RG-59 Radio-grade flexible coaxial cable. A: outer plastic sheath B: copper screen C: inner dielectric insulator D: copper core

Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer. It is used as a high-frequency transmission line to carry a high-frequency or broadband signal. 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.

An electrical transmission line comprising an inner, central conductor surrounded by a tubular outer conductor. The two conductors are separated by an electrically insulating medium which supports the inner conductor and keeps it concentric with the outer conductor. One version of coaxial cable has periodically spaced polyethylene disks supporting the inner conductor. This coaxial is a building block of multicoaxial cables used in L-carrier systems (see illustration). See also Transmission lines.

Construction of multicoaxial transmission line with twenty 0.375-in. (9.5-mm) coaxial units.

The symmetry of the coaxial cable and the fact that the outer conductor surrounds the inner conductor make it a shielded structure. At high frequencies, signal currents concentrate near the inside surface of the outer conductor and the outer surface of the inner conductor. This is called skin effect. The depth to which currents penetrate decreases with increasing frequency. Decreased skin depth improves the cable's self-shielding and increases transmission loss. This loss (expressed in decibels per kilometer) increases approximately as the square root of frequency because of the skin effect. See also Electrical shielding; Skin effect (electricity).

Coaxial cables can carry high power without radiating significant electromagnetic energy. In other applications, coaxial cables carry very weak signals and are largely immune to interference from external electromagnetic fields.

A coaxial cable's self-shielding property is vital to successful use in broadband carrier systems, undersea cable systems, radio and TV antenna feeders, and community antenna television (CATV) applications. Coaxial units are designed for different mechanical behavior depending upon the application. Widely used coaxials are classified as flexible or semirigid.

A high-capacity cable widely used in audio and video applications, commonly called "coax" (pronounced "co-ax"). Coax has also been used in earlier Ethernet networks (see 10Base2). Strong and flexible, coaxial cable contains an insulated solid or stranded wire surrounded by a solid or braided metallic shield, all of which is wrapped in a plastic cover. Fire-safe Teflon coating is optional.

There Are Many Types

Often similar in appearance, there are numerous types of coaxial cables. Typically with impedances of 50 or 75 Ohms, cables have different outside diameters and maximum capacities for operating voltage. Cables are also rated for signal loss (attenuation in dBs per 100 feet). Coaxial cable types are designated with an RG prefix such as RG-6, RG-59, etc. Coax is a two-wire cable. The inner wire is the primary conductor, and the metal sheath is used for ground.

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