NATL INST OF STANDARDS & TECH R.I.C.

A11 103081 542

Cruz, J. E/Alternative techniques tor so QClOO .U5753 NO.1320 1989 V198 C.I NISTNIST

PUBLICATIONS

NIST TECHNICAL NOTE 1320

U.S. DEPARTMENT OF COMMERCE / National Institute of Standards and Technology
NATIONAL INOTTUTE OF STANDARDS &

rtaiiur.rti' TECHNOLOGY

Research Mormafen^^er

Gaithersburg, MD 20899 ,

DATE DUE

Demco. Inc. 38-293
A/isfR

Alternative Techniques for Some ^ /^^

Typical MILSTD-461/462 j^^^^

Types of Measurements

J.E. Cruz

E.B. Larsen

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

Sponsored by

U.S. Army Aviation Systems Command

AMSAV-QE

St. Louis, Missouri 63120

^<^e^^o^co.

^^^rts o* ^

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

Ernest Ambler, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Acting Director

Issued March 1989
National Institute of Standards and Technology Technical Note 1320

Natl. Inst. Stand. Technol., Tech Note 1320, 44 pages (Mar 1989)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1989

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
contents

Page

List of Figures iv

1. Intrcxiuction 1

2. EM Field of a Single-Wire Transmission Line in a Partially Loaded Screenroom 3

3. Measurement of Antenna Fac±or in a Partially Loaded

Screenroom 5

4. Unifonnity of Electric Field Generated by a Transmitting

Biconical Antenna Within a Cubic Volume of a Partially

Loaded Screenroom 7

5. Uniformity of Electric Field Generated by a Transmitting

Biconical Antenna Within a Cubic Volume of a Fully

Loaded Screenroom 9

6. Electric Field Distribution on the Grounded Table in a Screenroom 9

7

.

Summary 11

8

.

Conclusions 12

9 Recommendations 13

10

.

References 15

111

List of Figures

Page

1. The long-wire antenna configuration in a screenroom 16

2. Calculated-to-measured field strength versus distance

below the long-wire for selected frequencies in a partially loaded screenroom 17

3. Test setu^) required by MIL-STD-461A. determination of

antenna factors 18

4. Placement of the rf loading (absorber material) in the

NIST shielded room 19

5. Sketch of three screenroom configmrations for measuring

the antenna factor of two identical antennas 20

6. Antenna factors measured in an unloaded screenroom

at a separation distance of 1 m, for three selected

positions 21

7. Antenna factors measured in a partially loaded

screenroom at a separation distance of 1 m, for three

selected positions 22

8. Comparison of antenna factors measured in four

different facilities, with aim separation between

antennas 23

9. Identification of the ei^t comers of a cube for

measuring the field strength differences with respect

to the center of the cube 24

10. Electric field differences of the top 4 comers of a cube with respect to the center of the cube, in a

particilly loaded screenrocm 25

11. Electric field differences of the bottom 4 comers of

a cube with respect to the center of the cube, in a partially loaded screenroom 26

12. Sketch of a cubic volume with an enclosed metal box,

for measuring the field distortion caused by the metal

box 27

13. Electric field differences of the top 4 comers of a cube with respect to the center of the eitpty cube,

caused by inserting a rectangular inetal box 28

V
Page

14. Electric field differences of the bottcm 4 comers of

a cube with respect to the center of the enpty cube,

caused by inserting a rectangular metal box 29

15. Electric field differences of the top 4 comers of a cube with respect to the center of the cube, in a fully

loaded screenroom (anechoic chamber) 30

16. Electric field differences of the bottom 4 comeirs of a cube with respect to the center of the cube, in a fully

loaded screenroom (anechoic chamber) 31

17. Measurement location in the screenroom for determination

of the electric field above the grounded table 32

18. Electric field differences on the grounded table for 10

positions, using the center of the table (position R) , with the wall loaded, as the dB reference 33

19. Electric field differences on the grounded table for 10

positions, using the center of the table (position R)

,

with the wall unloaded, as the dB reference 34

20. Electric field differences between position R and

position (of figure 17) on the groundai table, using

position R as the dB reference, with the walls either

loaded or unloaded 35

vi
Alternative Techniques for Some Typical

MIL-STD-461/462 Types of Measurements

J. E. Cruz and E. B. Larsen

Electromagnetic Fields Division

National Institute of Standards and Technology

Boulder, Colorado 80303

Most testing for MIL-STD- 461/462 is performed in a shielded enclosure (screenroom) which leads to uncertainty

in the measurement of emissions from electronic equipment,

or the susceptibility of equipment to electromagnetic

radiation. Possible alternative techniques for improved

measurements in a screenroom have been developed by the

National Institute of Standards and Technology (NIST)

.

These techniques are covered in this report.

This report presents antenna factors determined in a screenroom which was partially loaded with radio frequency

(rf) absorbing material, using the two-antenna insertionloss technique. These antenna factors are compared with the

antenna factors obtained in an unloaded screenroom, a fully

loaded screenroom (anechoic chamber) , and at an open field

site. In addition, measurements at the eight corners of a cube were made in the partially loaded and fully loaded

screenroom to determine the field deviation at the eight

corners of the cube with respect to its center. Also,

measurment improvements are quantified for the electric

field strength beneath a single-wire transmission line, in

a partially loaded screenroom. Finally, electric-field

measurements were made on top of the grounded table in a partially loaded screenroom to determine the field strength

variation above the table.

Key words: antenna factor; electromagnetic compatability

;

field strength standards; loaded screenroom; military

standards; screenroom measurements.

1. Introduction

This report describes a follow-on project to that which was covered in a previous National Bureau of Standards (NBS) report [1] to the same sponsor.

That report of October 1986, NBS Tech Note 1300, is titled "Assessment of Error

Bounds for Some Typical MIL-STD-461/462 Types of Measurements". The work was sponsored by the U.S. Army Aviation Systems Command (AVSCOM) in St. Louis,
Missouri. NBS Tech Note 1300 deals with the instrumentation and equations for

screenroom measurements which are used by the U.S. Army for electromagnetic

compatability (EMC) testing and calibrations. The report relates to Military

Standards 461 and 462, which are standards issued by the U.S. Department of

Defense (DOD) for use by all departments and agencies within the DOD . MIL-STD461A is titled "Electromagnetic interference characteristics, requirements for

equipment" and includes the specified limits for radiated and conducted radio

frequency (rf) emissions, both narrowband and broadband [2]. It is dated August

1, 1968 and supersedes MlL-STD-461. MIL-STD-462 was issued by DOD on July 31,

1967, and is titled "Electromagnetic interference characteristics, measurement

of" [3]. These two standards, MIL-STD-461A and MIL-STD-462, will generally be

referred to here collectively as MIL-STD-461/462

.

Most EMC testing for MIL-STD-461/462 is performed in a shielded enclosure

(screenroom) rather than at an open field site because of lower cost, shielding

from ambient fields, and convenience. This MIL-STD, which is used in the

procurement of many military and other government systems, has several

recognized problems. Possible errors and large uncertainties can occur in the

measurement of electromagnetic (EM) emissions from electronic equipment, or the

susceptibility of this equipment to EM radiation. The uncertainty is increased

when performing the measurements inside a screenroom where multiple reflections

and antenna impedance perturbations occur. An assessment and discussion of the error bounds is covered in NBS Tech Note 1300 [1].

In the present report, measurement improvements have been quantified for

generating standard (known) electric fields beneath a single-wire transmission

line in a partially-loaded screenroom. That is, a typical screenroom was loaded

with pieces of rf absorbing material at some of the surfaces of the screenroom.

The MIL-STD describes a laboratory method for producing a field having known E and H magnitudes in a shielded room using a long-wire antenna. The objective of

NIST testing was to evaluate the field level and uniformity in this partially
loaded screenroom, and compare these measurements with theoretically calculated

values and with measurements made previously [1].

Also, measurements have been performed in a partially loaded and a fully

loaded screenroom (anechoic chamber) and on the NIST ground screen to determine

antenna factors using the two -antenna method. Data of antenna factors

determined in the partially loaded screenroom and in the fully loaded screenroom

(anechoic chamber) are presented and compared with those determined in Tech Note

1300 [1] and at the NIST open field site.

In addition, measurements of electric field at the eight corners of a cube were made in the partially loaded and fully loaded screenroom to determine the

electric field deviation at the eight corners of a cube with respect to its

center. The electric-field strengths in the partially loaded screenroom were

measured with and without a metal box inside the cube. In the anechoic chamber

the electric -field strength was measured only on the eight corners of the empty

cube. The data are evaluated and compared.

Finally, electric-field measurements were made on top of the grounded table

of the partially loaded screenroom, to determine the field strength variations

which may occur above this type of table.

2. EM Field of a Single-Wire Transmission Line in a Partially Loaded Screenroom

The long-wire antenna, as illustrated in figure 1, is fabricated in a shielded enclosure for use when the test items are too large to be placed in a parallel-plate line or other standard- field chamber. The objective of the NIST

tests was to determine the electric field uniformity along the length of the

long-wire line in a partially loaded screenroom and also to compare the measured

field strengths with theoretically calculated values as a function of vertical

distance below the wire. This long-wire antenna is actually the center

conductor of a transmission line or TEM cell formed by the walls of the
screenroom acting as the outer conductor. The technique for matching the

generator source impedance to the characteristic impedance of the long-wire

transmission line is described in NBS Tech Note 1300 [1] on pages 3.1-3.3.

The equation for calculating the magnitude of the E field beneath a longwire antenna is given in MIL-STD-461 [ 2 ] . The theoretical field strength at a distance c (see figure 1) vertically below the center of the long-wire in a screenroom is given approximately by [1]

E=- 60 V

(V^g) c + 2a 2b c - 2a 2b

c + 2a + 2b c - 2a + 4b c + 4a + 2b 4a - 4b (1)

where E =electric field strength at a distance c below the center of the

long-wire, V/m,

V =RMS voltage at the input to the transmission line, V,

Z= characteristic impedance of the line, Q,

Z =impedance of the generator, fi, o

a =distance from the horizontal wire to the ceiling of the screen

room, m, b =distance from the wire to the floor, m, and

c =vertical distance from the wire to the field point P, m.

When measuring field strength using a calibrated transfer probe such as the NIST EFM-5 [4], the probe should not distort the field being measured.

Equation (1) was used to calculate the electric field strength and this was compared with the measured field for signal frequencies of 1, 2, 3, 5, 7.5, 15,

22, and 30 MHz. The vertical distance (c) to the wire ranged from 0.2 m to 0.8 m below the center of the long wire. The differences between calculated field
using eq . (1), and field strengths measured with the EFM-5 probe (in dB) , were

plotted versus distance to the wire. That is, the dB "differences" plotted in

figure 2 were obtained from the expression [20 log (calculated E/measured E) ]

.

Figure 2 is a graph of the calculated- to -measured field strength ratio as a function of distance below the center of the long-wire antenna, in a partially

loaded screenroom. The three lowest frequencies in the figure (1, 2 and 3 MHz)

are all close to the dB line and thus show the least difference between the

calculated and measured values. However, it can be seen that the variation of

field strength as a function of distance from the wire (slope of the graph) is

generally greater at these lower frequencies. The data indicate that the

electric-field strength is frequency dependent , but does not vary in a systematic manner versus frequency.

3. Measurement of Antenna Factor in a Partially Loaded Screenroom

The measurement of electromagnetic fields in a shielded enclosure using the

MIL-STD 461/462 approach has serious problems because of uncertain antenna

factors caused by multipath reflections from conductive surfaces. The accuracy

of measurements in a shielded enclosure and historical background dealing with

the reliability of measurements in screenrooms , error bounds of antenna factors

measured in a screenroom, and reliability of measured antenna factors for making

screenroom measurements are discussed in NBS Tech Note 1300 [1] on pages 4.1-

4.5.

In this report, measurements of antenna factors in partially loaded and

fully loaded screenrooms are presented and compared with the antenna factors

determined previously for the same antennas in an unloaded screenroom and at the

NIST open-field site. To measure antenna factors in a screenroom, the twoantenna method described in MIL-STD-461A was used. Figure 3 shows the

instrumentation and configuration used for making these measurements . The

screenroom was loaded with rf absorbing material at the two ends of the room,
with the pyramidal points facing parallel to the direction of propagation of the

electromagnetic wave. Figure 4 shows this absorbing material as placed in the

shielded room. The procedure described in the MIL-STD requires antennas with

"identical" characteristics. The equation used for determining antenna factor

is

'A^] -

where K = antenna factor, dB,

F = frequency, MHz,

Vj= measured voltage at the position of the

transmitting antenna when the antenna cables

are directly connected, V,

d = distance between the two antennas as shown in

figure 3, m, and

Vj^= measured voltage of the receiving antenna when

the radiated path is inserted, V.

Using eq (2) and the equipment configuration of figure 3, one can determine an

in situ antenna factor for two identical antennas at any location in a given

shielded enclosure. The derivation of equation (2) is given in reference [1],

pp 4.5-4.6.

Figure 5 is a sketch of the screenroom layout used for measuring the

antenna factors of two biconical antennas in the NIST screenroom. The two

antennas were located in three different positions as shown in the figure.

Figure 6 gives the results for the unloaded screenroom measurement, for the

three positions of figure 5, with a separation distance of 1 m between antennas

Figure 7 gives the results for the partially loaded screenroom, for the same three positions with the same separation distance of 1 m between antennas. A

comparison of figures 6 and 7 indicates that partial loading of the screenroom
achieves a reduction in the screenrcom (cavity) Q, thereby reducing the

screenroom resonance effects and miltipath reflections. As a consequence the

maximum excursions in measured antenna factor are reduced by about 27 dB.

Antenna factors were also measured in the NIST anechoic chamber to

determine the possible beneficial effect of measuring antenna factor in a fully

loaded screenroom. Figure 8 shows the antenna factors measured at the follov/ing

four sites: (a) in the partially loaded screenroom, (b) in the unloaded

screenroom at position 1, (c) in the fully loaded screenroom (NIST anechoic

chamber) , and (d) at the outdoor field site above the NIST groundscreen. All

these antenna factor measurements were made with an antenna separation of 1 m. As one progresses from an unloaded to a fully loaded screenroom, the antenna

factor values approach those v^ich were ctotained at the NIST open field site

above a groundscreen. These data indicate that a fully loaded screenroom (NIST

anechoic chamber) is more likely to produce results v^ich approach the freespace values, even though the rf absorbing material in the anechoic chamber is

not designed for frequencies below 300 MHz.

4. Uniformity of Electric Field Generated by a Transmitting Biconical Antenna

Within a Cubic Volume of a Partially Loaded Screenroom

One criterion for a good standard-field setup is the uniformity of the

generated field as a function of field point position and frequency. Because a conventional screenroom has measurement prctolems due to multiple reflections,

resonances, and distortion of the electromagnetic field, it is difficult to

predict the field generated by a transmitting antenna at a given receiving point

in the screenrocm. Using the NIST designed EIM-5 probe, after calibrating it in

a TEM cell [5] , we measured the electric field in the partially loaded

screenroom. The EFM-5 probe uses electrically short dipoles (5 cm long) that

have a known r^esponse over the frequency range of 0.5 to 1000 MHz. The probe

uses high resistance plastic leads and can be used to measure the field strength
at a point location as a function of frequency without perturbing or distorting

the field being measured.

To measure the electric field uniformity in the partially loaded screen

room, a cubical volume midway between the two ends of the room was defined by

the eight corners of an empty dielectric box. The selected position was with the

center of the box 1 m above the floor and at a distance of 1 m from the center

of the transmitting biconical antenna. The length of each side of the cubical

box was 0.6 m. The location of the transmitting antenna was the same as that

shown for the transmitting antenna in figure 5, position 1. Figure 9 shows the

cubical box with the top four corners designated as lA through 4A and the bottom

four corners designated as IB through 4B. A constant field strength was maintained at the center of the cube throughout the frequency range of 20 to 200

MHz; data were taken in 10 MHz increments. That is, with a constant field

amplitude applied at the center of the cube, as measured with the EFM-5 probe,

the field was then measured at the eight corners of the cube. Figure 10 shows

the measured electric field strength differences at positions lA through 4A,

with respect to the strength at the center of the cube. Figure 11 shows the

measured field differences at positions IB through 4B.

Field scattering by items being tested for EM susceptibility may introduce

additional error when determining the value of an electric field being

generated. In order to ascertain the severity of field distortion caused by a nearby conducting object, a small metal «-box of dimensions 0.21 m wide by 0.16 m high by 0.28 m deep was placed in the center of the cubic test volume (large

dielectric box) . An EM field with the same unperturbed magnitude as that

applied previously without the metal box was generated at the center of the test

volume. The metal box was then introduced, and field strength measurements were

made to determine the change in magnitude of the electric field at the 8 corners

of the cubical volume. Figure 12 is a pictorial representation of this metal

box in the center of the dielectric cube. Figure 13 is a graph of the electric

field differences between the empty and loaded cube, measured at the top 4
corners of the cube. Figure 14 shows the differences measured at the bottom 4 corners of the cube. The data show that for this size of metal box, the fieldstrength perturbations at the 8 corners of the large empty cube, produced by

introducing the small metal box in the center of the cube, were all less than ±

2 dB.

5. Uniformity of Electric Field Generated by a Transmitting Biconical Antenna

Within a Cubic Volume of a Fully Loaded Screenroom

To measure the electric field perturbations in a fully loaded screenroom

(anechoic chamber) , the dielectric cube was placed inside the chamber at a 1 m distance from the transmitting biconical antenna. This cubic volume is the same

as the one used in the partially loaded screenroom. A constant field was maintained at the center of the cube as measured with the EFM-5 probe. That is,

with a constant field amplitude applied to the center of the cube, the field was measured at the eight corners of the cube. Figure 15 shows the measured

electric field strength differences with respect to the center of the cube at

positions lA through 4A. Figure 16 shows the electric field differences at

positions IB through 4B. Comparison of the data of figure 10 with that of

figure 15, and figure 11 with figure 16, indicate that a fully loaded screenroom

has less perturbation than a partially loaded screenroom. Also, the measured

field strength differences with respect to the center of the cubic volume, (0 dB

reference) are much less for the fully loaded room, especially at frequencies

above about 100 MHz

6. Electric Field Distribution on the Grounded Table in a Screenroom

The purpose of the grounded table or "test stand" in a MIL-STD screenroom

is to provide a convenient bench on which to place test equipment. MIL-STD-462

requires that the minimum table size be 2.25 m (24 ft ) and the depth be at

least 0.76 m (2.5 ft). To assure electrical continuity of the ground plane of

the table with the shielded enclosure, MIL-STD-462 states that the bonding
points shall be placed at a distance no greater than 0.30 m (1 ft) apart. In

practice, it is common to bond the seam along the entire length of the bench.

There are three recommended procedures for making susceptibility tests

above the grounded table in a screenroom: (1) monitor the field strength with a receiving antenna at the test sample, (2) monitor at 180° or another angle from

the test sample, and (3) precalibrate and regenerate the electric field. To

monitor the field with a receiving antenna at the test sample, the antenna

factor of the receiving antenna must be known. Data were presented in a previous report to show that this technique is not very accurate and the error

bounds are quite large [1] . Monitoring the field strength at 180° has an even

greater uncertainty and should thus be avoided. To precalibrate the receiving

antenna after placing it in the position of the test sample, and then regenerate

the field strength with the test sample in place of the antenna, will change the

conditions of the field. Precalibration and regeneration of the electric fields

are feasible only if the test sample is in place and the receiving probe is

calibrated and does not perturb the electric field [4]

.

The NIST EFM-5 probe was used to plot the measured electric field strength

generated by a transmitting biconical antenna at various positions on top of the

grounded table. Figure 17 shows the locations of positions 1 through on the

table where field strength measurements were made. The transmitting biconical

antenna was placed with its center at a distance of

2 m from the reference position R. This setup was used to assure that a constant field strength was generated at position R throughout the frequency

range of 20 to 200 MHz. The EFM-5 probe was then placed at positions 1 through and the field strength measured, with the same constant field applied

at position R. These measurements were made under two conditions,

with and without rf absorbing material placed on the wall above the table. The

total area covered by the absorbing material was 3.25 m (35 ft ). The EFM-5

probe was placed at a height of 0.23 m (0.75 ft) above the top of the table.

10
The positions 1 through 5 and positions 6 through had successive displacement

distances of 0.5 m.

Figure 18 shows the measured electric field differences of positions 1

through with respect to position R, with the screenroom wall loaded with

absorber. Figure 19 shows the measured field strength differences with the wall

unloaded. Figure 20 is a graph for position in both the loaded and unloaded

configurations . The data indicate that there is a maximum difference of 16 dB

in field strength in the loaded configuration and a maximum difference of 20 dB

in the unloaded configuration. These data indicate that it is important to

carefully measure the field strength configuration when radiating a sample for

immunity tests, rather than rely on theoretically calculated values for the

field strength.

7 . Summary

The measured E field levels directly beneath the center conductor of a long-wire antenna vary both with frequency and distance from the wire, in either

an empty or partially loaded screenroom. Data were taken at distances varying

between 0.2 and 0.8m below the wire. This variation in field strength as a function of frequency, for a given measurement location, is up to 10 dB over

the frequency range of 1 to 30 MHz. The variation of field strength difference

between measured and calculated values, as a function of distance below the

center conductor of the long-wire, is greater at the lower frequencies (up to 4

dB at 1 MHz) than at the higher frequencies. However, the average field values

at a given distance below the wire are generally closer to the theoretical

values at the lower frequencies.

The two-antenna insertion-loss method as described in MIL-STD-461A was used

at NIST to determine in situ antenna factors in a partially loaded screenroom. Antenna factors were also measured in a fully leaded screenroom (anechoic

chamber) . The maximum antenna factor difference measured at a given frequency

11
and location between the unloaded and partially loaded screenroom, was about 30

dB . The maximum difference at a given frequency and location, between the

partially loaded and fully loaded screenroom, was about 12 dB . Comparison of

antenna factors between the fully loaded room and the NIST open field site

indicates that the maximum difference in this case is only 2 dB.

Measurements of the electric field strength at the 8 corners of a cubical

volume indicate a variability of about 24 dB with respect to the center of the

cube when measured in the partially loaded screenroom. However, the variation

of field strength when measured in a fully loaded room is reduced to about 14

dB . When the field strength is measured at the corners of the cube, and a metal

box is placed at the center of the cube, the difference with/without the metal

box is about ± 2 dB maximum over a frequency range of 20 to 200 MHz.

Measurements of the E field on top of the grounded table in a screenroom,

for positions 1 through 0, indicate that variations may be as great as 28 dB

with the wall above the table loaded with rf absorber. Without absorber, the

variations of E field may be up to 32 dB.

8. Conclusions

The equation given in MIL-STD-462 for calculating the electric field

magnitude beneath a single-wire transmission line in a typical screenroom is

inadequate for either an unloaded or a partially loaded screenroom. The data

obtained at NIST indicate that the field strength varies with frequency in a nonsystematic manner. The measured E field values beneath a single-wire line in

a partially loaded screenroom indicate that reflections cause field

perturbations which are frequency dependent. However, these variations of the

field strength as a function of distance below the center conductor are not as pronounced as those in an unloaded screenroom [1].

12
Measurements of antenna factor in a screenroom by the two -antenna

insertion- loss technique are not an accurate parameter for subsequent

measurement of field strength with the calibrated antenna, either in free space

or at another location in the screenroom. Large errors may occur even in a partially loaded screenroom because of room resonances, multiple reflections,

and distortion of the field. Antenna factors determined in a partially loaded

screenroom are slightly more accurate, relative to the open field site antenna

factors, but they are still subject to variations as a function of antenna

placement and size of the shielded enclosure. If the size of the screenroom

could be increased and the enclosure fully loaded with rf absorbing material,

the error bounds would diminish. In this case a measurement with the calibrated

antenna would lead to a more accurate field strength determination.

Our general conclusion is that it is very beneficial to add absorbing

material to the walls of a screenroom, both for measurement of emissions and for

susceptibility testing. It is even more beneficial to work inside an anechoic

chamber. This conclusion applies even when using microwave absorbers which are specified as "good" only above 500 MHz. The data indicate that the beneficial

effects of this partial loading by microwave absorbers are noticeable at

frequencies as low as 25 MHz,

Measurements of the E- field variation within a cubic enclosure and on top

of the grounded table within a screenroom indicate the magnitude of the problems

in properly determining field strength in a screenroom environment, both in the

unloaded and partially loaded configurations.

9. Recommendations

EMC testing in a shielded room having highly conductive surfaces is

accompanied by large measurement uncertainty, due to multiple reflections and

antenna impedance perturbations. All of the experimental data show large

variability of antenna factors and field strength in the screenroom as a

13
function of frequency and location. Although the error bounds are reduced by

partially loading the screenroom with rf absorbing material, there is still much

ambiguity in determining field strength within the enclosure when using standard

measurement procedures. The following recommendations with respect to the use

of a shielded room are made in order to achieve greater accuracy when making EMC

measurements

1. A shielded room should be used only when it is not feasible to use other facilities, such as an open field site or a fully loaded

screenroom.

2. To minimize the measurement uncertainty caused by making measurements

in different types of shielded enclosures, all screenrooms should be

standardized with respect to their dimensions and internal geometrical

configuration. The standard dimensions should be large enough that the

walls and other surfaces of the enclosure can be loaded with rf

absorbing material. A partially loaded screenroom will minimize the

uncertainty. However, the problem of reflections will still introduce

significant errors in the measurements.

3. Because the fields in a shielded room change with placement of the

equipment being tested, an E- field probe should be used as a transfer

standard for checking the calculated fields in a screenroom

environment. The E- field probe is a small electrically isolated

antenna that has a known response over a given frequency range. Such a probe can be used to measure the field strength as a function of

frequency at each desired test location.

14
10. References

[1] Cruz, J.E., and Larsen E.B. Assessment of Error Bounds for Some Typical MILSTD-461/462 Types of Measurements, U.S. Nat. Bur. of Standards, NBS Tech.

Note No. 1300, 108 pages Oct. 1986.

[2] MIL-STD-461A. Military Standard, Electromagnetic Interference

Characteristics, Requirements for Equipment; 1968 August 1. Also Notice 1,

1969 February 7; Notice 2, 1969 March 20; Notice 3, 1970 May 1; Notice 4,

1971 February 9; and MIL-STD-461B, Notice 1, draft, 1983 January 5.

[3] MIL-STD-462. Military Standard, Electromagnetic Interference

Characteristics, Measurement of; 1967 July 31. Also Notice 1; 1968

August 1, Notice 2; 1970 May 1, Notice 3; 1971 February 9, and MIL-STD-462A,

draft; 1970 June 29.

[4] Larsen, E.B., and Ries , F.X. Design and Calibration of the NBS Isotropic

Electric-Field Monitor (EFM-5), 0.2 to 1000 MHz, U.S. Nat. Bur. of

Standards, NBS Tech. Note 1033, 104 pages March 1981.

[5] Crawford, M.L. Generation of Standard EM Fields using TEM Transmission

Cells, IEEE Trans. Electromag. Compat. , EMC -16 No. 4, pp 189-195 November

1970.

15
Insulator

Long-Wire Antenna

t

C

I

T

Insulator

R2 = Zl

I

Figure 1. The Icng-wiie antenna cxxifiguraticxi in

a screenrocm.

16
7.5 MHz

PQ

-a

o

c

1-

OJ <+a

OJ

73

(U

L.

13

CO

rtJ (U z:

Io

+->

I a

(U -p

(n

U

rd U

0.2 0.4 0.G 0.8

Distance Belou Center Conductor, m

Figure 2. Calculated- to-measured field strength versus

distance below the long-wire for selected

frequencies in a partially loaded screenroom.

17
1 • • • • Transi

Ant€

nitting Rece

jnna Ante

d ri k

'iving

mna

• • • • 4

1 \

/ d =1 nneter \

/ (or 3 meters) \ /

Signal

Gen.

50 Q

Output

RCVR

50 Q

Input

10 dB

PAD

10 dB

1 1 PAD

1 1

• • • •

1 1

• • • • <

L--- —^ )

Cable Connector

•

1

1

1

J

Screenroom

Figure 3. Test setLp required by Mn/-STD-461A foa:

detjennination of antenna factors.

18
a. Rear end of screenroom

b. Front end of screenroom

Figure 4. Placement of the rf loading (absorber material) in

the NIST shielded room.

19
0.78 ra Copper Clad Table

\5

fm

Tripod Leg

Transm ItLlng ^^^^ ^:^^ ^^^\jB ^conical. i ^^^^ ^X*^ YAnLenna

d - 1 ra ^°^- ^ei^ht oC antenna

fc>r 3 m abcwe floor - 1 m

Receiving

Biconlcal Antenna

U m

Coax Cable Position 1 Position 2

i 1 0.78 m Y

Copper CLad Table

Coax Cable ^ >^\^^llL \ 0^ / CA^

--^^

^^^^^^^O'A ^^'^^toT ran sm itLing TVT \ yBiconical ^^ \ \ lAntenna

y^*** Coax Cabla

Position 3

Figure 5. Sketch of three screenroom configurations for

measuring the antenna factor of two identical

antennas

.

20
1 1 1 1 1 1 50 ._ n POSITION—

1

/ POSITION—

2

45 - / POSITION 3

40

J

1 ^

35

/a/\ 3 K I I -

J

30

o

•^ 25

o

fd

h 20

^

\ \ / \ \ A/Vo / /

fd

C 15

w

1 10

4f

"^ 5 •ff>''*^ *

e — II 1 1 1 1 20 40 60 80 100 120' 140

Frequency, MHz

1G0 180

Figure 6. Antenna factors measured in an unloaded

screenroom at a separation distance of 1 m, for

three selected positions.

21
50

1 1 1 ! POSITION-—!

POSITION—

! 1

45 POSITION—

—

40 —

59 35 a

__

-^ 30 ._ __

L-o

-^ 25 — , u A

fd / \

Ll. 20 - /A 3v

fd / \\ /\ A / i/X^ j

c 15 y2.-'^^^\\ A^ / a\^/nT^yX >^

c ^:^^ \i \ / /\ Q / / /y^^ ^--.^^^;A^

(D » \ Q^ / •\j>^ Vv /^'J / y^ tT +^ 10 ~ \ \ /I \^d^^^'^y —

C J^ \. / / /'^ 2^ v---'^^^ ^"^ ^

5 >^*^ —

1 1 1 20 40 G0 80 100 120

FVequency,

140 1G0

MHz

180

Figure 7. Antenna factors measured in a partially loaded

screenroom at a separation distance of 1 m, for

three selected positions.

22
i 1 1 50

/^

UNLOHDED SCREENROOMLOflDED SCREENROOM——U

45 / HNECHOIC CHRMBER —R -

40 - /

GROUNDSCREEN —

G

o

35 7

—

L- 30 ^ I o

+->u

(V

h

25

20 -A A /

C 15

10 ^\l Kp^ \^ i

""^-^

(1

5

^

1 1 1 1 1 20 40 60 80 100 120 140

F"requency, MHz

160 180

Figure 8. Comparison of antenna factors measured in four

different facilities, with aim separation

between antennas.

23
Figure 9. Identification of the eight" corners of a cube for

measuring the field strength differences with

respect to the center of the cube.

24
T5

UJ

2:

ci: UJ

b_

a

_j

UJ

»—

UI—

a:

hUJ

_j

UI

80 100 120

FREQUENCY MHz

Figure 10. Electric field differences of the top 4 corners

of a cube with respect to the center of the cube,

in a partially loaded screenroom.

25
D3

-a

Ld

u

2:

Ld

QlU

Li_

Li_

a

a

_j

bj

l-H Li_

On

I—

CJ

ioJ _J

LJ

80 100 120 140

FREQUENCY MHz

1G0 180

Figure 11. Electric field differences of the bottom 4

corners of a cube with respect to the center of

the cube, in a partially loaded screenroom.

26
Figure 12. Sketch of a cubic volume with an enclosed metal

box, for measuring the field distortion caused by

the metal box.

27
m

-a

bJ

Ld

UJ

O

a

_J

Lu

n

(J

UJ

_J

UJ

20 40 60 80 100 120 140

FREQUENCY MHz

1G0

Figure 13. Electric field differences of the top 4 corners

of a cube with respect to the center of the empty

cube, caused by inserting a rectangular metal

box.

28
CD

-a

z

bJ

q:u

u.

u.o

CD

_J

Ld

I—

Li- U

q:

HCJ

Ld

_JU

80 100 120 140

FREQUENCY MHz

Figure 14. Electric field differences of the bottom 4

corners of a cube with respect to the center of

the empty cube, caused by inserting a rectangular

metal box.

29
12

10

8

PQ

~o G

UJ 4

o

z:UJ Q

en

t_ LJ

HHO

-?

UJ

_J

UJ -4

U_

CJ -R n

Ql

-8

UJ

UJ -10

12

20 40 60 80 100 120 140

FREQUENCY MHz

1G0 180

Figure 15. Electric field differences of the top 4 corners

of a cube with respect to the center of the cube,

in a fully loaded screenroom (anechoic chamber)

30
u

z

UJ

q:

UJ

L_

Li_ o

a

_j

Ld

»—

o

LJ

UJ -

80 100 120 M0

FREQUENCY MHz

Figure 16. Electric field differences of the bottom 4

corners of a cube with respect to the center of

the cube, in a fully loaded screenroom (anechoic

chamber)

31
C)

o

C)

R

C)

9

8

7

6

Transmitting

bioonical anbeama

Figure 17. Measurenpait locaticn in the scaneenrocm for deteminaticai of the electric

field above the grounded table.

32
80 100 120 M0

FREQUENCY MHz

Figure 18. Electric field differences on the grounded table

for 10 positions, using the center of the table

(position R) , with the wall loaded, as the dB

reference

33
UJ

IG

rn

-D i;:^

UJ 8

u^

Ul 4 (Y

UJ

IjLjPI HH

C=^

1 -4

Ld

^-^ h -B

O

HH

CK -12

1—

CJ

hi

1 -IB

20 40 60 80 100 120 140 160 180

FREQUENCY MHz

Figure 19. Electric field differences on the grounded table

for 10 positions, using the center of the table

(position R) , with the wall unloaded, as the dB

reference

34
PQ

no

LJ

2:

LJ

LJ

Ll_

Lx_ l-H a

_j

i_d

I—

I

u

CK

I—

Ld

_J

Ld

1 lit > ' ;i ' 1

'

1

' ' /\ LORDED L

' 1

'

IG / \UNL0RDED—

U

—

12 — / w A8 u \\ K f^ / \

4 — B W l\f^~\ /

~

— \Y/

^ / -

-4 — —

-8 —

12 —

IG

1 1 1 1 1 ill ill 1 1 1 20 40 60 80 100 120 140 1G0 180

FREQUENCY MHz

Figure 20. Electric field differences between position R and

position (of figure 17) on the grounded

table, using position R as the dB reference,

with the wall either loaded or unloaded.

35

NBS-IUA (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NIST/TN-1320

2. Performing Organ. Report No. 3. Publication Date March 1989

4. TITLE AND SUBTITLE

Alternative Techniques for Some Typical MIL-STD-461/462 Types of Measurements

5. AUTHOR(S)

J.E. Cruz and E.B. Larsen

6. PERFORMING ORGANIZATION (If joint or other tlian NBS. see instructions)

National Institute of Standards and Technology

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

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

U.S. Army Aviation Systems Command

AMSAV-QE

St. Louis, MO 63120

10. SUPPLEMENTARY NOTES

\2^ Document describes a computer program; SF-185, FlPS Software Summary, is attached.

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

bibliography or literature survey, mention it here)

Most testing for MIL-STD-461/462 is performed in a shielded enclosure

(screenroom) which leads to uncertainty in the measurement of emissions from

electronic equipment, or the susceptibility of equipment to electromagnetic

radiation. Possible alternative techniques for improved measurements in a screenroom have been developed by the National Institute of Standards and

Technology (NIST) . These techniques are covered in this report.

This report presents antenna factors determined in a screenroom which was partially loaded with radio frequency (rf) absorbing material, using the

two-antenna insertion-loss technique. These antenna factors are compared with

the antenna factors obtained in an unloaded screenroom, a fully loaded

screenroom (anechoic chamber), and at an open field site. In addition,

measurements at the eight corners of a cube were made in the partially loaded

and fully loaded screenroom to determine the field deviation at the eight corners of the cube with respect to its center. Also, measurement improvements

are quantified for the electric-field strength beneath a single-wire transmission

line, in a partially loaded screenroom. Finally, electric-field measurements

were made on top of the grounded table in a partially loaded screenroom to

determine the field strength variation above the table.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate ke/ words by semicolons)

antenna factor; electromagnetic compatibility; field strength standards;

loaded screenroom; military standards; screenroom measurements

13. AVAILABILITY

gH Unlimited

I I For Official Distribution. Do Not Release to NTIS

[X3 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

\2Z\ Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

44

15. Price

(JPO 675-065/5058

USCOMM-DC 6043-P80


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