S9407-AB-HBK-010 Datasheet, PDF (158/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
The depth within the conducting material, where current is reduced by a factor of
e^-1, or by approximately 37 percent of its surface value, is referred to as one "skin depth," and is
determined from
δ = ¹ meters,
πfµ_mσ _m
(1)
where in mks units:
f = frequency in hertz
µm = permeability of material in henries/meter
sm = conductivity of material in siemens/meter.
The absorption loss in dB for one "skin depth" is
A (dB) = 20 log₁₀e¹= 8.686 dB .
(2)
For a shield of any thickness this becomes
A (dB) = 20 log₁₀e^t/δ= 8.686 t/δ dB ,
(3)
where t = thickness of material in the same units as d.
For calculations where relative values of conductivity and permeability are used, equation (3)
can be equated to
A (dB) = 3.34 t fµ_rσ_r ,
(4)
where relative values are:
µ_r
=
µ_m
µ_o
=
permeability
of
material
relative
to
that
of
(µo = 1.26 x 10-⁶ henries/meter)
σ_r
=
σ_m
σ _cu
=
conductivity
of
material
relative
to
that
(^σcu ⁼ 5.8 x 10⁷ siemens/meter).
Figure 7-1 is a plot of skin depth (equation 1) versus frequency for typical shielding materials
and for the ratio of t/d equal to 1 (where d is given in inches). Although the permeability of ferrous
materials is inversely related to frequency, it is plotted as a constant value in figure 7-1. As the
number of "skin depths" increases with frequency, adequate shielding can usually be achieved despite
this decrease in material permeability. If, however, the permeability of the material at the frequency of
the interference is known, the absorption loss can be calculated using equation (3) or (4).
CHANGE 1 of Revision 2
7-3