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Coaxial cable

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 - 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

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