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Thursday, April 28, 2011

Coaxial Cable Types

Coaxial cables are cables that consist of two conductors that share an axis. The inner conductor is usually a straight wire; the outer conductor is the wire's shield, which is usually a braided wire or a metal ribbon. It is used primarily for radio and television signal transference. The combination of the center conducting wire and the outer conducting shield keeps other cable noise out and the signal in.

Different types of coaxial cable rated by the U.S. government are designated by the letters RG and then a number. RG stands for "radio grade" and are from when the Army used most of its coaxial cable for radio transmission. The numbers were chosen arbitrarily. The most common RG designations are RG-6, RG-8, RG-11, RG-58, and RG-59. The main difference between them is the impedance. RG-8 and RG-58 have a 50-ohm impedance, while RG-6, RG-11, and RG-59 are 75 ohms.
Coaxial cable comes in many different types of impedance. Impedance is the resistance the cable puts on the signal flow. So, 50-ohm coaxial cable is used primarily in radio transmitters because its low resistance is able to handle the high transmitter power. Fifty-ohm cable replaced the 60-ohm version that was used for the same thing in the mid-20th century. Televisions use 75-ohm cables to carry video and audiosignals.

Shield Contruction
The outer conductor, or the shield, of the coaxial cable is constructed in different ways. Flexible, or braided, coaxial cable is a cable that has an outer conductor made from a braided wire shield. One of the drawbacks to this kind of cable is that if the braid isn't weaved tightly enough, a small amount of high-frequency energy can radiate and cause the signal to bleed over into other wiring. Semigrid coaxial cable has a solid tubular outer conductor that contains all the RF energy. Ribbon coaxial cable is the construction of many tiny coaxial cables each wrapped in a foil shield.

The right coaxial cable means nothing if they have the wrong connectors. Different connectors work better at different frequencies. Ultra high frequency (UHF) connectors were the standard used before World War II and have a frequency range of 0-300 megahertz (MHz). N connectors, developed after WW II, are the oldest of the high-performance coax connectors and have a frequency range of zero to 11 gigahertz (GHz). A BNC connector has a much narrower frequency range, zero to 4 GHz, than the N connector. The TNC connector is the improved version of the BNC with a frequency range of zero to 11 GHz.


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