The special application of micro coaxial cables also has certain peculiarities in material selection. This article will focus on the selection of inner conductor, insulation, and outer conductor materials for micro coaxial cables, and provide a detailed explanation.
1, Selection of Inner Conductor Materials
The inner conductors of micro coaxial cables are very thin, but the frequency of use and operating temperature of the cables are relatively high. So a single copper conductor is not an ideal inner conductor. Copper will oxidize in the air and produce black oxide skin, especially when the temperature is above 80 ℃, the oxidation rate accelerates. When the temperature reaches 176 ℃, the oxidation will be extremely rapid. The formation of black oxide skin will cause a decrease in its electrical performance.
The maximum operating temperature of copper conductors is generally limited to 100 ℃. The usage frequency is generally limited to below 3000MHz. Copper clad steel wire has high strength and fatigue resistance. At frequencies above 10MHz, the resistance of copper clad steel wire is almost the same as that of solid copper wire, while its mechanical strength is three times that of solid copper wire.
Copper clad steel, silver plated copper clad steel, or tin plated copper clad steel wires are commonly used as inner conductors for micro coaxial RF cables. Although tin has much lower electrical conductivity than copper, it has advantages such as oxidation resistance and corrosion resistance. The working temperature can generally reach 150 ℃, and in rare cases, it can also reach 200 ℃. Silver plated copper wire has better corrosion resistance and much higher conductivity than copper, making it widely used under high temperature and high frequency conditions. Its continuous working temperature can reach 200 ℃, and its short-term use temperature can reach 250 ℃.
2, Selection of insulation materials
The most commonly used insulation material for coaxial cables is polyethylene. The advantages of solid polyethylene insulation are high electrical and 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 operating temperature of polyethylene is generally 85 ℃, so when high temperature resistance is required, polytetrafluoroethylene (F4) and perfluoroalkoxy (F46) should be used. They have good thermal and chemical stability, and their electrical performance is also excellent. The long-term working temperature of polytetrafluoroethylene can reach 250 ℃, and the long-term working temperature of perfluoroethylene propylene can reach 200 ℃.
3, Selection of external conductor materials
The most commonly used outer conductor for micro coaxial lines is weaving, and the weaving density is generally above 95%. This is mainly due to its high transmission frequency, and the cable shielding attenuation is relatively large (compared with the tubular outer conductor, the attenuation will generally increase by 1.5 to 4 times, especially at high frequencies where the attenuation value will sharply rise).
The advantage of weaving is that the cable is relatively soft and easy to bend. The tubular (copper tube, aluminum tube) outer conductor has the advantages of low attenuation, good shielding, high mechanical strength, moisture resistance, and good sealing performance; The disadvantage is poor flexibility, allowing for a large bending radius and not suitable for mobile applications.
The last method is electroplating the outer conductor, which involves first coating a 0.05 micron thick copper layer on the insulation surface using chemical plating, and then electroplating it to a thickness of 0.025 millimeters. Electroplated coaxial cables with outer conductors have good flexibility, light weight, good shielding, low attenuation, low noise, and high corona voltage. They are an ideal outer conductor structure for small flexible RF cables.
