The same shield has different shielding properties for electromagnetic waves of different properties. Therefore, when considering the 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 impedances.

The wave impedance ZW of the electromagnetic wave is defined as: the ratio of the electric field component E and the magnetic field component H in the electromagnetic wave: ZW = E / H

The 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 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 a large current and low voltage (the impedance of the radiation source is low), the wave impedance of the generated electromagnetic wave is less than 377, which is called a 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 generated electromagnetic wave is greater than 377, which is called an electric field wave.

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

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