In the video transmission of video surveillance systems, unbalanced coaxial cable (ie single coaxial cable, while the dual-core is balanced) is the most widely used video cable, which represents the model is RG6 cable and RG11 cable. By the inner conductor coaxial cable copper wire (single strand solid wire or stranded wire), an insulating layer, an outer conductor and a protective outer layer of plastic shield mesh weave composed. Since the outer conductor shielding layer, the coaxial cable has a good anti-interference, it is widely used in data transmission and image transmission in a higher rate.
According to the standard, only using 75 ohm video transmission type. Although the appearance of other impedance coaxial cable with 75 ohm almost, but over a distance of ten meters, which transmit the image quality becomes particularly poor.
There are many different grades of coaxial cable. The main difference between different brands of cable is that the number and type of shielding, and the core wire and shield to isolate the insulating layer (dielectric). The most commonly used is copper shield layer single series, double braided layers or aluminum foil (video surveillance system can not use aluminum foil cable). Aspects of the insulating layer, materials commonly used in rubber, plastics and other solid; there is a spiral-type hollow insulating layer, and thus became part of the insulating layer of air. In this cable, the current flows from the camera when the monitor through the core line, and return the camera from the monitor through the shield, which would shield between the core wire and a voltage difference. Therefore, we call this cable unbalanced cable because the current (and voltage) having the circuit unbalanced (unbalance) role.
For best transmission effect, the ratio should be selected covers over more than 95% of the cable. Places copper wire copper wire package or better. Bag made of steel wire copper wire, wrapped with a layer of copper; tensile strength of this line is better adapted to long-distance through the laying tube type. Also covering the market share of 65% of the cable should not be used. Video transmission system requires coverage ratio greater than 95% of the cable, in order to prevent external electromagnetic interference through the barrier layer to penetrate into the video signal. Coverage ratio greater than 95% of most of the cables can interfere with the masked, thereby reducing the chance of interference picture. Coverage ratio greater than 95%, the use of copper as the loop resistance of the cable core wire approximately 50-200 ohms per kilometer (resistivity different models cables different from each other). Core conductor cross-sectional area is larger, the smaller the transmission loss, which can be transmitted over longer distances. Therefore, the outdoor transmission over longer distances, should be used thick wire cables, most outdoor coaxial cable diameter generally in more than 1.3cm.