The same shield has different shielding properties for different electromagnetic waves. Therefore, when considering electromagnetic shielding performance, it is necessary to have a basic understanding of the types of electromagnetic waves. There are many ways to classify electromagnetic waves, but when designing shielding, electromagnetic waves are divided into electric field waves, magnetic field waves, and plane waves according to their wave impedance.

The wave impedance ZW of an electromagnetic wave 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 wave impedance of the electromagnetic wave is related to the nature of the radiation source of the electromagnetic wave, the distance from the observation point to the radiation source and the propagation medium in which the electromagnetic wave is located.

When the distance from the radiation source is relatively close, the wave impedance depends on the radiation source characteristics. 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 the magnetic field wave. If the radiation source is high voltage and small current (the impedance of the radiation source is high), the wave impedance of the electromagnetic wave generated is greater than 377, which is called the electric field wave.

When the distance from the radiation source is far, the wave impedance is only related to the electric field wave propagating medium, and its value is equal to the characteristic impedance of the medium, and the air is 377Ω.

The wave impedance of the electric wave decreases with the increase of the propagation distance, and the wave impedance of the magnetic wave increases with the increase of the propagation distance.