SHIELDING EFFECTIVENESS

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SHIELDING EFFECTIVENESS
The THREE KEYS you need to know
to design an effective shield including EMP
protection.
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FOR A SHIELD TO BE EFFECTIVE, WE MUST BLOCK BOTH
ELECTRIC AND MAGNETIC FIELDS IN ANY COMBINATION
THEY MAY APPEAR.
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What is an Electric Field?
An Electric Field is a property in space where a
force is generated on a charged particle by
another charge.
If you place another positively charged particle
in the electric field at the left, it will
experience a force that pushes it away from the
first.
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What is Electric Field Shielding Effectiveness?
Imagine a sphere made of non-conductive material
with positive and negative charges locked in an
even distribution around it.
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If we add a positively charged electric field,
the electric lines of force pierce the sphere
easily. This is 0 shielding effectiveness.
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If we increase the materials conductivity a bit,
electrons in the sphere can now migrate under the
force of the electric field. One side becomes
positively charged, the other negatively and the
net electric field inside the sphere begins to be
reduced.
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If we remake the sphere out of sufficiently
conductive material, enough electrons can move to
where the charges balance out and the electric
field inside the sphere goes to zero. This is
100 Shielding Effectiveness.
This effect was discovered by Michael Faraday in
the 1830s. The Faraday effect provides shielding
ONLY for Electric Fields.
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What is a Magnetic Field?
Magnetic Fields are produced by flowing electric
currents that are are either macro in scale like
a current flowing through a wire or microscopic
in scale because of currents associated with
electrons in atomic orbits.
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What is Magnetic Field Shielding Effectiveness?
Magnetic Shielding can be achieved in one of two
ways First, by using a material with
magnetically permeable properties that offer a
path of least resistance to magnetic lines of
force. Magnetic fields flow around the shielded
area for both DC and Alternating Currents.
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Second, magnetic shielding can be achieved in low
permeability materials that have high
conductivity. An alternating magnetic field
induces circular electrical currents, known as
eddy currents (light blue), that tend to cancel
out the incoming magnetic field.
This only works for alternating frequencies. The
degree of magnetic shielding falls off
significantly as frequency drops.
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Eddy Currents Skin Depth
8.7 db of magnetic shielding results at one skin
depth. 10 skin depths develop 87db of magnetic
shielding.
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SUMMARY
THREE SHIELD MATERIAL FACTORS THAT AFFECT
ELECTRIC MAGNETIC SHIELDING EFFECTIVENESS

• Shield Conductivity
• Shield Magnetic Permeability
• Shield Thickness

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What external factors affect Shielding
Effectiveness?

1. Frequency of the incoming signal you want to
   shield from. For example Do you have just a
   single frequency or a spectrum of frequencies?
2. Location of the shield relative to the signal
   source. Example Is the source close enough to
   the shield to require Near Field treatment of
   electric or magnetic fields varying significantly
   or is it in the Far Field where the energy can
   be considered as a flat, Plane Wave that is
   propagating in a constant manner?

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Near and Far Fields
The Far Field line, where electromagnetic
radiation stabilizes into a plane wave, is 0.7
x Wavelength
EXAMPLE Far Field for 2 meters 2 X 0.7 1.4m
or 4.6 feet and farther
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To Simplify
If your shield is farther away from the
electromagnetic source than 0.7 of a wavelength,
then you are working with a stable wave. You can
use any Plane Wave Shielding Effectiveness
Calculator on the Internet to find out just how
good your shield is. (Clemson has an easy one
to use.)
http//www.cvel.clemson.edu/emc/calculators/SE_Cal
culator/index.html Or search for Plane Wave
Shielding Effectiveness Calculator
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What does radio frequency energy do when it hits
a shield?

• It is either
• reflected,
• absorbed or
• transmitted through

Plane Wave Shielding Effectiveness is measured in
deciBels (db) and is the sum of Absorption Losses
plus Reflection Losses.
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Lets run some practical numbers.Use Copper Foil
that is 0.001 thick or 1 mil.
100 kHz
10 MHz
1000 MHz
119 db
109 db
184 db
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Copper Foil Shielding Effectiveness Absorption
Reflection Loss
E-Field (electric) H-Field (magnetic) plots are
used to show near-field reflection losses Plane
Wave Reflection plots show far-field reflection
losses Absorption losses are resistive and not
related to E-field H-field ratios.
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If my shield has a seam for an opening, how does
this gap change Shielding Effectiveness?
Lets use a more sophisticated Internet Shielding
Calculator to compute for openings in the
shield. We will design in a 0.05 inch gap
(5/100ths) in the shield by putting 1000 square
holes across the shield space 0.0001 apart for
our model. Shield material is 1mil copper in the
far field. We will use a Shielding Effectiveness
Calculator from Laird Technologies to compute the
result.
http//www.lairdtech.com/ad/
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A tiny crack of five one hundredths of an inch
has defeated the high frequency effectiveness of
our copper foil shield.
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Lets Design a real EMP Shield. What MINIMUM
Shielding Effectiveness does our Military say is
needed in MIL-STD-188-125-1?
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BUT
No military, foreign or domestic, will give up
its strategy or information about its technology
strengths/weaknesses. So how do we develop an EMP
shield design??? Lets find out what the worst
case is as best we can.
Dec 2012 Infragard EMP Special Interest Group
Conference 250,000 volts/meter EMP Electric
Field Strength
Click Picture for video
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BUT
250,000 volts per meter is only a 5-fold increase
over the 1962 Starfish Prime EMP Test at
50,000v/m. http//www.youtube.com/watch?vKZoic9v
g1fw
Lets be safely conservative and estimate that in
50 years of engineering the improvement might be
200 times more or a worst case electric field of
10,000,000 volts per meter.
If that wild guess at a worst case number would
be acceptable as a design point, how much
Shielding Effectiveness do we need to drop 10
million volts/meter to a safe value of 1
volt/meter inside?
Our Internet db voltage ratio calculator says our
shield must reduce the electric field intensity
by 140 db to protect from a 200 times greater EMP
level than was produced in 1962.
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BUT
Can we afford that GOOD of a shield?
Lets go back to our Clemson Shielding Calculator
and toss out the 1 mil foil and use 50 mil
(0.05) thickness copper plate.
Clearly foil doesnt appear a wise choice, but a
thin copper plate will meet the hurdle of our
wild, high design point at six critical
frequencies for EMP and not break the bank.
_______________________________
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BUT
We still have two problems The thin soft copper
metal isnt very strong structurally and the
closure must essentially be air tight to avoid
the severe EM radiation leak problem that
absolutely kills our Shielding Effectiveness.
Lets go back and use a cheaper metal, but make
it even thicker so it can be structurally rugged.
Lets use Aluminum and raise the thickness to a
¼ wall.
Can the thicker Aluminum perform as well as
thinner Copper?
(Whip out that Internet Calculator!)
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YES! With far better shielding numbers to boot
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So where can you find an Aluminum container with
¼ inch thick walls that is solid and airtight?
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Grandmas All-American EMP Shield
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QA

• Bruce Cavender, WD8KVQ
  blcavender_at_gmail.com