2.2.2 The screening

The screen, to be operative, doesn't leave the entrances to the e.m. wave, how satisfy this condition if the bin must have an access entrance, entrances for the ventilation, the pass for the cables, etc?
The access entrance (cover or little door) when is closed must have some points of good electric contact distanced between them of 1/10 from the maximum length of the downing wave. For the frequencies which pass 500MHz in convenient to appeal to the RF mountings installed a long the side of entrance perimeter in order that to the closed little door are compressed assuring a good electric contact on all the perimeter.
The ventilation entrance is screened putting between this and the ventilator a filter "bees' nest" constituted from lots of holes with the rapport depth/diameter to assure the wave driving.
The input feeding and signal lines is screened involving the opportune filters and/or filtrated connectors.
The screen is choice on the strength of the field from which has to defend.
for the radiates fields (condition in the distant field) the attenuation is principality for reflexion to the low frequencies and for absorption to others frequencies (beyond 100MHz).
To the high frequencies reduce the penetration depth and are sufficient the fines screens to obtain the
wanted hit.
In the condition of the nearby field the electric and magnetic fields are hit like this:

• the electric field, principality for the reflexion to the low frequencies and the absorption to the high frequencies;
• the magnetic field, principality for the absorption to the lower frequencies where, for the increasing the reduction are involved materials to high permeability that convey easier the fluxed lines deviated them from the protected volume.

This materials but already at low frequencies below 100KHz are loosing their properties. Therefore at high frequencies, far for them source, is almost non influential the screening material type as long as metallic and to a high conductibility.
To the low frequencies, nearby the source, is needed to select the material more suitable after that from reducing the electric field, that magnetic or both of them.