Transcript Electric Field

SHIELDING EFFECTIVENESS
The THREE KEYS
you need to know
to design an
effective shield…
including EMP
protection.
FOR A SHIELD TO
BE EFFECTIVE,
WE MUST BLOCK
BOTH
ELECTRIC AND
MAGNETIC
FIELDS …IN ANY
COMBINATION
THEY MAY
APPEAR.
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.
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.
If we add a positively charged electric field,
the electric lines of force pierce the sphere
easily. This is 0% shielding effectiveness.
If we increase the material’s 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.
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.
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.
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.
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.
Eddy Currents & Skin Depth
8.7 db of magnetic shielding results at one skin depth.
10 skin depths develop 87db of magnetic shielding.
SUMMARY
THREE SHIELD MATERIAL FACTORS THAT
AFFECT ELECTRIC & MAGNETIC
SHIELDING EFFECTIVENESS
Shield Conductivity
 Shield Magnetic Permeability
 Shield Thickness

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?
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
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_Calculator/index.html
Or search for
“Plane Wave Shielding Effectiveness Calculator”
What does radio frequency
energy do when it hits a shield?
It is either
(1) reflected,
(2) absorbed or
(3) transmitted through
Plane Wave Shielding Effectiveness is
measured in deciBels (db) and is the sum of
Absorption Losses plus Reflection Losses.
Let’s run some practical numbers.
Use Copper Foil that is 0.001” thick or 1 mil.
100 kHz
119 db
10 MHz
109 db
1000 MHz
184 db
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.
If my shield has a seam for an
opening, how does this gap change
Shielding Effectiveness?
Let’s 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.
http://www.lairdtech.com/ad/
We will use a Shielding Effectiveness Calculator
from Laird Technologies to compute the result.
A tiny crack of five one hundredths of an inch has defeated
the high frequency effectiveness of our copper foil shield.
Let’s Design a real EMP Shield.
What ‘MINIMUM’ Shielding Effectiveness does our
Military say is needed in MIL-STD-188-125-1?
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???
Let’s 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
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?v=KZoic9vg1fw
Let’s 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.
BUT…
Can we afford that GOOD of a shield?
Let’s 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
doesn’t 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.
BUT…
We still have two problems:
The thin soft copper metal isn’t 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.
Let’s go back and use a cheaper metal,
but make it even thicker so it can be
structurally rugged. Let’s 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!)
YES!
With far better shielding numbers to boot…
So where can you find
an Aluminum
container with ¼ inch
thick walls that is
solid and airtight?
Grandma’s All-American
EMP Shield
Q&A
Bruce Cavender, WD8KVQ
[email protected]