Coaxial cable consists of two cylindrical conductors sharing the same axis (hence ‘co-axial’) and separated by a dielectric. For low frequencies (in flexible cables) the dielectric may be polyethylene or polyethylene foam, but at higher frequencies Teflon and other materials are used.
Also used in some applications, notably high powered broadcasting transmitters, are dry air and dry nitrogen.
Several forms of coaxial line are available. Flexible coaxial cable discussed earlier in this chapter is perhaps the most common form.
The outer conductor in such cable is made of either braided wire or foil. Again, television broadcast receiver antennas provide an example of such cable from common experience.
Another form of flexible or semi-flexible coaxial line is helical line in which the outer conductor is spiral wound. This type of coaxial cable is usually 2.5 or more centimetres in diameter.
Hardline is coaxial cable that uses a thin-walled pipe as the outer conductor. Some hardline coax used at microwave frequencies has a rigid outer conductor and a solid dielectric.
Gas-filled line is a special case of hardline that is hollow, the centre conductor being supported by a series of thin ceramic or Teflon insulators. The dielectric is either anhydrous (i.e. dry) nitrogen or some other inert gas.
Some flexible microwave coaxial cable uses a solid ‘air-articulated’ dielectric, in which the inner insulator is not continuous around the centre conductor, but rather is ridged. Reduced dielectric losses increase the usefulness of the cable at higher frequencies.
Double shielded coaxial cable provides an extra measure of protection against radiation from the line, and EMI from outside sources from getting into the system.
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