coaxial cable

coaxial cable. A coaxial cable is an electrical cable consisting of a central conductor, an insulating dielectric layer, a metallic shield, and an outer insulating jacket. Its key design feature is that the shield and the inner conductor share a common geometric axis, which minimizes electromagnetic interference and allows efficient transmission of high-frequency signals. This construction makes it a fundamental component in modern telecommunications, broadcasting, and computer networking.

Construction and design

The central element is typically a solid or stranded copper wire, though alternatives like copper-clad steel are used for strength. Surrounding this is a dielectric insulator, which may be solid polyethylene, foam polyethylene, or air spaced with helical supports, determining the cable's velocity factor. A critical component is the metallic shield, often a braid of aluminum or copper, sometimes combined with a solid aluminum foil layer to form a dual shield; this shield acts as the return path for the electrical signal and blocks external radio frequency interference. The entire assembly is protected by an outer jacket made of materials like polyvinyl chloride or polyethylene, providing environmental and mechanical protection. The precise geometry, maintained by the dielectric, ensures the transmission line has consistent impedance characteristics, crucial for signal integrity from the transmitter to the receiver.

Electrical characteristics

The primary electrical property is its characteristic impedance, with common standardized values being 50, 75, and 93 ohms; 50-ohm cables are standard in radio frequency power applications like connecting radio transmitters to their antennas, while 75-ohm cables are the norm for video signal and cable television distribution. Signal loss, or attenuation, increases with frequency and is measured in decibels per unit length, influenced by the dielectric material and conductor size. The shielding effectiveness determines the cable's immunity to external noise from sources like power lines and its ability to contain signals, preventing them from interfering with nearby equipment, a principle vital for compliance with regulations set by the Federal Communications Commission. The capacitance and inductance per unit length, along with the velocity of propagation, are determined by the physical dimensions and dielectric constant, affecting the phase of transmitted signals in systems like radar.

Applications

A dominant historical and continuing application is in cable television systems, where it forms the trunk and distribution network connecting the headend to subscriber homes. It is equally fundamental in broadband internet access, with standards like Data Over Cable Service Interface Specification enabling high-speed data over hybrid fiber-coaxial networks. In professional video production, cables like RG-6 carry composite video and serial digital interface signals for equipment from Sony and Panasonic. The radio communication industry relies on it to connect base stations, amateur radio equipment, and cellular network infrastructure, with low-loss types used in large installations like those for American Broadcasting Company broadcast towers. Before the widespread adoption of Ethernet over twisted pair, it was the medium for early computer networks such as 10BASE2, which used Thinnet cabling.

History

The principle was first patented in 1880 by Oliver Heaviside, the English engineer who also developed the theory of telegraphy transmission lines. Significant commercial development began in the 1920s at Bell Labs, particularly for long-distance telephone lines carrying multiplexed voice channels. Its use expanded dramatically during World War II, where it was crucial for radar and other radio equipment, with further refinements by the United States Army Signal Corps. The post-war boom in television broadcasting created massive demand for it as the backbone of community antenna television systems. The late 20th century saw its role cemented in cable television networks and early internet infrastructure, with standards evolving through the work of organizations like the Society of Cable Telecommunications Engineers.

Types and standards

Cables are categorized by RG standards, a designation originating from U.S. military specifications like MIL-C-17; common types include RG-6 for residential satellite television and broadband, and RG-59 for shorter-run closed-circuit television. For high-power and low-loss applications, semi-rigid cables with a solid aluminum outer sheath are used in aerospace and defense contracting. Flexible variants with high strand-count shields are employed in broadcast mobile production vehicles. Industry standards are maintained by groups such as the International Electrotechnical Commission, which defines test methods, and the Electronic Industries Alliance. Specific performance categories include Triaxial cable, which adds a second shield for enhanced noise rejection in professional video camera connections, and radiating cable, or leaky feeder, used in tunnels and mines for wireless communication. Category:Electrical cables Category:Telecommunications equipment Category:Broadcasting technology