Coaxial RF cable
The coaxial cable (Coaxial Cable) is a cable with two concentric conductors, and the conductor and shielding layer share the same axis. The common coaxial cable consists of insulated copper conductor. Inside the insulating layer is another layer of ring conductor and its insulator. Then the whole cable is covered by PVC or Teflon sheathed.
Coaxial cable can be used for analog signal and digital signal transmission. It is suitable for all kinds of applications, including TV transmission, long-distance telephone transmission, short distance connection between computer systems and LAN. Coaxial cable is developing rapidly as a means of transmitting TV signals to thousands of households. This is cable television. A cable television system can load dozens or even hundreds of TV channels, and its transmission range can reach tens of kilometers. Coaxial cables have long been an important part of long distance telephone networks. Today, it faces increasingly fierce competition from optical fiber, ground microwave and satellites.
Coaxial cable can be divided into two basic types, baseband coaxial cable and broadband coaxial cable.
Baseband coaxial cable
The shielding layer of baseband coaxial cable is usually made of copper mesh structure, and its characteristic impedance is 50 ohms. The cable is used to transmit digital signals. The commonly used models usually have RG-8 (Rough cable) and RG-58 (thin cable). The difference between the thick cable and the fine cable is that the diameter of the cable is different. The rough cable is suitable for larger local network. Its standard distance is long and its reliability is high. However, the cable network must be installed with transceiver and transceiver cable, so it is difficult to install. Therefore, the overall cost is high. On the contrary, the thin cable is relatively simple, and the cost is low. However, because the cable is cut off during the installation process, it is easy to produce hidden troubles when there are more joints.
The fault points often affect all the machines on the entire cable, no matter whether the cables are connected by thick or thin cables. The diagnosis and repair of faults are very troublesome. Therefore, the baseband coaxial cable has been gradually replaced by unshielded twisted pair or fiber optic cable.
Broadband coaxial cable
The shielding layer of broadband coaxial cable is usually punched by aluminum, and its characteristic impedance is 75 ohms. This kind of cable is usually used to transmit analog signals. The commonly used model is RG-59, which is the standard transmission cable used in cable TV network, and can transmit multi-channel TV signals in one cable at the same time. Broadband coaxial cable can also be used as a transmission medium for certain computer networks.
Coaxial cable is divided into four layers from inside to outside: central copper wire (single stranded solid wire or stranded wire), plastic insulator, mesh conductive layer and wire sheath. The central copper wire and the mesh conductive layer form a current loop. Because the central copper wire and the net conductive layer are coaxial.
Coaxial cable conducts alternating current instead of direct current, that is to say, there will be several reverses of current direction every second.
If a general wire is used to transmit high frequency current, this wire will be equivalent to an antenna transmitting outward radio, which will lose the power of the signal and reduce the intensity of the received signal.
Coaxial cable is designed to solve this problem. The radio emitted from the center wire is isolated from the conductive layer, and the conductive layer can control the transmitted radio by way of grounding.
There is also a problem with coaxial cable, that is, if there is a larger extrusion or distortion in a certain section of the cable, the distance between the center wire and the mesh conductive layer is not consistent, which causes the internal radio waves to be reflected back to the signal source. This effect reduces the signal power that can be received. To overcome this problem, a plastic insulator is added between the center wire and the mesh conductive layer to ensure the distance between them is consistent. This also causes the rigidity of the cable to be rigid and not easy to bend.
The shielding material of coaxial cable is essentially the improvement of external conductors, from the initial tubular outer conductor to the single layer braiding and double layer metal. Although tubular external conductors have excellent shielding performance, they are not easy to bend and are inconvenient to use. The shielding efficiency of single layer braiding is poor, and the transfer impedance of double-layer braiding is 3 times lower than that of one layer braiding. All cable manufacturers are constantly improving the outer conductor structure of cables to maintain their performance.
Advantages and disadvantages
The advantage of the coaxial cable is that it can support high bandwidth communication on a relatively long repeater free circuit, and its disadvantages are obvious: first, the diameter of the large cable is 3/8 inch thick, which occupies a lot of space in the cable duct; two, it can not withstand entanglement, pressure and severe bending, which will damage the cable structure and prevent the transmission of the signal; then it is the cost. High, and all these shortcomings are overcome by twisted pair. Therefore, in the current LAN environment, it has basically been replaced by twisted pair Ethernet physical layer specification.
Coaxial cables are usually installed between equipment and equipment. Each user location is equipped with a connector to provide users with an interface. The installation of the interface is as follows:
(1) thin cable: cut the thin cable, install BNC on both ends, then connect to the two ends of the T connector.
(2) coarse cable: coarse cable is usually installed with a Tap device similar to splint. It uses the guide pin on Tap to penetrate the insulation layer of the cable and directly connects with the conductor. The ends of the cables are equipped with terminal devices to weaken the reflection effect of signals. A network with a transmission rate of 10Mbit/s