Magnetostatic shielding

Static magnetic field is the magnetic field generated by steady current or permanent magnet. Magnetostatic shielding uses high permeability μ The ferromagnetic material is made into a shield to shield the external magnetic field. It is similar to but different from electrostatic shielding.

The principle of magnetostatic shielding can be explained by the concept of magnetic circuit. If the ferromagnetic material is made into a circuit with a cross section as shown in Figure 7, in the external magnetic field, most of the magnetic field is concentrated in the ferromagnetic circuit. The ferromagnetic material and the air in the cavity can be analyzed as a parallel magnetic circuit. Because the permeability of the ferromagnetic material is thousands of times greater than that of the air, the magnetic resistance of the cavity is much greater than that of the ferromagnetic material. Most of the magnetic induction lines of the external magnetic field will pass through the ferromagnetic material wall, and the magnetic flux entering the cavity is very small. In this way, the cavity shielded by ferromagnetic material basically has no external magnetic field, so as to achieve the purpose of magnetostatic shielding. The higher the permeability of the material, the thicker the cylinder wall, and the more significant the shielding effect. Because ferromagnetic materials with high permeability such as soft iron, silicon steel and permalloy are commonly used as shielding layers, magnetostatic shielding is also called ferromagnetic shielding.

Magnetostatic shielding is widely used in electronic devices. For example, the magnetic flux leakage generated by transformers or other coils will affect the movement of electrons and affect the focusing of electron beams in oscilloscope tubes or kinescope. In order to improve the quality of instruments or products, the components that produce magnetic flux leakage must be statically shielded. In a watch, a thin soft iron shell can be used to protect the movement from magnetism.

As pointed out earlier, the effect of electrostatic shielding is very good. This is because the conductivity of metal conductor is more than ten orders of magnitude larger than that of air, while the difference between the permeability of ferromagnetic material and air is only a few orders of magnitude, usually about several thousand times larger. So the magnetostatic shield always has some magnetic leakage. In order to achieve better shielding effect, multi-layer shielding can be used to shield the residual magnetic flux leaked into the cavity again and again. Therefore, the magnetic shielding with good effect is generally bulky. However, if we want to create an absolute "magnetostatic vacuum", we can use the Meissner effect of superconductors. That is, a superconductor is placed in an external magnetic field, and the magnetic induction intensity B in its body is always zero. Superconductors are completely antimagnetic and have the most ideal magnetostatic shielding effect, but they can not be widely used at present.

electromagnetic shielding 

When the electromagnetic field propagates in the conductive medium, the amplitudes of its field quantities (E and H) decay exponentially with the increase of distance. From the point of view of energy, there is energy loss when electromagnetic wave propagates in conductive medium. Therefore, it shows the decrease of field amplitude. The field on the conductor surface is the largest, and the deeper into the conductor, the smaller the field. This phenomenon is also called skin effect. Using skin effect can prevent high-frequency electromagnetic wave from penetrating into a good conductor to form an electromagnetic shielding device. It is more universal than electrostatic and magnetostatic shielding.

Electromagnetic shielding is an effective means to suppress interference, enhance equipment reliability and improve product quality. Reasonable use of electromagnetic shielding can suppress the interference of external high-frequency electromagnetic waves, and can also avoid affecting other equipment as an interference source. For example, in a radio, a hollow aluminum shell is used to cover the coil, so that it is not disturbed by the external time-varying field, so as to avoid noise. This is also the case with shielded wires for audio feeders. The oscilloscope tube is covered with iron sheet so that the stray electromagnetic field does not affect the scanning of electron rays. The high-frequency electromagnetic wave generated by the components or equipment inside the metal shielding shell can not pass through the metal shell without affecting the external equipment.

What material is used for electromagnetic shielding? Since the electromagnetic wave decays rapidly in a good conductor, the thickness decaying from the conductor surface to 1/e (about 36.8%) of the surface value is called the skin thickness (also known as the penetration depth), which is represented by D. with electromagnetic shielding, when the electromagnetic field propagates in the conductive medium, the amplitude of its field quantities (E and H) decays exponentially with the increase of distance. From the point of view of energy, there is energy loss when electromagnetic wave propagates in conductive medium. Therefore, it shows the decrease of field amplitude. The field on the conductor surface is the largest, and the deeper into the conductor, the smaller the field. This phenomenon is also called skin effect. Using skin effect can prevent high-frequency electromagnetic wave from penetrating into a good conductor to form an electromagnetic shielding device. It is more universal than electrostatic and magnetostatic shielding.

Electromagnetic shielding is an effective means to suppress interference, enhance equipment reliability and improve product quality. Reasonable use of electromagnetic shielding can suppress the interference of external high-frequency electromagnetic waves, and can also avoid affecting other equipment as an interference source. For example, in a radio, a hollow aluminum shell is used to cover the coil, so that it is not disturbed by the external time-varying field, so as to avoid noise. This is also the case with shielded wires for audio feeders. The oscilloscope tube is covered with iron sheet so that the stray electromagnetic field does not affect the scanning of electron rays. The high-frequency electromagnetic wave generated by the components or equipment inside the metal shielding shell can not pass through the metal shell without affecting the external equipment.

What material is used for electromagnetic shielding? Since the electromagnetic wave decays rapidly in a good conductor, the thickness from the conductor surface to 1/e (about 36.8%) of the surface value is called the skin thickness (also known as the penetration depth), which is represented by D, where μ and σ Respectively, the permeability and conductivity of the shielding material. If TV frequency f=100mhz, for copper conductor（ σ= five point eight × 107／·m， μ ≈ μ o＝4π × 10-7h / M), d=0.00667mm can be obtained. It can be seen that the electromagnetic shielding effect of good conductor is remarkable. If it's iron（ σ＝ 107 / · m), then d=0.016mm. If aluminum（ σ＝ three point five four × 107 / · m), then d = 0.0085mm.

In order to obtain effective shielding, the thickness of the shielding layer must be close to the electromagnetic wave length inside the shielding material（ λ= 2πd）。 If f = 500KHz in radio, d = 0.094mm in copper（ λ= 0.59mm）。 In aluminum, d = 0.12mm（ λ= 0.75mm）。 Therefore, good shielding effect can be obtained by using thinner copper or aluminum materials in the radio. Because the TV frequency is higher and the penetration depth is smaller, the required thickness of the shielding layer can be thinner. If the mechanical strength is considered, the necessary thickness is required. At high frequency, due to the large hysteresis loss and eddy current loss of ferromagnetic materials, the Q value of the resonant circuit quality factor decreases. Therefore, generally, the magnetic shielding with high permeability is not used, but the material with high conductivity is used as the electromagnetic shielding. In electromagnetic materials, the skin current is eddy current, so electromagnetic shielding is also called eddy current shielding.

At power frequency (50Hz), d = 9.45mm in copper and 11.67mm in aluminum. Obviously, it is not suitable to use copper and aluminum. If iron is used, d = 0.172mm. At this time, ferromagnetic materials should be used. Because the electromagnetic field attenuation in ferromagnetic materials is much greater than that in copper and aluminum. Because of the low frequency, the Q-value problem does not need to be considered. It can be seen that at low frequencies, electromagnetic shielding is transformed into magnetostatic shielding. Electromagnetic shielding and electrostatic shielding have similarities and differences. The same point is that they are all made of metal materials with high conductivity; The difference is that electrostatic shielding can only eliminate capacitive coupling and prevent electrostatic induction. The shielding must be grounded. The electromagnetic shielding is to make the electromagnetic field only penetrate into a thin layer of the shielding body, and eliminate the interference of the electromagnetic field by eddy current. This kind of shielding body can not be grounded. However, since the conductor used as electromagnetic shielding increases electrostatic coupling, even if only electromagnetic shielding is carried out, it is better to be grounded, so the electromagnetic shielding also plays the role of electrostatic shielding.

To sum up, the physical contents, physical conditions and shielding effects of electrostatic shielding, magnetostatic shielding and electromagnetic shielding are different, and the materials used should also be based on the specific conditions. But they are all shielding electromagnetic fields, which are intrinsically related.