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 (jacket). It is used as a high-frequency transmission line to carry a high-frequency or broadband signal. 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.

Significance of impedance

A question that is often asked is what the significance of a 52 or 75 Ω characteristic impedance is. The best coaxial cable impedances to use in high-power, high-voltage, and low-attenuation applications were experimentally determined in 1929 at Bell Laboratories to be 30, 60, and 77 Ω respectively.[citation needed] 30 Ω cable is exceedingly hard to make however, so a compromise between 30 Ω and 60 Ω was reached at 52 Ω, which has persisted; note this also corresponds very closely to the drive impedance of a half wave dipole antenna in real environments, and provides an acceptable match to the drive impedance of quarter wave monopoles as well. 73 Ω is an exact match for a centre fed dipole aerial/antenna in free space (approximated by very high dipoles without ground reflections), so 75 was adopted as a compromise between 73 and 77 ohms.