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A coaxial cable consists of a center conductor with an outer shield. In most cases, the shield consists of a metallic web of conductors with or without an additional metal-foil wrapping surrounding the center conductor. The entire assembly is then wrapped with a plastic covering, called a sheath.
In most cases, the shield is connected to an equipment ground, which is accomplished by connecting one end of the shield to the chassis of the equipment that it's connected to. The chassis, in turn, is connected to earth ground by the neutral connection of the power cord and receptical.
Ground Loops
In most cases, the shield on a coaxial cable is connected to only one ground to eliminate ground loops. Ground loops occur when there is more than one ground along the path of a video signal. It occurs because there's a sizable potential difference (voltage) between the two grounding points.
This potential difference causes electrons to flow between the two ground points--a current that is not associated with the video signal. As the ground current combines with the current on the common side of the video signal, it causes the flow of electrons (current) to vary according to the electrical interference present in the ground current.
Because the most common electrical current flowing through ground is 60 cycle power from the power line bus, a 60-Hz. component is added (modulated) onto the video signal. This, in turn, effectively corrupts the otherwise near-perfect video signal, causing any one of several effects. By eliminating one of the grounds, the ground loop current is essentially stopped.
Coaxial Cable Quality
The single most cause of problems in CCTV systems is low-quality coaxial cables. That's why it's important that installers use a quality coaxial cable. Although coaxial cable is probably the least expensive part of a CCTV system, it is by no means the least important. Good coaxial cable, for example, has a low DC resistance because its center conductor is large enough to effectively carry the signal.
A good quality coaxial cable also has a shield that's rated higher in its shielding capability than less expensive coaxial cables. This rating is commonly expressed as a percentage, reflecting the degree of shielding that's provided by a particular cable. It's not uncommon, for example, to see coaxial cables with shielding efficiencies of 80% to 95%.
A good coaxial shield is crucial to the proper, uninterupted and uncorrupted operation of a CCTV system. The shield around the center conductor of a good oaxial cable, for example, will stop stray electromagnetic radiation (EMR) that occurs in the environment from entering the cable and interfering with the video signal. Sources of stray EMR, for example, are common in homes and commercial buildings, as well as along the street. Examples of EMR sources are electric power lines, motors, power transformers, and radio broadcast transmitters (i.e. television and commercial radio transmitters as well as citizen band and amateur ham radio transmitters in the neighborhood).
The type of center conductor inside the coaxial cable an installer uses is also important. Experts say that nothing less than a 100% copper center conductor will eventually result in the replacement of the cable, especially in outdoor applications where the cable is constantly exposed to EMR sources of all types. The issue of whether to use a solid or stranded center conductor must also be considered. Cable with a solid center conductor is less expensive than stranded, but this type of cable should never be used on a camera that's mounted to a pan & tilt mechanism. Instead, stranded cable should be used in pan and tilt applicatons, as well as any other situation where the camera must move from time to time. Using a cable with a solid core usually results in a broken center conductor at some point in the future.
The type of cable sheath used is also important to the longevity of a video installation. Installers should not use indoor-type coaxial cable in outside applications, for example. When they do, ultraviolet radiation from sunlight can cause cracks to develop in the cable sheath. This enables moisture and contaminants to penetrate the sheath, changing the impedance of the cable. The net effect will be disruption of the video signal that flows along the center conductor.
Choosing The Right Coaxial Cable
Installers must select the right coaxial cable for each application that they encounter.
The first consideration is cable impedance. Coaxial cable is available in several impedances. This is because the coaxial cable must match the output impedance of a video device to the input of another. An impedance match makes it possible for the optimum exchange of energy between the two devices.
It's the dielectric insulator between the center conductor and braided shield (ground) of a coaxial cable that ultimately determines impedance. There are a number of dielectric materials available for this purpose. The type of dielectric material and its thickness determine the impedance of a particular coaxial cable.
Like the CCTMA industry, the video industry uses an impedance of 75 ohms. Thus, installers must be certain to use a 75-ohm coaxial cable when installing CCTV systems. Otherwise, the quality of the video signals carried by the coaxial cable that they do use will likely be less than desirable. This is because the attentuation of the video signal will vary with the instantaneous frequency of the signal. In other words, the quality of the output video signal will vary from one moment to the next because the coaxial cable will transport some portions of the signal with more ease than others. This can cause ghosting, snow, and a generally poor video picture.
In addition, the type of coaxial cable selected will also determine the distance that video signals will travel. Coaxials, for example, are categorized according to size and their distance- carrying capabilities. For example, the most common coaxial cable used today is RG59/U. This coaxial cable will carry signals for distances of up to 1,000 feet. The next most popular coaxial is RG6/U, which is capable of carrying video signals up to 1,500 feet. RG11/U cable will likewise carry video signals for up to 3,000 feet without any appreciable degradation of signal. |
