|Tuesday, 15 August 2006|
Coaxial cables are a preferred means for transmitting signals. The use of coaxial cable has become more prevalent in communications and other systems because of its advantages over other wire systems. Effective data transmission between electronic equipment is dependent upon the successful utilization of such cables. Coaxial cables are used in many areas such as transmission and computer cables, computer networking, video signal transmission, instrumentation cables, and broadcast cables. Compared with other transmission lines, coaxial cables have the advantage that they are suitable for broadband radio frequency signals, even microwave-frequency signals. Coaxial cables transmit energy in the TEM (transverse electromagnetic) mode, and have a cutoff-frequency of zero. A coaxial cable can propagate a signal with as little attenuation as possible because it comprises a two-conductor transmission line having wave impedance and propagation constant of an unbounded dielectric, and the phase velocity of the energy is equal to the velocity of light in an unbounded dielectric. A coaxial cable has excellent interference suppression properties due to its structure. Electromagnetic fields outside the coaxial conductor disturb the signals to be transferred in the coaxial conductor less than in other transmission lines. This advantage makes coaxial cables particularly suited for efficient operation in the HF (high frequency) and UHF (ultra high frequency) regions of the electromagnetic spectrum. It is a perfectly shielded line and has a minimum of radiation loss. The nearby metallic objects and electromagnetic energy sources have minimum effect on the cable as the outer conductor serves as a shield for the inner conductor.
A typical coaxial cable generally consists of a centrally located electrical conductor, usually made of copper, which is surrounded by a first dielectric insulator or sheath. This dielectric insulating layer can be made of plastic or foam and forms an annular ring of substantially uniform thickness around the centrally located electric conductor. The central conductor may be composed of a single wire or bundle or wound wires, also known as litz. An outer conductor or conductive shield surrounds the inner dielectric sheath and typically is a metal braid, or optionally a metallic foil, or further optionally a multilayered combination of either or both. The conductive shield is surrounded by an outer dielectric sheath of the cable. This combination of braided metallic strands and/or metallic foil serves as a second, outer conductive shield. Several different types of outer conductors have been employed in coaxial cables. These commonly used outer conductors include braided wire employed for the outer conductor providing excellent flexibility, flat tape with braid applied over it providing lower attenuation and better RF shielding, corrugated copper or aluminum tubes providing excellent shielding and low loss, and smooth wall copper or aluminum tubes providing the lowest loss and excellent shielding. The coaxial cable be made with a braided outer conductor for increased flexibility and it is generally impervious to weather.
Coaxial cables are generally of two types. Each has an inner conductor, surrounded by an outer conductor, with the space between the inner conductor and the outer conductor being filled with air, or a dielectric material, either a solid dielectric, or a foam dielectric. Coaxial cables of the air dielectric type of construction have very good signal propagation characteristics. Coaxial cables of the foam dielectric type of construction possess significantly better bending properties than air dielectric cables. The cables filled with air are the most effective in preventing signal loss, however the space left between the inner conductor and outer conductor must be kept dry in order to avoid loss of electrical performance caused by intrusion of moisture. Cables which use a solid polymer dielectric are less expensive, but they are less efficient since air is a superior dielectric. Foam dielectrics provide good performance at lower cost than cables, which require that dry air be supplied to the annular space, and they are more efficient than cables, which employ solid dielectrics. Coaxial cables containing polyethylene or another resin in the dielectric layer usually require antioxidants to provide protection against loss of physical properties over time caused by oxidative degradation. Coaxial cables with dielectric insulation are typically stabilized with primary antioxidants. Coaxial cables are generally susceptible to moisture migration between the insulation and the conductor. This moisture reacts with the metallic surface of the conductor and causes corrosion to develop on the conductor.