Coaxial cable is a cable with a central copper conductor wrapped by layers of insulated wires. It is characterized by good anti-interference ability, stable data transmission, and cheap price, and is also widely used, such as closed-circuit television 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 interval 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, up to 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 greatly attenuated. Therefore, coaxial cables are only suitable for transmitting image signals at close distances. When the transmission distance reaches about 200 meters, the image quality will be significantly reduced, especially the colors will become dull and distorted.

In engineering practice, in order to extend the transmission interval, a coaxial amplifier is used. The coaxial amplifier can amplify the video signal to a certain extent, and can also compensate different frequency components with 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, only 2 to 3 coaxial amplifiers can be connected in a point-to-point system. Otherwise, the video transmission quality cannot be guaranteed, and it is difficult to adjust. Therefore, when using coaxial cables in the monitoring system, in order to ensure better image quality, the transmission interval is generally limited to about four or five hundred meters.

In addition, there are some shortcomings 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.