The center conductor layer is a thin conducting wire, either solid or braided copper. A dielectric layer, made up of an insulating material with very well-defined electrical characteristics, surrounds the wire. A shield layer then surrounds the dielectric layer with metal foil or braided copper mesh. The whole assembly is wrapped in an insulating jacket. The outer metal shield layer of the coax cable is typically grounded in the connectors at both ends to shield the signals and as a place for stray interference signals to dissipate.
A key to coaxial cable design is tight control of cable dimensions and materials. Together, they ensure the characteristic impedance of the cable takes on a fixed value. High-frequency signals are partially reflected at impedance mismatches, causing errors.
Characteristic impedance is sensitive to signal frequency. Above 1 GHz, the cable maker must use a dielectric that doesn't attenuate the signal too much or change the characteristic impedance in a way that creates signal reflections.
Electrical characteristics of coax are application-dependent and crucial for good performance. Two standard characteristic impedances are 50 ohms, used in moderate power environments, and 75 ohms, common for connections to antennas and residential installations.
Types of coaxial cables
Hard-line coaxial cable relies on round copper tubing and a combination of metals as a shield, such as aluminum or copper. These cables are commonly used to connect a transmitter to an antenna. Triaxial cable has a third layer of shielding that is grounded to protect signals transmitted down the cable. Rigid-line coaxial cables are made up of twin copper tubes that function as unbendable pipes. These lines are designed for indoor use between high-power radio frequency (RF) transmitters. Radiating cable mimics many components of hard-line cable, but with tuned slots in the shielding matched to the RF wavelength at which the cable will operate. It is commonly used in elevators, military equipment, and underground tunnels.
Uses of coaxial cables
In the home and small offices, short coaxial cables are used for cable television, home video equipment, amateur radio equipment and measuring devices. Historically, coaxial cables were also used as an early form of Ethernet, supporting speeds of up to 10 Mbps, but coax has supplanted by the use of twisted pair cabling. However, they remain widely in use for cable broadband internet. Coaxial cables are also used in automobiles, aircraft, military and medical equipment, as well as to connect satellite dishes, radio and television antennae to their respective receivers.