FAQ
		What is the typical dB loss of a cable?

Cable performance is usually defined as having a certain loss of signal. This performance is measured in units called decibels or (dB). The Decibel is a means of measuring the ratio of two signals, in CCTV terms its what we put into the cable at the camera end and what we got out of it at the monitor end.

Cable specifications quote signal loss as being a certain dB loss at 5MHz (High frequency end of our video signal) per 100 meters. Ideally it would be preferred if the video signal had no loss, however in practice an acceptable loss would be around 6dB.

If you equate this to RG59, which has a quoted loss of 2.25dB per 100 meters at 5MHz, we can calculate the maximum distance we can run before exceeding 6dB.

(6/2.25) X 100 = 267 meters

FAQ
		How do I decide which type of cable to use when designing a CCTV system?

The following factors should always be considered when coosing your cable:

1) Length of cable runs - measured in meters(m)
2) Route of cable runs, indoors or outdoors, protected (run in ducting) or unprotected (clipped to buildings or buried).
3) Interference from other cables and equipment. (Mains cables etc.)

Whatever cable you use will act as a resistance to the signal you wish to transmit down it due to the nature of its construction. The resistance encountered increases as the frequency of the signal increases. Although a Video signal transmits over a wide frequency spectrum, it is the high frequency end 5Mhz (highest loss) which is used to calculate which cable you use and how far you can run it.

There are two types of cable; Coax (unbalanced) cable and TP - Twisted Pair (balanced) cable. Coax cable is used for shorter distances (e.g. RG59 - up to 250m, CT125 - up to 500m) and TP on longer runs (up to 2.4km).

Coax cable is the standard in the CCTV industry (although TP is becoming more popular) with a 75 ohms rating, it is available in varying degrees of quality and cost and in a variety of lengths. The higher the quality (measured in loss of Decibels (dB) per 100 meters) the more expensive the cable.

Two additional considerations when buying coaxial cable:

1) The quality of the screen. The screen protects the signal from electrical interference, so before deciding which quality of screen is required you need to assess each cable run e.g. is it travelling in a duct with other load bearing cables?

2) The quality of the cable jacket. The jacket protects the cable from the environment. For example RBS jacketed cable protects against moisture. If you're trenching and burying cable than a steel armoured as well as a waterproof jacket should be considered.

TP (balanced) cable has a different impedance to coaxial cable; 100 and 150 Ohms are the most commonly used. As opposed to having one center core, the TP is made up of a pair of cables twisted at approximately one to two twists per meter. TP is widely used over longer cable runs, as the high frequency losses are smaller than coaxial cable. The twisting construction of TP cable also provides better immunity to electrical inteference.

CCTV equipment is designed to cater for 75-OHM coax cable termination, additional equipment is therefore required to launch and recover video signals over TP cables. The equipment can be Powered (Active) or Non-powered (Passive) this will determine the distance the signal can go.