1. What range of electromagnetic waves are usually used to shield aluminum plates?
2. In the electrical cabinet, the installation board of the main electrical components such as the filter, what special advantage is there if the aluminum plate is used?
3. We know that the magnetic permeability of aluminum is relatively low. If a space is sealed with a cover made of aluminum and does not leak any gaps, will the magnetic field lines of the external magnetic field not pass through this enclosed space?
(1) For high-voltage, low-current interference sources, the near field is dominated by electric field, and its magnetic field component can be ignored;
(2) For low-voltage and high-current interference sources, the near field is dominated by magnetic field, and its electric field component can be ignored;
(3) For higher frequencies or in places far away from the interference source (far-field conditions), regardless of the characteristics of the interference source itself, it can be regarded as a plane electromagnetic field. At this time, the electric and magnetic fields cannot be ignored. For the aluminum plate, the shielding effect of the electric field wave is very good in the whole frequency range; the shielding effect of the plane wave is also good in the whole frequency range. (The 0.5mm aluminum plate has at least the shielding effect in the whole frequency range Greater than 120dB); shielding magnetic field waves, the shielding effectiveness of low frequency bands (less than 100KHz) is poor, greater than 1MHz, the shielding effectiveness is significantly increased.
2. Use aluminum or copper plates, mainly to shield electromagnetic waves above 100KHz. As for the special benefits, there may be other considerations (personally think). Because aluminum or copper plates have low magnetic permeability, they are suitable for low-frequency magnetic fields. The shielding effect is very poor.
3. The magnetic permeability of aluminum or copper plates is very low. If the space is covered by an aluminum shielding cover, due to its low magnetic permeability, it plays little role in bypassing, that is, materials with low permeability cannot effectively shunt flow The magnetic circuit leads to a decrease in its shielding effectiveness against the magnetic field, so it is exactly the opposite of what you said.
In order to obtain a closed space with better shielding effectiveness, it is necessary to improve the shielding with a steel plate with high permeability. In this way, when the external magnetic field passes through the space, most of the magnetic field is guided away by the steel plate with high permeability (bypass). Make the space cleaner.
4. The shielding cover made of aluminum plate is not as effective as steel plate in shielding the magnetic field, because the magnetic permeability of aluminum is not as good as that of ferromagnetic materials such as steel. However, in some occasions, someone said that aluminum plate can isolate magnetic field, which mainly refers to the characteristics of aluminum plate What? I also want to ask, the magnetic permeability of aluminum is low, can the lines of force of the magnetic field pass through the aluminum plate?
The first question: What do you mean by the so-called certain occasions, which are actually many actual situations? In many cases, it is not possible to simply distinguish whether the magnetic wave component is large or the radio wave component is the main component. That is the situation above.
(3) For this situation, what I may say is simpler (for easy understanding). In fact, the shielding effectiveness of a certain material against electromagnetic waves not only depends on the magnetic permeability, but also affects the electrical conductivity in the case of plane waves. A factor in its shielding effectiveness.
In short: there are three factors that affect shielding effectiveness.
(2) Magnetic permeability;
(3) The frequency of the plane electromagnetic wave; although the magnetic permeability of aluminum or copper is small, its electrical conductivity is greater than that of steel, so:
a. In the case of low-frequency electromagnetic waves, the steel plate has a good shielding effect on plane waves (the skin depth of the steel plate is less than that of aluminum (copper));
b. For plane waves with a frequency greater than 0.01MHz, aluminum or copper has a good shielding effect. The frequency of electromagnetic waves in many occasions is greater than this frequency range, so in many occasions, you will hear that the shielding effect of aluminum plate is good.
In fact, whoever is better depends on the frequency band of the electromagnetic wave you want to shield. do you understand? For different materials, the frequency of the electromagnetic wave shielded is different.
The second problem: the magnetic permeability of the aluminum plate is low, and there are few lines of magnetic force passing through the aluminum plate. Think about it, magnetic permeability, as the name suggests, is the size of the magnetic permeability. The magnetic permeability of the aluminum plate is low. Of course, its magnetic permeability is poor. Many magnetic lines of force pass directly through the aluminum plate and affect the inner aluminum enclosure. The space affects the internal space of the aluminum cover, and thus the magnetic shielding effect is not achieved.
If it is a steel plate, when the magnetic line of force hits the steel plate, due to the strong magnetic permeability of the steel plate, most of the magnetic line of force is led away by the steel plate, so that the steel plate will not pass through the steel plate and affect the internal space of the steel plate to achieve a shielding effect.
Note: The above analysis only considers the magnetic permeability. That is to say, for the case where the low frequency is mainly magnetic.