Coaxial cable small knowledge
First, the project commonly used coaxial cable type and performance:
1.Coaxial Cable SYV75-3,5,7,9 ..., 75 ohm, polyethylene insulated solid coaxial cable. In recent years it was called "video cable";
2.Coaxial Cable SYWV75-3,5,7,9 ... 75 ohm, physical foam polyethylene insulated coaxial cable. Some people call it "RF cable".
Second, Coaxial Cable the basic performance:
1.Coaxial Cable SYV physical structure is 100% polyethylene insulation; SYWV is the foam ratio of 70-80% of the physical foam polyethylene insulated cable;
2.Coaxial Cable Due to dielectric loss, SYV solid cable attenuation is significantly greater than SYWV physical foam cable; in the commonly used engineering cable, the current physical foam cable is still the best transmission performance of the lowest cable, in the video, radio frequency, microwave Band is like this;
3. Coaxial cable can be in the DC, RF, microwave band applications. According to the "RF" / "video" to distinguish the cable, not only based on inadequate, but also prone to misleading: it seems that the video transmission must or can only choose a solid cable; from the engineering point of view, or "solid" and "foam" To distinguish the type more practical;
4. Coaxial Cable High (128) and low (64) cable characteristics of the difference: in the following frequency band 200KHz, high cable shield "low resistance" play a major role, so low-frequency transmission attenuation is less than low cable. However, in the 200-300KHz above the video, radio frequency, microwave band, due to "high-frequency skin effect" played a major role, high-cable has lost the "low resistance" advantage, so the high-frequency attenuation of the two cables are basically the same.
Third, the coaxial cable video transmission characteristics
1. Coaxial Cable The finer the cable, the greater the attenuation: such as 75-7 cable 1000 meters attenuation, and 75-5 cable more than 600 meters attenuation roughly the same, or 1000 meters 75-7 cable transmission with 75-5 cable cable more than 600 meters Transmission effect roughly equal;
2. Coaxial Cable The longer the cable, the greater the attenuation: 75 meters, such as 75-5 cable, 6M frequency attenuation "decibel", for 1000 meters attenuation "decibel" 75%, that is 15db; 2000 m (1000 + 1000) attenuation For 20 + 20 = 40db, the other frequency points of the same calculation. According to the above 1000 m cable test data, calculate the attenuation of different lengths of cable, please remember that "decibel number is alkali-related relationship" or "decay decibel number can be calculated according to the percentage change in length", can be used flexibly;
3. Coaxial Cable Frequency distortion characteristics: low frequency attenuation, high frequency attenuation. High / low side of the difference between the attenuation, can be called "side frequency difference", which is a very important parameter. The longer the cable, the greater the "side frequency difference"; it is important to understand and master the "frequency distortion characteristics" of the coaxial cable, which is the most important feature of the image quality. The most overlooked problem in the project.