Assuming that there is a strong external field, when absorption plays a major role, the external field can pass through the shielding layer, but it is greatly attenuated; when reflection plays a major role, the external field is reflected back by the shielding layer.
The absorption method is usually used in a magnetic field below 1000 Hz, using ferromagnetic materials such as steel and a specific magnetic shielding μ metal alloy (nickel-iron high-permeability alloy). At high frequencies, especially when the electric field is more important than the magnetic field, the comparative shielding materials are copper, brass and aluminum.
The hole problem in the shielding box Ideally, the shielding box should have no holes, but it is actually impossible. The input, output, and power supply must pass through the shielding box. Moreover, the circuit generates a lot of heat and requires ventilation and heat dissipation. The size of the hole should be much smaller than the shortest wavelength of the electromagnetic signal to be shielded. For screw holes and other mounting holes, a general rule is that the pitch cannot exceed 1/20 of the shortest wavelength. The screw hole spacing is less than 1/20 of the shortest wavelength. Wide screw holes or installation hole spacing has a great influence on the shielding effect. For a transmitter shielding box that is not determined according to the above principles, the shielding is basically invalid
Another type of shielding box, the bottom shell is completely closed except for the top and the top shell has an RF "finger structure" that penetrates into the bottom shell metal to create a tight RF joint and form at the junction of the two parts Lower impedance.