Electromagnetic waves are the main method of electromagnetic energy propagation. When high-frequency circuits work, electromagnetic waves will be radiated outwards, which will cause interference to other nearby equipment. On the other hand, various electromagnetic waves in space will also be induced into the circuit, causing interference to the circuit. The purpose of the electromagnetic shielding box is to cut off the propagation path of electromagnetic waves, thereby eliminating interference. Among the many means to solve the problem of electromagnetic interference, the electromagnetic shielding box is the most basic and effective. The biggest advantage of using electromagnetic shielding box to solve the electromagnetic interference problem is that it will not affect the normal operation of the circuit, so there is no need to make any changes to the circuit. The same shielding body has different shielding performances for electromagnetic waves of different properties.
Therefore, when considering electromagnetic shielding performance, it is necessary to have a basic understanding of the types of electromagnetic waves. There are many classification methods for electromagnetic waves, but when designing a shielding box, electromagnetic waves are divided into electric field waves, magnetic field waves, and plane waves according to their wave impedance.
The electromagnetic wave impedance ZW is defined as: the ratio of the electric field component E to the magnetic field component H in the electromagnetic wave: ZW = E / H
The electromagnetic wave impedance is related to the nature of the electromagnetic wave radiation source, the distance from the observation point to the radiation source, and the propagation medium where the electromagnetic wave is located.
When it is close to the radiation source, the wave impedance depends on the characteristics of the radiation source. If the radiation source is high current and low voltage (the impedance of the radiation source is low), the wave impedance of the electromagnetic wave generated is less than 377, which is called a magnetic field wave. If the radiation source is high voltage and small current (the radiation source has a high impedance), the electromagnetic wave impedance is greater than 377, which is called an electric field wave.
When the radiation source is far away, the wave impedance is only related to the electric field wave propagation medium, and its value is equal to the characteristic impedance of the medium, which is 377Ω for air. The wave impedance of the electric field decreases with the increase of the propagation distance, and the wave impedance of the magnetic field increases with the increase of the propagation distance.
The difference between the near-field area and the far-field area varies with frequency, and is not a fixed number. This should be paid attention to when analyzing the problem. For example, when considering the shielding box, the shielding box may be in the far field area relative to the high-speed clock signal on the circuit board, while for the lower operating frequency of the switching power supply, it may be in the near field area. When designing shielding in the near-field area, the shielding box should be shielded by electric field and magnetic field respectively.