Coaxial cable refers to a cable with two concentric conductors, and the conductor and the shielding layer share the same axis. The most common coaxial cable consists of a copper wire conductor separated by an insulating material, on the outside of the inner layer of insulating material is another layer of ring conductor and its insulator, and then the entire cable is wrapped by a sheath of PVC or Teflon material . It is characterized by good anti-interference ability, stable data transmission and low price, and is also widely used, such as CCTV lines. The coaxial thin cable is generally priced at a few yuan per meter in the market, which is not too expensive. The coaxial cable is used to connect with the BNC head. The coaxial cables sold on the market are generally finished products that have been connected to the BNC head, and you can use them directly.
However, according to the analysis of the characteristics of the coaxial cable itself, the attenuation of the signal when it is transmitted in 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 part of the image is modulated at the high frequency. In this way, 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 very different. , especially the color part is attenuated the most. 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 reduced, especially the color will become dim and there will be a sense of distortion.
In engineering practice, in order to extend the transmission distance, a coaxial amplifier is used. The coaxial amplifier can amplify the video signal to a certain extent, and can also compensate different frequency components in different sizes through equalization adjustment, so as to minimize the distortion of the video signal output by the receiving end. However, coaxial amplifiers cannot be cascaded indefinitely. Generally, in a point-to-point system, only 2 to 3 coaxial amplifiers can be cascaded at most. Otherwise, the video transmission quality cannot be guaranteed, and it is difficult to adjust. Therefore, when using a coaxial cable in a monitoring system, in order to ensure better image quality, the transmission distance is generally limited to about four or five hundred meters.
In addition, there are some disadvantages in the transmission of image signals by coaxial cable in the monitoring system:
1) The coaxial cable itself is greatly affected by climate change, and the image quality is affected to a certain extent;
2) The coaxial cable is thick, and the wiring is not convenient in intensive monitoring applications;
3) Coaxial cables generally 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 used in strong interference environment;
5) The coaxial amplifier also has the disadvantage of being difficult to adjust.
