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Coax cable)
Coaxial cable is an electrical cable consisting of a round, insulated conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer.
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 thin copper wire. 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. Connections to the ends of coaxial cables are usually made with RF connectors.
Electromagnetics
Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, twisted or otherwised shaped without changing their characteristic impedance. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line.
Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The line itself forms a coaxial waveguide, and the transmission of energy in the line occurs totally through the wave that propagates inside the cable between the conductors. Coaxial lines can therefore be bent and twisted without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them.
Coaxial lines are filled with a dielectric material that maintains the spacing between the center conductor and shield. Unfortunately, all dielectrics have loss associated with them, which causes most coaxial lines to be lossier than open wire lines.
Important parameters
- The Characteristic impedance in ohms (Ω) is calculated from the ratio of the inner and outer diameters and the dielectric constant. Assuming the dielectric properties of the material inside the cable does not vary appreciably over the operating range of the cable, this impedance is frequency independent.
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Capacitance, in farads per metre.
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Resistance, in ohms per metre.
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Attenuation or loss, in decibels per metre. This is dependent on the loss in the dielectric material filling the cable, and resistive losses in the center conductor and shield. These losses are frequency dependent, the losses becoming higher as the frequency increases. In designing a system, engineers must consider not only the loss in the actual cable itself, but also the insertion loss in the connectors.
- Outside diameter, which dictates which connectors must be used to terminate the cable.
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Velocity of Propagation, which depends on the type of dielectric.
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:
Uses of coaxial cable
Short coaxial cables are commonly used to connect home video equipment, or in ham radio setups. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications.
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). It is still common for carrying cable television signals.
Types of coaxial cable
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. Hard lines 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 or triax is coaxial cable with a third layer of shielding, insulation and sheathing. The outer shield, which is earthed, protects the inner shield from electromagnetic interference from outside sources.
Twin-axial cable or twinax is a balanced, twisted pair within a cylindrical shield. It allows a nearly perfect differential signal which is both shielded and balanced to pass through. Multi-conductor coaxial cable is also sometimes used.
Biaxial cable or biax is a figure-8 configuration of two 50-ohm coaxial cables, used in some proprietary computer networks.
Timeline
Last updated: 05-03-2005 09:00:33
