The shield box is a shield body made of various conductive or magnetic materials, which limits the electromagnetic capacity to a certain spatial range and is used to suppress the radiated metal body. What is the mechanism to be followed by the electromagnetic shielding of the shielding box?
First, electromagnetic field theory
Electromagnetic waves at the interface of different propagation media, because of the abrupt changes in wave impedance, electromagnetic waves will reflect. In addition, in the transmission medium, for example, inside the metal material, electromagnetic waves are attenuated. This reflection and attenuation just illustrates the shielding mechanism of metallic materials.
Second, the principle of electromagnetic induction
When the alternating electromagnetic field passes through the surface of the metal material, the metal material may form a vortex due to the induced electromotive force. The magnetic field generated by this eddy current is just opposite to the original magnetic field, which cancels out part of the original magnetic field and acts as a shield. In addition, because the metal material has a certain resistance, the eddy current generates heat inside the metal material that consumes some of the incident electromagnetic waves, and also plays a role in shielding.
Third, the transmission line theory
In the lossy non-uniform transmission line, the electromagnetic wave will reflect because the impedance of the transmission line does not match the impedance of the electromagnetic wave. In addition, the transmission line is lossy, and the electromagnetic wave will be attenuated during transmission. This is very similar to reflection and attenuation in electromagnetic wave theory analysis, but this method is much simpler than electromagnetic wave analysis method, and it is also the most widely used method in electromagnetic shielding analysis.
2018-03-30