188.8.131.52 Measured Data.
In contrast to the theoretical shielding effectiveness presented thus far, Table 8-8 and Figures 8-15 and 8-16
present actual measured data. Figure 8-15 illustrates representative shielding effectiveness data taken for a
variety of high-permeability sheet materials. Loop sensors were located 0.3 cm (1/8") from each sheet. The
figure shows the typical leveling off in shielding effectiveness as frequency is decreased, with the breakpoint
occurring in the 1-kHz range. Low frequency magnetic shielding is essentially achieved by establishing a low
reluctance path in which the magnetic field is contained. The variation of shielding effectiveness as a function
of loop sensor separation is shown in Figure 8-16 for one of the materials plotted in the previous figure. A
change in effectiveness of about 5 dB over the range of the test at a particular frequency is indicated.
A difficulty with most magnetic shielding materials is their tendency to change permeability when formed,
machined, subjected to rapid or extreme temperature changes, or dropped. These processes change the
orientation of the magnetic domains in the material, and it is necessary to reorient the domains by annealing to
restore the initial magnetic properties. A typical annealing process involves heating the material to about
2000 F (sometimes in an inert gas environment), holding it at that temperature for approximately two hours,
and letting it slowly cool to room temperature.
The shielding effectiveness in dB for a shield is calculated as the sum of three terms: absorption loss (A),
reflection loss (R), and a correction term (C). The absorption loss is independent of the distance from the EM
source. It depends upon the shield thickness and the shielding material's conductivity and permeability, as well
as upon the frequency of the incident EM wave. However, the reflection loss (like that of a junction of two
types of transmission lines) depends upon the ratio of the EM wave impedance to the shield impedance and is
therefore dependent upon both the EM source type and the distance between the source and shield. It is also
dependent upon the EM source frequency and the shield material's conductivity and permeability but does not
depend upon the thickness of the shield. The multi-reflection correction term is essentially zero for shields
with absorption losses greater than 10 dB; for shields with less absorption loss the correction factor should be
used. It is dependent upon the EM wave impedance classification and the absorption loss, as well as the
frequency, conductivity, and permeability. Table 8-9 summarizes the shielding equations.
Equations, tables, and graphs, have been presented for evaluation of the components of the shielding
effectiveness. The choice of which form to use will be influenced by the time available to the user and the
accuracy to which the data is needed.