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MILHDBK419A
where
131.4 if
is expressed in meters,
3.34 if
is expressed in inches,
shield thickness,
wave frequency, Hz,
relative permeability of shield material, and
conductivity of shield material relative to copper.
Note that the absorption loss (in decibels) is proportional to the thickness of the shield and also that it increases
with the square root of the frequency of the EM wave to be shielded against. As to the selection of the
shielding material, the absorption loss is seen to increase with the square root of the product of the relative
permeability and conductivity (relative to copper) of the shield material.
Table 81 contains a tabulation of electrical properties of shielding materials
is frequency
dependent for magnetic materials, it is given for a typical shielding frequency of 150 kHz. The last two
columns of Table 81 evaluate Equation 85 to give the absorption loss at 150 kHz for both a one millimeter and
a one mil (0.001 inch) thick sheet for each of the listed materials. The absorption loss for other thickness can
be calculated by simply multiplying by the shield thickness in millimeters or mils. Shield thicknesses are
commonly expressed in either millimeters (mm) or milliinches (mils); these two units are related as follows:
1 mm = 39.37 mils or 1 mil = 0.0254 mm
The variation of absorption loss with frequency, as well as a comparison of the absorption loss of three common
shielding materials one mm thick, can be seen in Table 82. Also included is a listing of the relative
permeability, as a function of frequency, for iron. Figure 83 presents the data of Table 82 in graphical form.
Remember that the absorption loss is just one of three additive terms which combine to give the attenuation
(shielding efficiency) of the shield. At this point, the absorption loss has been presented in equation form
(Equation 85), tabular form (Tables 81 and 82), and graphical form (Figure 83). The tabular and graphical
forms are easytouse sources of accurate results when the shield material and frequency of interest are
included in those tables and graphs. Quick results for almost any material and frequency combination can be
obtained from an absorption nomograph (see Vol II), but the results are generally less precise; nomographs are a
good source of data for initial design purposes. Once a shielding material and thickness are tentatively
selected, one may wish to compute a more precise value of the absorption loss by evaluation of Equation 85.
8.3.2 Reflection Loss.
According to Equation 83, the reflection loss portion, R, of the shielding effectiveness, SE, is given by:


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