S9407-AB-HBK-010 Datasheet, PDF (157/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
7.3 SHIELDING ENCLOSURES
Placing sensitive circuits in metal enclosures protects them from interference caused by
electromagnetic fields. The type, thickness, and electromagnetic properties of the metal used in
enclosure design determine the extent of such protection. Energy leakage through gaps and
discontinuities in the enclosure reduce the amount of desired protection and must be minimized during
design. Magnetic saturation of the shielding material can further reduce protection and must also be a
design consideration. The following paragraphs provide guidance for the design and installation of
effective shielding enclosures.
7.3.1 Magnetic Saturation of Materials
Shielding materials are often spoken of as magnetically hard or magnetically soft, indicating a
low or high permeability characteristic respectively. The magnetizing force required for saturation is
usually much greater for materials possessing low permeabilities. As the applied magnetizing force is
increased beyond saturation, the shielding effectiveness of a material is reduced. This condition is to
be avoided if at all possible.
The flexible shielding conduit described in section 3 and the mumetal enclosures discussed in
this section can be considered magnetically soft. They saturate at a relatively low magnetizing force
that is dependent upon the condition of the material. Rolled sheet and low carbon steel shielding
materials are not quite as soft and possess much lower permeabilities than flexible conduit or
mumetal and therefore require a much greater magnetizing force for saturation.
The magnetizing force, or field intensity, of the interference source to be shielded against is
always a factor to be considered in shielding problems. The magnitude of the interference source
should not exceed published values given for saturation of a particular material.
7.3.2 Shielding Effectiveness of Materials
The absorption loss (or penetration loss as it is sometimes called) is that loss due to the
attenuation experienced by an alternating, tangential magnetic field as it penetrates the shielding
material. Absorption loss is usually the dominant factor in the design of shielding enclosures and, as
such, dictates selection of the type and thickness of the material necessary to provide adequate
shielding effectiveness.
A changing magnetic field tangentially incident on a conducting surface causes current to flow
in the material; the current, in turn, tends to reduce the field at increasing depths within the material. If
the field is alternating, there is never time for the field to become completely established, and the
higher the frequency the more the current will be concentrated near the surface of the material. This
phenomenon is called "skin effect."
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