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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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SHIELDING EFFECTIVENESS

Jan 20, 2016

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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?. - PowerPoint PPT Presentation
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Page 1: SHIELDING EFFECTIVENESS

SHIELDING EFFECTIVENESS

The THREE KEYS you need to know

to design an effective shield… including EMP protection.

Page 2: SHIELDING EFFECTIVENESS

FOR A SHIELD TO BE EFFECTIVE,

WE MUST BLOCK

BOTH

ELECTRIC AND MAGNETIC

FIELDS …IN ANY COMBINATION

THEY MAY APPEAR.

Page 3: SHIELDING EFFECTIVENESS

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.

Page 4: SHIELDING EFFECTIVENESS

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.

Page 5: SHIELDING EFFECTIVENESS

If we add a positively charged electric field, the electric lines of force pierce the sphere easily. This is 0% shielding effectiveness.

Page 6: 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.

Page 7: SHIELDING EFFECTIVENESS

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.

Page 8: SHIELDING EFFECTIVENESS

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.

Page 9: SHIELDING EFFECTIVENESS

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.

Page 10: SHIELDING EFFECTIVENESS

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.

Page 11: SHIELDING EFFECTIVENESS

Eddy Currents & Skin Depth

8.7 db of magnetic shielding results at one skin depth.

10 skin depths develop 87db of magnetic shielding.

Page 12: SHIELDING EFFECTIVENESS

SUMMARY

Shield Conductivity Shield Magnetic Permeability Shield Thickness

THREE SHIELD MATERIAL FACTORS THAT AFFECT ELECTRIC & MAGNETIC SHIELDING

EFFECTIVENESS

Page 13: SHIELDING EFFECTIVENESS

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?

Page 14: SHIELDING EFFECTIVENESS

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

Page 15: SHIELDING EFFECTIVENESS

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”

Page 16: SHIELDING EFFECTIVENESS

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.

Page 17: SHIELDING EFFECTIVENESS

Let’s 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

Page 18: SHIELDING EFFECTIVENESS

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.

Page 19: SHIELDING EFFECTIVENESS

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.

We will use a Shielding Effectiveness Calculator from Laird Technologies to compute the result.

http://www.lairdtech.com/ad/

Page 20: SHIELDING EFFECTIVENESS

A tiny crack of five one hundredths of an inch has defeated the high frequency effectiveness of our copper foil shield.

Page 21: SHIELDING EFFECTIVENESS

Let’s Design a real EMP Shield.

What ‘MINIMUM’ Shielding Effectiveness does our Military say is needed in MIL-STD-188-125-1?

Page 22: SHIELDING EFFECTIVENESS

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

Page 23: SHIELDING EFFECTIVENESS

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.

Page 24: SHIELDING EFFECTIVENESS

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.

_______________________________

Page 25: SHIELDING EFFECTIVENESS

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!)

Page 26: SHIELDING EFFECTIVENESS

YES! With far better shielding numbers to boot…

Page 27: SHIELDING EFFECTIVENESS

So where can you find an Aluminum

container with ¼ inch thick walls that is solid and airtight?

Page 28: SHIELDING EFFECTIVENESS

Grandma’s All-American EMP Shield

Page 29: SHIELDING EFFECTIVENESS

Q&A

Bruce Cavender, WD8KVQ [email protected]