Historical development
The development of coaxial cables is mainly divided into four generations: the first generation is the use of polyethylene as a solid insulation medium in the mid-19th century; the second generation is the use of chemical Foamed PE material is used as the insulating medium; the third generation uses the lotus core longitudinal hole PE material as the insulating medium; the fourth generation uses the physical foamed PE material as the insulating medium. According to the structure, coaxial cables can be divided into: leaky coaxial cables, multi-core coaxial cables, narrow-diameter coaxial cables, and composite coaxial cables.
The development of the coaxial cable industry has gone through a series of changes. As the global electronics industry entered its peak in 2000, as part of the electronics industry, the size of the coaxial cable market has also reached its historical peak. In the following three years, as the global economic growth rate entered a trough, the coaxial cable industry also entered a downturn with the shrinking downstream demand. It was not until the second half of 2003 that there were signs of recovery. Since 2004, the global coaxial cable industry has entered a new round of growth. With the continuous expansion of mobile communication signal coverage, the expansion of the number of base stations, and the continuous improvement of mobile signal requirements in the transportation, energy, and medical fields, the market development prospects of the global RF coaxial cable industry are still promising.
Development Overview
The sustained and rapid growth of the Chinese economy has provided huge market space for cable products. The strong temptation of the Chinese market has made the world focus on the Chinese market In just a few decades of reform and opening up, the huge production capacity formed by China's wire and cable manufacturing industry has made the world look at it with admiration. With the continuous expansion of China's power industry, data communication industry, urban rail transit industry, automobile industry, shipbuilding and other industries, the demand for wires and cables will also grow rapidly, and the wire and cable industry still has huge development potential in the future.
Main classification
Coaxial cables can be divided into two basic types, baseband coaxial cables and broadband coaxial cables.
Baseband coaxial cable
The shielding layer of baseband coaxial cable is usually a mesh structure made of copper, and its characteristic impedance is 50Ω. This cable is used to transmit digital signals, and the commonly used models are generally RG-8 (thick cable) and RG-58 (thin cable). The most intuitive difference between a thick cable and a thin cable is the cable diameter. Thick cable is suitable for relatively large local networks. It has a long standard distance and high reliability; however, the thick cable network must be equipped with transceivers and transceiver cables, which is also difficult to install, so the overall cost is high. On the contrary, thin cables are relatively simple and cost less; however, since the cables have to be cut during installation, it is easy to cause poor contact when there are many connectors.
Whether it is a network connected by thick or thin cables, the point of failure often affects all the machines on the entire cable, and it is troublesome to diagnose and repair the fault. Therefore, baseband coaxial cables have been gradually replaced by unshielded twisted pairs or optical cables.
Broadband coaxial cable
The shielding layer of broadband coaxial cable is usually stamped with aluminum, and its characteristic impedance is 75Ω. This kind of cable is usually used to transmit analog signals. The commonly used model is RG-59. It is a standard transmission cable used in cable television networks. It can transmit multiple channels of TV signals in one cable at the same time. Broadband coaxial cable can also be used as the transmission medium of some computer networks.
Working principle
The coaxial cable is divided into four layers from inside to outside: central copper wire (single-strand solid wire or multi-stranded wire), plastic insulator, mesh Conductive layer and wire sheath. The central copper wire and the mesh conductive layer form a current loop. It is named because of the coaxial relationship between the central copper wire and the meshed conductive layer.
The coaxial cable conducts alternating current instead of direct current, which means that the direction of the current is reversed several times every second.
If a general wire is used to transmit high-frequency current, this wire will be equivalent to an antenna that emits radio outwards. This effect loses the power of the signal and reduces the strength of the received signal.
The design of coaxial cable is precisely to solve this problem. The radio emitted by the central wire is isolated by the mesh conductive layer, and the mesh conductive layer can be grounded to control the emitted radio.
There is also a problem with coaxial cables. If a certain section of the cable is squeezed or twisted, the distance between the central wire and the mesh conductive layer will not be consistent. This causes the internal radio waves to be reflected back to the source of the signal. This effect reduces the receivable signal power. In order to overcome this problem, a layer of plastic insulator is added between the center wire and the mesh conductive layer to ensure that the distance between them is consistent. This also causes this kind of cable to be relatively rigid and not easy to bend.
The shielding material of the coaxial cable is essentially the improvement of the outer conductor. From the initial tubular outer conductor, it has been developed into a single-layer braided and double-layered metal. Although the tubular outer conductor has very good shielding performance, it is not easy to bend and inconvenient to use. The shielding efficiency of single-layer braiding is the worst, and the transfer impedance of double-layer braiding is 3 times lower than that of one-layer braiding. It can be seen that the shielding effect of double-layer braiding has been greatly improved than that of single-layer braiding. Major coaxial cable manufacturers are constantly improving the outer conductor structure of the cable to maintain its performance.
Advantages and Disadvantages
The advantage of coaxial cable is that it can support high-bandwidth communication on relatively long lines without repeaters, and its disadvantages are also obvious: First, it is large in size. , The diameter of a thin cable is 3/8 inches thick, which takes up a lot of space in the cable duct; second, it cannot withstand tangles, pressure and severe bending, which will damage the cable structure and prevent signal transmission; the final cost is high. All these shortcomings can be overcome by twisted-pair cables. Therefore, in the current LAN environment, they have basically been replaced by Ethernet physical layer specifications based on twisted-pair cables.
Installation method
The coaxial cable is generally installed between the equipment. Each user position is equipped with a connector to provide an interface for the user. The installation method of the interface is as follows:
(1) Thin cable: Cut the thin cable, install BNC headers on both ends, and then connect to both ends of the T-connector.
(2) Thick cables: Thick cables are generally installed with a tap device similar to a splint, which uses the guide pin on the Tap to penetrate the insulation layer of the cable and directly connect to the conductor. There are terminator at both ends of the cable to weaken the reflection of the signal. Used for networks with a transmission rate of 10Mbit/s.
Parameter index
Electrical parameters
(1) The characteristic impedance of the coaxial cable: the average characteristic impedance of the coaxial cable is 50±2Ω, the periodic change of the impedance along a single coaxial cable is a sine wave, the center average value is ±3Ω, and its length is less than 2 meters.
(2) Coaxial cable attenuation: generally refers to the attenuation value of a 500-meter-long cable section. When measuring with a 10MHz sine wave, its value does not exceed 8.5db (17db/km); when measuring with a 5MHz sine wave, its value does not exceed 6.0db (12db/km).
(3) Propagation speed of coaxial cable: The minimum propagation speed required is 0.77C (C is the speed of light).
(4) Coaxial cable DC loop resistance: the sum of the resistance of the center conductor of the cable and the resistance of the shielding layer does not exceed 10 milliohms/meter (measured at 20°C).
Physical parameters
The coaxial cable is composed of a central conductor, an insulating material layer, a shielding layer composed of a mesh fabric, and an outer insulating material layer.
The coaxial cable is flexible enough to support a bending radius of 254mm (10 inches). The center conductor is a solid copper wire with a diameter of 2.17mm±0.013mm. The insulating material must meet the electrical parameters of the coaxial cable. The shielding layer is composed of a metal strip or sheet that meets the transmission impedance and ECM specifications. The inner diameter of the shielding layer is 6.15mm and the outer diameter is 8.28mm. The external insulation material is generally selected from polyvinyl chloride (such as PVC) or similar materials.
Quality inspection
1. Check the straightness of the insulating medium
The cross-section of the standard coaxial cable is very round, and the cable outside The conductor and aluminum are attached to the outer surface of the insulating medium. The more round the outer surface of the medium, the smaller the gap between the aluminum foil and its outer surface, and the less round the gap, the larger the gap. Practice has proved that the smaller the gap, the better the performance of the cable. In addition, the large gap air is likely to invade the shielding layer and affect the service life of the cable.
2. Measuring the consistency of the coaxial cable insulation medium
The fluctuation of the diameter of the coaxial cable edge media mainly affects the echo coefficient of the cable. This check The insulation medium of a section of the cable can be cut out, and the outer diameter of each point can be carefully checked with a micrometer to see if it is consistent.
3. Measuring the braided net of coaxial cable
The centralized power supply cable TV line is still the loop line of the power supply. Therefore, the quality inspection of the coaxial cable must check whether the textile network is tight and flat. The number of nets is evaluated, and if it matches the given index value, it is qualified. In addition, a single textile net cable is measured with a spiral micrometer. At the same price, the thicker the wire, the better the quality.
4. Check the quality of aluminum foil
The important shielding role of coaxial cable is aluminum foil, which prevents external open-circuit signal interference and cable TV signal Confusion plays an important role, so the quality of aluminum foil should be checked for new coaxial cables. First, cut open the sheath layer to observe whether the surface of the braided wire and aluminum foil layer maintains a good luster; secondly, take a piece of cable, tightly wind it on a small metal shaft, straighten it and turn it in the opposite direction, repeat it several times, and then cut it. Observe whether the aluminum foil is cracked or not on the cable sheath. You can also cut out a small piece of aluminum foil by repeatedly rubbing and stretching in your hands. After repeated rubbing and stretching, it has not broken. The product with a certain degree of toughness is qualified, otherwise it is inferior. .
5. Check the tightness of the outer sheath
The outer sheath of high-quality coaxial cable is tightly wrapped, so that it can be reduced The gap in the shielding layer prevents oxidation caused by air ingress, and prevents the relative sliding of the shielding layer from causing electrical performance drift. However, squeezing the package too tightly will cause inconvenience to stripping and increase the difficulty of construction. The inspection method is to take a 1m long cable and remove the protective layer at the end. It is appropriate that the core cannot be pulled out with force.
Application
In cable television transmission, coaxial cable has been widely used in medium and small transmission systems due to its low cost and easy construction. Especially in the HFC (Hybrid Fiber-Coaxial, hybrid fiber coaxial) network "last 1 km" transmission, it cannot be replaced by other cables. Many passive devices, active devices and users need cable connections, and all devices connected by coaxial cables need to achieve impedance matching. If it is not matched, the signal will be reflected between the components and the cable, which will increase the influence of noise and ghosting on the transmitted image. The quality of cables of the same specification produced by different manufacturers on the market now varies greatly. However, as long as you choose cables produced by regular manufacturers (the price is relatively high), their raw materials, electrical properties and production processes can be guaranteed, and the quality is trustworthy.