SAMPE Journal - January/February 2022 - 19

of two mediums. Impedance is the ratio of electric
field to magnetic field (E/H) of a wave propagating
in a medium. Impedance of a medium is a function
of its frequency, permeability, and electrical
conductivity. In general, the impedance of metal
is much lower than the free space impedance.
This is the reason for metals being good reflectors
of radio waves at the lower end of microwave
frequencies. Also, reflection loss is independent of
material thickness. This gives enormous potential
for thin film coated fibers to be used in EM
shielding. Absorption losses follow from electric
and magnetic dipoles of the material interacting
with the incident EM wave. Multiple reflection are
secondary reflections inside the material where
there is an impedance mismatch. It is relevant only
when the shield thickness is lower than the skin
depth3
.
It is now clear that the shielding mechanisms
change according to the frequency and electrical
properties of the medium.
Therefore, the
characterization of an EM shield material is
generally done for a specific frequency range.
Apertures
The equations given in the Table 1 quantifies the
SE for a solid sheet. However, in the case of woven
fabric there are small gaps at the interface of two
orthogonal yarns. These gaps are called apertures.
If the apertures are large enough, they act as a
source of EM leakage and reduce the overall SE of
a material. At higher frequencies, the intrinsic SE
of the material is of less concern than the leakage
through apertures. Faraday's law of induction can
be used to understand this phenomenon. When a
shield is placed in front of incident EM wave, the
magnetic field of EM wave causes an electric field
on the surface of the conductor. This electric field
produces an additional field which cancels the
original incoming EM field. For the cancellation of
EMF to happen, the current must flow freely on the
surface. Any obstruction to the flow of current, like
in the case of apertures, would lead to insufficient
cancellation causing leakage6
. Figure 4 shows the
2D representation of woven fabric with apertures.
Table 3 shows the equation for determination
of SE for woven fabric with multiple apertures.
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Table 3. Shielding effectiveness for apertures5
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.
JANUARY/FEBRUARY 2022
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SAMPE JOURNAL |
19
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SAMPE Journal - January/February 2022

Table of Contents for the Digital Edition of SAMPE Journal - January/February 2022
