Contributor: Video Experts
The most common call we receive on a daily recurrence is that of the « possessed » installation, the installation that chugged along perfectly, without a glitch for over a year, but now has fallen prey to the CCTV Gremlins. The system exhibits an intermittent operation, working fine for days, and then suddenly acting as though a spirit has taken over. Cameras mysteriously controlling themselves, monitors fading in and out, erratic keyboard control, you know the rest. Yet when you send out any component for service, they come back without any problem being confirmed. You’re convinced that it’s the component since the unit had been working fine for some time previously, it can’t be the installation.
Cables as well as the equipment they are attached to age, as with humans some more gracefully than others, but everything does age. As electronic equipment ages it requires more « juice » to operate at the same standard as when it was first installed. Where as cabling ages to the extent that if not properly specified at the installation, when it weakens, it can’t provide the signal/power required by the system.
Today we will discuss coaxial cables.
In Closed Circuit Television (CCTV) installations, the most common means of conducting video signals from one piece of equipment to another is via coaxial cable. Coaxial cable is often referred to simply as « coax ». Not only is coax the most commonly used cable, but also the least expensive, most reliable, most convenient, and easily maintained way of transferring electronic images in a CCTV system.
Coax is available from many manufacturers and comes in a variety of sizes, shapes, colours, specifications and capabilities. The most commonly recommended « coax’ type is RG59/U, but this designation actually represents a family of cables with widely varying electrical characteristics. Other varieties like RG59/U; are RG6/U and RG11/U; these are used predominately in CCTV and video work.
Though similar in many ways each cable has various physical and electrical characteristics, which must be taken into consideration before installation.
All three ‘coax » cable groups are included in the same general family. The RG reference is the cable specification for use as a ‘radio guide’, while the numerical value helps differentiate the specifications of each individual cable. Although each cable has its own number, characteristics, and size, there’s no difference in the way these different numbered cables work.
Common ‘coax » cable RG59/U, RG6/U, and RG11/U is circular. Each has a center conductor surrounded by dielectric insulating material, which in turn is protected by a shield or braid to prevent against electromagnetic interference (EMI). The outer covering is the ‘jacket ». A dielectric material separates the coaxial cable’s two conductors. The outer conductor acts as a shield and helps isolate the inner conductor from spurious electromagnetic interference. The outer covering helps to physically protect the conductors.
The center conductor is the primary means of carrying a video signal. The center conductor comes in varying diameters, usually ranging from 14 gauge to 22 gauge. The structure of the center conductor generally is solid copper or copper-clad steel, designated as bare copper weld, or BCW.
For CCTV applications, solid copper conductors are required! Copper clad, copper weld, or BCW cables have much greater loop resistance at baseband video frequencies and should never be used for CCTV. To determine the type, look at the cut end of the center conductor, copper clad cable will be silver in the center instead of copper all the way through. Variation in the size of the center conductor has an overall effect on the amount of DC resistance offered by cable. Cables that contain large diameter center conductors have lower resistances than cables with smaller diameters. This decreased resistance of large diameter cable enhances the ability of a cable to carry a video signal over a longer distance with better clarity, but it’s also more expensive and harder to work with.
For applications in where a cable may be required to move up/down or side-to-side, the selected cable should be one in which the center conductor consists of many small strands of wire. As the cable moves, these strands flex and resist wear due to fatigue better than a cable with a solid center conductor.
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.
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.