Shielding materials are mainly used in transformer laminated cores, filters, relays, recording (image) heads, deflection coils and focusing coil yokes, magnetic amplifiers, speakers, magnetic shields, etc.
Electromagnetic shielding is the use of shielding materials to block or attenuate the propagation of electromagnetic energy between the shielded area and the outside world. The principle of electromagnetic shielding is to use the shielding body to reflect, absorb and guide the electromagnetic energy flow, which is closely related to the charge, current and polarization induced on the surface of the shielding structure and inside the shielding body. Shielding is divided into electric field shielding (electrostatic shielding and alternating electric field shielding), magnetic field shielding (low-frequency magnetic field and high-frequency magnetic field shielding) and electromagnetic field shielding (electromagnetic wave shielding) according to its principle. Generally speaking, electromagnetic shielding refers to the latter, that is, shielding both electric and magnetic fields.
The shielding effect is evaluated by shielding effectiveness ~g (SE, Shielding effectiveness), which shows the attenuation degree of the shielding body to electromagnetic waves. Shielding effectiveness is defined as the ratio of the electromagnetic field intensity at that point before and after shielding, that is: SE=2OIg (Eo/Es) or SH=2Olg (HdHs) where:, are the electric field and magnetic field strength at the point before shielding,, are The electric field strength and magnetic field strength at that point after shielding. The evaluation of shielding effect is based on the size of shielding effectiveness.
According to the principle of shielding action, the contribution of shielding body to shielding effectiveness is divided into three parts: (1) The reflection loss caused by impedance mismatch on the surface of the shielding body; (2) When electromagnetic waves are transmitted inside the shielding material, electromagnetic energy is absorbed and causes transmission loss Or absorption loss; (3) Multiple reflection loss caused by multiple reflections of electromagnetic waves between the inner walls of the shielding material. From this, three basic factors that affect the shielding effectiveness of the material can be obtained, namely the electrical conductivity, magnetic permeability and material thickness of the material. This is also a problem and a breakthrough that must be paid attention to in the research of shielding materials. Of course, for the structure of the electromagnetic shielding body, its shielding effectiveness is also related to the structure, shape, airtightness, etc. For specific issues, it is also necessary to consider the electromagnetic wave frequency to be shielded and the nature of the field source.
Typical components that need to be shielded are: transformers, motors, reed relays, power supplies, amplifiers, vacuum tubes, photomultipliers, magnetrons, amplifiers, meters, speakers, recording heads, instruments, cables, transistors, and electric waves Filters etc.
The magnetic permeability of the shielding material, the geometry of the shielding cover, and the thickness of the material all affect the effect of magnetic shielding. Therefore, the selection of magnetic shielding materials should master the following principles and factors:
1) To maximize the attenuation of the external magnetic field, materials with high permeability μ should be selected;
2) The shielding effect is also a function of the ratio (t/D) of the wall thickness of the shield material to the diameter of the shield (or its diagonal in the case of a rectangular shield);
3) If the external magnetic field is too high, the attenuation in the shielding material will decrease. This is because when the external magnetic field is too high, the shielding material has begun to close to saturation.