Material Selection for Micro-Coaxial Cables: Inner Conductor, Insulation and Outer Conductor

Due to the particularity of its applications, micro-coaxial cables have certain special requirements in material selection. This article will elaborate on the material selection of the inner conductor, insulation, and outer conductor of micro-coaxial cables in detail.

I. Material Selection for Inner Conductor

The inner conductors of micro-coaxial cables are very thin, while the operating frequency and temperature of the cables are relatively high. Therefore, a single copper conductor is not an ideal inner conductor. Copper oxidizes in the air to form black oxide scale, especially when the temperature is above 80°C, the oxidation rate accelerates, and when the temperature reaches 176°C, the oxidation becomes extremely rapid. The formation of black oxide scale will degrade its electrical performance.

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Therefore, the maximum operating temperature of copper conductors is generally limited to 100°C. In terms of operating frequency, it is generally limited to below 3000 MHz. Copper-clad steel wire has the characteristics of high strength and fatigue resistance. Under the condition of frequency higher than 10 MHz, 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 copper wire.

Therefore, micro-coaxial RF cables often use copper-clad steel, silver-plated copper-clad steel, or tinned copper-clad steel wires as inner conductors. Although the conductivity of tin is much lower than that of copper, it has advantages such as oxidation resistance and corrosion resistance. The operating temperature can generally be up to 150°C, and in very individual cases, it can also be up to 200°C. Silver-plated copper wire has better corrosion resistance and much higher conductivity than copper, so it is widely used under high-temperature and high-frequency conditions. Its continuous operating temperature can reach 200°C, and the short-term operating temperature can reach 250°C.

II. Material Selection for Insulation

The most commonly used insulation material for coaxial cables is polyethylene. Solid polyethylene insulation has the advantages of high dielectric strength, high mechanical strength, low thermal resistance, and stable structure; its disadvantage is that the dielectric constant is large, and the attenuation is greater when the frequency is very high. The maximum operating temperature of polyethylene is generally 85°C, so polytetrafluoroethylene (PTFE, F4) and perfluoroethylene-propylene copolymer (FEP, F46) should be used when high-temperature resistance is required. They have good thermal stability and chemical stability, and their electrical performance is also excellent. The long-term operating temperature of PTFE can reach 250°C, and the long-term operating temperature of FEP can reach 200°C.

III. Material Selection for Outer Conductor

The most commonly used outer conductor for micro-coaxial cables is braided, and the braiding density is generally above 95%. This is mainly because it transmits high frequencies, and the cable shielding attenuation is relatively large (compared with tubular outer conductors, the attenuation will generally increase by 1.5 to 4 times, especially at high frequencies, the attenuation value will rise sharply).

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The advantage of braiding is that the cable is relatively soft and easy to bend. Tubular (copper tube, aluminum tube) outer conductors have the advantages of low attenuation, good shielding performance, high mechanical strength, good moisture resistance, and good sealing; the disadvantage is poor flexibility, large allowable bending radius, and unsuitability for mobile occasions.

Finally, there is an electroplated outer conductor. First, a 0.05-micron-thick copper layer is plated on the insulation surface by electroless plating, and then thickened to 0.025 mm by electroplating. Coaxial cables with electroplated outer conductors have good flexibility, light weight, good shielding performance, low attenuation, low noise, and high corona voltage. They are an ideal outer conductor structure for micro flexible RF cables.