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MINIMIZING THE EFFECT OF OPENINGS
When access panels, doors, ventilation, displays, etc. are required in an otherwise solid enclosure, precautions must be taken to eliminate the transmission of radio frequencies through such openings or slots. The shielding effectiveness of an enclosure is a function of the wavelength of the frequencies involved versus the length of the longest dimension of openings. One common example is the seam opening occurring between the door and the sash of an electronic cabinet. If the mating seams of a shield form a slot, the slot is the analog of a dipole antenna. One-half wavelength apertures make excellent radiators, but shorter lengths radiate also. The longest dimension of an opening is the most critical (slot length, diagonal of a rectangle, diameter of a circle, etc.). Shielding gaskets are very low resistance connectors of mating surfaces which, when installed, make the surfaces appear as one continuous RF absorbing plane. The graph below displays the maximum shielding effectiveness possible from a given size opening in an enclosure.
It can be seen from the information above that the shielding effectiveness increases as the size of the opening is reduced. In order to achieve the same performance as a small opening, the slot must be coupled electrically across its opposing surfaces. This can be accomplished by using conductive fasteners or gaskets. Gaskets are usually the most common choice. They offer a continuous coupling along the full length of the slot while permitting the opposing surfaces to be separated easily for access to the components contained within. See examples in photos.
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