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Choosing the correct coaxial cable

By using cheap coaxial cable, one could be adding costs to one's installation and also settling for poor image quality.

For example, if a video signal is sent down 750 m coaxial cable RG59 (rated at 305 m max), the video signal received at the monitor will be smaller than the original signal and will have noise within it causing a distorted picture, if any. If a video signal is sent down 350 m through a RG-66 (computer rated cable of wrong impedance), the resulting signal will include echo signals (ghost pictures).

Cable selection

In order to appreciate the coaxial cable one must have a little understanding of the video signal.

* The video signal is measured at one volt peak to peak (1 Vpp). If there is a significant amount of video signal (0,1 V or more), pictures will appear flat with little or no contrast. If one loses enough of the amplitude of the signal, there could be a loss of horizontal synchronisation causing the picture to tear or be unstable.

* The video signal has a frequency range of 400 Hz to 7,8 MHz. This means that 400 to 7,8 million things happen in each video picture that is flashed on the screen. If an outside radio frequency (RF) or electromagnetic induction (EMI) is allowed to penetrate into a video signal, it will result in noisy pictures. Noise can appear in the form of snow, hum bars - dark or white, or cross lines.

* The video signal is attenuated at 75 Ω impedance. If this impedance is increased or decreased, any and/or all of the above symptoms could appear in video images.

* Distance determines the cable size. The most common coaxial cables used with CCTV systems are:

* URM70 (RG59) - a short run cable with average acceptable runs of up to 305 m.

* CT100 (RG-6) - a medium run cable with average acceptable runs of up to 450 m.

* CT125 (RG-11) - a long distance cable with average acceptable runs of up to 900 m.

All coaxial cable used in industrial CCTV must be rated at 75 Ω of impedance for the following reasons:

1. All CCTV cameras, regardless of manufacturer, are designed to emit a video signal that is balanced to 75 Ω impedance.

2. All monitors, switchers, time lapse recorders, amplifiers, multiplexers etc are designed to receive a balanced composite video signal at 75 Ω impedance.

It is important to remember that a coaxial cable must:

1. Have a proper jacket for the job assigned.

2. Have a proper shield 100% copper material, braided, 90% to 95% efficiency rating.

3. Have a dielectric rated at 75 Ω impedance.

4. Have a centre core 100% copper material solid or stranded.

For further details contact Abie Ali, Frank Street on +27 (0) 11 838 4515.

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