The most common form of cable used in CCTV applications is known as coaxial cable, often shortened to “coax”. This is the most effective and cheapest method of signal transmission and is still used in the majority of situations. Coaxial cable is available with different characteristic impedances. For radio work, cable with 50 ohms impedance is usually used, while for CCTV, 75 ohms cable is the norm.
Coaxial cable consists of two conductors, a centre core that is insulated by a non-conductive or dielectric material, and an outer screening that surrounds the inner core. A final covering of PVC or similar material protects the conductors from damage.
Video coaxial cable is designed to transmit maximum signal energy from a 75-ohm source to a 75-ohm load with minimum signal loss. Excessive signal loss and reflection occurs if cable rated for other than 75 ohms is used. Cable characteristics are determined by a number of factors (core material, dielectric material and shield construction, among others) and must be carefully matched to the specific application. Moreover, the physical environment through which the cable is run and the method of installation will influence the transmission characteristics of the cable.
RG-6, RG-59 and RG-11 are all 75-ohm cable types, with RG-59 being the smallest, RG-6 in between, and RG-11 being the largest. RG-11 is very large, not very flexible, and its main uses are in very long runs where low signal loss is of paramount importance. RG-6 and RG-59 are both available in many different types, with different shields, jackets, dielectrics, and centre conductor materials. The origin of the RG designations is in old, long-obsolete military specifications; RG stands for “Radio Guide,” and the RG numbers are completely arbitrary.
There are two factors that govern the selection of cable: the location of cable runs, either indoor or outdoor, and the maximum length of the individual cable runs.
Use only high quality cable and be careful to match the cable to the environment (indoor or outdoor). Solid core, bare-copper conductor is best suited to video applications, except where flexing occurs. In locations where the cable must be continuously flexed (i.e., when used with pan & tilt mechanisms), use cable intended for such movement. This cable will have a stranded wire core. Use only cable with pure copper stranding. Do not use cable with copper-plated steel stranding because it does not transmit effectively in the frequency range used in CCTV.
The preferred dielectric material is foam polyethylene. Foam polyethylene has better electrical characteristics and offers the best performance over solid polyethylene, but it is more vulnerable to moisture. Use cable with solid polyethylene dielectric in applications subject to moisture.
In the average CCTV installation, with cable lengths of less than 220m, RG59/U cable is a good choice. Having an outside dimension of approximately 6mm, it is usually available in 100m rolls.
For short cable runs, use RG59/U with a 22-gauge (AWG) centre conductor, which has a DC resistance of about 16 ohms per 300m). For longer runs, the 20-gauge (AWG) variety, which has a DC resistance of approximately 10 ohms per 300m, will work well. In either case, cables with polyurethane or polyethylene as the dielectric material are readily available.