly used to identify compatible connectors that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables. For example:

  • RG-6/U - 75 Ω dielectric diameters, O.D. (outside diameter or ) 0.180-0.185 inches, low loss at high frequency for satellite television and cablemodem (this last, with an F connector).
  • RG-6/UQ - This is 'quad' shield RG 6. Apparently it has twice as much shielding as regular RG-6/U
  • RG-11
  • RG-58 - 50 Ω - 0.2" (5 mm)
  • RG-59/U - 75 Ω - 0.25" (6.5 mm)
  • RG-62/U - 92 Ω
  • RG-178
  • RG-179 - 75 Ω - 2.8 mm
    ( = diameter, Ω = ohms)

References for this section

  • H. P. Westman et al, (ed), Reference Data for Radio Engineers, Fifth Edition, 1968, Howard W. Sams and Co., no ISBN, Library of Congress Card No. 43-14665

Uses of coaxial cable

Short coaxial cables are commonly used to connect home video equipment, or in ham radio setups. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications.

Long distance coaxial cable is used to connect radio networks and television networks, though this has largely been superseded by other more high-tech methods (fibre optics, T1/E1, satellite). It is still common for carrying cable television signals.

Types of coaxial cable

In broadcasting and other forms of radio communication, hard line is a very heavy-duty coaxial cable, where the outside shielding is a rigid or semi-rigid pipe, rather than flexible and braided wire. Hard line is very thick, typically at least a half inch or 13 mm and up to several times that, and has low loss even at high power. It is almost always used in the connection between a transmitter on the ground and the antenna or aerial on the tower. Hard lines are often made to be pressurised with nitrogen or desiccated air, which provide an excellent dielectric even at the high temperatures generated by thousands of watts of RF power, especially during intense summer heat and sunshine. Physical separation between the inner conductor and outer shielding is maintained by spacers, usually made out of tough solid plastics like nylon.

Triaxial cable or triax is coaxial cable with a third layer of shielding, insulation and sheathing. The outer shield, which is earthed, protects the inner shield from electromagnetic interference from outside sources.

Twin-axial cable or twinax is a balanced, twisted pair within a cylindrical shield. It allows a nearly perfect differential signal which is both shielded and balanced to pass through. Multi-conductor coaxial cable is also sometimes used.

Biaxial cable or biax is a figure-8 configuration of two 50 ohm coaxial cables, used in some proprietary computer networks.

Interference and troubleshooting

Despite being shielded, interference can occur on coaxial cable lines. Eventually, the insulation degrades and the cable must be replaced, especially if it has been exposed to the elements on a continuous basis. The copper screen is normally grounded, and if even a single thread touches the inner copper core, the signal will be shorted out. This most often occurs at improperly installed end connectors and splices. Also, the connector or splice must be properly attached to the copper screen, as this provides the return electrical path for the signal.

For cable television it is important to use the correct type of coaxial cable. RG-59/U should be avoided, and only RG-6/U, or in cases of severe interference, RG-6/UQ (quad-shield) used. Many consumers have purchased the cheaper RG-59/U to use as an extension for cable television, only to find it causes severe interference. Also, unknown to most cable television customers, leakage of signals can cause interference to aircraft communications which operate on the same frequency as several cable channels. This may even be a violation of the law.

In the United States and some other countries, cable channels 2-13 share the same frequency as those from television broadcast towers. If the cable consumer is too close to a television tower and the cable company provides the same station on the like channel, interference and 'ghosting' may result. The solution is to make sure the cable signal is at the maximum allowed strength (especially if splitters are use for multiple TV sets), as this will increase the signal-to-noise level (the "noise" being the pickup of the broadcast tower). Using the more expensive quad-shield coaxial cable also helps reduce interference. Only industrial-quality cable TV amplifiers (generally not available at retail) should be used to amplify weak signals. Cheaper ones, sold at consumer electronics stores, often cause more problems than they solve.

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External links

es:Cable coaxial fr:Cble coaxial it:Cavo coassiale ms:Kabel coaxial nl:Coaxkabel pt:Cabo coaxial fi:Koaksiaalikaapeli

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