Contributor: Video Experts

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Dielectric Insulating Material

Surrounding the center conductor is an evenly formed dielectric material, manufactured in some form of either polyurethane or polyethylene. This dielectric insulator helps determine the operating characteristics of the coax cable by maintaining uniform spacing between the center conductor its outer elements throughout the entire length of the cable. Dielectric manufactured from cellular polyurethane or foam is less likely to weaken a video signal than that made with solid polyethylene. This lower attenuation is desirable when calculating the loss/length factor of any cable. Foam also gives a cable greater flexibility, which may make an installer's job easier. Although foam provides the best performance, it can absorb moisture, which will change its electrical behavior.

Because of its rigid properties, solid polyethylene maintains its shape better than foam and withstands the pressures of accidental pinching or crimping, but this cable is more difficult to handle during installation. In addition, its loss/length attenuation factor is not good as foam, which should be considered in long cable runs.

Braid or Shield

Wrapped around the outside of the dielectric material is a woven copper braid or shield, which acts as a second conductor or ground between the connected equipment. It also acts as a shield against unwanted external signals, known as electromagnetic interference or EMI, which will adversely affect a video signal.



The amount of copper or wire strands in the braid determines the cable's resistance to EMI fields. Commercial grade coax cables containing loosely woven copper braid have a shielding of 80 percent. These cables are suitable for general-purpose use in installations where interference is known to be low. They also work well when the cable is enclosed in a metal conduit or pipe, which also aids in shielding.

If you are not sure of the present conditions and want to safeguard against any future possibilities of a EMI producing device being added near to your installation, use coax with a 'maximum shield' or heavy braid-type shield containing more copper than a commercial grade coax. This extra copper obtains a higher "shielding" by being manufactured with a tighter weave. For CCTV applications, copper conductors are required. Cables using aluminum foil shielding or foil wrap material are not suitable for CCTV installations, they usually are intended to transmit radio frequency signals such as those found in transmission systems or in master antenna distribution systems. Aluminum or foil shielded cable may distort a video signal to such a point that the signal is way below the level required for proper system operation, especially over long installation runs.

Outer Jacket

The last component comprising a coax cable is the outer jacket. Although other materials are used, the most common is polyvinyl chloride, or PVC. Available in many colours, black, tan, white and gray, it's suitable for indoor and outdoor applications. Be sure to check on your local fire code before choosing a cable for any particular installation since Teflon and other fire retardant materials are becoming quite popular and even required.