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SHIELDING EFFECTIVENESS

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?.

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SHIELDING EFFECTIVENESS

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  1. SHIELDING EFFECTIVENESS The THREE KEYS you need to know to design an effective shield… including EMP protection.

  2. FOR A SHIELD TO BE EFFECTIVE, WE MUST BLOCK BOTH ELECTRIC AND MAGNETIC FIELDS …IN ANY COMBINATION THEY MAY APPEAR.

  3. 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.

  4. 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.

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

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. Eddy Currents & Skin Depth 8.7 db of magnetic shielding results at one skin depth. 10 skin depths develop 87db of magnetic shielding.

  12. SUMMARY • Shield Conductivity • Shield Magnetic Permeability • Shield Thickness THREE SHIELD MATERIAL FACTORS THAT AFFECT ELECTRIC & MAGNETIC SHIELDING EFFECTIVENESS

  13. What external factors affect Shielding Effectiveness? • Frequency of the incoming signal you want to shield from. For example: Do you have just a single frequency or a spectrum of frequencies? • 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?

  14. 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

  15. 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”

  16. 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.

  17. 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

  18. 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.

  19. 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/

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

  21. Let’s Design a real EMP Shield.What ‘MINIMUM’ Shielding Effectiveness does our Military say is needed in MIL-STD-188-125-1?

  22. 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

  23. 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.

  24. 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. _______________________________

  25. 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!)

  26. YES! With far better shielding numbers to boot…

  27. So where can you find an Aluminum container with ¼ inch thick walls that is solid and airtight?

  28. Grandma’s All-American EMP Shield

  29. Q&A Bruce Cavender, WD8KVQ blcavender@gmail.com

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