Custom Search
|
|
|
||
 MIL-HDBK-419A
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 8-1 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 8-1 evaluate Equation 8-5 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 milli-inches (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 8-2. Also included is a listing of the relative
permeability, as a function of frequency, for iron. Figure 8-3 presents the data of Table 8-2 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 8-5), tabular form (Tables 8-1 and 8-2), and graphical form (Figure 8-3). The tabular and graphical
forms are easy-to-use 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 8-5.
8.3.2 Reflection Loss.
According to Equation 8-3, the reflection loss portion, R, of the shielding effectiveness, SE, is given by:
|
 
|
|
|
||
