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Adams. John W/Shieldlng effectiveness me

QC100 U56 NO 85-3035 VI 986 C.1 NBS-PUB- SHIELDING EFFECTIVENESS

MEASUREMENTS OF PLASTICS

Reference

NBS

PUBLICATIONS

John W. Adams

Eric J. Vanzura

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

January 1986

QC

100

U56

85-3035

1986

NBS RESEARCH INFORMATION CENTER NBSIR 85-3035

Xu

SHIELDING EFFECTIVENESS

MEASUREMENTS OF PLASTICS

John W. Adams

Eric J. Vanzura

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

January 1986

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige. Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

1. Background 1

2. Materials 3

3. Problems 4

4. Types of Coaxial Holders for SE Measurements M

5. Automated Measurements With the NBS Coaxial Holder 7

6. Observations and Implications of Data Shown in Figures 8 and 9 9

7. Conclusions 10

8. Acknowledgments 10

9. References 10

Appendix 21

ili

Shielding Effectiveness Measurements of Plastics

John W. Adams and Eric Vanzura

Electromagnetic Fields Division

National Bureau of Standards

Boulder, Colorado

Measurement of shielding effectiveness (SE) of plastic materials

may give serious problems due to the insulating nature of many

plastics. A method of making these measurements using a flanged

coaxial holder overcomes these limitations.

Key Words: ASTM SE measurements; flanged coaxial holder; SE

measurements of plastics; shielding effectiveness measurements

1 . Background

A number of methods are either in use or are being developed and

evaluated for measuring shielding effectiveness (SE) of materials [1-6].

There are also a number of problems both in making the measurements and

interpreting results [1,7]. The main difficulty is to obtain repeatable

results on the same material using different techniques and sometimes with

the same technique. It is difficult to match the measurement conditions

with application conditions. The increased use of plastic binders with

insulating characteristics is a substantial change in application technology

and also creates additional problems in measurement technology. Federal

Communication Commission (FCC) incidental radiation regulations require

expensive testing on whole products or systems. Reliable SE measurements

may aid design decisions and reduce need for redesign after final FCC

testing.

The definition, SE = 10 log (P

? /P^), is deceptively easy to state,

where P^ and P^ are power levels without and with a sample of material

present. Conditions that exist during measurement such as angle of

incidence and polarization of waves are often omitted or are not known t^i

are essential factors influencing P

? and P,

1
The five methods of measurement covered in references [1] and [2] are:

(1) a variation of MIL-STD 285, (2) a dual transverse electromagnetic (TEM)

cell, (3) a flanged coaxial holder, (4) a time domain measurement system in

free space, and (5) a time domain measurement system through an aperture of

a shielded room. There is substantial disagreement among the measured

values of SE on the same material using these five methods. The measured

results are shown in figure 1 . There is at least a partial understanding of

problems causing these variations, and some improvements may allow more repeatable measured results under specific conditions.

There is no practical solution to the use of continuous wave (CW)

through an aperture of a shielded room (modified use of MIL-STD 285). The

multitude of resonances within the room and the resonances of the aperture

cause such a wide range of measured values over short frequency intervals

that only by taking a tremendous number of measurements and averaging the

data can any meaningful results be obtained. This is prohibitively time

consuming and costly.

The dual TEM cell [2,3,4] is useful for measurements with the electric

field (E) normal to the surface of the material and for frequencies up to

occurrence of modes of higher order than the TEM mode. Thus, the upper

frequency is a function of the size of the TEM cell [8], The major

practical problem with this method is the variation in measured data due to

contact resistance. Many materials, especially those made of plastic, give

considerable variation in contact resistance. This is also a problem for

some other measurement systems.

There are various designs of coaxial holders for making SE measurements

[1,5,6]. Some require electrical contact with both inner and outer

conductors since the measurement relies on conduction current. Others that

allow insertion of a sample between flanges of a transmission line can rely on displacement current. This is more effective for measurement on

insulating samples, but extra steps must be taken to compensate for the

transmission line perturbation caused by this insertion of sample material.

2
This method seems promising between 10 and 1000 MHz. At frequencies below

10 MHz, compensation for transmission line perturbation becomes increasingly

difficult. At frequencies above 1 GHz, higher order modes cause problems.

More discussion of the flanged coaxial holder follows since it seems to have

the most potential for giving repeatable measured values that compare

favorably with theoretically predicted values.

Two time domain techniques under study use subnanosecond pulses to

provide SE information from 100 MHz to 4 GHz [9,10]. For both systems,

received time domain pulses are first windowed to eliminate unwanted

diffracted and reflected signals. After performing Fourier transforms on the received pulse shapes with and without the sample present, a comparison

is made of the spectral responses.

The first technique requires samples more than one meter square so the

direct path signal can be separated from the diffracted signal coming around

the material (see figure 2). The direct difference in spectral responses

with and without the material gives SE values. Unfortunately, for SE

applications a large sample size is often a limiting factor.

The second method involves placing a small (10 cm diameter) sample over an aperture of either a wall of a shielded room or of a large plane metal

sheet (see figure 3). Measured SE values are confounded by effects of

aperture loading, contact impedance and aperture resonances at higher

frequencies. These effects are not well understood at present.

2. Materials

A proliferation of shielding materials are now available. Metal such

as nickel and copper are being plated onto plastic substrates, or conductors

such as carbon and graphite are being embedded into a plastic binder. A measurement system should give good measured values regardless of the type

of construction or content of a shielding material. This is certainly not a fully accomplished fact as of this writing.

3
3. Problems

Another problem in interpreting measured data is that specific

measurement conditions, usually plane wave incidence with E and magnetic

field (H) tangent to the sample surface, are very seldom satisfied in

applications. Typically, a source is inside an enclosure, and near field

waves impinge on surfaces at a multitude of different angles. Transmission

and reflection factors change drastically as angle of incidence and

polarization change. There may not be a tractable solution other than the

final FCC test [11]; even the measurement of SE of the enclosure material

may be a minor factor compared to a large, relatively open aperture such as

a video screen. However, decisions must be made to select materials for

shielding, and these may be done on a relative basis.

A very enlightening theoretical study [7] of coupling through a small

loaded aperture indicates how antenna position, contact impedance and the

angle of incidence of an arriving plane wave can significantly affect the

measured SE of a material. The specific coupling due to either E or H is

shown. Typically, any conductive material severely attenuates the electric

field. The magnetic field is, however, shown to penetrate much more

strongly through most samples. The particular example shows the shielding

of the magnetic field to be 103.6 dB for 0.0 ohm contact resistance, 54.1 dB

for 0.01 ohm, and 14.3 dB for 1.0 ohm.

4. Types of Coaxial Holders for SE Measurements

Two types of coaxial transmission line holders will be discussed for SE

measurements. Two circuits with lumped elements given in figures 4a and 4b are shown to illustrate the problem caused by contact resistance.

Figure 4a shows a sample represented by Z connected to the inner and

Lj outer conductors of a coaxial transmission line by a parallel resistor and

capacitor. The resistor represents contact resistance (conduction current)

and the capacitor represents capacitive coupling (displacement current).

4
f

This may simulate the case of the American Society for Testing Materials

(ASTM) coaxial holder. Figure 4b shows the same sample Z connected to

Lj similar parallel R and C circuits which are now in series with the coaxial

line. This may simulate the case of the flanged National Bureau of

Standards (NBS) holder. Although measured numbers may be used to analyze

these circuits, the concepts are illustrative and distributed circuits or field analysis should be used for rigorous calculations. Erratic and

erroneous measurements are caused when both susceptance and conductance are

too small (poor coupling). For conductive samples, conductance will be

large and either circuit will give good data. For insulating materials,

conductance will be small and variable, and displacement current through

capacitive coupling must be greater than conduction current in order to

obtain repeatable data. This may be achieved by increasing capacitance by

enlarging the capacitive coupling area by use of flanges as shown in figure

4b. Thin samples will also increase this capacitance. The ratio of

displacement to conduction current will determine how low in frequency the

coaxial fixtures may be used. For insulating materials, the conductance

will be very low, and the more stable capacitance will carry the dominant

portion of the current. Unfortunately, as frequency decreases the

susceptance also decreases to the point that the relatively unrepeatable

conductance carries the larger proportion of current. Measured data at low

frequencies show this instability. The "corner" frequency may be calculated

by f = 1 / 2-ttRC ; the range of instability is below this corner frequency.

The holder proposed by ASTM committee D-9.12.14 in Emergency Standard

ES 7 - 83 is described by Simon [6] and shown in figure 5. The coaxial line

has continuous inner and outer conductors of diameter 4.35 and 9.9 cm,

respectively (see figure 4a). The sample should be an annular disk with

matching dimensions to fit between the inner and outer conductor. The

sample must make resistive contact with the walls. Silver paint is

suggested, but even with a good quality silver paint, any variation in the

contact resistance causes substantial change in measured SE values. Samples

with imperfectly conducting edges inherently give unrepeatable results. Th<- capacitive coupling for thin samples is so small that it is insufficient tc

5
overcome poor contact resistance, especially for samples which are basically

good insulators. Two runs are needed; one is taken with an empty holder,

while the second is taken with the sample present.

NBS has a coaxial transmission line holder with flanges in the

midsection and with inner conductor diameter of 3-2 cm and outer conductor

diameter of 7.6 cm. See the Appendix for engineering drawings of this

holder. The flange has an outer diameter of 13-3 cm, as shown in figure 6.

The sample should be a solid disk with diameter of 13.3 cm. The

transmission line is perturbed by this sample since it adds series

admittances to both inner and outer conductors (see figure Mb). This

perturbation depends on dielectric constant, conductivity, and thickness of

the test sample. The perturbation caused by these series admittances may be

isolated by making a reference annular disk that matches the dimensions of

the outer conductor and a solid disk that matches the dimensions of the

inner conductor. This leaves the space between inner and outer conductor

open. These reference disks are the same thickness as the sample disk to be

used for the load. Measured transmission values with these reference disks

give insertion loss caused by the series admittances. These values are then

used as reference values to be compared with values obtained when the solid

disk of the sample material is clamped between the flanges. Nylon screws

should be used to clamp the flanges together to minimize erratic conductive

contact with edges of samples. This means that each set of measurements

must be made twice, first with the reference and then with the sample.

With an automated measurement system, the time required to make the

needed measurements is still reasonable. The values obtained with the NBS

sample holder are repeatable and agree with calculated values, except for

inhomogeneous samples. Many of the insulating plastic samples give erratic

data when measured in the ASTM holder; conductive samples give more

consistent results if contact resistance is minimized with silver paint.

Inhomogeneous samples give more difference in capacitance due to positioning

and pressure variations than do homogeneous samples.

6
5. Automated Measurements with the NBS Coaxial Holder

Figure 7 shows the test configuration of the NBS shielding

effectiveness measurement system. The NBS system uses a computer-controlled

data acquisition setup which measures incident, reflected and through powers

at any number of frequencies in the 1 MHz to 1 GHz range. The operator

controls the measurement through the computer by answering prompts. First

the signal generator power level and desired frequencies are input to the

computer. The computer sets the signal generator to the specified power

level and frequency. It also sets the digital spectrum analyzer to this

same frequency. Power from the signal generator then goes through a 10 dB

attenuator to reduce reflections going back into the generator. A

calibrated bidirectional coupler gives the incident and reflected powers at

the 50-ohm coaxial sample holder input port (port 1). When the rf reaches

the surface of the test sample, the incident power is partially reflected

due to impedance mismatch and partially enters the sample. Once inside the

sample, rf power is absorbed as it passes through. On the other side of the

test sample the incident power is again partially re-reflected back into the

sample and partially coupled through as transmitted power. This transmitted

power passes through a calibrated 9 dB attenuator and a set of automated

coaxial rf switches. The computer controls a relay actuator that configures

the rf switches so that either incident, reflected or through power passes

to the spectrum analyzer. The two rf lines that are not being measured are

terminated in 50 ohm loads. Power from the switches goes through a calibrated 3 dB attenuator and is then measured by the spectrum analyzer.

Under computer control the spectrum analyzer performs many functions.

To provide a good measurement range the spectrum analyzer is first set to 10

dB/division and its reference level is set 13 dB higher than the highest

possible signal level that could get through. This reference level is

usually the signal generator level minus the total attenuation of the three

pads (22 dB) plus 13 dB. Then, at each frequency, the spectrum analyzer

sets the frequency span to the current center frequency divided by 100. For

example, if the frequency were 500 MHz the frequency span would be 5 MHz.

7
The spectrum analyzer then takes eight sweeps using the "max hold" function,

and uses the "peak search" to measure the frequency and power level. It

then moves off the center frequency and measures the noise floor of the

spectrum analyzer. If this measured signal-to-noise ratio is less than 10

dB, the computer will make the spectrum analyzer go through a high

resolution subroutine in which the reference level, frequency span,

resolution bandwidth and video bandwidth are reduced to pull the signal out

of the noise as much as possible. If the signal is still too small, a flag

is set indicating that the measured value is too near the noise floor. The

computer then resets the spectrum analyzer to its normal operation. Once

the incident, reflected and transmitted powers are measured, the computer

sets the spectrum analyzer and signal generator to the next frequency and

the measurement process is repeated through the specified number of

frequencies.

The computer stores and displays data in different ways. As each

measurement is taken, the power level is displayed on the computer's CRT.

If the signal is sufficiently higher than the noise, that level is stored in an array. When the three power levels at any particular frequency have been

made, the computer sends the information to the printer where it is written

along with any other information about that measurement. When a run is

completed the computer prompts the user to store the information on disk and

asks for a filename. The frequencies and corresponding power levels are

then written to disk, and measurements which were too small are marked

appropriately. As discussed earlier, both reference and load runs are

needed to calculate shielding effectiveness of the sample. Once information

from both runs have been collected, the computer calculates the difference

between the through power of the load and reference runs, using incident

power measurements to normalize both runs to the same input power level.

This difference is then written to disk where it can later be plotted. Some

of these plots are shown in figures 8 and 9.

Automating the shielding effectiveness measurement system not only

speeds up the measurement procedure but also decreases human error and makes

8
data processing and presentation effective and easy. A typical run

measuring incident, reflected, and through powers at 41 different

frequencies usually takes about 15 minutes. The same run before automation

took more than eight hours. The information may be stored directly to

floppy disks and retrieved and plotted.

The dynamic range achieved by the NBS system is about 95 dB . This is

more than adequate for measuring the SE of most materials. This number can

be pushed beyond 100 dB, but additional care is needed for measurements that

require more than 90 dB of dynamic range to avoid contamination of data by

leakage. The repeatability of measured SE values is within 2 dB from 10 MHz

to 1 GHz. The low frequency end will vary depending on the sample and is

determined by the ratio of conduction to displacement currents for the

particular sample.

Current investigations at NBS are to determine the accuracy of these

and other types of shielding effectiveness measurement techniques.

6. Observations and Implications of Data Shown in Figures 8 and 9

The three curves shown in figure 8 are indicative of several potential

problems. The sample is a thin gold film on a plastic substrate. Even if

the edges are silver painted before the sample is inserted into the ASTM

holder, poor conductive contact gives grossly pessimistic values of SE as shown by the silver-painted-before-insertion curve. Silver painting in

place assures good contact, but some mismatch and resulting standing wave

are caused by a combination of variations of characteristic impedance of the

transmission line holder and by reflections from the silver paint along the

edges of the sample. The flanged NBS holder which relies on displacement

current at frequencies above 30 MHz gives the expected flat response. At

the low frequencies it too gives erratic values as conduction current

becomes dominant.

9
Figure 9 shows results of an insulating sample. Even though the edges were silver painted in place, the results in the ASTM holder are grossly

erroneous. The flanged coaxial holder gives reasonable results down to

about 30 MHz where displacement current falls off.

7. Conclusions

There are serious pitfalls for the unwary caused by measurement-system

limitations in making SE measurements on insulating materials. This paper

shows that flanged coaxial holders offer a viable means of making plane

wave, normal incidence SE measurements on insulating materials as well as on conducting materials.

8. Acknowledgments

Mark Ma, A. R. Ondrejka, and Perry Wilson have made numerous helpful

suggestions which are greatly appreciated.

9. References

[1] Ondrejka, A. R.; Adams, J. W. Shielding effectiveness (SE)

measurement techniques. 19 8-4 IEEE National Symposium on

Electromagnetic Compatibility; San Antonio, Texas: 1984 April 24-26.

249-256.

[2] Wilson, P. F.; Ma, M. T. Small aperture analysis of the dual TEM

cell. 1984 IEEE National Symposium on Electromagnetic Compatibility,

San Antonio, Texas; 1984 April 24-26. 365-369.

[ 3 ] Wilson, P. F.; Ma, M. T. Small aperture analysis of the dual TEM cell

and an investigation of test object scattering in a single cell. Nat.

Bur. Standards (U.S.) Tech. Note 1076; 1984 October.

10
[4] Faught , A. N. , Jr.; Dowell, J. T. ; Scheps, R. D. Shielding material

insertion loss measurement using a dual TEM cell system. 1983 IEEE

International Symposium on Electromagnetic Compatibility; Washington,

DC; 1983 August 23~25. 289-290.

[5] American Society for Testing and Materials Emergency Standard ES 7 - 83,

Electromagnetic shielding effectiveness of planar materials.

Philadelphia, PA; 1983 December.

[6] Simon, R. M. High accuracy in EMI measurement. Modern Plastics;

1983 August. 78-80.

[7] Wilson, P. F. ; Ma, M. T. Factors influencing material shielding

effectiveness measurements. 1985 IEEE International Symposium on Electromagnetic Compatibility; Wakefield, MA, 1985 August. 29~33-

[8] Tippet, J. P. ; Chang, D. C.; Crawford, M. L. An analytical and

experimental determination of the cutoff frequencies of higher-order

modes in a TEM cell, Nat. Bur. Stand. (U.S.) NBSIR 76-841, 1976 June.

[9] Lawton, R. A.; Ondrejka, A. R. Antennas and the associated time

domain range for measuring impulsive fields. Nat. Bur. Stand. (U.S.)

Tech. Note 1008; 1978 April.

[10] Kanda, M. The effects of resistive loading of TEM horns. IEEE

Transactions on Electromagn. Compat . EMC-24, No. 2: 245~255; 1982 May.

[11] FCC Rules, Part 15, Subpart J (47 CFR 15.801-15.840), Federal Register

at 45 FR 42347, U.S. Government Printing Office, 1980 June 24.

11
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12

Figure

1.

Measured

SE

data

of

the

same

material

using

five

different

measurement

techniques.
Material

Under

Test

13

Figure

2.

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time

domain

measurement

system.
Shielded

Room

14

Figure

3.

Aperture

time

domain

measurement

system.
Figure Ha Figure Hb

Figure H . Circuit models for showing effect of contact resistance.

15
Figure 5. ASTM coaxial holder for SE measurements

16
17

Figure

6.

Flanged

coaxial

holder

for

SE

measurements.
18

Figure

7.

Block

diagram

of

automated

SE

measurement

system

that

uses

coaxial

holders.
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19

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20

Figure

9.

Measured

data

of

laminated

(insulating)

sample

using

flanged

and

ASTM

holder.
Appendix

Engineering Drawings of Flanged Coaxial Holder

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U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 85-3035 January 1986

4 . TITLE AND SUBTITLE

Shielding Effectiveness Measurements of Plastics

5. AUTHOR(S)

John W. Adams and Eric Vanzura

6. PERFORMING ORGANIZATION (If joint or other than N BS. see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10.

SUPPLEMENTARY NOTES

] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

Measurement of shielding effectiveness (SE) of plastic materials

may give serious problems due to the insulating nature of many

plastics. A method of making these measurements using a flanged

coaxial holder overcomes these limitations.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s ASTM SE measurements; flanged coaxial holder; SE measurements of

plastics; shielding effectiveness measurements

13.

AVAILABILITY

S

Unl imited For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

36

15. Price