Coaxiais

Coaxiais, commonly called coaxial cables, are a type of electrical transmission line designed for high-frequency signal transmission. They consist of a central conductor surrounded by a dielectric insulator, a conducting outer shield, and an outer jacket. The shield is usually made of braided or foil conductors and serves to return current and shield the signal from external noise. The geometry confines the electromagnetic field to the region between the inner conductor and the shield, supporting a transverse electromagnetic (TEM) mode that allows low-loss transmission over a range of frequencies.

Characteristic impedance is determined by the conductor dimensions and the dielectric material. Common values are 50 ohms and 75 ohms, with 50-ohm cables preferred for most radio-frequency systems and 75-ohm cables common in television and broadband applications. The impedance is given by Z0 = (60/√ε_r) ln(b/a), where a is the inner conductor radius, b is the inner radius of the outer conductor, and ε_r is the relative permittivity of the dielectric.

Loss and bandwidth: Attenuation increases with frequency due to conductor and dielectric losses. The velocity of signals depends on the dielectric constant, typically allowing speeds around two-thirds to nearly the speed of light in the medium. Applications include RF circuits, cable television, satellite and broadband internet, CCTV, and instrumentation. Variants include foam-dielectric and solid-dielectric types; common field types include RG-59, RG-6 (75 Ω) and RG-11 (low loss).