Coaxial cable: Difference between revisions
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Important parameters of a coaxial cable include: |
Important parameters of a coaxial cable include: |
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*characteristic impedance, in [[ohm]]s - this enables it to be matched to the equipment at either end. |
*characteristic impedance, in [[ohm]]s - calculated from the ratio of the inner and outer diameters, is independent from the frequency of the signal it carries; this enables it to be matched to the equipment at either end. |
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*attenuation, in [[decibel]]s per [[metre]] - this depends on the frequency of the signal. |
*attenuation, in [[decibel]]s per [[metre]] - this depends on the frequency of the signal. |
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*capacitance, in [[farad]]s per metre - this is important only for low-frequency applications |
*capacitance, in [[farad]]s per metre - this is important only for low-frequency applications |
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Revision as of 00:23, 28 July 2003
Coaxial cable is an electrical cable consisting of a round, insulated conducting wire surrounded by a round, conducting sheath. The sheath is usually surrounded by a final insulating layer. Short cables are commonly used in home video equiptment; long distance coaxial cable is used to connect radio networks and television networks.
The cable is designed to carry a high-frequency or broadband signal, usually at radio frequencies. Sometimes DC power is added to the signal to supply the equipment at the other end. Because the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors (at least in an ideal cable), 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 copper. 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. Coaxial cables are usually terminated with RF connectors.
Important parameters of a coaxial cable include:
- characteristic impedance, in ohms - calculated from the ratio of the inner and outer diameters, is independent from the frequency of the signal it carries; this enables it to be matched to the equipment at either end.
- attenuation, in decibels per metre - this depends on the frequency of the signal.
- capacitance, in farads per metre - this is important only for low-frequency applications
- resistance, in ohms per metre
- outside diameter - dictates which connectors must be used to terminate the cable.
Standard cable types
Most coaxial cables have a characteristic impedance of either 50 ohms or 75 ohms. The RF industry uses standard type-names for coaxial cables. The US military uses the RG or RG/U prefix (probably for "radio grade, universal", but other interpretations exist). For example:
- RG-58 - 50 ohms, 0.2" / 5 mm diameter
- RG-59 - 75 ohms, 0.25" / 6.5 mm
- RG-178
- RG-179
Timeline
- 1884 - Coaxial cable patented in Germany by Ernst Werner von Siemens, but with no known application. [unverified: more details needed]
- 1894 - Waveguide transmission demonstrated to the Royal Institution by Oliver Lodge.
- 1929 - First practical 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.