A Handbook on Electromagnetic Shielding Materials and Performance

Table of contents :
Page No.
ACKNOWLEDGEMENT iid
OTHER BOOKS BY THE AUTHOR i1d
PREFACE iv
TABLE OF CONTENTS vi
LIST OF TABLES viii
LIST OF ILLUSTRATIONS ix
LIST OF SYMBOLS AND ABBREVIATIONS Xiii

CHAPTER 1 SHIELDING THEORY
1.1 FIELD THEQORY 1.1

1.2 WAVE IMPEDANCE 1.5

1.3 METAL IMPEDANCE 1.8
1.3.1 Barrier Impedance of Metals (t >> §) 1.9
1.3.2 Barrier Impedance of Metals (t < 3¢) 1.11

1.4 SHIELDING EFFECTIVENESS 1.14
1.4.1 Absorption Loss 1.19
1.4.2 Reflection Loss 1.19
1.4.3 Re-Reflection Correction 1.29
1.4.4 Total Losses for K »> 1) 1.29
1.4.5 Low-Frequency Magnetic Shielding Effectiveness 1.32
1.4.6 Performance Degradation 1.35

CHAPTER 2 SHIELDING MATERIALS AND TESTING
2.1 SHIELDING MATERIALS 2.1
2.1.1 Homogeneous Metals 2.1
2.1.2 Pseudo-Homogeneous Metals 2.10
2.1.3 Small-Aperture Metals 2.19
2.1.4 Shielded Optical Display Windows 2.24
2.2 SHIELDING DENSITY FOR WEIGHT-SENSITIVE APPLICATIONS 2.28
2.3 MIL-STD-285 2.34

CHAPTER 3 APPLICATIONS AND EXAMPLES
3.1 HOW TO USE THE DESIGN GRAPHS 3.1
3.2 ILLUSTRATIVE EXAMPLES
3.3 HP-65 PROGRAM FOR SHIELDING EFFECTIVENESS
3.3.1 User Program Instructions
3.3.2 I1lustrative Examples

CHAPTER4 REFERENCES
APPENDICES
APPENDIX A COPPER A T-A6
APPENDIX B MONEL B.1-B.6
APPENDIX C NICKEL C.1-C.6

APPENDIX D IRON D.1-D.6
APPENDIX E HYPERNICK E.1-E.6
APPENDIX F 78 Permalloy F.1-F.6
APPENDIX G HIGH PERMEABILITY G.1-G.6

INDEX

Citation preview

A HANDBOOK ON

ELECTROMAGNETIC SHIELDING MATERIALS AND PERFORMANCE By

Donald

R.

J.

White,

MSEE/PE

DON

WHITE CONSULTANTS, INC, State Route 625 P.0. Box D Gainesville, Virginia 22065 Phone: 703-347-0030 TLX: 89-9165 DWCI GAIV

(:)Copyright Second

1980

Edition

All rights reserved. This book, or any thereof, may not be reproduced in any form the written permission of the publisher. Library Printed

of

Congress in

the

Catalog

United

Card

States

of

No.

parts without

75-16592

America

ACKNOWLEDGEMENT The author wishes to thank the many people who encouraged him to He expresses his appreciation to write this handbook on shielding. the individuals and companies who have furnished several of the illustrative figures, for which acknowledgements in this handbook have been made.

The author expresses his appreciation to his wife Colleen and Muriel M. Moeller for their assistance in typing, and in the many facets of logistics involved in preparation of the manuscript; to Luis F. Longoria III and Jane Backstrom for their drafting, HP-65 calculator computations, and the many others who have helped produce this publication.

ii

OTHER BOOKS PUBLISHED BY DWCI Design and Applications; Electrical Filters-Synthestis, (1) Reprinted December Inc. 1963, by White Electromagnetics,

published 1970. published

Electrical

Volume

2,

Electromagnetic

Interference

Volume

3,

Electromagnetic

Compatibility

Volume

4,

Electromagnetic

Interference

Test

5, Electromagnetic 1972.

Interference

Prediction

(3)

and

(4)

published

Techniques;

(5)

and Systems; Techniques;

Specifications;

EMI

1,

and Procedures; Methods

and

Noise

Volume (2) 1971.

(6)

published

1974.

published

1973,

1971.

Volume published

Standards,

Volume 6, Electromagnetic Interference (7) and Regulations; published 1975.

published

1971.

Frequency

Interference

Inc.

(8)

A Glossary

(9)

Mertel,

Encyclopedia

Series;

II

Noise,

(11) Volume

1977.

published

1979.

Volume

of

V

and

the

Control

Instruments

Specifications,

and Symbols; National

Radio

EMC

Multi-Volume

1978. Spectrum Management Techniques, Encyclopedia Series; published

M., EMC

Hart, William C. and Malone, Edward W., Lightning anda

Protection,

1979.

published

Series;

published

I of

Volume

Methods

Herman, John R., Electromagnetic Ambients and Man-Made III of the Multi-Volume EMC Encyclopedia Series;

(12)

Lightning

International

K.,

Herbert

Regulations,

Jansky, Donald (10) of the Multi-Volume

Volume

Abbreviations,

of Acronyms,

Test

Keiser, Bernhard Multi-Volume EMC

(13) the

EMC

the Multi-Volume

IV of

Volume

Encyclopedi

E., EMI Control in Aerospace Systems, Encyclopedia Series; published 1979

Feher, Kamilo, Digital Modulation Techniques in an (14) Interference Environment, Volume IX of the Multi-Volume EMC EncySeries;

in Medical Series;

Gard,

Electronics,

1979.

published

Procedures

Ships,

(15)

published

White, (16) (EMC Design (17)

Volume

published

1980.

Michael

Volume

of

F.,

Electromagnetic

X of

Control

Interference

the Multi-Volume

EMC

Encyclopedia

Donald R. J., EMI Control Methodology Synthesis); published 1978.

and

in Boats

and

Carstensen,

XXIV

1977.

the

Russell

V.,

Multi-Volume

[ 5 e e

clopedia

EMI Control

EMC

Encyclopedia

Series;

PREFACE There

exists

substantial

material

in

the

literature

on

the

subject

For Chap. 4 presents many references. of electromagnetic shielding. either an individual who has only recently been introduced to shielding or to a design engineer, however, much of the literature appears to be Missing in the either confusing or poorly organized for design use. all the princiincluding graphs design useful of series a are literature Thus, this mamner. understandable clear, a in presented variables pal handbook on Shielding was conceived to fill these voids.

This

handbook

does

not

cover

the

topics

of where

and when

to

shield,

These topics are covered in Vol. 3 of the and where to ground a shield. Rather, this handbook explains shielding theory EMC Handbook Series. and performance and presents many design graphs of shielding effective~ ness vs frequency as a function of shield metal and its characteristics, and E and H-fields and plane waves.

Regarding the impedance of the fields (E, H, or plane waves), tie For exliterature and manufacturers' data are often very misleading. ample, since the wave and circuit impedance which produced the field .are interlocked and since a circuit impedance is not infinite, E-field shielding effectiveness data are generally optimistic (too high) relaIn a converse manner, H-field shielding tive to actual performance. effectiveness data are pessimistic (too low) since a magnetic source This handbook clarifies and quantifies circuit impedance is not zero. these points, Another example of possibly misleading information is the use of MIL-STD-285 to measure and report the shielding effectiveness of test The reference test distance per MIL-STD-285 items to E and H-fields. Thus, for installations located in the is one foot (0.305 meters). near field which are greater than one foot from an interfering source, actual E-field shielding performance will be less and H-field performance will be greater than that reported by MIL-STD-285 measurements. The converse applies for application distances between sources and metal barriers which are less than one foot away as illustrated in this handbook.

The discussions and design data on shielding effectiveness in this In fact no real handbook are not restricted to homogeneous metals. room is homogenor cabinet, box, life and useful shielded compartment, configuration shield six-sided a of penetrations many eous since usually a shielded of integrity the reinstate to used Techniques necessary. are Shielding enclosure are discussed in Vol, 3 of the EMC Handbook Series. materials and performance of non-homogeneous metals are discussed in Some examples are pseudo-homogeneous shields this handbook on Shielding. Shields made of made from metal deposition and flame-spray processes. Examples include screens, small-aperture metals are also presented.

iv

PREFACE wire meshes, cable braids discussed herein together

and metalized textiles, with design data.

all

of

which

are

The appendices of this handbook are perhaps the most important of all material presented. They contain 42 pages of design shielding effectiveness graphs for several metals whose thicknesses range from 0.0001 mil (2.54 pm) to 1 inch (2.54 cm). For both near and farfield calculations and associated frequencies, the design graphs cover source-to-metal distances ranging from 10 cm to 10 km. Frequency coverage is from 10 Hz to 30 GHz. All data were run-off on the HP-65 programmable calculator. For those who have an HP-65, the program is presented so that they can develop and use their own magnetic card. There also exist many design graphs other than direct shielding effectiveness which the reader should find useful. The author of this handbook invites the user to communicate him. He especially invites comments, questions, or requests for further elucidation. December 1975 Germantown, Maryland

January 1980 Gainesville,

Donald

R.

White

USA

Second Virginia

J.

with

USA

Edition

TABLE OF CONTENTS ELECTROMAGNETIC SHIELDING MATERIALS AND PERFORMANCE Page

ACKNOWLEDGEMENT OTHER BOOKS BY THE AUTHOR PREFACE TABLE OF CONTENTS LIST OF TABLES LIST OF ILLUSTRATIONS LIST OF SYMBOLS AND ABBREVIATIONS

CHAPTER 1

SHIELDING THEORY

1.1

FIELD THEQORY

1.2

WAVE

1.3

METAL 1.3.1 1.3.2

1.4

SHIELDING 1.4.1 1.4.2 1.4.3

1.4.4

1.4.5 1.4.6

CHAPTER 2 2.1

2.1.3 2.1.4

1.1 1.5

IMPEDANCE

1.8

IMPEDANCE Barrier Barrier

Impedance Impedance

of Metals of Metals

(t >> §) (t < 3¢)

1.19 1.19 1.29

Absorption Loss Reflection Loss Re-Reflection Correction

Total

Losses

for

1.9 1.11 1.14

EFFECTIVENESS

K »> 1)

Low-Frequency Magnetic Shielding Performance Degradation

Effectiveness

1.29

1.32 1.35

SHIELDING MATERIALS AND TESTING 2.1

MATERIALS

SHIELDING 2.1.1 2.1.2

No.

iid i1d iv vi viii ix Xiii

Homogeneous Metals Pseudo-Homogeneous

Small-Aperture Metals Shielded Optical Display

2.2

SHIELDING

2.3

MIL-STD-285

DENSITY

2.1 2.10

Metals

FOR WEIGHT-SENSITIVE

APPLICATIONS AND EXAMPLES CHAPTER 3 3.1 HOW TO USE THE DESIGN GRAPHS

2.19 2.24

Windows APPLICATIONS

2.28 2.34

3.1

TaBLE oF CONTENTS 3.2 3.3

ILLUSTRATIVE HP-65 3.3.1 3.3.2

EXAMPLES

PROGRAM

FOR SHIELDING

User Program I1lustrative

EFFECTIVENESS

Instructions Examples

CHAPTER4

REFERENCES

APPENDICES APPENDIX A

COPPER

A T-A6

APPENDIX

B

MONEL

B.1-B.6

APPENDIX

C

NICKEL

C.1-C.6

APPENDIX

D

IRON

D.1-D.6

APPENDIX

E

HYPERNICK

E.1-E.6

APPENDIX

F

78

F.1-F.6

APPENDIX

G

HIGH

Permalloy

PERMEABILITY

G.1-G.6

INDEX

vii

LIST OF TABLES Page

CHAPTER2 2.1

SHIELDING MATERIALS AND TESTING Relative Metals

Conductivity

and

Permeability

of

2.2

Weight per Unit Area Some Metals

2.3

Applicable

2.4

Relative Thickness and Weights of Some Metals for Yielding the Same Shielding

2.19

Line

per Unit Thickness

Selection

for

Use

in

Fig.

Effectiveness

CHAPTER 3

APPLICATIONS AND EXAMPLES

3.1

Definition of Permeability to Copper

3.2

Metal Use

3.3

Applicable Specified

3.4

HP-65 Shielding Steps

Class

Metal Class Based on and Conductivity Relative

for

Choice

Appendix Distance

of

Design

Appendix Graph

Effectiveness

viii

to for

Program

of

No.

LIST OF ILLUSTRATIONS

Fig.

3 4 5

Electric-Field Strength vs. Source Distance Conceptual Illustration of Field Strengths vs, Source Type and Distance Wave Impedance as a Function of Source Distance Wave Impedance for Saveral Circuit Impedances Surface Impedance and Skin Deptl: of Various Metals vs. Frequency Barrier Metal Impedance Error in Zp Expression by

Assuming

.10 11 12 13 .14 .15 .16 17 .18 .19 .20

CHAPTER 2

Page

Title

No.

1 .1 1 .2 —_—

SHIELDING THEORY

t/s

Surface Impedance of Copper and Iron vs. Freguency and Skin Depth in Units of t/é Ratios Representation of Shielding Phenomena for Plane Waves Geometry of Metal Barrier Used in Explaining Shielding Effectiveness Absorption Loss vs. Freguency and Thickness for Copper Absorption Loss vs. Frequency and Thickness for Aluminum Absorption Loss vs. Frequency and Thickness for Brass Absorption Loss vs. Frequency and Thickness for Beryllium Absorption Loss vs. Frequency and Thickness for Monel

Absorption

vs.

Loss

and Thickness

Frequency

for

Iron Absorption Loss vs. Frequency and Thickness for Stainless Steel Absorption Loss vs. Frequency and Thickness for High-Permeable Metals Re-Reflection Correction vs. VSWR and Material Absorption Loss Shielding Effectiveness vs. Metal-to-Emission Distance and Surface Resistances Low Frequency, Shielding Effectiveness to Magnetic Fields

SHIELDING MATERIALS AND TESTING

2.1

Magnetization

Curve

2.2

Permeability

Curves

Some

H ard

Important B.

I and

(Solid)

Magnetic B-H

and Hysteresis

Quantities

of Iron, with are

also ix

used

are

u Plotted as

Loop

ITlustrated

Against

Abscissae

No.

s

CHAPTER 1

.12 13 .16 .16 .20 .21 .22 .23 .24 .25 .26 .27 .30 .33 .34

2.16 2.17 2.18 2.19 2.20

APPENDIX A Al

A.2 A.3 A4 A5 )

APPENDIX B B.1

B.2 B.3 B.4

Both

of Plastic

EMI

and

Static

COPPER

Shielding Effectiveness of Copper Source~to-Metal Distance Shielding Effectiveness of Copper Source-to-Metal Distance of Im Shielding Effectiveness of Copper

Distance

of 10m

Shielding Effectiveness of Copper Source-to-Metal Distance of 100m Shielding Effectiveness of Copper Source~to-Metal Distance of lkm Shielding Effectiveness of Copper Source-to-Metal Distance of 10km

MONEL

NN NN [a]

13 .15

Equipment Bleed

Shielding Effectiveness of Source-~-to-Metal Distance Shielding Effectiveness of Source~to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance Shielding Effectiveness of

Monel of 10cm Monel of Im Monel of 10m Monel

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

Mo N

Flame Spray Wire Metallizing Gun Thermo Spray, Metal Powder Metallizing Gun Plasma Flame Spray Metallizing Gun Shielding Effectiveness of Screen Wire to Plane Waves Light Transmission of Conductive Glass Shielding Effectiveness of Gold vs. Frequency for Source-to-Shield Distance of 1m Shielding Effectiveness of Gold vs. Freguency for Source-to-Shield Distance of lkm Aluminum Thickness and Weight vs. Frequency Correction in Shielding Effectiveness to Convert MIL-STD-285 Results to Another Distance

Source-to-Metal

no

for

Surface

~n

Interior

vs.

M

Coatings

on

Copper

vs.

Freguency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

.15 17 17

™

Functions

Coating

to

.21 2.25

N

2.11 2.12 2.13 2.14

Enclosure Conductive

Thicknesses

Relative

.26 NN

2.10

Conductive

Metal

.27 .31 .37

I

2.9

for Various

Conductivity Resistances

I

2.8

X

2.4 2.5 2.6 2.7

Minor Hysteresis Loops Shown on Magnetization Curve Portrayal of Real World Situation Relative Permeabiiity vs. Frequency Relative Permeability vs. Magnetic-Flux Density Surface Resistance of Copper vs. Volume Resistivity

=

2.3

o~

LisT oF [LLUSTRATIONS

C.1

c.2 C.3 C.4

C.5 C.6

APPENDIX D D.1 D.2 D.3 D.4 D.5 D.6

APPENDIX E E.T

E.2 E.3 E.4 E.5 E.6

NICKEL

Shielding Effectiveness of Nickel Source-to-Metal Distance of 10cm

Shielding

Effectiveness

of Nickel

Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance

Shielding

Effectivenass

of Im Nickel of 10m Nickel of 100m

of Nickel

Source-to-Metal Distance of Tkm Shielding Effectiveness of Nickel Source-to-Metal Distance of 10km

IRON

Shielding Effectiveness of Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance Shielding Effedtiveness of Source-to-Metal Distance

HYPERNICK

for

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

vs.

Frequency

for

Iron vs. of 10cm Iron vs. of Im Iron vs. of 10m Iron vs. of 100m Iron vs. of lkm Iron vs. of 10km

Frequency

for

Frequency

for

Frequency

for

Frequency

for

Frequency

for

Frequency

for

Shielding Effectiveness of Hypernick for Source-to-Metal Distance of 10cm Shielding Effectiveness of Hypernick for Source-to-Metal Distance of Im Shielding Effectiveness of Hypernick for Source-to-Metal Distance of 10m Shielding Effectiveness of Hypernick for Source-to-Metal Distance of 100m Shielding Effectiveness of Hypernick for Source-to-Metal Distance of Tkm Shielding Effectiveness of Hypernick for Source-to-Metal Distance of 10km

xi

Freguency

vs.

Freguency

vs.

Frequency

vs.

Frequency

vs.

Frequency

vs.

Freguency

vs.

Frequency

oo

APPENDIX C

of 100m Monel vs. of lkm Monel vs. of 10km

W

B.6

Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance Shielding Effectiveness of Source-to-Metal Distance

4w ™

B.5

N

L1sT OF ILLUSTRATIONS

LisT OF [LLUSTRATIONS

F.4 F.5 F.6

APPENDIX G G.1

G.2 G.3 G.4 G.5 G.6

HIGH PERVEABILITY

Shielding Frequency Shielding Frequency Shielding Frequency Shielding Frequency Shielding Frequency Shielding Frequency

Effectiveness of High for Source-to-Metal Effectiveness of High for Source-to-Metal Effectiveness of High for Source-to-Metal Effectiveness of High for Source-to-Metal Effectiveness of High for Source-to-Metal Effectiveness of High for Source-to-Metal

xii

vs. 10cm vs. 1Im vs. 10m vs. 100m vs. 1km vs. 10km

FreFreFreFreFreFre-

Permeability vs. Distance of 10cm Permeability vs. Distance of Tm Permeability vs. Distance of 10m Permeability vs. Distance of 100m Permeability vs. Distance of Tkm Permeability vs. Distance of 10km

o

F.3

78 Permalloy Distance of 78 Permalloy Distance of 78 Permalloy Distance of 78 Permalloy Distance of 78 Permalloy Distance of 78 Permalloy Distance of

O

F.2

Shielding Effectiveness of quency for Source-to-Metal Shielding Effectiveness of quency for Source-to-Metal Shielding Effectiveness of quency for Source-to-Metal Shielding Effectiveness of quency for Source-to-Metal Shielding Effectiveness of guency for Source-to-Metal Shielding Effectiveness of quency for Source-to-Metal

o

F.

78 PERVALLOY

@

APPENDIX F

LIST OF SYMBOLS AND ABBREVIATIONS dB dB

absorption

loss

re-reflection velocity

of

cm

centimeter

Cu

copper

dB

decibel

Napierian

dB

loss

in

0.01

0.1

Bel

base

=

electric-field frequency

dB

electromagnetic =

=

in

meter =

=

10

wave

in

0.3937

air

=

1//ue=

3x108m/sec.

inches

loglo(power

ratio)

2.718

strength

in volts/meter

in Hertz

iron frequency

in MHz

magnetic-field

current

in

imaginary A/2nr

strength

=

=

E fields;

wave-to-metal meter

amperes/meter

amperes operator

for

in

100

angle

m/2

=

90

2nr/i

for

H

fields;

impedance cm =

1000

ratio, mm

=

degrees

Zw/Zm

39.37

=

= VSWR

inches

1

for

=

3.28

39.37

mils

plane

for

K21 feet

0.001 inch = 2.54x10 >cm = 25.4 um millimeter

=

nanometer

=

0.1

cm

10_9m

=

distance

from

distance

r

shielding

time

0.001

10-6mm

meter =

emission

=

10~3um

source

to

=

39.37x10_6 metal

barrier

loss

in

dB

(loss) (excludes

in

dB

re-reflection

thickness in

ratio

seconds

of

metal-thickness

voltage

in

voltage

standing

impedance

mils

in meters

effectiveness

reflection

metal

EMI

=

to

skin-depth

volts in

wave

ratio

ohms

barrier

metal

impedance,

circuit

impedance

in

Zm

ohms

xiii

for

any

t/$§

ratio

loss)

waves

LisT oF SyMBOLS AND ABBREVIATIONS

/fi;7€;

=

=

a+jB

NN

=

plane-wave

N

1

E/H

R

>»

attenuation

W

t/§

phase

X

for

propagation

1

metal

voltage

3

of

transmission

coefficient

from

air

3

impedance

transmission

coefficient

from

metal

impedance

377

=

1207

ohms

constant

constant

o

skin

™

or

permitivity

m

ma

= wave

impedance

absolute

wave

depth

in

1§

constant

transmission

medium

=

permitivity =

permeability

absolute

air

to

air

medium

permeability relative

micrometer

=

voltage

wave

or

10—6m

=

=

of

air

= 47x

to

air

(or

10—3mm

reflection

=

to

coefficient

from

metal

of

medium

conductivity

relative

frequency

in

in to

henrys/m

mils

coefficient

reflection conductivity

lO—7

0.3937

air

surface

farads/m

copper)

from

in

interface

pour

coefficient

impedance

air

to

= 1/367%x10°

reflection

radial

interface

c/f of

permeability

metal

€8,

of

relative

wavelength

to

metal

of

permitivity

coefficient

mhos

metal

interface

air

interface

to

per

unit

distance

copper

radians/sec

= 2nf

ohms

impedance

(or

resistance)

xiv

in

ohms/square

(HAPTER 1 SHIELDING THEORY Eighty per cent of this handbook is design data of shielding effectiveness vs, several variables including metal type, metal properties, thickness, distance, frequency, etc. Most of the design and applications data are presented in the appendices. Basics and fundamentals of shielding are presented in Chaps. 1 and 2. Chap. 3 covers information on how to use the design graphs in the appendices including constraints and illustrative examples. This chapter, Chap. 1, presents tutorial information on shielding theory and materials. This includes field theory, near and far-field definitions, wave impedance, metal barrier impedance, absorption loss, reflection loss, and overall shielding effectiveness.

1,1

FIELD THEORY

The purpose of this section is to present some relations about magnetic, electric, and electromagnetic fields as pertinent background to understanding and applying field theory. Since the literature is replete with discussions of Maxwell's equations and field theory, only a few aspects are presented here. The

oscillating from

E

electric

applying

8

doublet

(tiny

Maxwell's

ZoID7T sind _—__;E—___ 2ZoIDT

(Eg,

A

A

Er = T[(z‘?) H¢

where,

2 IDr sing|{_ [(2“1) = —-—)\2

and

dipole

equations:

ol

cos6

E,)

\3

)3

magnetic

(Hy)

in which

cosy

A Tnr

-

V2

fields

its

length

. siny

A + Tur

existing

(D

(1.10) :

Z521T

ZOZWr

Fig,

1

—3

(1.11)

> z, 1.4

impedances

(1.12) for

of

several

50,

To the extent that these conditions exist, the ohms. ion line impedances, then, never permit either a very wave impedance condition to exist when r >we and t>>8%

Q/sq.,

air

for frequency

(Eq.

(1.22))

is

in MHz

a purely

stant, whereas the intrinsic impedance of a metal resistive and inductive component. Consequently, the permeability and conductivity of the metal. Eq.

(1.24)

may

be

expressed

|Zm|= where,

Eq.

0

The

skin

369"urfMHz/or

conductivity

of

copper

o,

=

conductivity

of

metal

is

plotted

barrier

depth,

§:

in

Fig.

impedance

_a+) =3

Egqs.

1.5

of

oz

As

described

which

quency

is

approaches

a metal

=

later,

very

%% Two skin depths current flow. For

Often

con-

to

copper:

(1.25)

and

much

zero,

g

is

(1.26),

to

value)

copper

metals

sometimes

the

L

Y/Tfuo

surface

greater

&+,

various

mhos/meter

expressed

t,

terms

ohms/sq.

(1.26)

and

skin

Z 0.

the

is

defined:

>t

impedance

than

depth

skin

(1.27) is

based

depth,

on

t>>§.

a metal

= 86.5% and three skin depths = 95.0% of 99% of the current flow, 4.6 skin depths

thickness,

in

the surface thickness of a metal at any 63.2%%* of the current is flowing therein.

H

the

= 5.80x107

(absolute

as or

(1.23)

for

a=_/g'=l\/%§:= *

relative

relative

metals

/2=

The skin depth is defined frequency for which 1-1/e

ness

resistive

contains an equal Z; depends upon both

uQ/sq.

=

combining

(1.24)

=0.X0,

Zm

By

terms

o,

(1.25)

of

in

(1.23)

is

considered

1.9

to be

adequate

As

the are

when,

thick-

the

fre-

total required.

t>36.

METAL IMPEDANCE Sec. 1.3

So g B

z1901

L0000 20000

° €0000°gr

o

5

2000

Looo*

10000

210"

S 50000 fs C

S

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o3

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55

-

2aao

5

8

S 2

10"

20

)/2m), but do not result in the same SEdB in the near field since the wave impedances are different.

Fig. 1.8 shows the conceptual mechanism for determining shielding effectiveness. It is detailed in Fig. 1.9 and may be explained as follows. The incident field strength (an electric field is illustrated here) is considered as unity relative to itself. The reflected field

is:

- 1-K"_ P=Ix

-1

=0

for K >>

Zw

= wave

1 in Eq. applies if Eq. (1.50)

of

to

Eq.

(1.51)

as

the

voltage

standing-wave

(1.50). When Z,/Zy < 1, is defined as Eq. (1.51)

The relative transmitted field, Fgm»> metal-to-air barrier material is:

T of

0 > K

K = ZW/Zm

=

the

for

(1.47)

_ 1K Pam = T4

E,

The

(1.46)

forK=1

=41

where,

1

=1-p

just

the VSWR in which

inside

ratio

(VSWR)

concept still Im/Zy > 1.

the

left

edge

(1.52)

This field undergoes an attenuation in traversing the thickness the metal barrier which turns the associated loss into exothermic

1.15

of

Sec, L.4

SHIELDING EFFECTIVENESS

INSIDE

Reflected

Wave

Barrier

Thickness,

Air

of

1 Pam

11—

Pamt— | T

(1

=

-

|Pha(1-Pam)e

Wave

2yt

e

2

.

Incident

-

T T e

Pmall-Pam'e

-2yt \\

\

Outside

Shielding

/

a1

e

P.ml€

T

Plane

(1-p

-

\

oo

2

\\pma(] Pam)®

Waves

-

) (1=

)e

-

‘t

e | ————————

Emerging Wave Beyond Shielding

'

Barrier

Pam

) -yt

——

/P

VAR

Wave

Air

—n—

Barrier

Metal

._T-

Reflected

for

Phenomena

Shielding

of

nternal

—

t

Representation

-

1.8

Figure

t

Wave

Attenuated Incident Wave ~

H, —t

QUTSIDE WORLD Metal

Transmitted

Wave

2

ENCLOSURE

\

Incident

OF

-3yt

(1-p ) (1= —_—

/—1—————’

Barrier

)08

-3yt

etc.

s=——

»

Metal

Propagation

Figure 1.9 - Geometry of Effectiveness (See Text)

Thickness, Constant Metal

t——

y = o + Jj8

Barrier

1,16

Used

=1_

in

Explaining

Shielding

i

Sec, 1.4 heat. at the

SHIELDING EFFECTIVENESS

The arriving right inside

Ty

= Pame‘yt

= e—(a+j8)tfam

propagation

comstant

o = attenuation

constant

=

Y

where,

field results in a lower edge of the barrier:

B = phase

constant

t = metal

thickness

strength

Trr where,

Pma

Pmalar =

= metal-to-air

The relative barrier is:

pma(l

reflection

transmitted

field,

Tpp == 10 1- T,0 == e Eq. (1.55) is the tive number) when

shielding y>>1,

at

the

inside

pam)

to

the

(Eq.

right

(1.51)). just

outside

(1 pam)(l _ pma) expressed

as

a gain

(a nega-

When the propagation round-trip re-reflections

constant is not significant, one or more must be considered. For example, the re-

shift in propagating back interface of Fig. 1.9:

to

the

FRRe

e Vo

reflected

field

of

Eq.

(1.54)

=7

PLR The re-reflected barrier is:

field =

FLL field

tive

undergoes

e

YE,

strength, I IR -=

p

a second

inside

©

edge

pma(l T LL?

-2yt

P 2

attenuation

of

the

left

(1 - P

strength,

Tpp,

v

FAR

the

left

Finally, metal barrier

the is:

transmitted _

réT,_

=Yt

-

3

=3Yt

component

T (l_pma)rAR

=

e

1.7

»

Pra of

-3yt

1

_ 1

this re-reflected edge, the rela-

(1.58)

to

1

-

inside (1.57)

©am

Tpp 2 Pra

edge

)

becomes:

FLLe

phase

(1.56)

Undergoing a third attenuation and phase shift of in arriving back at the inside face of the right

field

and

metal-to-air

[ pam)

from

the

(1.55)

YE[(_

effectiveness

right

(1.54)

coefficient

Ipp,

(1.53)

jB8

+

a

=

©

impinging

= e_(a+j6)t(1-93m)

The re-reflected relative field strength I'gp, of the metal-to-air barrier of Fig, 1.9 is:

edge

metal

field

the

fam

right

!-0

outside

the

(1.59)

Sec, 1.4

SHIELDING EFFECTIVENESS

Since the re-reflected the direct transmitted

field field

component of Eq. (1.59) of Eq. (1.55), they are

ry = e—Yt(l—pam)(l~pma)[%

. zYtpéa S

is coherent with coherently added:

4Yfg;a + ....‘]

(1.60)

First Multiple RoundRound trip Re-Reflections trip ReReflections The terms in the bracket constitute an infinite series (i.e., an infinity of re-reflections). The bracket expression can be simplified by writing this series in terms of its reciprocal. Thus, Eq. (1.60)

becomes:

r,o=e V10 T

)1-p

am

)(1-02ma 727t o

ma

(1.61) °

Eq. (1.61) may be expressed in terms of the impedance ratio of metal and metal-air interfaces by substituting Eqs. (1.50) and therein:

_

—atf2k

2

_ —at 4K _i~m

[l

Ip = e

Expressing

rather

SEdB

than

_

20

a

Eq.

gain,

(1.63)

and

loglo(l/TT)

where,

Re-Reflection

K-112 -2y¢ |71

(11'15) (m)[l - (R:q) e S——

as

_

20

K-1)2

_(E‘T'T)

e

]

(1.62)

-2vt|~! ]

(1-63)

S

Re-Reflection Correction Reflection Term (R) Absorption Term (A)

a loss

converting

=

the air(1.51)

loglo{}

(i.e.,

shielding

o t{(1+K) 2

K-1\2

it

to

decibels,

(B)

effectiveness)

there

P—ZK——-[%—(KII>

Term

e

results:

-2yt

(1.64)

Absorption

Loss,

AdB

= 8.686aut

(1.65)

Reflection

Loss,

RdB

=

(1.66)

Correction,

BdB

=

20

20

loglo(l+K)2/4K

loglotl-éKrl)z/(K+l)%k_zyt

(1.67)

Eq. (1.64) is plotted in graphs in the appendices in this handbook for several metals, metal properties, metal thicknesses, distances, and frequencies, This question will now be examined in further detail of its three loss components: ( 1) absorption loss, (2) reflection loss, and (3) re-reflection loss correction.

1,18

Sec, 1.4 1.4,1

SHIELDING EFFECTIVENESS ABsorpTION Loss

Eq.

(1.65)

may

be

expanded:

AdB where,

Yy =

a +

jB

a

=

(1+j)

=

B8 =

since

g

>>

where,

(1.4)

for

metals

(1.69)

¥Ynfuo

(1.70) for

metals in in

(1.71) terms cm it

of t in becomes

mils (thousandths of for both the English

fMquror

dB,

English

=

1314.3tcm

fMHz“rcr

dB,

metric

and

(1.72)

o,

are

and

permeability

(1.73)

are

and

plotted

units

(1.72)

units

(1.73)

conductivity

in

Figs.

1.10

relative

to

through

1.17

copper, aluminum, brass, beryllium, monel, the exotic high-permeability metals¥*,

iron,

reflection loss relations are predicated upon at the metal-barrier interfaces. Thus, it is

the

for

Zy

impedances and

of

Eq.

(1.23)

Eq.

for

(1.49)

by

Zp:

Z,

where,

k = A/2mr

= 1/2nrf

k =

2rr/A

=

1

far

= Combining

For

copper

for

stain-

their

an impedance useful to sub-

equivalents

from

K = EE _ e kK A uo/so O/ A O N

*

an and

RerLecTioN Loss

The mismatch

stitute

we

3.338tmils

various metals: less steel, and

1.4,2

(1.68)

=

M

Egs.

8.686t vV mfuo

Vnfuc

If Eq. (1.68) is defined inch) and f in MHz, and for t metric system of units: AdB

=

= Yjwu(o+tjwe)

= Yjwuo

or,

= 8.6860t

Eq.

magnetic

stipulated

(or flux 2.1.4).

uy

for (1.74)

2nrf

materials and

r >

H

fields

1.74

:

(1.75) (1.76) (1.77)

yields:

the

varies

frequency,

low-impedance,

E fields

\/2m

(1.77)

(u,>1),

which

for high-impedance, for

fields,

through

condition

density)

AN

(

(+y)/mEu/o

Vuoao

Eq.

graphs

with

especially

ll]-g

are

both

accurate

only

magnetic-field

above

several

kHz

for

the

(see

Sec.

strength

SHIELDING EFFECTIVENESS N

J49ddo)

Jo0i

ZHY00L

Aauanbau 4

ssauydoLyl

WN

pue

Adusnbau4

"SA

ss07

uolriduosqy

L

ydedn

siyp

- QL |

Ajp

is

Eq.

1.4.4

given

(1.92)

in Eqs.

is

(1.72)

plotted

in

and

Fig.

metal-barrier

~2Y I

(1.91)

. -3sin0.23A45 )| (1.92)

1

- 20 l°glo(l _ e—ztwhfuoe-jzt/wfuo) where,

and

(1.93)

(1.94)

(1.73)

1.18.

ToraL Losses For K >> 1

It may be a bit misleading to think of a reflection loss and reflection correction as separate terms. After all, a reflection should be the entire loss including re-reflection.

For

certain

_

SEdB

=

conditions,

20

loglo {e

where, wave

For

conditions

]

in which

mismatch

=

e

20 log,

When in

Eq.

vt

(or

(1.96)

t/§)

may

is

be

(1.64)

for

(1.95)

metal

K >>

1,

for

barrier

Eq.

K

- e—Zt/de‘th/6) Total

small

expanded

simplified:

(R'—"T)

t/s ’4_Ig( 1-e _ -2Yt)

very

greatly

= o = Jrfuo

i.e.,

¢/¢ %y(l

be

K-1\2 -2yt e

-

a substantial

Absorption Loss

term

may

[l

v//f

exists,

SE a8 = 20 log10

(1.64)

t/8| (K+1)2 7K

1/8 impedance

Eq.

1.29

a

impedance

(1.64)

>>

1

for

and

becomes:

(1.96)

K >>

1

(1.97)

Reflection Loss

(i.e.,

in

reloss

yt

power

—

X

o

§

g§a

s

.

zWiol

§=

S€

SL

zZ o3 2z

02

25

0

08 2E €0 g s

0§

s

0L

@

ZWiooL

S

002

2,31

Sec, 2.2 (475

SHIELDING DENSITY

(2)

3 m distance;

t = 6.8 mils

grams/mz).

Thus, aluminum away since the applications.

(175

um)

and W/A = 1,55

oz/ft?

will not work for magnetic sources which are only 10 cm shield would be too thick and heavy for weight-sensitive

It remains to compare the options in the above example for metals other than aluminum in order to determine if a lighter metal can be found with the same shielding effectiveness. The answer is obtained by using Eqs. (2.19) and (2.20) in which the ratio of tpj; is formed from

the

two

(or

more)

candidate

metals:

_ 9r1 (tmil)ratio

Eqs.

using

Table

(2.24)

and

(2.25)

the

information

2.4

- Relative

Metal Copper Monel Brass Steel S3 Netic Titanium Aluminum Magnesium

are

in

(W/A)ratio

=

computed

for

Tab.

2.2.

Thicknesses

the Same

Shielding

(Efiii)ratio

(2.24)

- 92

and

(W/A/mil)2

91

(w/A7mil)l

99

Tab.

Weights

2.4 of

Effectiveness

relative Some

(W/A/mil) yatio

1 24 .4 2.13 50 5.81 27.8 1.59 2.63

1 0.989 0.953 0.877 0.868 0.507 0.304 0.193

(2.25)

(see

to

Metals

copper for

by

Yielding

Constraints)

(W/A) ratio 1 24,1 2.03 43.9 5.05 14.1 0.483 0.509

The above table shows that steel is a very poor low-frequency magnetic shield as long as it is operated under t/§ 10), Eq. (1.64) becomes: =20

loglo(0.707Kt/6)

SE,. dB = 20 log where,

K = k =

Zw/zn1=

10

for

loss

for

is

significant

t/81

k x constant

A/27nr

reflection

(1.101)

Eq.(1.74))

E-fields

= 27r/A

for H-fields

=1

for

plane

waves

When Eq. (1.100) or (1.101) is applied for MIL-STD-285 for any two distances an error is developed which is a function of k alone. Thus, for any measurement distance, rp, and any user applied distance of ry, the correction in shielding efficiency, ASEqp, becomes:

ASE . = 20

loglo(rm/ru)

for

E-fields

(2.26)

=-20

loglo(ru/rm)

for

H-fields

(2.27)

for

plane

(2.28)

=0 in which

it

is

understood

that

both

rj

2,35

and

r;

are

waves in

the

near

field.

MIL-STD-285

Sec, 2.3 When one of the distances is the far field, Eqs: (2.26) and

in

ASEdB

=20

loglO(Zflrm/A)

in the (2.27)

for

E-fields,

r, =

20

loglO(A/ZWru)

in

for

loglO(anu/A)

in

for

log10

O/Zflrm)

for

(2.26)

distance

through

of

r;

ITlustrative

=

(2.32)

0.305

Example

m(12

are

in near

other

(2.29) r,

in

near

and (2,30)

and

r,

in

near

and (2.31)

and

r

in

near

and

fields

plotted

in

and

(2.32)

Fig.

any

is

and

field

far

inches)

the

field

far

in

r

and

H-fields,

r,

Eqs.

far

in

and

field

H-fields,

r 20

far

and

E-fields,

r = 20

near field become:

2,20

user

for MIL-STD-285

distance,

r,,

as

shown.

2.5

A manufacturer's literature states that per MIL-STD-285 a metalized silicone elastomer offers at least 90 dB of shielding effectiveness to E-fields at 10 MHz, 30 dB shielding effectiveness to H-fields at 10 MHz Determine the likely shielding efand 70 dB to plane waves above VHF, fectiveness at a distance of 10 meters,

From

and =

to

67

the

H

dB

Fig.

and

far

2.20,

fields

is

SEgp

field

+

the

23

correction

dB.

(H-field)

(ry>)\/2m)

at

ends at 4.8 MHz (ry = A/2m). (plane waves) = 70 dB, [1lustrative

Example

=

53

10

dB.

foil Since

(see at

a

curve

both

5

cm

a 10

Note

m

that

the

there

at

ry = 10

distance the

ry

is no

SEyg

10

= 10

m

m

m is

-23

(E-field)

distance

line

correction

in

for

is

Fig.

SEgp

dB in

2.20

2.6

this

dB

E fields

since

Thus,

The shielding effectiveness of aluminum was measured at

50

for

MHz,

foil

be

to

Thus,

M,

Fig.

distance

conditions

ASEgp = 20 logjg(5cm/100cm) 26 dB = 24 dB.

2.19). from

are

in

to H-fields of a 1 m distance a

What

protection

hostile

the

= -26 dB.

2.%

two mils (51 um) sheet at 42 kHz and found to

near

magnetic

field,

would

Eq.

be

field?

(2.27)

offered

applies,

Thus, SEgg (H-field) = 50 dB -

by

MIL-STD-28 Sec. 2.3

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apoL Z Ps prats-3 pazoaudooun (1) 10.2) magnetic metals, Appendix A (copper) is used, while for 0r < 0.2 magnetic metals, Appendix B (monel) is used. After selecting the appendix letter to use from either Tabs 3.1 or 3.2, as applicable, it remains to select the correct design graph within the appendix. Each apperdix subset is based on the distance between the EMI emission source and the metal barrier., Six such subsets are presented in each appendix as shown in Tab. 3.3 (the X corresponds to any appendix letter, viz., A.B.C, etc.). Having selected the applicable graph within the appendix, it remains to apply the graph to the problem, The graph may be used to determine either: (1) the resulting shielding effectiveness given the

metal

thickness,

required

metal

operating

thickness

frequency,

given

the

and

desired

3.2

E or H-field shielding

problem,

(2)

effectiveness,

the

Sec, 3.1 Table

How To Use THE DESIGN GRAPHS

3.2

- Metal

Class

for

Choice

of Appendix No.

>300kHz

tal

1,

1

Co

Use

Metal Mumet

e Brass 917% 66% Brass Cadmium Chromium

to

Nicke Pe

Cu, Cu

34% %

1

Permall ermallo

Z

78 -

r

Steel,cold-rolled

rnick

H

Hiperco

Iron, ron Iron, Lead

=

e

commercial e ur cone 4% S

Su

Magnesium

anganese

Merc Monel

Table

3.3

Figure

operating lower metal

Appendix

- Applicable Number

Design

Nominal

Ry = 10cm

X.2

Ry

X.3

Rp = 10m

X.4

Rp

=

100m

X.5

Ry

=

lkm

X.6

R,

=

10km

and

useful frequency thickness, and E

for

Distance

X.1

frequency,

Graph

=

Applicable 30cm < Ry £

3m< Ry < 30m < R < 300m
p

06

KEYS

Ags £

7

gx$y

80

31;(1)3 31

71

CHS

RCL

40

CHS £ N

X

04

3407

02

80

Registers: Rl Frequency

R2.g, R3

ur

in

MHz

R4

¢t in

mils

R5 Ry in meters R6

Eor

H

1

or

3.8

Steps

71 3401 3402 09 71

CHS STO 7 RCL 8 1 = £ Vx RCL 7 £ LN

Program

EFFECTIVENESS

71

RCL 3

17,{

EEX

40

40

3401 81

7

SHIELDING

Title:

L R 42

LBL

30

0

Effectiveness

Shielding

f## 1513Program.

Card

0

- HP-65

3.4

Table

2

R7

working

R8

working

R9

80

42 32 07

35

08 02

50

100

Sec, 3.3

ILLUSTRATIVE EXAMPLES

3.3.2

ILLUSTRATIVE EXAMPLES

The following examples will serve to illustrate the use of the HP-65 shielding effectiveness program, The results may be compared with those in the appendices* or in Sec. 3.2.

Example

#1

A sheet

of

1/16

inch

(i.e.,

62.5

to shield a box from a strong magnetic generator located 5 feet (1.52 meters) effectiveness in dB,

mils)

field away.

iron

is

being

considered

originating from a 60-Hz Compute the shielding

After loading the magnetic program card into the HP-65 calculator, key in fyp, = 60 x 10~ MHz = 60 EEX 6 CHS (sTO 1), 0y = .17 for iron (STO 2), uy = 1000 for iron (STO 3), t = 62,5 mils (STO 4), Ry = 1.52 meters (STO 5), and 2 for H-field (STO 6). To see shielding effective-

ness,

key

Example

label

"A"

and

see

24

dB,

#2

A piece of sensitive electronic equipment is located near (100 m) an A-M broadcast station antenna transmitting at 1250 kHz, Determine the shielding effectiveness of a 1/32 inch (31.25 mils) sheet metal aluminum box enclosure to E-fields or plane waves, as applicable. After loading the magnetic program card, key in fva, = (8TO 1), Oy = .61 for aluminum (STO 2), p, = 1 (STO 3), t = 4), Ry = 100m (STO 5), and 1 for E-field or plane wave (STO

see

shielding

effectiveness,

key

label

"A"

and

see

196

dB.

1.25 MHz 31.25 (STO 6). To

Note

that

196 dB would never be obtained in practice because of the penetrations required into and out of the box (see Chap. 11, Vol. 3 EMC Handbook Series). Also note that the near/far-field interface (Rp = A/2m) exists at 38.2 m. Thus, the box is located in the far field of the transmitter and plane-wave conditions apply rather than E-field conditions.

Example

#3

A ground-level nuclear detonation produces a broadband electromagnetic pulse (EMP) with most of its energy distributed in the 10 kHz band. From a distance of 5 km, the blast center looks like a magnetic source having billions of amperes. To protect electromagnetic equipment, specify the thickness of both aluminum and sheet steel to provide 300 dB isolation,

Key fyp, = .01 MHz (STO 1), o, = .61 (STO 2), uy = 1 (STO 3), t *

1If you like this program, other HP-65 EMC programs.

contact

S

Don

White

Consultants,

Inc.

for

Sec. 3.3

HP-65 SHIELDING EFFECTIVENESS

(this is a 2 (STO 6).

try

Again

mils,

(this

guess) = Key "A"

t = 1 inch try

t

it

=

is

For

a

(1000

800

seen

the

is

250 mils (STO to see SEgg =

sheet

guess)

mils

mils

that =

- STO

(STO

4)

SEgp

steel, 100

4),

key

o,

(STO

dB. =

Ry, = dB,

and

and

= 308

mils

4), 191

key

.17

4).

5000 m (STO 5), and H/PW = Since this is inadequate,

again

"A"

key

to

(STO 2),

Key

"A"

see

to

334

dB,

u, = 1000

"A"

to

see

SEgg

see

387

For

(STO =

526

1300

MHz

Next try t = 1/16 inch (62.5 mils) to see SEgg = 362 dB. mils, the shielding effectiveness is 308 dB, the same as aluminum,

dB.

t =

3)

700

and

dB.

t

For t = 50 700 mils of

Examgl e #4 A nearby an

(500

electric-field

m)

L-Band

strength

weather of

100

radar

V/m

in

operating

an

at

internal

room

in

a

creates

build-

ing where a computer is to be located. It is known that the computer, its peripherals, and all interconnecting cables will not be susceptible to radiation below 3 V/m, Thus, determine a relatively inexpensive shield for the computer room.

The required shielding effectiveness is the ratio of the electricfield strengths or 100/3 which equals about 31 dB, Consider the shielding effectiveness of 1 mil (25.4 um) household aluminum foil as one

possible

solution,

(STO

oy

lired

with

1),

Here

overlapping

=

.61

(STO

the

joints

2),

uy

walls,

taped

= 1

ceiling, in

(STO

place.

3),

and

flooring

Key

fyg,

t = 1 mil

=

(STO

would 1300

4),

be

MHz

so

R, = 500

m (STO 5), and E/PW = 1 (STO 6). Key "A" to see SEgqg = 169 dB, Thus, the aluminum provides an enormous shielding effectiveness and it remains to insure that door seams, power entrance, and the like do not create the mode of EMI entry.

3.10

CHAPTER 4 REFERENCES (1) Abramowitz, M,; Stegun, I.A,, "Handbook of Mathematical tions", Boulder, Colorado; National Bureau of Standards; 1964,

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4,12

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