Coaxial cable refers to a cable with two concentric conductors, an annular conductor and its insulator outside the inner insulating material, and the conductor and shielding layer share the same axis. Common coaxial cables are composed of copper wire conductors isolated by insulating materials. The entire cable is wrapped by a polyvinyl chloride or Teflon material sheath. So, let’s learn about the principles and classification of coaxial lines!

Coaxial lines can be used for the transmission of analog signals and digital signals. As a means of transmitting television signals to thousands of households, coaxial lines have rapidly developed as cable TV. A cable TV system can carry dozens to hundreds of TV channels, suitable for various applications, and its transmission range can reach tens of kilometers. Coaxial cable has long been an important component of long-distance telephone networks. Today, important among them are television broadcasting, long-distance telephone transmission, short-distance connections between computer systems, local area networks, etc. We face fierce competition from fiber optics, terrestrial microwave and satellite.

The development of coaxial lines is mainly divided into four generations: starting from the mid-19th century, polyethylene materials were used as solid core insulation media; the second generation used chemical foaming PE materials as the insulation medium; the third generation used lotus core vertical hole PE materials as the insulation medium. Insulating medium; the fourth generation uses physical foaming PE material as the insulating medium. Coaxial cables are divided into leaky coaxial cables, multi-core coaxial cables, thinned coaxial cables, and composite coaxial cables according to their structures.

Working principle of coaxial line:

The coaxial line is divided into four layers from the inside to the outside, which is a central copper wire (a solid wire or multiple stranded wires), a plastic insulator, a mesh conductive layer, and a wire sheath. The central copper wire and the mesh conductive layer form a current loop. It is named because the central copper wire and the mesh conductive layer are in a coaxial relationship. Coaxial wires transmit alternating current instead of direct current. That is to say, the direction of the current is reversed many times in one second. If an ordinary wire is used to transmit high-frequency current, this wire is equivalent to an antenna that emits wireless energy outwards. This effect It will lose the power of the signal and reduce the strength of the received signal.

The coaxial line is designed to solve this problem. In coaxial lines, if part of the cable is greatly compressed and twisted, the distance between the center wire and the mesh conductive layer is not always constant, and there is also the problem of internal electric waves being reflected by the signal source. The radio radiated from the center wire is isolated by a mesh conductive layer that controls the radiated radio through grounding, an effect that reduces the receivable signal power. To overcome this problem, a plastic insulator is added between the central wire and the mesh conductive layer, ensuring that the distance between them is always maintained. As a result, the cable is stiff and less prone to bending.

The coaxial line shielding material is essentially an improvement on the outer conductor. The outer conductor has very good shielding performance, but it is not easy to bend and is inconvenient to use. It can be seen that the shielding efficiency of single-layer braiding is poor. The transfer resistance of double-layer braiding is reduced by 3 times compared with single-layer braiding. From the tubular outer conductor to single-layer braiding and double-layer metal, the shielding effect of double-layer braiding is compared with single-layer braiding. has been greatly improved. Large coaxial cable manufacturers continue to improve the outer conductor structure of their cables in order to maintain performance.

Main categories:

There are two basic types of coaxial cables: baseband coaxial cable and broadband coaxial cable.

Baseband coaxial line

The shielding layer of the baseband coaxial cable is usually a copper mesh structure with a characteristic impedance of 50. This cable is used to transmit digital signals. Generally, RG-8 (thick cable) and RG-58 (thin cable) are commonly used. The intuitive difference between thick cables and thin cables is the diameter of the cables. However, thick cable networks require the installation of transceivers and transceiver cables, which is also very difficult to install, so the overall cost is high. On the contrary, thin cables are relatively simple and low-cost; however, since the cables will be cut during installation, poor contact may occur if there are many connectors. Whether it is a network connected with thick cables or a network connected with thin cables, the point of failure is often Will affect all equipment along the entire cable. Thick cables are suitable for larger local networks, with long standard distances and high reliability; diagnosis and repair of faults are troublesome. As a result, baseband coaxial cable has been replaced by unshielded twisted pair and fiber optic cable.

Broadband coaxial line

The shielding layer of broadband coaxial cable is usually made of aluminum stamping. It is a standard transmission cable used in cable TV networks and can transmit multiple TV signals simultaneously in one cable. Its characteristic impedance is 75Ω. This kind of cable is usually used to transmit analog signals, and the commonly used model is RG-59. Broadband coaxial cable can also be used as the transmission medium for some computer networks.