NISTIR 89-3927

RECENT IMPROVEMENTS IN

TIME-DOMAIN EMC MEASUREMENT

SYSTEM

J.W. Adams

A.R. Ondrejka

K. H. Cavcey

J.E. Cruz

H.W. Medley

J.H. Grosvenor, Jr.

National Institute of Standards and Technology

U.S. Department of Commerce

Boulder, Colorado 80303-3328

November 1989

NISTIR 89-3927

RECENT IMPROVEMENTS IN

TIME-DOMAIN EMC MEASUREMENT

SYSTEM

J.W. Adams

A.R. Ondrejka

K. H. Cavcey

J.E. Cruz

H.W. Medley

J.H. Grosvenor, Jr.

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

November 1989

Sponsored by

U.S. Army Aviation Systems Command

St. Louis, Missouri 63120

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

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

CONTENTS

Page

1.0 Background 1

2.0 Measurement Systems 2

3.0 Measured Data 5

4.0 Needed Improvements 6

5.0 Acknowledgments 9

6.0 References 9

iii

RECENT IMPROVEMENTS IN TIME -DOMAIN EMC MEASUREMENT SYSTEM

J.W. Adams, A.R. Ondrejka, K.H. Cavcey, J.E. Cruz,

H.W. Medley, and J.H. Grosvenor, Jr.

Improved techniques for determining critical resonant frequencies

and the current response of internal wiring due to external fields

for rotary-wing aircraft are given. The measurement method uses a train of low-level, radiated pulses. These do not disturb other

spectrum users, nor do other spectrum users significantly disturb

these measurements. The fields are low, a distinct advantage from

both cost and personnel hazard standpoints. The problems that

should be addressed before the full potential of the technique can

be realized are discussed.

Key words: induced currents, natural resonant frequencies, TEM horn

antennas, time-domain measurement system, weak source fields.

1 . 0 Background

The National Institute of Standards and Technology (NIST, formerly

NBS) has been developing time-domain techniques and calibration facilities

for a number of years [1]. More recently, AVSCOM has supported our efforts

to apply these techniques to whole-system measurements for aircraft

electromagnetic compatibility (EMC) work [2], an area that is weakly covered

by MIL- STD 6051.

The basic idea is to transmit an impulse over a large enough area to

expose a complete aircraft to a radiated electromagnetic field. The

scattered or reflected response of the aircraft can be sensed with a

receiving antenna, digitized, recorded, and processed by any of several

mathematical algorithms to give the spectral response. These data contain

information on the natural resonant frequencies of the airframe, frequencies

that will significantly enhance currents induced on internal wiring within

the airframe. These frequencies should be examined carefully in any EMC

testing

.

An extension to this basic idea is to use another receiving system to

directly measure current waveforms induced on internal wiring by this

external field. If these measured current-time functions can be related to

the external electromagnetic (EM) field and if several other criteria can be

satisfied, it may be possible to substitute conducted injection testing for

radiated susceptibility testing. This has great potential since wholesystem radiated susceptibility testing cannot be done at present.

There are several essential components required for such a system.

Many of these have now been developed and tested. ' Some additional key steps

need further work. These issues will be discussed later.

Working in the time domain offers some very important advantages over

working in the frequency domain. In the frequency domain, it is necessary

to measure both amplitude and phase in order to reconstruct a waveform.

Amplitude is easy to measure, but phase is difficult to measure. If we work

only with amplitudes, for example, we may assume that the intervening

networks are minimum-phase in order to be able to deduce phase from

amplitude [3]. This assumption is hard to justify. In the time domain, the

recorded time series contains all the information in the waveform.

1
Conversions to the frequency domain are subject only to limitations of the

mathematical algorithm selected.

Another important advantage of working in the time domain is the

capability to avoid interference with other spectrum users. This can be

implemented by use of a train of low-power, periodic pulses and fiber-optic

triggering to achieve selective coherence. Fiber-optic trigger lines from a clocked pulse generator to the transmitter and to each of the receiving

systems allow these receiving systems to be synchronized with the

transmitted pulses and incoherent with the rest of the electromagnetic

environment. This allows averaging -- effectively trading time for power.

The train of pulses are of such low power that they cannot be detected 100 m

away from the transmitting antenna. External fields do not interfere with

our measurement system since they are not coherent with«our transmitted

pulses

.

Advantages of working with small signals as opposed to strong signals

are: first, wave shapes of small signals have better repeatability than

those of strong signals needed for single-event time domain systems such as

those used presently in nuclear electromagnetic pulse (EMP) testing, and

second, possible hazards to personnel are less for lower fields than for

higher fields.

There are limitations to the applicability of these time-domain

techniques. The spatial arrangements among source antennas, aircraft and

receiving systems must remain constant during the measurement time;

otherwise the repeatability needed for the averaging is lost. Therefore,

for the same reason, moving aircraft, even with the rotor turning, cannot be

characterized this way. The periodicity of the train of pulses (small time

jitter) must be maintained to great accuracy, but this is easy to achieve

with currently available equipment, so it is not a practical limitation.

The scattered signals and induced currents must be strong enough to be

detected within a practical time interval for averaging. The area where the

measurement system and aircraft are located must be clear of reflective

obstructions to minimize distortions of the desired received signal. Since

the received waveforms are analog, they must be filtered adequately,

relative to the sampling rate, before being digitized to prevent aliasing.

Aliasing is the contamination of low frequency signals by the "folding down

about the Nyquist frequency" of higher frequency signals when data are

converted from analog to digital form. Special, linear-phase filter designs

are needed so as not to distort the waveforms.

These techniques have been applied at three locations using

helicopters. The first measurement system was used at Ft. Rucker, Alabama,

in 1986. An improved measurement system was used at Western Army Area

Training Site (WAATS) near Tucson, Arizona, and at Ft. Huachuca, Arizona, in

1988. This document discusses the more recent work.

2 . 0 Measurement Systems

The complete measurement system consists of one transmitting system

and two receiving systems. All systems usually operate simultaneously.

Both receiving systems record time-series waveforms which are generated in

response to the train of impulses from the transmitting system. A block

diagram of these systems is shown in figure 1. A photograph, figure 2,

shows a helicopter, both transmitting and receiving antennas, and an edge of

the van where the balance of the system is located. The fiber optic line is

2
protected by a rope and cinder blocks. The source consists of a step

generator that drives a transverse electromagnetic (TEM) horn antenna. This

step generator is triggered via a fiber optic line by the same pulse

generator that synchronizes the receiving systems. The antenna radiates a series of fast rise time (2 ns) impulses of electromagnetic energy. A

variable time delay proportional to separation distances is added to the

receiving systems so that coherent detection of the transmitted signal is

possible

.

The transmitting and receiving antennas have flat amplitude and

linear phase characteristics. This allows radiation and reception of pulses

with very little distortion of the waveform. Their design is essentially a tapered TEM horn [4]

;

they are about 2 m in length. This is sufficient to

extend their low- frequency range down to about 5 MHz. These antennas are

supported on non-metallic tripods and guyed with nylon ropes to add

stability during windy conditions such as may be caused by a hovering

aircraft. These support structures are essentially transparent to EM

fields. The antennas can be mounted on the tripods to give either vertical

or horizontal polarization.

The first receiving system (outside the aircraft) detects the signal

reflected from the aircraft. This waveform contains a combination of damped

sinusoids which have the natural resonant frequencies of the aircraft

excited by the transmitted impulse. A time -averaging sampling oscilloscope

is used as the receiver. Reflections from objects other than the aircraft

may be rejected by 'time windowing,' that is selecting portions of a received time waveform.

If a reflecting object other than the aircraft is located so that its

reflected signal arrives at the same time as a signal reflected from the

aircraft, the waveform contain both signals, and the analysis is much more

difficult. This sets the requirement for an unobstructed test area. This

clear area should be approximately a circle of radius 4 times the separation

distance from the transmitting antenna to the aircraft under test and

centered on the aircraft's axis of rotation.

The received signals are filtered, digitized, and stored on disk for

subsequent processing. The digitized, time-domain waveforms are checked at

time of recording to see that proper scale settings, for example, are used

on the digitizing oscilloscopes, so as to adequately record the waveforms.

Usually, the same settings are maintained throughout a set of measurements.

The oscilloscope settings are recorded as part of the data file.

The second receiving system (inside the aircraft) measures the

current induced on a particular wire or bundle of wires. Again, the

response is a time series of damped sinusoids. This system consists of a current clamp, amplifier, filter, and a sampling digitizing oscilloscope. A

laptop computer is used to record the data on disk. Data are transferred

from the oscilloscope to the laptop computer using an RS-232 link and stored

on disks. These data are then processed later using the same procedures

that are used for data from the first receiving system.

Since these data are digitized before they are recorded, the input

signals must be filtered before recording to prevent aliasing errors. A

special set of linear-phase filters which have cutoff frequencies of 64, 128

or 256 MHz is used. The current clamp has a frequency response only

3
slightly higher than the 128 MHz filters, so in this case the 128 MHz

filters are used for most digitizing.

A reference waveform should be recorded with the aircraft not

present; this can be subtracted from other waveforms recorded when the

aircraft is present in order to remove direct antenna- to -antenna coupling

as well as other specific characteristics of the measurement site. This

technique imposes a requirement that may be difficult to meet -- maintaining

relative physical position of all objects over the duration of the

measurements. This may be difficult during windy conditions or if movement

of extraneous objects occurs in the immediate vicinity. It also requires

stability of the triggering circuit and pulse generator, which is usually

not a problem. It is not possible to take a reference signal for the second

receiving system; what is recorded (using the antenna from the first system)

is the unperturbed field strength when the aircraft is removed. This may be

used to develop a transfer function which relates induced current inside the

aircraft to external field strength.

An exception to the use of time windowing occurs when analyzing

induced currents inside the aircraft using the second receiving system. The

waveform is determined by the effects of intervening media, structure, and

cabling -- usually no part of the waveform should be discarded since there

is not sufficient spatial separation to allow us to distinguish what object

or effect is causing which portion of the time signal.

The measurement systems used in Arizona incorporates several

improvements over those used previously in Alabama. The antennas are

larger, allowing lower frequencies; they are mounted on fiber-glass supports

and can be rotated to give either horizontal or vertical polarization of the

E field. The antennas can be raised or lowered over a several -meter range.

They are structurally stronger, and can be used during substantial wind

conditions. Use of optical fiber lines to allow triggering of all systems

from our pulse generator is another improvement. This prevents stray

coupling along conductive trigger lines.

A special step generator was designed and built on a card attached to

the transmitting antenna. The peak voltage across the antenna elements is

over 250 V. This is much higher than we could achieve at Ft. Rucker, but

the pulse repetition rate had to be sharply reduced. The trade-off of peak

voltage versus repetition rate will be considered on future designs. The

transmitting antenna differentiates the step function to produce a radiated

impulse

.

An 80386 personal computer was used to record and process the data.

It is a part of the first receiving system. Data from the digitizing

oscilloscope of the external receiving system were transferred directly to

the computer through the IEEE-488 bus, while data from the portable system

inside the aircraft were read from disks into the 80386 computer for

processing. The 80386 computer was used to obtain processing speed

necessary to perform the mathematical algorithms nearly in real time,

without having to use a mainframe computer with a typical one -day delay in

obtaining results due to the need to transfer data by modem, process the

data in batch mode, and transfer the results back by modem. This enabled us

to perform a preliminary assessment of our measured data to see whether

results were reasonable. The 20 MHz, 80386 computer with an 80387

coprocessor that we used is about 12 times faster than an 80286 computer

with an 80287 coprocessor, yet about 9 times slower than the actual cpu rate

4
of our mainframe computer. The processing time of our computer is

approximately 2 seconds for 1024-point fast Fourier transform (FFT) and 4

min for a 46 -pole run using the Prony algorithm. Newer computers are even

faster

.

The response of the receiving antenna is determined using the NIST

time -domain range [1].

3 .

0

Measured Data

The FFT and Prony transforms are two commonly used algorithms that

can be used to convert time -domain waveforms into the frequency domain.

The mathematical processing can be done with any of a number of

algorithms, but the FFT should be tried first to obtain an overview of the

spectral content of the data. Other algorithms can be used in combination

with preprocessing to determine other characteristics of the data. The

Prony algorithm gives either Q or damping factor, important characteristics

for EMC analysis.

The nonlinear techniques suggested by Dudley and Goodman [5] are

certainly more powerful than the techniques we used, but in view of the

fairly good signal- to-noise ratio we are able to obtain, the increased time

and computer power needed may not give substantially better results.

Sometimes it is advantageous to preprocess the data. An example is

decimation as suggested by Dudley and Goodman [5]. This consists of using

every second, third, or nth data point and padding the end of the waveform

with zeros. This enhances the low-frequency resolution of data processed

with the FFT at the expense of high-frequency information. The data are

also preprocessed to remove any dc offset that might be present in the data.

The Prony algorithm fails if there is a substantial dc offset.

The response of the current clamps is determined by using cw currents

of known amplitude at various frequencies. The frequency response is fairly

flat from 10 to 50 MHz, is down about 3 dB at 80 MHz, and takes a sharper

drop above 120 MHz. The system calibration is a function of all of the

components of the receiving system in addition to the current clamps. This

includes the recording oscilloscope, filter, amplifier, laptop computer,

mathematical algorithms, and the required interface parameters of the

personal computer. Some care must be used in converting power spectral

density into units of current. The level at which the cw calibration is

performed is valid only at that level on a linear current scale. By using a logarithmic scale (dB) , we can adjust a current density scale to match this

calibration point, and the nonlinear current 7 squared relation to power

reflected through the 201og( ^/i-^) and 101 og(P

2

/P^) allows direct readings

of current density values. A second vertical scale for cw current is

needed. Current induced from a cw (discrete frequency) source should be

used to determine this logarithmic current scale. The two scales may be

made identical by selecting the frequency interval used by the FFT and

giving the current spectrum density function in terms of dB relative to 1

/iA/Jx Hz, where x is the width of the FFT frequency interval.

Two recorded waveforms show raw and preprocessed data. Figure 3

shows a raw waveform from the receiving antenna while figure 4 shows the

same waveform decimated using a factor of 4. Figure 5 shows the spectral

5
response of the decimated time waveform obtained from the FFT . Examples of

measured power spectrum density (related to square of current) from various

wires within the helicopter were obtained from processed data and are shown

in the remaining figures. Figure 6 shows the null response of the current

clamp, that is, when it was not coupled over any wire. Some clamps respond

to electric field, thus confounding the data unnecessarily. Figure 7 shows

power spectrum density in a particular wire, while figure 8 shows the power

spectrum density in this same wire, but with the rotor at a different

position. Figure 9 shows a much stronger power spectrum density induced in

a different single wire; the aircraft was excited by a vertically polarized

E field. Figure 10 shows power spectrum density in another wire. Figure 11

shows power spectrum density induced in a bundle of wires. Figure 12 shows

power spectrum density variations in one wire at 6 different static

positions of the rotor.

We tried to address a number of measurement conditions of the

aircraft under test. Some of these conditions are elevation of aircraft,

position of rotor, effect of power cords, whether the door(s) are open or

closed, and effect of a turning rotor. All of the measurements gave

inconclusive results for a number of reasons. Some differences were

observed due to each of these variations. More carefully controlled

experiments with larger variations of conditions are needed in order to

resolve their effects. Some of our observations include these:

The effect of a power unit or cord can be eliminated by use of battery

operated equipment.

The effects of elevation were measured for 0 and 15 cm elevations. This

increase in elevation was achieved by rolling the helicopter up wooden

ramps onto plywood on top of foam blocks 15 cm thick. There are some

repositioning errors in addition to this change in elevation; the

measured effects were clearly discernible, but produced variations in

amplitude of 3 dB or less in most cases.

The effect of an open door versus a closed door caused very minor

changes, 2 dB , in measured results but this may be case- and doorspecific .

The effect of different rotor positions was easiest to see (figure 12)

.

Large changes in the waveform were apparent as the rotor was moved

through 6 different angles of rotation. Over most of the spectrum of

interest, the changes in measured current were less than 4 dB , but at a few frequencies, were as much as 10 dB as shown in figure 13. The

frequencies where results were most affected seem to occur at multiples

of half -wavelengths of principal dimensions of the helicopter such as

tip-to-tail of body or rotor diameter. For example, 45.75 MHz gives a 5/2A of 53.8 f (16.4 m) , the approximate diameter of the main rotor, and

58.25 MHz gives a 3 A of 50.75 f (15.5 m) , the length from the nose- tip

to the center of the tail rotor.

4 . 0 Needed Improvements

There are a number of practical improvements that are easy to make

and will speed up the measurement considerably. One example is the use of

more current clamps so that only connectors need to be switched rather than

relocating a single current clamp to many locations. Another example is to

use two sets of antennas so that polarizations could be changed without

6
having to remount one set of antennas. The effect of azimuthal variations

will have to be evaluated to see whether the small displacement necessary

for using two sets of antennas has any significant effect. How effective

this will be depends on the number of measurements to be mad£

.

The trade-off between peak pulse power radiated and repetition rate

of the impulses needs to be weighed carefully. Probably it is more

advantageous to increase the repetition rate than the pulse power.

Stability of waveform can probably be maintained better with lower-power

pulses

.

The method of processing and/or preprocessing the data to convert

from time to frequency domain may be improved if poorer signal- to -noise

ratios are encountered in future applications, but the present techniques

are adequate when applied judiciously. This is an area where improvements

may be desirable but are not essential to the implementation of the

measurement technique to present applications.

These are relatively simple problems with obvious solutions. But

there are several areas where improvements are needed in order to achieve

the full potential of this measurement technique and perhaps fill this gap.

Consider that whole-system, radiated, electromagnetic susceptibi lity

testing cannot be done effectively on an aircraft. Electromagnetic (EM)

fields cannot be radiated across the spectrum at high enough power for

susceptibility testing without disrupting other spectrum users.

Electromagnetic fields generated inside an enclosure such as an anechoic

chamber or mode-stirred chamber can be used to immerse. the aircraft in

specified fields, but the aircraft cannot be operational. This means that

avionics are not completely operational, so key failure criteria are not

available, and their response to these EM fields remains unknown. This

leaves a critical inability to make effective radiated susceptibility tests

on large, whole systems. This leaves a crucial gap for newer weapon systems

that rely heavily on the proper functioning of sensitive and complex

electronic systems to perform their missions.

Can energy be injected into parts of the whole system as a viable

substitute for radiated susceptibility testing? The time-domain system

can be used to obtain current response of internal wiring due to external

fields over the frequency range allowed by the antennas and other ancillary

equipment. But this is for one orientation of the aircraft relative to the

transmitting antenna. For each of an infinite possible number of relative

orientations, there will be a different current response, since fields are

vector quantities and couple energy differently depending on these relative

orientations. The aircraft is a receiving antenna of unknown

characteristics: patterns vary with frequency; differing delay times cause

variations in waveshapes. But the aircraft resonances are independent of

variations of azimuth. We may be able to develop statistical methods that

give an upper bound for currents that will not be exceeded regardless of the

relative orientation. An experiment must be designed to take data at an

extensive number of azimuths such as may be obtained using a turntable.

Methodology for determining the minimum number of azimuths would then be

needed. Then conductive injection could be used to reprodd.ce this current

bound (which would still be a function of frequency) as a conducted EM

susceptibility test that would be equivalent to worst-case radiated EM

7
susceptibility testing, regardless of the relative orientation of source and

the aircraft. This current must be tied to the unperturbed field strength

in order to relate the injection current to the equivalent field strength.

This can be done by measuring the unperturbed field strength with no

aircraft present. Once the relationship between external field strength and

induced (upper bound) current is established, the levels can be scaled

upward as long as the coupling mechanisms between the airframe and the

internal wiring are linear, that is, capacitive, resistive, or inductive.

The avionics may not be linear, but the injection can linearly simulate

increasing field strength. This process could significantly reduce cost of

testing since current injection costs much less than radiated

susceptibility testing.

Bulk current injection is a method of injecting current into a bundle

of wires that has been espoused by Nigel Carter [6] and analyzed with

considerable care by M.F Sultan [7]. This method has tremendous advantages

for both recording and injecting in that the wire bundles often contain many

conductors. Measuring the current in each conductor takes n times as much

time as a single bulk measurement where the current clamp is placed over the

whole bundle and net current is measured instead of measuring current in n

individual wires. Of even more potential importance is the use of bulk

current when injecting, since not only is the same time saving achieved but

also the problem of interaction is avoided -- injecting one wire may not

have the same effect as injecting 2, 3, or n wires simultaneously. But are

bulk current techniques valid? Current measurements have been made on

individual wires where the current in the individual wire exceeded the

current measured in bulk. The question whether the current induced by

external fields can be measured and simulated by injection must be studied

by theoretical and experimental methods.

If bulk current injection is not valid, or is only valid under a limited number of conditions, another question must be resolved. Does

current injected on n wires one at a time have the same effect as injecting

all wires simultaneously? Probably not, especially for cases where several

conductors have identical resonance frequencies. This undoubtedly will

depend on the specific characteristics of each avionics unit, particularly

its linearity. This information is usually unknown or very difficult to

determine

.

In one special case, testing electroexplosive device (EED) response

to nuclear EMP , only linear scaling and current bounding problems are

relevant, since current response is needed in only one wire.

Another question is determining the amount of perturbation caused by

the measurement system used to make the current measurements. The present

system required ac power; battery operation would reduce the additional

wiring needed to provide power.

For applications where space is too limited to contain the second

receiving measurement system, such as inside an attack helicopter or small

missile, additional work is needed to develop a fiber optic telemetry system

to allow recording the induced current waveforms at some out-of-the-way,

nearby site. The current probes and amplifiers could fit inside a much

smaller volume than is needed for the present system.

8
Each type of aircraft or model of aircraft with different wiring is

unique. Each should be tested separately.

5 . 0 Acknowledgments

This work was supported by the US Army Aviation Systems Command

(AVSCOM) for improvements to the measurement system. Arrangements for

field measurements at Western Army Aviation Training Site, Arizona, were

also made by AVSCOM. Gilmore Krener of AVSCOM has provided support on a

continuing basis. Arrangements for measurements at Ft. Huachuca, Arizona,

were made and supported by Byron Dean of US Army Intelligence Center and

School and by Grady Banister of US Army Electronic Proving Ground. Dennis

Friday of NIST has offered suggestions concerning use of statistics.

6 . 0 References

[1] Lawton, Robert A.; Ondrejka, Arthur R. Antennas and the associated

time-domain range for the measurement of impulsive fields. Nat. Bur.

Stand. (U.S.) Tech Note 1008; 1978 November.

[2] Adams, John W. ; Ondrejka, Arthur R. ; Medley, Herbert W. Time-domain

system for identification of the natural resonant frequencies of

aircraft relevant to electromagnetic compatibility testing. Nat.

Bur. Stand. (U.S.) NBSIR 87-3077; 1987 November.

[3] Papoulis, A. The Fourier integral and its application". McGraw

Hill; 1962.

[4] Kanda, Motohisa The effects of resistive loading of "TEM" horns.

IEEE Trans. Electromagnetic Compatibility, EMC 24, No. 2, 1982 May.

[5] Goodman, D.M.; Dudley, D.G. An output error model and algorithm for

electromagnetic system identification. Circuit Systems Signal

Process, Vol. 6, No. 4, 1987.

[6] Carter, N.J.; Bull, D.A. Evolution of aircraft clearance techniques.

IERE International Conference on Electromagnetic Compatibility; 1984

September 18-21, Guildford, England.

[7] Sultan, M.F. Modeling of a bulk current injection setup for

susceptibility threshold measurements. IEEE International Symposium

on Electromagnetic Compatibility; 1986 September 16-18, San Diego,

CA.

9
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(relative)

obtained

by

FFT

from

current

time

waveform

in

yet

another

single

wire

from

FFT.
r*

0

CD

D o o o o

O o o o o

o o o o o

in CM o> CO m

N

as

o

c

Q)3

cr

a)u

fc.

( zH/fl) X^TSuap amJ^oads jartod aAj:jex ad

20

Figure

11

.

Power

spectrum

density

(relative)

obtained

by

FFT

from

current

time

waveform

of

a

bundle

of

wires

.
(ZH/f\) Xn-psuap unloads j*t\od aAj:i*-[aa

21

Frequency

(Hz)

Figure

12.

Power

spectrum

density

(relative)

obtained

by

FFT

from

current

time

waveform

in

same

wire

at

several

azimuths

of

rotor

position,

static

measurements.
0

0

00

O 00 CO^

CM O

(ap) Ojpy U I [Aj /x D [/\|

22

Ratio

of

maximumto

-minimum

power

spectrum

densities

in

dB

for

different

rotor

positions

shown

in

figure

12.
NIST-1 14A

(REV. 3-89)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

PUBLICATION OR REPORT NUMBER

NISTIR 89-3927

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBLICATION DATE

November 1989

4.

TITLE AND SUBTITLE

Recent Improvements in Time—Domain EMC Measurement System

5.

AUTHOR(S)

J.W. Adams, A.R. Ondrejka, K.H. Cavcey, J.E. Cruz, H.W. Medley, and J.H. Grosvenor, Jr.

6.

PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURG, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

U.S. Army Aviation Systems Command

St. Louis, Missouri 63120

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 BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

Improved techniques for determining critical resonant frequencies and the current

response of internal wiring due to external fields for rotary-wing aircraft are

given. The measurement method uses a train of low-level, radiated pulses. These

do not disturb other spectrum users, nor do other spectrum users significantly

disturb these measurements. The fields are low, a distinct advantage from both

cost and personnel hazard standpoints. The problems that should be addressed

before the full potential of the technique can be realized are discussed.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

induced currents; low-level source fields; natural resonant frequencies; TEM horn

antennas; time-domain measurement system; weak source fields.

13. AVAILABILITY 14. NUMBER OF PRINTED PAGES

X

UNLIMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). 28

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402.

15. PRICE

—X_ ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161. ELECTRONIC FORM

if U S GOVERNMENT printing OFFICE 1989-Jacket 773-929/25026 Region 8
.

*

"