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Coaxial cable is a widely used transmission medium. Coaxial cables are including large coaxial, medium coaxial, small coaxial, and micro coaxial according to the size of OD. Mini coaxial cables are mainly used in mobile communication systems, satellite communications, and key defense projects.
According to the characteristic impedance, there are usually two types: 75Ω and 50Ω. 75Ω coaxial cable is used in the CATV network for analog transmission, and its transmission bandwidth is above 1GHz. 50Ω coaxial cables are mainly used for digital transmission and are mostly used for baseband transmission. The current transmission bandwidth is below 1GMHz.
1. The inner conductor is multi-strand silver-plated copper-clad steel, the outer layer is solid PTFE, the outer conductor is braided with silver-plated copper wire, and the outer diameter of the sheath is 1.30mm (SFF-50-0.8 high-temperature wire). Although this cable has a simple structure, it has strict processing requirements, especially for outer diameter control. Therefore, the production equipment is crucial, and the tension of the take-up line must be adjusted through the constant tension adjustment system. And the pay-off tension should be able to meet the conductor requirements of solid copper wire, copper-clad steel wire, copper-clad aluminum wire, etc. Because the outer diameter tolerance is not well controlled during the manufacturing process, it has a great influence on the uniformity of wave impedance. Generally speaking, the smaller the tolerance range of the characteristic impedance test of the cable, the better the process control.
2. The inner conductor is a single bare copper wire, the insulation layer is foamed skin, the outer diameter is 1.30mm, the outer conductor is woven aluminum foil, and the outer diameter of the sheath is 2.8mm (RG-174 low-loss cable). The characteristic impedance of this cable is 50Ω, and its structure is the same as the ordinary solid RG-174 (insulation outer diameter of 1.55mm) with a 7/0.16CCS conductor. The difference is that the insulating layer uses foamed skin, which reduces the equivalent dielectric constant and dielectric loss tangent of the insulating layer. Therefore, under the same wave impedance condition, if the inner conductor remains unchanged, the outer diameter of the insulation can be made smaller, and the attenuation can be greatly reduced. Especially under high-frequency conditions, the effect of reducing attenuation is more obvious.
The use of foam insulation has the advantages of lightweight, low attenuation, high power, and wide frequency. The disadvantage is that the withstand voltage is low and the mechanical strength is poor. To make up for this shortcoming, it is necessary to add a layer of solid PE to the outermost surface of the foamed insulator. This layer of solid PE can be used for insulation and coloring, as well as moisture and moisture-proof, increasing the mechanical strength of the insulating layer and improving the service life of the cable. In the manufacturing process of the small coaxial skin foam, the requirements from the selection of insulating materials, inner skinning, and foaming molds to the production process are relatively high.
Inner skinning extrusion: To effectively bond the foam to the conductor wire, a layer of LDPE (because the viscosity of LDPE is relatively high) must be extruded as the inner skinning before the conductor is foamed and extruded. Its thickness and concentricity have a great influence on the performance of the final product (the thickness is generally controlled at about 0.05mm). The equipment generally adopts a control system composed of an AC motor and an AC frequency conversion speed regulating device, the main control system, and a diameter measuring device to ensure a stable and uniform extrusion of the extremely thin inner junction layer.
Foaming extrusion: Because it has a high degree of foaming (the degree of foaming can reach 70~80%), all the foaming materials should be made of HDPE to improve the mechanical strength of the insulation layer and be able to withstand sufficient external pressure. The extrusion host should use a DC motor and a fully digital DC speed regulating device to achieve speed regulation to ensure a stable extrusion volume. The screw length to diameter ratio is not less than 34:1 to ensure that the extruded insulation layer is fully plasticized and foamed to meet the process requirements. Ole measurement and control feedback should have an alarm device, and the input outer diameter tolerance should not be too large (according to the process capability index of the equipment and the process level). Besides, it is best to use high-pressure gas injection to ensure stable pressure.
Outer sheath extrusion: Generally, HDPE is also used for sheath material, and the sheath screw temperature is set as high as possible, especially the outlet mold temperature because the quality of the appearance is directly related to the surface quality of the cable. The thickness of the outer skin of a small coaxial cable should not be too thick, generally controlled at about 0.05mm. For cables with colored outer skins, the eccentricity must be good, otherwise, the surface color will affect the surface quality.
It is particularly pointed out that when producing small coaxial cables with foamed leather, the on-line inspection system should have the outer diameter and capacitance measuring devices. The outer diameter measuring device should include the outer diameter of the inner crust and the outer diameter of the insulation layer (generally manufacturers do not pay attention to the thickness control of the inner crust). Both the outer diameter and capacitance measurement should have feedback functions to make the production line form closed-loop feedback, and automatically control the insulation outer diameter and foaming degree of the cable to fully ensure the quality of the cable.
We all know that the inner conductor of the micro coaxial cable is very thin, but the frequency and working temperature of the cable are higher, so a single copper conductor is not an ideal inner conductor. Copper will oxidize in the air to produce a black oxide scale, especially when the temperature is above 80℃, the oxidation speed will increase, and when the temperature reaches 176℃, the oxidation will be extremely rapid. The generation of a black oxide scale will reduce its electrical performance. Therefore, the maximum operating temperature of copper conductors is generally limited to 100°C. The frequency of use is generally limited to below 3000MHz. Copper-clad steel wire has high strength and fatigue resistance. Under the condition of frequency higher than 10MHz, the resistance of copper-clad steel is almost the same as that of solid copper wire, while the mechanical strength is three times that of solid. Therefore, small coaxial radio frequency cables often use copper-clad steel, silver-plated copper-clad steel, or tin-plated copper-clad steel wire as the inner conductor.
Although the conductivity of tin is much lower than that of copper, it has the advantages of oxidation resistance and corrosion resistance. The working temperature can generally be up to 150°C, and in very rare cases it can also be used up to 200°C. Silver-plated copper wire has better corrosion resistance and its electrical conductivity is much higher than copper, so it is widely used in high temperatures and high-frequency conditions. Its continuous working temperature can reach 200℃, and the short-term use temperature can reach 250℃.
The most commonly used insulation material for coaxial cables is polyethylene. The advantages of the solid polyethylene insulation layer are electrical strength, high mechanical strength, low thermal resistance, and stable structure. The disadvantage is that the dielectric constant is large, and the attenuation is greater when the frequency is high. The maximum use temperature of polyethylene is generally 85°C, so when high-temperature resistance is required, polytetrafluoroethylene (F4) and polytetrafluoroethylene propylene (F46) should be used. They have good thermal and chemical stability, and their electrical properties are also excellent. The long-term working temperature of PTFE can reach 250℃, and the long-term working temperature of PTFE can reach 200℃.
The braid density of the most commonly used outer conductor of the micro coaxial cable is generally above 95%. This is mainly due to the large attenuation of the cable shielding (compared with the tubular outer conductor, the attenuation will increase to 1.5~4 times under normal circumstances, and the attenuation value will increase sharply especially at high frequencies). The advantage of braiding is that the cable is relatively flexible and easy to bend. Tubular (copper tube, aluminum tube) outer conductor has the advantages of low attenuation, good shielding, high mechanical strength, moisture-proof and good sealing. The disadvantage is that it has poor flexibility and allows a large bending radius, which is not suitable for mobile occasions.
Finally, there is the electroplating of the outer conductor, which is to coat a layer of copper with a thickness of 0.05 microns on the insulating surface by electroless plating, and the thickness is increased to 0.025 mm by electroplating. The electroplated outer conductor coaxial cable has good flexibility, lightweight, good shielding, low attenuation, low noise, and high corona voltage. It is an ideal outer conductor structure for micro-small flexible radio frequency cables.
Low-attenuation cables are required in various radio frequency power distribution, receiving, and measurement systems. Because when communication and electronic equipment goes underground, the distance between the cable and the whole machine must be increased. As the transmission distance increases, the requirement for low attenuation becomes more prominent. In particular, the miniaturization, multi-function, and frequency of use of the whole machine are getting higher and higher, and the contradiction between the attenuation value and size of the cable and the frequency of use is becoming more and more prominent. Therefore, coaxial cable manufacturers must resolve the contradiction between miniaturization, low attenuation, and high frequency, and low attenuation requirements often conflict with the flexibility of the cable. Various light pipe outer conductor cables have the least attenuation structure, but they have poor flexibility. However, ordinary flexible cables with braided structures have poor electrical performance, especially large attenuation, especially at frequencies of several gigahertz.
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