When routing cables in harsh environments, it is necessary to consider interference from magnetic fields. Generally, efforts to reduce interference are made on the sheath. What are the mthodes to reduce the electromagnetic interference(EMI)?
1. Aluminum foil Mylar
Aluminum foil Mylar is made from soft aluminum foil and polyester film, which are laminated through gravure coating. After the aluminum foil Mylar is cured, it is cut into rolls. The adhesive can be adjusted, and after die-cutting, the aluminum foil Mylar can be used for assembling shielding and grounding.
Aluminum foil Mylar tape is primarily used in the interference shielding of communication cables. Aluminum foil Mylar includes: single-sided aluminum foil, double-sided aluminum foil, expanded aluminum foil, hot-melt aluminum foil, aluminum foil tape, and aluminum-plastic composite tape. The aluminum layer provides excellent conductivity, shielding effectiveness, and corrosion resistance, adapting to various requirements in different scopes. The shielding range is mainly from 100K to 3GHz. The hot-melt aluminum foil Mylar has a layer of hot-melt adhesive applied on the side where the aluminum foil contacts the cable. Under high-temperature preheating, the hot-melt adhesive can tightly wrap around the cable core insulation, enhancing the shielding performance of the cable. Ordinary aluminum foil does not have adhesiveness and is simply wrapped around the cable core insulation, resulting in poor shielding performance of the cable.
Aluminum foil Mylar is primarily used to shield high-frequency electromagnetic waves, preventing them from coming into contact with the conductors of cables and generating induced currents, which can increase crosstalk. According to Faraday's law of electromagnetic induction, when high-frequency electromagnetic waves come into contact with aluminum foil, they tend to adhere to the surface of the foil and generate induced currents. At this point, a conductor is needed to direct the induced currents to the ground, avoiding interference with the transmitted signals. Wires using aluminum foil as a shielding layer generally require a repetition rate of no less than 25% for the aluminum foil. The largest number of applications is currently in network cabling, which is mainly used in hospitals, factories, and other places with strong electromagnetic radiation or a large number of high-voltage equipment; it is also used in government and other areas with high requirements for network security.
2. Copper foil
Copper foil is a good conductor that primarily shields electromagnetic waves through reflection. However, at high frequencies, reflection alone may lead to multiple reflections of electromagnetic waves, which may eventually leak through gaps. Although copper foil has excellent conductivity, its poor flexibility can easily cause gaps or poor contact when attached to irregular surfaces, forming channels for electromagnetic leakage. Copper foil has excellent shielding effect on low-frequency electromagnetic waves, but at high frequencies, its surface current (skin effect) can lead to a decrease in shielding effect.
3. Conductive fabric
Conductive fabric is typically woven from conductive fibers, which not only reflect electromagnetic waves but also possess a certain absorption capability. Its multilayer structure and gaps between fibers aid in absorbing some high-frequency electromagnetic wave energy, reducing the impact of secondary radiation. Conductive fabric exhibits good flexibility and elasticity, allowing it to conform closely to irregular surfaces, minimizing poor contact issues and enhancing shielding effectiveness. Additionally, conductive fabric typically boasts a broader frequency response range, particularly at high frequencies, where its interwoven fiber structure contributes to improved shielding efficiency.
4. Braided mesh (metal shielding) made of copper/aluminum-magnesium alloy wire
Metal shielding is achieved by weaving metal wires into a specific woven structure using a weaving device. The materials commonly used for shielding include copper wire (tin-plated copper wire), aluminum alloy wire, copper-clad aluminum, copper tape (copper-plastic tape), aluminum tape (aluminum-plastic tape), steel tape, and other materials. Corresponding to metal weaving, different structural parameters exhibit varying shielding performance.
The shielding effectiveness of the braided layer is not only related to structural parameters such as conductivity and permeability of the metal itself, but also depends on the number of layers, coverage rate, and braiding angle. The more layers, the higher the coverage rate, and the smaller the braiding angle, the better the shielding performance of the braided layer. The braiding angle should be controlled between 30-45°. For single-layer braiding, the coverage rate is preferably above 80%. In this way, it can convert unwanted energy into other forms of energy such as heat and potential energy through mechanisms such as hysteresis loss, dielectric loss, and resistance loss, achieving the effect of shielding and absorbing electromagnetic waves.
Braided mesh is generally made of tinned round copper wire or aluminum-magnesium gold wire, primarily to prevent low-frequency electromagnetic wave interference. Its working principle is similar to that of aluminum foil. For shielded network cables using braided mesh, the density of the braided mesh is generally required to be at least 80%. This type of braided mesh is mainly used in places where a large number of network cables are laid in the same cable tray, reducing external crosstalk generated between a large number of network cables. In addition, it can also be used for inter-pair shielding, thereby increasing the twisted length of the pairs and reducing the requirements for cable lay length.
