Coaxial cable receives its name because all its parts — outer jacket, grounded return conductor, dielectric layer and inner metallic pin — share a common axis. Many installers assume the purpose of the dielectric layer is to insulate the center pin from the braided or foil shielding, but in reality the way coax works is somewhat more complex. The signal that it conveys depends upon an electrostatic charge in the dielectric layer.
Coax can be used to carry modest amounts of electrical power, but its primary application is as a radio frequency transmission line. It can perform both functions simultaneously, even if the electrical energy travels in a direction opposing the RF. An example is when the cable is used to establish the connection between a satellite TV dish outdoors and the receiver, typically sitting 50 ft inside the building. The radio-frequency signal — carrying audio, video and synch information — comes from the dish to the receiver. But semiconductors on the dish receive their dc bias from a power supply built into the indoor modem.
Coax is capable of conveying higher frequencies than conventional cable such as twisted-pair telephone wire or audio speaker cable. As the frequency of the signal rises, capacitive and inductive losses assume greater importance. Capacitive reactance drops and inductive reactance rises at higher frequencies. Because inductive reactance in a transmission line is a series phenomenon and capacitive reactance is a parallel phenomenon, the two circuit parameters conspire to attenuate the signal.
Coaxial cable successfully deals with this problem up to a frequency, depending upon the type, in the neighborhood of one gigahertz. Above this limit frequency related losses become unacceptable and a waveguide is required.
An important parameter of coaxial cable is its characteristic impedance. Commonly used values are 50 and 75 Ω. The characteristic impedance of coaxial cable depends on the effective diameter and material composition of inner and outer conductors and thickness and dielectric constant of the material that separates them. Characteristic impedance cannot be measured by an ohmmeter (a time-domain reflectometer is required) and it is not related to the length of the cable run. For most work, it is not necessary to measure the characteristic impedance because this is set by the manufacturer. At that level strict quality control is necessary, particularly in regard to conductor spacing as determined by dielectric layer thickness.
At RF frequencies, coaxial cable must be precisely matched to source and load impedance except where mismatch is intentionally introduced for the purpose of attenuation. Any mismatch will give rise to out-of-phase waveforms, making for reflected energy back toward the source, data loss and signal corruption.
Since its invention by Oliver Heaviside, who described and patented the cable in 1880, coax has been a constant in the industry. It has been partly eclipsed by category cable in recent decades, but remains the best choice for numerous applications including instrumentation wiring, antenna leads and audio/video links.