Coaxial cable is a cable that has two concentric conductors, and the conductor and shield share the same axis. The most common coaxial cable consists of a copper conductor isolated from an insulating material. On the outside of the inner insulation is another layer of ring conductor and its insulator, and then the entire cable is covered by a sheath of polyvinyl chloride or Teflon material. It is characterized by good anti-interference ability, stable transmission data and low price, and is also widely used, such as closed-circuit television lines. The coaxial thin cable line generally sells for a few yuan per meter, which is not too expensive. Coaxial cable is used to connect with the BNC head. The coaxial cable sold on the market is generally a finished product that has been connected with the BNC head. You can use it directly.
However, according to the analysis of the characteristics of the coaxial cable itself, the attenuation of the signal as it travels within the coaxial cable is related to the transmission distance and the frequency of the signal itself. In general, the higher the signal frequency, the greater the attenuation. The bandwidth of the video signal is very large, reaching 6MHz, and the color portion of the image is modulated at the high end of the frequency, so that when the video signal is transmitted in the coaxial cable, not only the overall amplitude of the signal is attenuated, but also the attenuation of each frequency component is greatly different. In particular, the color portion is most attenuated. Therefore, the coaxial cable is only suitable for transmitting image signals at a short distance. When the transmission distance reaches about 200 meters, the image quality will be significantly degraded, especially the color becomes dim and has a sense of distortion.
In engineering practice, in order to extend the transmission distance, a coaxial amplifier is used. The coaxial amplifier has a certain amplification of the video signal, and can also compensate different frequency components by different adjustments by equalization adjustment, so that the distortion of the video signal outputted by the receiving end is as small as possible. However, coaxial amplifiers cannot be cascaded without limitation. Generally, in a point-to-point system, coaxial amplifiers can only cascade up to 2 to 3, otherwise the quality of video transmission cannot be guaranteed and adjustment is difficult. Therefore, when using a coaxial cable in a monitoring system, in order to ensure a good image quality, the transmission distance is generally limited to about four or five hundred meters.
In addition, coaxial cable has some disadvantages in transmitting image signals in the monitoring system:
1) The coaxial cable itself is greatly affected by climate change, and the image quality is affected to some extent;
2) The coaxial cable is thicker, and the wiring is not convenient in dense monitoring applications;
3) Coaxial cable can only transmit video signals. If the system needs to transmit control data, audio and other signals at the same time, additional wiring is required.
4) Coaxial cable has limited anti-interference ability and cannot be applied to strong interference environment;
5) Coaxial amplifiers also have the disadvantage of difficulty in adjustment.