Coaxial cable refers to 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. The entire cable is then covered by a sheath of polyvinyl chloride or Teflon. .
Coaxial cables can be divided into two basic types, baseband coaxial cable and broadband coaxial cable. At present, the baseband is a commonly used cable, and the shielded wire is meshed with copper, and the characteristic impedance is 50 (such as RG-8, RG-58, etc.); the shield of the cable commonly used for broadband coaxial cable is usually aluminum. Stamped, the characteristic impedance is 75 (such as RG-59, etc.). 
Coaxial cables can be divided into thick coaxial cables and thin coaxial cables according to their diameter. The thick cable is suitable for relatively large local networks. It has a long standard distance and high reliability. Since it is not necessary to cut the cable during installation, the computer can be flexibly adjusted according to the needs of the network. However, the thick cable network must be equipped with a transceiver cable. It is difficult, so the overall cost is high. On the contrary, the installation of the thin cable is relatively simple and the cost is low. However, since the cable is cut during the installation process, the basic network connector (BNC) must be installed at both ends, and then connected to the T-type connector, so that when the connector is long, it is easy to cause a bad Hidden danger, this is one of the most common failures that occur in Ethernet currently in operation.
Whether the thick cable or the thin cable is a bus topology, that is, a cable is connected to multiple machines. This topology is suitable for machine-intensive environments, but when a contact fails, the fault will affect the entire cable in series. All the machines on it. The diagnosis and repair of the fault is cumbersome and will therefore be gradually replaced by unshielded twisted pair or fiber optic cable.
Variety introduction editor
Coaxial cables are divided into thin cables: RG-58 and thick cable RG-11. And use very few semi-rigid coaxial cables and feed tubes.
The diameter of the thin cable is 0.26 cm and the maximum transmission distance is 185 meters. When used, it is connected with 50Ω terminating resistor, T-connector, BNC connector and network card. The wire price and connector cost are relatively cheap, and there is no need to purchase equipment such as hubs. It is very suitable for setting up a small Ethernet network with more centralized terminal equipment. Do not exceed 185 meters in total cable length, otherwise the signal will be severely attenuated. The impedance of the thin cable is 50Ω.
The thick cable (RG-11) has a diameter of 1.27 cm and a maximum transmission distance of 500 m. Due to its relatively large diameter, it is less flexible and is not suitable for erection in indoor narrow environments. The RG-11 connector is also relatively complicated to manufacture and cannot be directly connected to a computer. It requires a transfer. The device is turned into an AUI connector and then connected to the computer. Because the strength of the thick cable is strong and the maximum transmission distance is longer than the thin cable, the main purpose of the thick cable is to play the role of the network backbone, which is used to connect several networks formed by thin cables. The impedance of the thick cable is 75Ω.
Semi-rigid coaxial cable
This type of cable is rarely used and is usually used for module connections inside communication transmitters because of the small transmission loss of the line, but it also has some disadvantages such as high hardness and difficulty in bending. In addition, the transmission frequency of such cables can reach 30Ghz for the most part. Model CXJ--50--3 The typical structure of this type of cable is shown in the following table. Structure Material Diameter (mm) 1. Inner conductor Silver plated copper wire 0.93 2. Insulator polytetrafluoroethylene (PTFE) 3.00 3. Outer conductor
1 seamlessly annealed copper tube 3.58
2 tinned (alloy) seamless copper tube
3 silver plated seamless copper tube
At present, the process is gradually improving, and some such wires with a large bending range have appeared. However, the author recommends that such a wire of a conventional copper tube outer conductor be used as much as possible to ensure stability in a place where flexibility is not required.
English abbreviation SYV, there are often 75-7, 75-5, 75-3, 75-1 and other models, the characteristic impedance is 75 ohms, to adapt to different transmission distances. It is the main medium for transmitting video signals in an asymmetric baseband manner.
Main application range
The main application areas are: device bracket connection, closed circuit television (CCTV), shared antenna system (MATV) and color or monochrome RF monitor transfer. These applications do not require the selection of precision video coaxial cables with particularly tight electrical tolerances. The characteristic resistance of the video coaxial cable is 75 ohms, which is not optional. Physics has proven that the attenuation characteristics of video signal optimization occur at 77 ohms. In low power applications, the material and design determine the optimum impedance of the cable to 75 ohms.
Standard video coaxial cable has both solid and multi-conductor designs. It is recommended to use multi-strand conductor designs in applications where the cable is to be bent, such as the internal connection of a CCTV camera to a tray and stand unit, or the transfer cable of a remote camera. Also includes monitoring equipment