同轴电缆

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同軸電纜 (Coaxial cable)是一種電線, 通常是由四層物料造成: 最內是一條導電線, 線的外面有一層電介質(絕緣體)圍攏, 絕緣體外面又有一層簿的導電體, 然後導電體外面是最外層的絕緣物料.

一條普通的同軸電纜 (Radio-grade flexible coaxial cable)
A:最外層的絕緣物料
B:導電層
C:線中的電介質(絕緣體)
D:中心的銅線

導線是設計用作傳送高頻和寬頻信號, 是作為高頻傳送線。 有時候直流電會加進導線內提供另一端的電器使用, 例如直接寬頻衛星接收器。因為他傳送信號的電場理論上只在內外導體間傳送, 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.

[编辑] Signal propagation

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 transmission of energy in the line occurs totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore be bent and twisted (subject to limits) without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them. The inner conductor can be made of braid and the outer conductor can be made of corrugated tube for greater flexibility, but this comes at the cost of increased ohmic losses and lower phase velocity. The outer conductor can also be made of (in order of increasing leakage) wound foil, woven tape, or braid.

In radio-frequency applications up to a few gigahertz, the wave propagates only in the transverse electric magnetic (TEM) mode, which means that the electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certain frequency called the cutoff frequency, transverse electric (TE) and/or transverse magnetic (TM) modes can also propagate, as they do in a waveguide. It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with different phase velocities to propagate, interfering with each other. The outer diameter is roughly inverse proportional to the cutoff frequency.

Coaxial cables require an internal structure of an insulating (dielectric) material to maintain 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. In typical applications the loss in polyethylene is comparable to the ohmic loss at 1 GHz and the loss in PTFE is comparable to ohmic losses at 10 GHz. Most cables have a solid dielectric; others have a foam dielectric which contains as much air as possible to reduce the losses. Foam coax will have about 15% less attenuation but can absorb moisture - especially at its many surfaces - in humid environments, increasing the loss. Stars or spokes are even better, but more expensive. Furthermore the lower dielectric constant of air allows for a greater inner diameter at the same impedance and a greater outer diameter at the same cutoff frequency lowering ohmic losses.

[编辑] Connectors

From the signal point of view, a connector can be viewed as a short, rigid cable. The connector usually has the same impedance as the related cable and probably has a similar cutoff frequency although its dielectric may be different. High-quality cables are often silver plated, while high-quality connectors are usually gold plated with lower-quality connectors using silver or tin plating.

[编辑] 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 do 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.
• cutoff frequency

[编辑] Standards

Most coaxial cables have a characteristic impedance of either 50, 52, 75, or 93 ohms. The RF industry uses standard type-names for coaxial cables.

A series of standard types of coaxial cable were specified for military uses, in the form "RG-#" or "RG-#/U" (RG from Radio Guide, /U indicates multiple uses). They go back to World War II and 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:

[编辑] References for this section

• RF transmission lines and fittings. Military Standardization Handbook MIL-HDBK-216, U.S. Department of Defence, 4 January 1962. [1]
• Withdrawal Notice for MIL-HDBK-216 2001
• Cables, radio frequency, flexible and rigid. Details Specification MIL-DTL-17H, 19 August 2005 (superseding MIL-C-17G, 9 March 1990). [2]
• Radio-frequency cables. International Standard IEC 60096.
• 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

[编辑] Uses

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.

[编辑] Types

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.

Semi-rigid cable is a coaxial form using a solid copper outer sheath. This type of coax offers superior screening compared to cables with a braided outer conductor, especially at higher frequencies. The major disadvantage is that the cable, as its name implies, is not very flexible, and is not intended to be flexed after initial forming.

[编辑] 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. Low frequency signals (below 100MHz) can penetrate the shield while high frequency signals cannot.

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.

[编辑] Timeline

• 1884 - Coaxial cable patented in Germany by Ernst Werner von Siemens, but with no known application. [unverified: more details needed]
• 1894 - Oliver Lodge demonstrates waveguide transmission at the Royal Institution. Nikola Tesla receives 美國專利 514167, Electrical Conductor, on February 6.
• 1929 - First modern coaxial cable patented by Lloyd Espenschied and Herman Affel of AT&T's Bell Telephone Laboratories.
• 1934 - First transmission of TV pictures on coaxial cable, from the Berlin Olympic Games to Leipzig.
• 1936 - AT&T installs experimental coaxial TV cable between New York and Philadelphia.
• 1936 - Coaxial cable laid by the Post Office (now BT) between London and Birmingham, providing 40 telephone channels. [Source: archives at http://www.bt.com]
• 1941 - First commercial use in USA by AT&T, between Minneapolis, Minnesota and Stevens Point, Wisconsin. L1 system with capacity of one TV channel or 480 telephone circuits.
• 1956 - First transatlantic coaxial cable laid, TAT-1.

[编辑] External links

• Video: Types of Coaxial cabling used in 2.4 GHz networks
• Coaxial Cable Specifications
• How to attach connectors to a coax cable

[编辑] References

• Discussion of using coaxial cable above cutoff, on "Towertalk" mailing list