Principle of Electromagnetic Shielding

Electromagnetic shielding can generally be divided into three types: electrostatic shielding, magnetostatic shielding and high-frequency electromagnetic field shielding. The purpose of the three types of shielding is to prevent the external electromagnetic field from entering a certain area that needs to be protected. However, due to the different characteristics of the field to be shielded, the requirements for the material of the shielding shell and the shielding

1. Electrostatic shielding

The purpose of electrostatic shielding is to prevent external electrostatic fields from entering an area that needs to be protected. The principle of electrostatic shielding is: under the action of the external electrostatic field, the surface charge of the conductor will be redistributed until the total field strength inside the conductor is zero everywhere. A grounded closed metal case is a good electrostatic shield. The grounded closed metal shell divides the space into two areas inside and outside the shell, and the metal shell is maintained at zero potential. According to the uniqueness theorem of electrostatic field, it can be proved that the electric field inside the metal shell is only determined by the charged body inside the shell and the potential of the shell, and has nothing to do with the charge distribution outside the shell. When the charge distribution outside the shell changes, the charge distribution on the outer surface of the shell changes accordingly, so as to keep the electric field distribution in the shell unchanged. Therefore, the metal shell has a shielding effect on the inner area. The electric field outside the shell is determined only by the potential of the charged body and the metal shell outside the shell and the potential at infinity, and has nothing to do with the charge distribution inside the shell. When the charge distribution in the shell changes, the charge distribution on the inner surface of the shell changes accordingly, so as to keep the electric field distribution outside the shell unchanged. Therefore, the grounded metal shell also has a shielding effect on the external area. In electrostatic shielding, it is very important to ground the metal shell. When the charge distribution in the shell or outside the shell changes, through the ground wire, the charge is redistributed between the shell outer surface and the earth to keep the shell potential constant. From the physical image, because there is no electric field inside the metal during electrostatic equilibrium, the electric field lines inside and outside the shell are cut off by the metal and have no connection with each other. Therefore, the conductor shell has the effect of isolating the electrostatic interaction inside and outside the shell.

If the metal shell is not completely closed, there are holes or slits on the shell, which also have the effect of electrostatic shielding. In many practical applications, the electrostatic shielding device often replaces the closed metal shell with a metal mesh woven from metal wires. Even a metal plate or a metal wire has a certain electrostatic shielding effect, but the shielding effect is not as good as the metal shell. .

Under the action of the external electric field, the redistribution of the electric charge on the conductor can be completed in the order of 10-19 seconds. Therefore, for the electric field with low frequency variation, the electric charge on the conductor has a long enough time to ensure that the internal field strength is zero. Therefore, the electrostatic shielding device also has a shielding effect on the slowly changing electric field. In order to improve the shielding effect of the changing electric field, the conductivity of the shield should be large, the grounding wire should be short, and the contact with the ground should be good.

For people wearing high-voltage work clothes, because they are wrapped by copper wire woven clothes, the field strength in the human body remains zero, so there is no current flowing through the human body, and the human body is safe. However, at the moment when the operator just touches the high-voltage line, the electric charge on the high-voltage suit has an instantaneous distribution process. In this extremely small period of time, the human body will have a short-term weak electric field effect, and the general operator can withstand this test. The characteristic of electrostatic shielding is that generally only the shielding of the electrostatic field is considered, and the shielding effect of the closed conductor is complete (that is, the internal field strength can be truly equal to zero), and there is no requirement for the thickness and conductivity of the screen wall shell. Only when the shielding of the low frequency alternating electric field is included in the electrostatic shielding, it is always desirable to have the shielding case as high in conductivity as possible.

2. Magnetic shielding

The purpose of magnetostatic shielding is to prevent the external static magnetic field and the magnetic field of low-frequency current from entering a certain area that needs to be protected. At this time, a magnetic medium must be used as the shell. The principle on which magnetostatic shielding is based can be explained with the help of the concept of parallel magnetic circuits. When a spherical shell made of a material with high magnetic permeability is placed in an external magnetic field, the wall of the iron shell and the air in the cavity can be regarded as a parallel magnetic circuit. Since the permeability of air is close to 1, and the permeability of the iron shell is at least several thousand, the reluctance of the cavity is much larger than that of the wall of the iron shell. In this way, most of the magnetic induction flux of the external magnetic field will “pass” along the wall of the iron shell, and there is very little magnetic flux “entering” the cavity, which achieves the purpose of magnetic shielding.

The thickness and permeability of the shell have a significant impact on the shielding effect: the thicker the shell and the higher the permeability, the better the shielding effect. Therefore, when the weight and volume are limited, permalloy with a magnetic permeability of up to tens of thousands is often used as a shielding shell, and each part of the shell should be combined as closely as possible to make the magnetic circuit unobstructed.

If you want to create an absolute “magnetostatic vacuum”, you can use the “Meisner effect” of superconductors. When a superconductor is placed in an external magnetic field, the magnetic induction in its body is always zero. Superconductors are completely diamagnets and have the most ideal magnetostatic shielding effect, but they are not yet widely used.

In order to prevent the watch from being magnetic, it is also a shielding effect to install an iron bushing outside the movement.

3. High-frequency electromagnetic field shielding

In the train car, when the semiconductor radio is turned on, there is almost no radio broadcast. This is because the skin of the car is mostly made of iron, which acts as a shield.

High-frequency electromagnetic field shielding is to prevent external high-frequency electromagnetic fields from entering a certain area. Due to the high frequency of change in the electromagnetic field (such as megahertz or higher), the induced charge on the conductor in the field can no longer be regarded as static (the conductor is no longer in electrostatic equilibrium), so it is necessary to use electromagnetic waves in the conductor. “Through the depth” to illustrate the principle of shielding: when a high-frequency electromagnetic wave strikes the surface of a conductor and enters the surface, it will induce a high-frequency alternating current in the conductor, and this current will excite a new electromagnetic wave. The phase of the electromagnetic wave inside the conductor is opposite to the incident electromagnetic wave. At the same time, the generation of current in the conductor also leads to the consumption of the incident wave field energy. As a result, the total electromagnetic field inside the conductor basically decays exponentially with the depth, which can be expressed as “penetration depth”. degree of attenuation. The “penetration depth” is related to the frequency of the incident electromagnetic wave, the electrical conductivity and magnetic permeability of the conductor: the higher the frequency, the greater the electrical conductivity, and the greater the magnetic permeability, the smaller the “penetration depth”. When the thickness of the shell wall is greater than the penetration depth, the shell has a good electromagnetic shielding effect. A shell made of high electrical conductivity or high magnetic permeability material is a good electromagnetic shielding device. Improving the electrical conductivity or magnetic permeability of the shell material and increasing the thickness of the shell wall can improve the effect of electromagnetic shielding.

For metals such as aluminum, steel, and iron, the “penetration depth” of electromagnetic waves of about 1 MHz is about a few percent of millimeters, so these metals can basically shield electromagnetic waves of 1 MHz as long as they are as thick as a sheet of paper. Iron, in particular, has a particularly good shielding effect because of its high permeability. For example, in a radio, a hollow aluminum shell is used to cover the outside of the coil, so that it is not disturbed by the external electromagnetic field and thus avoids noise. The same is true for shielded wires for audio feeders. The oscilloscope tube is covered with iron, so that the stray electromagnetic field does not affect the scanning of the electron rays. The high-frequency electromagnetic waves generated by the components or equipment inside the metal shielding shell cannot penetrate the metal shell without affecting the external equipment.

Shields made of high-conductivity materials have poor shielding effect on low-frequency magnetic fields. For example, when the power frequency is 50 Hz, the penetration depth of copper is about 9.4 mm, and the shielding effect of the thin-walled copper shell is very small. In practical applications, magnetostatic shielding measures are often used to shield low-frequency magnetic fields. Electromagnetic shielding can also shield electrostatic interference after grounding. The electromagnetic shield cannot be opened at will, because the electromagnetic shield also uses the effect of eddy current. If the gap cuts off the path of the eddy current, the shielding effect will be reduced.

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