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.

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 inversely 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 have more loss 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.

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 connectors are usually gold or rhodium plated, with lower-quality connectors using nickel or tin plating. Silver is occasionally used in some high-end connectors due to its excellent conductivity, but it usually requires extra plating of another metal since silver readily oxidizes in the presence of air.

One increasing development has been the wider adoption of micro-miniature coaxial cable in the consumer electronics sector in recent years. Wire and cable companies such as Tyco, Sumitomo Electric, Hitachi Cable, Fujikura and LS Cable all manufacture these cables, which can be used in cellular phones.

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