First, what is shielded wire?
A shielded wire is a transmission line that wraps a signal wire with a mesh braided wire.
The wire wrapped around the conductor is called a shielded wire. The wrapped conductor is called a shield layer. It is usually a braided copper mesh or copper poise (aluminum mooring, etc.). The shield layer needs to be grounded to prevent interference signals from entering the inner conductor and reducing the transmission signal. Loss. Commonly woven wire, wrapped wire is a shielded wire.
Second, the role of shielding
Shielding is the separation of metal between two spatial regions to control the induction and radiation of electric, magnetic and electromagnetic waves from one region to another. Specifically, the shield is used to surround the components, circuits, assemblies, cables, or interference sources of the entire system to prevent interference with the electromagnetic field to the *********; use the shield to receive the circuit, device or system Surrounded to prevent them from being affected by external electromagnetic fields. Because the shield absorbs energy (eddy current loss), reflected energy (interface reflection of electromagnetic waves on the shield) and cancels energy (electrical waves) from external electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems. The magnetic induction generates a reverse electromagnetic field on the shielding layer, which can cancel the effect of partially interfering electromagnetic waves, so the shielding body has the function of reducing interference.
(1) When the frequency of the interfering electromagnetic field is high, the eddy current generated in the metal material of low resistivity is used to form a canceling effect of the external electromagnetic wave, thereby achieving the shielding effect.
(2) When the frequency of the interference electromagnetic wave is low, a material with a high magnetic permeability is used, so that the magnetic field line is confined inside the shield to prevent diffusion into the shielded space.
(3) In some cases, if high-frequency and low-frequency electromagnetic fields are required to have good shielding effect, different metal materials are often used to form a multilayer shielding body. Many people don't understand the principle of electromagnetic shielding. They think that if they use metal as a box and then ground the box, they can act as electromagnetic shielding. The result is a failure under the guidance of this concept. Because the electromagnetic shielding is not related to the grounding of the shield. There are only two factors that really affect the shielding effectiveness of the shield: one is that the entire shield surface must be electrically conductive and the other is a conductor that does not directly penetrate the shield
There are many conductive discontinuities on the shield. The most important one is the non-conductive gap formed by the joints of different parts of the shield. These non-conductive gaps create an electromagnetic leak as if the fluid would leak from the gap in the container. One way to address this leakage is to fill the gap with a conductive elastomer to eliminate non-conducting points. This is like the principle of filling rubber in the gap of a fluid container. This elastic conductive filling material is an electromagnetic sealing gasket. In many literatures, the electromagnetic shield is likened to a liquid-tight container, and it seems that electromagnetic wave leakage can be prevented only when the gap is sealed with a conductive elastic material to the extent that the drip does not leak. In fact this is not exact.
Whether the gap or hole leaks electromagnetic waves depends on the size of the slit or hole relative to the wavelength of the electromagnetic wave. When the wavelength is much larger than the opening size, no significant leakage will occur. Therefore, when the frequency of the interference is high, when the wavelength is short, an electromagnetic sealing gasket is required. Specifically, when the frequency of the interference exceeds 10 MHz, it is necessary to consider the use of an electromagnetic sealing gasket.
Any material that is elastic and electrically conductive can be used as an electromagnetic sealing gasket.
Conductive rubber: The silicon rubber is filled with metal particles in a proportion of 70 to 80% of the total weight, such as silver powder, copper powder, aluminum powder, silver-plated copper powder, silver-plated aluminum powder, silver-plated glass ball, and the like. This material retains a portion of the silicone rubber's good elasticity properties while having good electrical conductivity.
Metal woven mesh: braided into a tubular strip with bismuth copper wire, monel wire or stainless steel wire, which looks like a shielded shielded cable. However, its weaving method is different from the cable shielding layer, which is braided with a plurality of wires, and the shielding gasket is woven from a single thread. The analogy of an image is like the sleeves of a sweater. In order to enhance the elasticity of the metal mesh, a rubber core is sometimes added to the mesh tube.
Finger reed: Reed made of beryllium copper with good elasticity and electrical conductivity. Electrical conductivity and elasticity. Multi-conductive rubber: composed of two layers of rubber, the inner layer is ordinary silicone rubber, and the outer layer is conductive rubber. This material overcomes the shortcomings of the poor elasticity of the conventional conductive rubber, and the elasticity of the rubber is fully reflected. Its principle is somewhat like a wire mesh strip with a rubber core.
There are four factors to consider when choosing which type of electromagnetic seal gasket to use: shielding effectiveness requirements, environmental seal requirements, installation structural requirements, and cost requirements. Compared with the characteristics of different gasket materials, the shielding mechanism can be divided into electric field shielding, magnetic field shielding and electromagnetic field shielding.
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