CCTV video signals are commonly transmitted using coaxial cable. Coaxial cable is designed to transmit the complete video frequency range with minimum distortion or attenuation, making it an excellent choice for CCTV. In order to transmit this full spectrum of frequencies with little distortion or attenuation, it is important to select the correct cable that meets the specifications for CCTV transmission.
Coaxial cables are also available in different RG types. RG stands for Radio Guide and is a term used when sending Radio Frequency (RF) signals down a coaxial cable. The 75-ohm coaxial cable comes in several sizes with the most common types being RG59, RG6, and RG11. An RG59 cable is the most commonly used because it is smaller in diameter and easy to work with. The RG11 cable is the largest in diameter and harder to work with. The RG6 size is between the two.
The parameters to consider are mechanical characteristics such as the center conductor material, dielectric material, shield type and material, and jacket material. The electrical characteristics such as resistance, capacitance, impedance and attenuation are critical for proper transmission. The following explains each component:
Center Conductor material made of bare copper is recommended for optimum performance in CCTV signal transmission. Because a CCTV video signal is a baseband composite video with fairly low-frequency components compared to a CATV video signal, the low D.C. resistance that copper provides will greatly improve the video signal transmission. If the cable is going to be used on a CCTV camera that will be in a fixed position, then a solid conductor is acceptable. However, if the cable will be used in a pan and tilt application, then you should choose a stranded conductor because a solid conductor construction will break with continuous flexing.
Coaxial cable is also available with a copper-covered steel center conductor. A copper-covered steel center conductor has a much higher D.C. resistance than bare copper and greatly attenuates the lower frequency components of a CCTV video signal (refer to table). Although it may be less expensive than pure copper, it is not suitable for CCTV and is not recommended.
Dielectric Material of a coax cable is also another key area that should be addressed. The dielectric material and its composition are critical as it sets up the electrical characteristics such as capacitance, the velocity of propagation, impedance, and attenuation of the cable. These parameters will determine signal strength and transmission distance. It is recommended to choose a dielectric with excellent electrical properties such as polyethene or FPE. Such material will give you lower capacitance and a higher velocity of propagation.
A braided shield is the proper type of shield for CCTV and has two key purposes. One is to provide a low D.C. resistance ground path and the second is to provide shielding of outside interference from distorting the video signal. The shielding should be constructed of bare copper to provide a low D.C.R. return path. It should have a 95% or better braid coverage in order to provide adequate shielding from outside electrical interference’s. Anything less is usually not acceptable for CCTV. To provide increased shielding in the RFI range, a construction with an aluminium foil is acceptable as long as a high percentage copper braid is used to provide the low D.C.R. return path.
A cable with a combination aluminium foil shield and low coverage aluminium braid commonly used for CATV is not acceptable! Because aluminium shields have a much higher D.C. resistance return path and the braided portion of these type of cables provide only a low percentage of coverage, they do not provide the type of shielding required for CCTV video transmission.
Jacket choice is mainly determined by the environment where the cable will be installed. PVC is a good choice for most applications located inside a building. Polyethene is recommended for outdoor applications where the cable may be subjected to the elements and a high degree of moisture, sunlight, and abrasion resistance is required. Do not install indoor rated cable in aerial, direct burial or underground pipe. An indoor-rated cable is not designed for these harsh environments and the electrical and mechanical characteristics will degrade over a period of time and will need to be replaced. Always follow the applicable code for the proper cable type for your installation.