The cable is designed to carry a high-frequency or broadband signal, usually at radio frequencies. Sometimes DC power (called bias) is added to the signal to supply the equipment at the other end, such in direct broadcast satellite receivers. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.
Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, both usually of copper. The inner insulator, also called the dielectric, has a significant effect on the cable's properties, such as its characteristic impedance and its attenuation. The dielectric may be solid or perforated with air spaces. Coaxial cables are usually terminated with RF connectors.
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2 Standard cable types 3 Uses of coaxial cable 4 Timeline |
Important parameters
Standard cable types
Most coaxial cables have a characteristic impedance of either 50 or 75 ohms. The RF industry uses standard type-names for coaxial cables. The U.S military uses the RG-# or RG-#/U format (probably for "radio grade, universal", but other interpretations exist). For example:
Long distance coaxial cable is used to connect radio networks and television networks, though this has largely been superseded by other more high-tech methods (fibre optics, T1/E1, satellite).
In broadcasting and other forms of radio communication, hard line is a very heavy-duty coaxial cable, where the outside shielding is a rigid or semi-rigid pipe, rather than flexible and braided wire. Hard line is very thick, typically at least a half inch or 13mm and up to several times that, and has low loss even at high power. It is almost always used in the connection between a transmitter on the ground and the antenna or aerial on the tower. Even more efficient is a product called wave guide, which consists of hollow metal (usually steel or aluminum) sections with flanges on the ends, which are bolted together to form a continuous length. Both hard line and wave guide are often made to be pressurised with nitrogen or desiccated air, which provide an excellent dielectric even at the high temperatures generated by thousands of watts of RF energy, especially during intense summer heat and sunshine. Physical separation between the inner conductor and outer shielding is maintained by spacers, usually made out of tough solid plastics like nylon.
Triaxial cable also exists, in which a third layer of insulation and sheathing is included. This allows a nearly perfect signal which is both shielded and balanced/differential to pass through. Multi-conductor coaxial cable is also used sometimes.
Biaxial cable or biax is a figure-8 configuration of two 50 ohm coaxial cables, used in some proprietry computer networks.
Timeline