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

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Kanda, M/Evaluatlon of otf-axis measurem QC100 U5753 NO.1305 V1986 C.2 NBS-PUB-C

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..*' NBS TECHNICAL NOTE 1305

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
7^

m he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The

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Ciaiihcrsburg. MI) 20899. '.Some divisions wiihin the ccnicr are located ai Boulder. CO 80303.

'l (Kaicd ai Boulder, CO. with some cicmenis ai Ciaiiticrsburg, MD.
RESEARCH

iUFORMATlON CENTER

Evaluation of Off-Axis IVIeasurements

Performed in an Anechoic Chamber

M. Kanda

J.C. Wyss

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

O

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

NATIONAL BUREAU OF STANDARDS, Ernest Annbler, Director

Issued October 1986
National Bureau of Standards Technical Note 1305

Natl. Bur. Stand. (U.S.), Tech Note 1305, 40 pages (Oct. 1986)

CODEN:NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1986

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

Page

1

Introduct i on 1 2. Experiment Setup 2 3. Results and Conclusions 4

4. References 6

5. List of Figures 7

m

Evaluation of Off-Axis Measurements Performed

in an Anechoic Chamber

M. Kanda and J. C. Wyss

Electromagnetic Fields Division

National Bureau of Standards

Boulder, CO 80303

Field strength versus distance from various source antennas

is measured in a rectangular rf anechoic chamber on axes parallel

to the boresight axis. An electrically small field probe is

repeatedly scanned longitudinally away from the launch antenna and

into the chamber. With each scan various parameters are changed,

including: 1) horizontal and vertical position of the probe with

respect to the center line of the launch antenna; 2) frequency,

and 3) type of launch antenna. With the probe located 1 m off the

center line and scanning between 2 and 6 m from the launch horn,

the uncertainty due to being off the center line ranges from il dB

at 250 MHz to ±5.0 dB at 800 MHz and above. If the probe is within ±50 cm of center line, the uncertainty is no more than ±1.5 dB

at 800 MHz; and, for ±25 cm from center line the uncertainty is

further reduced to ±0.5 dB at 800 MHz.

Keyv^ords: anechoic chamber; open-end waveguide; probe; pyramidal

horn

1. Introduction

A side view of the rf anechoic chamber room is shown in figure 1. The

chamber is illuminated by placing a pyramidal horn or an open-end waveguide

(OEG) in a doorway with their apertures in the plane of the absorber points on the chamber wall. A receiving probe is placed on a computer controlled cart.

Riding on precision tracks, the cart can move up to 6 m into the chamber, using

stepper motors.

In a previous paper, FitzGerrell describes an evaluation of the anechoic

chamber where measurements are made down the center line of the source antennas

[1]. In that experiment electrically small dipoles or OEG transmitting an- tennas were located on the cart while source antennas, used as receivers, were

placed in the door. Comparing the experimental results to theoretical calculations of the center-line field, FitzGerrell showed excellent agreement between

the two. In addition he determined anechoic chamber errors due to reflections

to range from -0.6 to +0.5 dB at 229 MHz up to ±0.04 dB at 18 GHz. These

reflection errors appear as smooth oscillations in the experimental data.
In the experiment reported in this paper, the anechoic chamber errors due

to off-axis misalignment of the receiving antenna with respect to the source

antenna are determined. The error is measured by comparing center-line data

with data taken at various distances off the center line. By quantifying this

source of error, we will be able to provide criteria for alignment of the

probes. This measurement will also be included in the anechoic chamber errorbudget analysis that is presently being performed [2].

2. Experiment Setup

In the present chamber-evaluation test, the source antennas are located

at the doorway and the receiving probe is located on the movable cart (fig.

1). The receiving probe, an electric-field monitor designated EFM-3, consists

of a spatially isotropic, broadband (100 to 3,000 MHz) antenna developed by

NBS [3]. The probe is connected to a receiver box by a nonperturbing, highresistance cable. The receiver output ranges from to 2 Vdc representing a field strength of to 20 dB above 1 V/m at the probe. This probe output is

put into the cart's on-board computer, where it is converted to a digital

signal. The cart's computer also controls the stepper motors which move the

cart back and forth along two precision guide rails. A laboratory computer,

through a fiber optical data link, communicates with the cart's computer

telling it to 1) digitize and transmit the probe output to the lab computer

for storage on a floppy disk; and 2) move the cart a predetermined distance.

For a typical scan, the probe would initially be manually set to 1.00 ± 0.005 m from the source antenna aperture. The source field would then be set to

provide a 20 dB V/m (or 10 V/m) field reading on the receiver probe. The

laboratory computer would then instruct the cart computer to move away at a constant rate from the source antenna and to simultaneously digitize a reading

from the probe after each 5 cm movement. As each reading is taken and

digitized, it is sent over a fiber-optic line back to the laboratory computer

for storage and later manipulation.

The receiving probe is mounted on a 2-m wide by 2-m high wood frame that

has holes drilled into it for holding the probe at eighteen different locations. As shown in figure 2, the center location (position 1) of the frame is

along the center line of the source antenna. At each frequency, the probe

would first be placed in position 1 for the first scan. To check the

2
reproducibility of the measurement system, the position 1 scan would typically

be repeated once or twice. For the same source frequency and field intensity,

the probe would then be moved to other positions on the frame and the measurement scan repeated. By comparing the off-axis measurements (positions 2-9,

21-23, 41-43, 61-63) with the center line measurement (position 1), we are able to determine the anechoic chamber measurement error due to off-axis misalignment of source and receiver antennas.

To make the off-axis error evaluation meaningful, it is necessary to con- sider other variables of the measurement system that could contribute to or "mask" the off-axis error measurement. Those other variables will be discussed here: 1) The intensity stability of the field generation system was measured as a function of time for periods ranging from 0.5 to 2.0 h. In the

first 30 min of warmup of the system, the power-level maximum variation is

±0.5 dB (one standard deviation). During the next 1.5 h, the power-level

variation dropped to less than ±0.1 dB (one standard deviation). 2) The EFM-3

probe was calibrated versus electric-field strength at the beginning and

ending of each day, and the daily variation between these measurements was found to be ±0.2 dB. Over nine days of measurement (where the probe batteries

are recharged each night) the calibration error varied ±0.56 dB (one standard

deviation). The calibration error from adjacent scan to scan (separated by a few minutes) should be no more than ±0.1 dB. 3) The uncertainty in the cart

position was measured to be ±2 cm. 4) The source antenna is held in place

vertically by a hydraulic lift which sags or drops about 1 cm/h. Before each

scan the source antenna height was remeasured and adjusted to be within ±1 cm

of the initial height on the first scan. 5) The uncertainty due to the

placement of absorber material around a horn, transmitting at 800 MHz, and in

the rest of the door was measured to be no more than ± 0.15 dB (one standard

deviation) as shown in figures 3 and 4. Four separate scans are shown in

figure 3: a) horn aperture plane aligned with tips of wall absorbers, no absorber in door area, b) horn moved 0.79 m into chamber, no absorber in door

area, c) absorber added to side of horn, filling half of door area, and d)

additional absorber added above and below the horn (the horn remained 0.79 m into the chamber in the last two scans). The variation of these measurements

is shown in figure 4 where the standard deviation between the four scans is

plotted at each point versus the distance from the horn. The worst value
occiirring after the 5 m mark is ±0.15 dB. These measurements are made with

the receiving probe at the center line of the source antenna.

The overall reproducibility of a scan, determined for consecutive scans

and for repeated scans on different days, is a measure of the total uncer- tainty due to all of the possible error sources already mentioned. With the source antenna transmitting at 800 MHz and the receiving probe placed on the

center line of the source antenna (position 1 of fig. 2), four scans were made

on one day. The probe was then removed and recalibrated. On the following

day the probe was returned to the center line position and four additional

scans were taken. In figure 5 the eight scans are plotted versus distance

from the source antenna. Note that the eight scans form two groups of four

scans each where the groups are separated by about 1.5 dB. The lower group

represent the four scans on the first day while the upper group are the four

scans on the second day. To measure the variability of the scans consider

figure 6. The lov/er curve represents the point-by-point repeatability for the

first four scans (the lower group of fig. 5) and does not exceed ±0.15 dB (one

standard deviation). The upper curve is the point-by-point calculated

standard deviation using all eight scans from both days. The upper curve

gives a measure of the day-to-day repeatability of a measurement and equals

±0.5 dB at its worst. Where more precise measurements are needed on a day-today basis, the field can be set manually using a standard dipole antenna as a test probe to monitor the absolute field strength.

For the present work, the center-line scan is to be compared to off -axis

line scans. Since the compared scans are taken one after another without

changing the initially set field strength, the error from lack of reproducibility should be the same as the adjacent scan-to-scan reproducibility of

±0.15 dB.

3. Results and Conclusions

In figure 7a, the center-line scan (position 1 on frame) is plotted along

with scans v/here the probe has been moved horizontally and vertically -tl ^

(fig. 7b). An open-ended v/aveguide with 250-MHz output is used. In figure 8,

a standard deviation is calculated for these five scans and shows that the

off -axis error is less than ^,o.8 dB from 2.3 to 5.2 m from the horn. In figure 9a, five more scans at 250 MHz are taken with the probe spacing now

4
±1.41 m (fig. 9b), and figure 10 shows that the off -axis error now increases

to ±1.0 dB from 2.4 to 4.8 m.

With pyramidal horns instead of OEGs , the off -axis error gets larger as the frequency increases. At 600 MHz and 700 MHz only two off-axis scans are taken. These show that the off-axis error is now +2.5 dB (one standard

deviation) or less for 600 MHz, 2.5 to 5.9 m; and ±2.5 dB for 700 MHz at 3.0

to 5.9 m (see figs. 11 through 14.) At 800 MHz (figs. 15 and 16), the error

was ±2.5 dB for 2.5 to 6.5 m; at 900 MHz (figs. 17 and 18), the error was i2.5 dB for 2.5 to 6.5 m; and, at 1000 MHz (figs. 19 and 20), the error was ±3.0 dB for 2.5 to 6.5 m.

Clearly, the pyramidal horns at higher frequencies have a greater offaxis error contribution. Another way of putting this is to say that for equal error contributions, a smaller volume is available at higher frequencies. To

investigate this smaller volume at higher frequencies, additional measurements

were made at 800 MHz using a smaller offset on the probe. Figures 21 and 22

show that for a 75-cm offset, the error is less than ±2.0 dB; figures 23 and

24 show that a 50-cm offset yields an error value of ±1.4 dB; and, figures 25

and 26 show that a 25-cm offset yields an error value of ±0.6 dB (where error

values are for 2.5 to 5.3 m distances from horn).

The results are summarized in table 1. In general a 1.0-m offset from

center line can contribute an error of ±3.0 dB at high frequencies and only a ±0.8 dB error at lower frequencies. If the offset volume is reduced to less

than ±25 cm from center line, the error will be less than ±1.0 dB at all

frequencies.

In figure 27 the offset error (one standard deviation) is plotted versus offset distance at 800 MHz. To reduce the offset error to ±1.0 dB, the center

line offset must be less than ±50 cm. This should be readily attainable in

the present anechoic chamber arrangement.
Table 1. Error due to off-axis alignment.

Offset distance from center line (m) 0.25

(d6)

0.50

(d6)

0.75

(dB)

1.00

(dB)

Frequen

(MHz)

cy Soijrce ant

type

enna

250 OEG ±0.8

600 Horn ±2.5

700 Horn ±2.5

800 Horn to.

±1.2 ±2.0 ±2.5

900 Horn ±2.5

1000 Horn ±3.0

4. References

[1] FitzGerrell, Richard G. Using free-space transmission loss for

evaluating anechoic chamber performance. IEEE Trans. Electromagn.

Compatib. EMC-24: 356; 1982.

[2] Kanda, M., to be published.

[3] Larsen, E. B. and Ries, F. X. Design and calibration of the NBS

isotropic electric-field monitor (EFM-5), 0.2 to 1000 MHz. Nat. Bur,

Stand. (U.S.) Tech Note 1033; 1981 March. 104 p.
List of Figures

Page

1. A side view of the NBS-Boulder Anechoic Chamber. 9

2. Frame used for holding probe on cart. Positions 1 through 9 are separated by 1 m intervals. The

numbered positions (21-24, 41-44 and 61-64) are each

separated by 25 cm. 10

3. Field strength is plotted versus distance from horn

for four separate scans at 800 MHz. The probe is at

the center position (number 1 in fig. 2). For each

scan, additional absorber was placed in doorway to

the chamber and around the horn. 11

4. Standard deviation versus distance from horn is

plotted. Using the four scans plotted in figure 3, • a standard deviation is calculated at each point and

plotted here. 12

5. Field strength versus distance from horn is plotted

for eight different runs taken on two different days.

Four scans from the first day make up a lower group

while the four scans on the second day form the

upper group. 13

6. Standard deviation versus distance from horn is plotted.

The lower curve is calculated from the four scans of

the first day plotted in figure 5 (lower group). The

upper scan results when all eight scans from both days

are included. 14

7. a) Field strength versus distance from OEG. Five scans

are plotted for 250 MHz and include the center line,

position 1 (solid line) and the 1 m off-axis scans,

positions 2, 4, 6, 8 (dashed lines), b) Shows positions

of probe on frame. 15

8. Standard deviation versus distance from OEG using scans

of figure 7. 16

9. a) Field strength versus distance from OEG. Five scans

are plotted for 250 MHz and include the center line,

position 1 (solid line) and the 1.4 m off-axis scans,

positions 3, 5, 7, 9 (dashed lines), b) Shows positions

3, 5, 7, 9 (dashed lines). 17

10. Standard deviation versus distance from OEG calculated

from scans shown in figure 9. 18
Page

11. a) Field strength versus distance from horn for 1 m

off -axis position at 600 MHz. b) Locations of probe on frame for scans. 19

12. Standard deviation versus distance from horn for scans

shown in figure 11. 20

13. a) Field strength versus distance from horn for 1 m off-axis position at 700 MHz. b) Locations of probe

for these scans. 21

14. Standard deviation versus distance from horn for scans shown in figure 13. 22

15. a) Field strength versus distance from horn for 1 m off-axis positions at 800 MHz. b) Locations of probe. 23

16. Standard deviation versus distance from horn for scans in figure 15a. 24

17. a) Field strength versus distance from horn for 1 m off-axis positions at 900 MHz. b) Locations of probe. 25

18. Standard deviation versus distance from horn for scans

shown in figure 17a. 26

19. a) Field strength versus distance from horn for 1 m off-axis positions at 1000 MHz. b) Locations of probe. 27

20. Standard deviation versus distance from horn for scans

shown in figure 19a. 28

21. a) Field strength versus distance from horn for 75 cm off-axis positions at 800 MHz. b) Locations of probe. 29

22. Standard deviation versus distance from horn for scans

of figure 21a. 30

23. a) Field strengths versus distance from horn for 50 cm off-axis positions at 800 MHz. b) Locations of probe. 31

24. Standard deviation versus distance from horn for scans shown in figure 23a. 32

25. a) Field strengths versus distance from horn for 25 cm off-axis positions at 800 MHz. b) Locations of probe. 33

26. Standard deviation versus distance from horn for scans

of figure 25a. 34

27. Standard deviation versus offset from center line at

800 MHz. In each case, the worst case standard deviation

is taken for distances more than 2 m from the launching horn. 35
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Off- Set from On -Ax is (m)

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Figure 27. Standard deviation versus offset from center line at 800 MHz. In each case, the worst case standard deviation is taken for

distances more than 2 m from the launching horn.

35
NBS-n4A IREV. 2-8C1 U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions^

1. PUBLICATION OR

REPORT NO.

NBS/TN-1305

2. Performing Organ. Report No, 3. Publ Ication Date

October 1986

4. TITLE AND SUBTITLE

EVALUATION OF OFF-AXIS MEASUREMENTS PERFORMED IN AN ANECHOIC CHAMBER

5. AUTHOR(S)

M. Kanda and J.C. Wyss

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struct/on sj

NATIONAL BUREAU OF STANDARDS

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)

10. SUPPLEMENTARY NOTES

[^n Document describes a computer program; SF-185, PIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual sun^nr)ary of most significant infornnation. If document includes a significant

bib/iogrophy or literature survey, mention it here)

Field strength versus distance from various source antennas

is measured in a rectangular rf anechoic chamber on axes parallel

to the boresight axis. An electrically small field probe is

repeatedly scanned longitudinally away from the launch antenna and

into the chamber. With each scan various parameters are changed,

including: 1) horizontal and vertical position of the probe with

respect to the center line of the launch antenna; 2) frequency,

and 3) type of launch antenna. With the probe located 1 m off the

center line and scanning between 2 and 6 m from the launch horn,

the uncertainty due to being off the center line ranges from ±1 dB

at 250 MHz to ±5.0 dB at 800 MHz and above. If the probe is within ±50 cm of center line, the uncertainty is no more than ±1.5 dB

at 800 MHz; and, for ±25 cm from center line the uncertainty is

further reduced to ±0.5 dB at 800 MHz.

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

anechoic chamber; open-end waveguide; probe; pyramidal horn

13. AVAILABILITY

[Xl Unlimited

I I For Official Distribution. Do Not Release to NTIS

[X] 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

40

15. Price

•U.S. GOVERNMCrJT Pr?INTING OFFICE: t 9 8 60-7 7 3-59 6/65 1 9 USCOMM-DC 6043-P80
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