When selecting coaxial cables, we should not only pay attention to the properties of the connectors but also have a preliminary understanding of the cable part, especially the shielding properties. This article will provide a brief analysis focusing on the shielding properties of coaxial cables.
I. Types of Coaxial Shielding Layers
Shielding layers generally adopt braiding and winding methods. Braiding and winding usually use materials such as bare copper, tinned copper, silver-plated copper, silver-plated copper-clad steel, and aluminum-magnesium alloy wires. In addition, semi-flexible and semi-rigid cables are generally made in the forms of corrugated tubes, tinned braids, copper or aluminum tubes; the cable shielding layer can also adopt the method of aluminum foil + ground wire. The main function of shielding is to prevent crosstalk of transmitted signals, block external noise from entering to cause interference, and enhance the mechanical strength of the cable.
1. Conductivity and Permeability of Shielding Material Wires
Conductivity: The higher the conductivity, the better the shielding effectiveness, in the order of silver - copper - aluminum - brass - iron - steel;
Permeability: The permeability of iron and steel is much higher than that of copper and aluminum, so their shielding effect is better.
Steel shielding is mostly used in situations with strong magnetic fields and low frequencies; aluminum and copper are mostly used for shielding of medium and high-frequency cables; while copper-clad steel has good shielding effectiveness at any frequency.
2. Shielding Coverage Rate of the Shielding Layer (i.e., braiding rate, winding rate, etc.)
In general, the shielding effectiveness of metal wires increases with the increase of braiding density and decreases with the increase of frequency. This is because when the frequency is high enough, its wavelength will be close to the size of the mesh aperture, and these apertures will act like slot antennas, thereby reducing the shielding effectiveness. In low-frequency (usually below 100KHz) magnetic fields, the shielding effectiveness not only changes with frequency but also increases with the increase of braiding density and material permeability.
In addition, the actual shielding effectiveness of metal wires is also related to the contact resistance between bare wires. Because electromagnetic shielding works by the current flowing through the shield, if the contact resistance between bare wires is large, the current is not easy to pass through. Therefore, for old shielded cables, the surface oxidation and rust of the metal mesh increase the resistance, which will also cause a sharp drop in shielding effectiveness.
3. What are the main factors affecting cable braiding density?
3.1. Number of Spindles: That is, the number of braided strands.
3.2. Number of Wires per Spindle: That is, the number of metal wires in each braided strand. Under the same conditions, the more the number of wires, the higher the braiding density.
3.3. Diameter of Braided Copper Wire: That is, the outer diameter of each copper wire in the braid. Under the same conditions, the thicker the copper wire, the higher the braiding density. However, it is generally not advisable to increase the shielding rate by increasing the outer diameter of the copper wire, as this will increase costs and lead to larger cable outer diameter or poor appearance.
3.4. Braiding Angle: That is, the angle formed by the intersection of every two spindles of braiding and the axial vertical line, which is generally required to be 30~70°. The larger the braiding angle, the looser it will be, resulting in fewer meshes and larger pitch. It is more likely to produce bell mouths during tin plating.
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