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ContinuousWave: Small Boat Electrical
Primer on Coaxial Transmission Line Cables
|Author||Topic: Primer on Coaxial Transmission Line Cables|
posted 08-10-2008 09:52 AM ET (US)
Boaters use coaxial cable transmission lines in several applications on their small boats. The most common is to connect a VHF Marine Band radio to its antenna. We briefly discuss several properties of coaxial cable.
Coaxial cable is used as a transmission line and is made to have a characteristic or surge impedance. The transmission line impedance is generally chosen to match the terminating impedance of the antenna so as to reduce the occurrence of standing waves on the transmission line. In the transmission of radio frequency power an impedance of 50-ohms is practically in universal use. The impedance of a coaxial transmission line is determined by the ratio of the diameters of the inner and outer conductors and is affected by the dielectric constant of the medium that separates and insulates them. The impedance of flexible coaxial cables is usually rated with some variation, and a range of plus or minus two ohms is typical.
The material between the inner and outer conductors of a coaxial transmission line can be of any good insulating material with low radio-frequency absorption. Typical materials are air, nitrogen, plastic, and foamed-plastic. The lowest loss occurs with a vacuum, but since this is impractical to implement, nitrogen or de-humidified air is often used. Some cables use a combination of air and a spiral insulated plastic carrier. The trade name Heliax is one such product. Solid polyethylene is a good dielectric material for cables that need flexibility. Foamed polyethylene is also used. It is claimed to initially provide slightly lower loss. Coaxial cables using foam dielectric often have problems with displacement of the center conductor, particularly when the cable flexes or bends.
The conductors of a coaxial cable are typically copper or tinned copper. For non-rigid cables the outer conductor is typically made from a tightly woven braid. The braid density is often expressed as a percentage. The higher the percentage of coverage the better. The inner conductor is made from a stranded copper wire, or, in some cases, from a solid copper wire. Cables made from bare copper are subject to more problems with oxidation than those made from silver or tinned-copper wire.
Outer Vinyl Jacket
The outer conductor of a flexible coaxial cable is typically covered with a plastic jacket made from vinyl plastic. There are two distinct grades of vinyl material. A Type-I vinyl is known to leach plasticizers into the outer conductor of the cable, causing increased attenuation. A Type-II vinyl jacket is known as a non-contaminating vinyl and will not degrade the cable performance. Sunlight and weather exposure contribute to contamination of the outer conductor from the vinyl jacket.
Coaxial cable is purchased by the military in great quantities and the initial control of its design and quality was specified by a standard. Most coaxial cable of high quality is still manufactured and sold to this standard. The current standard is Mil-C-17-G.
Initially there were two major types of 50-ohm transmission line, RG-8/U and RG-58/U. These designators have been applied to many products which actually did not meet the specifications. As a result, new designators were developed to prevent ambiguities and misidentification.
RG-8/U cable is now generally sourced as RG-213/U. This is a half-inch 50-ohm transmission line with very high braid coverage, a solid dielectric, a non-contaminating vinyl jacket, and bare copper conductors. RG-213/U is a Mil-Spec cable.
RG-58/U is now generally sourced as RG-58C/U. This is a quarter-inch 50-ohm transmission line with very high braid coverage, a solid dielectric, a non-contaminating vinyl jacket, and tinned conductors. RG-58C/U is a Mil-Spec cable.
RG-8X is a cable grade invented by some cable manufacturers. It typically has a very loosely woven and low density braid, a foam dielectric, bare copper conductors, and a contaminating vinyl jacket. RG-8X is not a Mil-Spec cable.
Coaxial cable for marine use should be chosen with consideration for its ability to tolerate movement, sunlight, exposure to moisture, and high percentage of braid coverage (to prevent leakage and interference).
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