Electromagnetic shielding uses 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. According to its principle, shielding is divided into electric field shielding (electrostatic shielding and alternating electric field shielding), magnetic field shielding (low frequency magnetic field shielding and high frequency magnetic field shielding) and electromagnetic field shielding (shielding of electromagnetic wave). Generally speaking, the electromagnetic shielding refers to the latter, that is, shielding the electric field and the magnetic field at the same time.

According to the shielding principle, the shielding body's contribution to shielding effectiveness is divided into three parts:

(1) Reflection loss due to impedance mismatch on the shield surface;

(2) When electromagnetic waves are transmitted inside the shielding material, electromagnetic energy is absorbed to cause transmission loss or absorption loss;

(3) Multiple reflection loss caused by multiple reflections of electromagnetic waves between the inner walls of the shielding material. Thereby, three basic factors affecting the shielding effectiveness of the material, namely the electrical conductivity, magnetic permeability and material thickness of the material, can be obtained. This is also a problem and breakthrough that the shielding material research itself must pay attention to. Of course, for the structure of the electromagnetic shielding body, the shielding effectiveness is also related to the structure, shape, air tightness, etc. For specific problems, the frequency of the electromagnetic wave to be shielded, the nature of the field source, etc. need to be considered.