PUBLICATIONS

NBSIR 88-3083

AIRCRAFT FIELD DEGRADATION AND

ELECTROMAGNETIC COMPATIBILITY

Kenneth H. Cavcey

Dennis S. Friday

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303-3328

January 1988

NBSIR 88-3083

Research Information Center

IValional Bureau of Standarda

Gaithersburg, Maryland 20899

AIRCRAFT FIELD DEGRADATION AND

ELECTROMAGNETIC COMPATIBILITY

Kenneth H. Cavcey

Dennis S. Friday

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

January 1988

Sponsored by

U.S. Army Aviations Systems Command (AVSCOM)

St. Louis, Missouri 63120-1798

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

CONTENTS

LIST OF TABLES

LIST OF FIGURES

ABSTRACT

1. INTRODUCTION

2. TEST CONFIGURATION

3. EXPERIMENTAL DESIGN CONSIDERATIONS

4. READING THE PLOTS

5. OBSERVATIONS and DISCUSSION . . .

6.1 KIOWA

6.2 COBRA

6 . 3 BLACKHAWK

6.4 CHINOOK

6.5 MOHAWK FIXED WING AIRCRAFT . . . .

6.6 THE THREE HUEYS

7. CONCLUSIONS

8. REFERENCES

Page

V

V

1

1

3

4

7

8

9

10

11

12

12

13

14

17

iii

LIST OF TABLES

Page

Table 1. Aircraft Tested 18

Table 2. Active Frequencies for CAAF 19

Table 3. A comparison of noise floors in dBm taken . with the inside E-field monop o 1 e ( he 1 i c op t e r

running) 20

LIST OF FIGURES

Page

Figure 1. Field Test Configuration 21

Figure 2. Block Diagram of Electronic System Hardware

used for Data Acquisition 22

Figure 3. Transfer Function for the Receiving

Up-Converter measured at 1000 Hz 23

Figure 4. Kiowa Helicopter 24

Figure 5. Kiowa Helicopter 25

Figure 6. Kiowa Helicopter 26

Figure 7. Kiowa Helicopter 27

Figure 8. Kiowa Helicopter 28

Figure 9. Kiowa Helicopter 29

Figure 10. Kiowa Helicopter 30

Figure 11. Kiowa Helicopter 31

Figure 12. Kiowa Helicopter 32

Figure 13. Kiowa Helicopter 33

Figure 14. Kiowa Helicopter 34

Figure 15. Kiowa Helicopter 35

Figure 16. Kiowa Helicopter 36

Figure 17. Cobra Helicopter 37

Figure 18. Cobra Helicopter 38

Figure 19. Cobra Helicopter 39

Figure 20. Cobra Helicopter 40

Figure 21. Cobra Helicopter 41

Figure 22. Cobra Helicopter 42

Figure 23. Cobra Helicopter 43

Figure 24. Cobra Helicopter 44

Figure 25. Blackhawk Helicopter 45

Figure 26. Blackhawk Helicopter 46

Figure 27. Blackhawk Helicopter 47

Figure 28. Blackhawk Helicopter 48

Figure 29. Blackhawk Helicopter 49

Figure 30. Blackhawk Helicopter 50

Figure 31. Blackhawk Helicopter 51

Figure 32. Blackhawk Helicopter 52

Figure 33. Blackhawk Helicopter

Figure 34. Chinook Helicopter -u Figure 35. Chinook Helicopter

Figure 36. Chinook Helicopter ‘ Figure 37. Transfer Function for a Small Size H-field

Current Clamp - Serial No. 156

V
Figure 38. Transfer Function for a Small Size H-field

Current Clamp - Serial No. 143 58

Figure 39. Transfer Function for a Medium Size H-field

Current Clamp - Serial No. 539 59

Figure 40. Transfer Function for a Medium Size H-field

Current Clamp - Serial No. 55 60

Figure 41. Transfer Function for a Large Size H-field

Current Clamp - Serial No. 22 61

Figure 42. Chinook Helicopter 62

Figure 43. Chinook Helicopter 63

Figure 44. Chinook Helicopter 64

Figure 45. Chinook Helicopter . 65

Figure 46. Chinook Helicopter 66

Figure 47. Chinook Helicopter 67

Figure 48. Mohawk Airplane 68

Figure 49. Mohawk Airplane 69

Figure 50. Mohawk Airplane 70

Figure 51. Mohawk Airplane 71

Figure 52. Mohawk Airplane 72

Figure 53. Mohawk Airplane 73

Figure 54. Mohawk Airplane 74

Figure 55. Mohawk Airplane 75

Figure 56. Mohawk Airplane 76

Figure 57. Mohawk Airplane 77

Figure 58. Mohawk Airplane 78

Figure 59. Mohawk Airplane 79

Figure 60. Mohawk Airplane 80

Figure 61. Mohawk Airplane 81

Figure 62. Mohawk Airplane 82

Figure 63. Mohawk Airplane 83

Figure 64. Huey Helicopter No. 1 84

Figure 65. Huey Helicopter No. 2 85

Figure 66. Huey Helicopter No. 2 86

Figure 67. Huey Helicopter No. 2 87

Figure 68. Huey Helicopter No. 2 88

Figure 69. Huey Helicopter No. 3 89

Figure 70. Huey Helicopter No. 3 90

Figure 71. Huey No. 2 91

Figure 72. Huey No. 3 92

Figure 73. A comparison of the NOISE floor for Hueys . No . 2 and No . 3 93

vl
AIRCRAFT FIELD DEGRADATION AND ELECTROMAGNETIC COMPATIBILITY

Kenneth H. Cavcey and Dennis S. Friday

Electromagnetic Fields Division

National Bureau of Standards

Boulder , Colorado 80303

This paper discusses the first tests undertaken to study the

problem of field degradation in army aircraft (helicopters and

one fixed wing airplane) due to the deterioration of electronic

and electrical systems. The electromagnetic compatibility (EMC)

of such systems was investigated by passive measurement of the

aircraft as a collection of radio frequency sources. Methods for

detection of these sources were developed that included

sensitivity to both stationary and nonstationary noise that

existed.

The collected data were studied to see if there existed any

obvious factors derived from the data that one could use to

correct potential problems that might affect flight safety.

Emphasis was placed upon making such test methods appropriate,

inexpensive, and easily performed by army field personnel. In

addition, applications to quality control or acceptance testing,

as related to the Environmental Stress Screening (ESS) program,

are examined.

Key words: bandwidth; data acquisition; electromagnetic

compatibility (EMC); electromagnetic interference (EMI);

electromagnetic spectrum; field probes (or antenna); noise floor;

noise sources; spectrum analyzer.

1. INTRODUCTION

The problem of aircraft (helicopter) field degradation due

to deterioration of the aircraft's electrical and electronic

systems was studied via experimental field testing. Radiated

emissions from the aircraft under test were detected over the

spectrum of frequencies ranging from 100 Hz to 500 MHz using

three different kinds of sensing probes. The data that were taken

suggest a possible future technique that if successful could

alert U.S. Army product assurance personnel of potential

problems. Correction of these electrical and/ or electroni=

systems would mitigate the interference thus assuring aircrtift

integrity and safety.

The problem of field degradation of electromarn* r‘ compatibility (EMC) can be studied from several viewpoints * aircraft's electronic hardware system should function ind.p' r

'•

1
of other outside or inside sources of electromagnetic radiation

that might impair its ability to complete its basic mission. This

is of course based upon the premise that its original design is

such that each subsystem will not affect any of the others

irrespective of the number and type activated (example:

communications and fire control). In almost all aircraft

electrical systems, the design is such that specific components

can be added or deleted without affecting the overall EMC

integrity. As discussed later, this has not always been the case.

Noise from cables and connectors were studied by Shands and

Woody [1] . They found that intermodulation (IM) noise as strong

as 40 dBm was generated when a cable network was radiated with EM

energy in the 22 to 425 MHz region. Connectors were found to be

the major offenders when subjected to mechanical vibration. Noise

can also be generated by defective ground or multiple tie points.

In a later study, the same investigators [2] found that MetalInsula t ive - Me t a 1 (MIM) junctions produced even more noise than

the cable system. Watson [3] studied the same phenomena with

relation to naval radio systems. He also found MIMs induced noise

on ships which may be mitigated by:

1) cleaning, reassembly and welding short flexible

shorting straps across the junction or

2) applying a special chemical compound to quench the

rectifying action of the MIM.

Other investigators [4] have used acoustics and other forms of

radiation, but the work was limited to laboratory efforts on

simulated structures with results that are not directly

comparable to the work of this report.

This report covers work that was performed on seven

helicopters and one fixed wing aircraft to see whether EMC

degradation could result from:

a) penetration from outside known or unknown sources;

b) interaction of subsystems from a static and dynamic

test (avionics on - engines off or on)

c) sources with and without the presence of the aircraft;

d) the generation of unknown sources due to radiation from

any known source impinging on the airframe skin due to

faulty connections (ground or otherwise).

All of these potential problems were studied to determine a method or technique of locating potentially dangerous

electromagnetic emissions. This interference leads to system EMC

degradation. If it can be detected by U.S. Army Product Assurance

2
Personnel and be corrected, the corrective action will insure

electrical/electronic systems integrity and aircraft safety.

The tests were performed at the U.S. Army Aviation Development

Test Activity Center (AVNDTA) located at Garins Army Air Field

(CAAF), which is part of Ft. Rucker, Alabama. This report covers

the measurement techniques, the test configuration and the

experiments that were conducted. Data that were collected

(spectrum plots and time series) are presented and interpreted in

light of the constraints placed upon those tests. In addition,

the validity of the methods are discussed and suggestions for

further work are proposed.

2. TEST CONFIGURATION

All of the measurements at Carins Army Air Field were made

at a location known as "Chinook Hill". The hill is an elevated

flat grassy hill overlooking the flight line and field where most

of the helicopters are parked and are flown. Figure 1 shows the

relative location of the van containing the measurement

electronics, interconnecting cables, and the aircraft under test.

The figure also shows the relative locations of the

electromagnetic field probes and current clamps. Except for the

probe directly in front of the aircraft nose (kept at a constant

1 meter distance), the other field probes and field cable clamps

were located as permitted depending upon the specific aircraft

under test.

Figure 2 shows a block diagram of the electronics hardware

used for all tests. Data acquisition was accomplished as follows:

a) A commercially available package written in BASIC version

3.0 for the MS-DOS operating system was used for obtaining

the spectrum plots.

b) A special program written in the "C" language and linked to

the object code written for the IEEE-488 interface card in

the computer was used for obtaining the time series data.

The hardware performed the two tasks corresponding to (a)

and (b) above. In the task under (a) , an additional piece of

hardware was built so that frequencies below 50 kHz (the lower

limit of the spectrum analyzer) could be covered. This hardware

was built so mechanical vibrations, that might result in low

frequency radiation, could be sensed. To accomplish this

measurement, a 100 kHz up-convertor was used that provided an 8

dB gain. Figure 3 shows the voltage input to output transfer

function of the convertor at a frequency of 1 kHz. The unit is

linear in response and allows coverage down to frequencies of 200

Hz or less. The local oscillator in this circuit has to be stable

3
and spectrally pure so that frequency determination is accurate

and spurious products from the mixer are minimized. The circuit

that was used incorporates a crystal oscillator, filter and

buffer with the second harmonic (200 kHz) being attenuated over

40 dB with reference to the fundamental. The higher harmonics are

attenuated in excess of 60 dB

The mixer part of the circuit uses a commercial integrated

circuit (designed for radio communications) that results in a linear voltage transfer function. For example, switching in the

convertor after setting up the spectrum analyzer to cover 100 kHz

to 600 kHz results in a frequency coverage range of nearly dc to

500 kHz with a span of 50 kHz per division. In the range 200 kHz

to 150 MHz the mixer provides an additional gain of 8 dB over the

gain of the spectrum analyzer.

Under (b) which is the second task (time series) the data

were acquired in the following manner. First, the signal of

interest was tuned in with the spectrum analyzer at a selectable

bandwidth up to 1 MHz (at lower frequencies this bandwidth must

be less) . Next, the 10 MHz IF signal (obtainable from the

spectrum analyzer) is detected or converted from ac to a time

varying dc value. Finally, by means of a system digital voltmeter

the dc signal is sampled as programmed and transferred through

the IEEE-488 bus to the computer. It is important to note that

the spectrum analyzer is used in the zero swept mode, when taking

time series measurements at a predetermined tuned frequency.

The data were taken in strings of 7 ASCII characters up to

9000 points in length (63 kB) and then stored in the virtual "D"

memory space of the computer. The data files were then

transferred onto floppy disks for analysis at a later time.

3. EXPERIMENTAL DESIGN CONSIDERATIONS

It must be emphasized that this study was exploratory in

nature and the intent was to acquire whatever information was

present in the electromagnetic fields being emitted from the

aircraft. A crucial consideration in designing these experiments

was that they not be compromised by any bias toward any prior

theories on the nature of the emissions. This approach was

validated as our data exhibited information in ways that we had

not preconceived, nor theorized. More on this later in the

report

The experimental procedures changed between the first

aircraft (Kiowa) and later aircraft. Preliminary examinations of

the data provided insights by which the procedures were modified

to obtain the information more efficiently. This was a prime

consideration since some of the aircraft were available only for

limited times. Testing for all of the aircraft was performed in

4
as uniform a manner as possible, considering all of the

constraints under which the tests were performed.

Three different types of probes were used for sensing the

aircraft's electromagnetic emissions; E-field probes, H-field

probes and H-field current clamps. The E and H probes were placed

in and around the aircraft and the current clamps were attached

to strategically located wiring harnesses inside the airframe.

Three loop H-field probes were used at first. One probe, which

is referred to as "the large H-field probe," was approximately

0.763 meter in diameter and operated over a single frequency

band. The remaining two loops were smaller, and are referred to

as the small H - f i e 1 d ' p r ob e s . These were adjustable over eight

smaller frequency bands. Five H-field current clamps were used;

one of which was large (0.067 meter in diameter) and somewhat

heavy. It could be used only where adequate space, cable size and

physical support were available. Two of the clamps were of medium

size (0.032 meters in diameter) and two were relatively small

(0.019 meters in diameter). These dimensions are the inside

dimension of the toroidal shaped clamps and indicate the largest

size of cable they can be attached to. In the text to follow they

are referred to as the large, medium and small clamps

respectively. Placement of the probes inside the aircraft was

constrained by the geometry of the aircraft by the need for a pilot (and sometimes a copilot) to operate the aircraft, and by

safety considerations. The safety requirement dictated the

aircraft and surrounding area be free of any loose cables,

probes, or other equipment when the rotors or propellers were

turning

Two E-field probes were used in the experiment. Each probe

was a broad band vertically mounted monopole of adjustable

length. The larger one ( 1.00 meter maximum length) was used only

for monitoring the field external to the aircraft because its

large base and length, when extended, prevented its use in

several of the aircraft. The small E-field probe (0.356 meters

maximum length) was selected for internal use and was mounted on

a small block of foam. It was difficult to find a consistent

location on all the aircraft for placement of the internal probes

because of the differing aircraft geometries.

In the cases of the three Hueys, the Chinook, and the

Blackhawk helicopters, there was a large personnel/cargo area

behind the cockpit, and the small E-field probe was mounted

vertically on the floor, or on a seat, as far away from

surrounding conductive surfaces as possible. In the cases of thn

Cobra helicopter and the Mohawk airplane, the cockpit was

extremely confined and there was no personnel/cargo area. On the

Cobra the E-field probe was placed on a shelf behind the pilots

seat (the rear of the two tandem seats) . On the Mohawk rh*

5
cockpit was so constrained that it was not possible to place the

probe in the same cavity as the crew. This was also the case for

the other aircraft. There were a series of chambers in the main

part of the fuselage, which were inter-connected by openings in

the longitudinal walls. The internal E-field probe was placed

centrally in this cavity. The Kiowa was the only aircraft tested

without an internal E-field probe.

The large E-field probe was placed on the ground in front of

the aircraft, on center, exactly 1 meter from the end of the

pitot tube or, the extreme point of the nose. This monopole was

fully extended for all of the tests. With the varying topologies

of the aircraft this was a location which could be replicated.

The probe had to be fixed (probes were not allowed to be moved

around an operating aircraft) for safety reasons. A more

important feature of this location was that all of the aircraft

had plexiglass canopies or bubbles in the forward fuselage. This

was often the major skin material in this region. Any fields

generated inside the aircraft cavity would therefore be likely to

leak through the acrylic plastic ports and be stronger in this

region than at any other location relative to all of the

aircraft

The choice of location of the H-field probes was similarly

constrained. The large H-field probe was placed centrally in the

relatively small personnel/cargo area. Since there were two small

identical H-field probes (electrostatically shielded loops), we

decided to use one internal and one external to the aircraft. The

placement of two such probes in electrically distinct locations

inside all of the aircraft was not always possible. The large Hfield probe (another loop) was used only internally. Its light

weight and large size and surface area rendered it unsafe for

external use anywhere within or near the radius of the rotor

blades. This probe was used only on the Kiowa when later it was

determined that the same information was better obtained from the

small E-field monopole. The small external H-field probe was

mounted on a wooden tripod 30 meters in front of the aircraft.

This probe was high enough that it had to be out of range of the

rotors and the resulting air downwash. Placing it in this

location put it in the same optimal region relative to the

helicopter as the external E-field probe.

The H-field current clamps, the third type of probe, were

used on all of the helicopters in different configurations. Their

placement was constrained by cable location, cable size, the

space surrounding the cable, the weight of the current clamp and

safety considerations. Within these limitations, the clamps were

dispersed, as much as possible throughout the airframe on

different cables. The objective was to identify any strong

localized emissions from sources interior to the aircraft.

6
Functional labeling of the harnesses was nonexistent and

electronic technicians on the base had only limited knowledge of

the function of some of the harnesses. Distributing the clamps to

accessible harnesses in different regions of the aircraft was the

only reasonable alternative within these limitations.

4. READING THE PLOTS

Important to the understanding of this report is the correct

interpretation of each spectrum plot included herein. The

following discussion is to assist the reader in this task.

For example, refer to plot D06067 (Figure 20) acquired at

time 14:09:48 on December 6, 1986. The top contains information

on the reference level, the center frequency of the frequency

range (abscissa) and the span or frequency per horizontal

divis ion

If, in the text of this report, the up-convertor is not

used, figure 20 is read in the following manner:

Plot D06067 (Figure 20)

1) All horizontal scale lines start at the top at a reference

level of -60 dBm.

2) One division down would be -70 dBm as stated by the VERT

DISPLAY statement of 10 dB/div in the lower left hand corner

o f the plot.

3) In this example the ordinate range is from -60 dBm at the

top to -140 dBm at the bottom. Note that the average noise

floor is about -93 dBm for this plot.

4) The span is 5 kHz/div, the center frequency of 125 kHz is

shown in the top center, and the abscissa range is from 100

kHz to 150 kHz

5) The video filter (VID FILTER) is off and the RES BANDW (or

bandwidth of the spectrum analyzer) is set to 10 kHz. This

information is important because the amount of noise

recorded is a function of the received bandwidth.

6) The bottom statement of 0-1.8 FREQ RANGE can be ignored

because it is just a statement of range for the instrument

(the 1.8 is GHz) . This is somewhat misleading because in

reality the lowest frequency that can be covered without a loss in gain is 50 kHz according to the specifications of

the instrument.

When the up-convertor was switched on (for all aircraft observed

7
after the Kiowa) the two things that changed were the frequency

range and the gain. The purpose of the convertor is to up-convert

from lower frequencies. Subtract 100 kHz from all horizontal

readings. Statement (4) now reads as follows:

4a) The span is 5 kHz per horizontal division, and the abscissa

range is from 0 to 50 kHz. In addition, since the up

convertor exhibits an 8 dB gain, the RF ATTN statement

should read +8 instead of 0 as is shown and the REF LEVEL

would be interpreted as -52 dBm.

At higher frequency spans the 100 kHz conversion frequency

is insignificant. At lower frequencies the interpretation is

reversed. Consider figure 7 and its abscissa. For plots like

this, subtract the 100 kHz; thus the 95 to 105 kHz range is

shifted to -5 to 5 kHz range, folded about zero in frequency,

similar to a double sided Fourier spectrum plot. Plots like this

are read only on the right side of zero frequency. Remember that

the up-convertor was used on all aircraft after the Kiowa.

5. OBSERVATIONS and DISCUSSION

Over 700 spectral and time series observations were recorded

during a seven-day testing period on the various aircraft

available. It should be stated that this study is not an

exhaustive one. The large amounts of data recovered were a first

look at radiated emissions in and around the aircraft under all

the given test conditions.

As testing progressed, experimental conditions were changed

in an effort to eliminate redundancy and obtain the most relevant

data. Equipment configurations as well as antenna and probe

selection and placement played an important part in obtaining

usable results. As might be expected in this type of evolutionary

process, other factors will be considered in future tests, now

that the first study has been completed.

The sequence of helicopters tested was determined by the

base flight operations group. Each is discussed separately except

for the three Hueys which are discussed as a group. Consequently,

the report will depict what was measured in a chronological

fashion. Discussion of the Hueys is carried out jointly because

their data may exhibit similarities due to their common geometry.

In the measurement scheme or experimental plan we hoped that

several helicopters of the same type but of differing age and/or

flight time could be tested so that one could look for EMC

degradation as a function of use. As the discussion progresses it

should become obvious that this plan was not fully achieved due

to circumstances beyond our control. However, this did not detere

the basic exploratory goals of the test.

8
Table 1 is a list of every aircraft tested and is arranged

in the order they were made available for testing. Table 2 is a list of radio frequencies that the army uses and that were

observed across the electromagnetic spectra. Examination of the

plots referenced in the aircraft topics should be made keeping

these frequencies in mind.

6.1 KIOWA

This was the first aircraft tested, and we stepped through

the entire frequency domain (dc-500 MHz) in small increments

without the avionics or engine running to acquire a look at the

ambient background noise and to observe those radio services that

were supposed to be present in the environment. The probe used

for this purpose was the external E-field unit located off the

aircraft's nose. A typical plot recorded from this series is that

of figure 4 which shows the 50-100 MHz region with local FM radio

stations clearly visible above a -82 dBm background. This is a typical plot. Only plots of special interest are shown, because

of the many that were taken.

On all aircraft, plots were taken under the following

conditions: (1) quiet, or (2) engine and avionics on. Two small

and one large H-field loops were used to acquire the data.

Comparing data acquired by the H-field loops and the E-field

probes implies that the loops were unnecessary. In addition the

loops are bandwidth limited and required tuning when switching

between frequency ranges. Since they were physically large

compared to the monopoles and somewhat insensitive, their use was

discontinued on all aircraft after the Cobra.

The first unidentifiable signal detected in the spectrum

measurements was referred to by our army liaison officer as a classified low frequency source. This signal as depicted in

figures 5, 6, 7 and 8 appears as a cluster of pulses moving

through the lower frequencies varying in both frequency and

amplitude. This signal was first seen on the external E-field

probe. It was later decided to see whether the aircraft itself

would pickup any signals by way of it's conductive surface. A

coaxial cable was attached from the nose of the Kiowa via the

Pitot tube and another was attached to a tie-down bracket located

on the tail boom. The cables were connected to the spectrum

analyzer via a 4:1 balun. As seen in figure 9 (helicopter

running) the airframe itself is an excellent antenna. This

phenomenon helped explain plots on other aircraft in which this

same signal was present on certain cables, wiring between

systems, and an internal E-field antenna that was added on the

Cobra helicopter. It is believed that the physical mechanism

responsible for the coupling to the field is either by ground

connections to cable shields (in which radiated currents can

flow) or capacitive coupling (as in the case of any conductor

9
located close to the airframe) or by physical contact with a suitable dielectric.

The Kiowa was considered to be an electrically noisy

aircraft when compared to the others tested. Even though an

internal E-field probe was not used until later, data from the Hfield probes and the external E-field monopole confirm this fact.

Noise can be categorized under several different definitions

which have been observed in the electromagnetic spectrum. The

most general noise definition is a non - s t a t i onary process emitted

from either a known or unknown source of a random amplitude and

bandwidth. In the case of the Kiowa, the noise detected seemed to

depend on whether or not the avionics were turned on. Figures 10

and 11 from the external E-field probe suggest a repetitive

process which may be an electronic switching circuit associated

with the on-board power supplies. These plots show the noise

extending up to 6 MHz. Figures 12, 13 and 14 are from the medium

size H-field current clamp located on a cable near the left front

windshield. Figures 15 and 16 show the noise on a small cable

located in the left rear bay of the aircraft (0-1 and 0-5 kHz).

Even though the noise floor is relatively low (-110 dBm in figure

15) the actual signal variation is plus or minus 10 dB for a 100

Hz bandwidth. This would show a higher level of noise if the

bandwidth were increased. This increase in bandwidth was

implemented later in the testing process. As the work continued,

measurement techniques were changed, but references were chosen

and adhered to for comparison purposes.

6 . 2 COBRA

An Internal E-field probe was first used on the Cobra. Since

the cockpit is a tandem arrangement, placement of the probe was

restricted to a location behind the pilots seat. Figures 17, 18

and 19 are spectra taken with this probe showing the OP

communications transmitters on board the helicopter at 36.4,

138.8 and 242.1 MHz. Note that due to the strong near field, 30

dB of rf attenuation was switched in at the spectrum analyzer to

avoid overloading.

The spectral plots for the E-field probes did not reveal any

significant EMI with the exception of the low frequency jamming

signal (previously discussed) present on the internal probe

( f i gur e 20).

Two medium clamps were located in the rear section of the

cockpit. Only one showed a change between the helicopter running

and quiet. Behind the pilot's seat, a clamp was placed near three

control boxes marked in one location as attitude control. Figure

21 displays electronic switching noise 10 dB higher that the

noise floor with a repetition rate of approximately 275 kHz.

10
Figure 22 shows the quiet or ambient noise floor. All other

features appeared normal.

The Cobra was the last ship to use the H-field loop probes.

The small external loop displayed the same noise floor at a 10

kHz bandwidth as the other probes (-95 dBm) . When the same kind

of probe was placed internal, the first apparent feature was

shielding from outside sources such as the ADF signal at 560 kHz

(see Figures 23 and 24 for comparison purposes). Other plots

seemed to be normal.

6 . 3 BLACKHAWK

All of the E-field plots seemed normal with the exception of

those viewed in the 10 to 50 MHz region. The noise shown in

figures 25 and 26 indicates that for some reason the environment

itself was the problem since the noise was most apparent on the

external probe.

Inside the aircraft, the H-field current clamps revealed

another picture. On the large clamp to the left behind the

copilot, electronic switching noise was detected with a repetition rate of about 75 kHz. The spectrum affected ran from

about 1.8 to 9.5 MHz. The noise peaks were raised 25 dB in the

running case as compared to the common noise floor of -90 dBm.

These results can be viewed on figures 27 and 28. The medium

clamp located behind the right side of the instrument panel

showed the same general result with a slightly lower amplitude

change (figures 29 and 30)Another interesting observation was

made with the clamp located behind the left instrument panel. A

signal or noise was observed when the avionics and or engines

were shut down. Figure 31 shows the signal as some type of

switched electronic source over the 1 to 10 MHz region. A check

of the external E-field probe at the time when the noise was

detected inside the airframe proved negative (no signal; see

figure 33). No conclusion can be made whether the signal should

or should not be present under what appeared to be reversed

conditions. Up to this point, noise and/or helicopter generated

signals were detected under "avionics on or engine running"

conditions. The signal was also detected on a medium sized field

clamp located under the right side of the instrument panel.

Because the amplitude was reduced by about 30 dB we might

conclude that the clamp was located further away from the source

than the previous clamp (see Figure 29). The signal is again seen

on figure 27 which is the output from a large clamp that was

positioned behind the left copilot's seat. Here the signal is

stronger than that seen behind the right instrument panel, but

still weaker than the left instrument location. When the

helicopter was powered up, the amplitudes in the same frequency

region dropped considerably as seen in figure 30. Further

comments on what was seen in this figure will be made later.

11
6.4 CHINOOK

The spectra collected by the E-field probes (both internal

and external) showed very little information with the helicopter

running or quiet. As a result, the discussion will concentrate on

the H-field current clamps.

The medium sized clamp located under the left cockpit

instrument panel demonstrated an increase in the noise floor

(0-50 kHz) with the helicopter running. The average noise power

can be calculated from equation (1) with reference to 1

milliwatt. This calculation is done with reference to the

50-ohm input impedance of the spectrum analyzer.

dB. = 10

] (1)

As shown in figures 34 and 35 the noise floor

increases only 10 dB from -105 to -95 dBm. The noise power at

this level is equivalent to 3.162 x 10"^^ watts In reality this

is a very weak field and the measurement system exhibits

sensitivity in making the measurements and its ability to see all

emissions. Examination of figure 36 shows the noise up to

frequencies of 600 kHz.

In the front cockpit a small clamp was attached to a cable

entering the compass. The noise floor over the spectrum of 500

kHz to 10 MHz increased 10 dB or so with the helicopter running.

Above 10 MHz little of Interest was observed. All of the H-field

clamps operate over only a specific range of frequencies. All the

H-field clamps used in these tests cut off at the high frequency

end at approximately 100 MHz. Figures 37-41 are the transfer

functions (transfer impedance in dB as a function of frequency in

MHz) for the five clamps used.

The CH-47HC Chinook was built primarily for transporting

troops and material. In the main bay in the right forward section

of this volume, circuitry was found near a heating unit. Here the

large H-field clamp was slipped over a large cable bundle. In the

dc to 50 kHz region the noise floor was found to be over 40 dB

higher when the heating unit was activated. From 50 kHz up to 900

kHz the noise floor dropped by 30 dB . From 900 kHz to 5 MHz the

noise continued to drop until it leveled off at -90 dBm. Compare

figures 42, 43, and 44 with figures 45, 46 and 47. The source

feeding this heater or nearby circuitry is unknown. The spectrum

plots clearly indicate a potentially harmful source of EMI.

6.5 MOHAWK FIXED WING AIRCRAFT

When the engines and avionics were activated, the noise in

12
the rear belly bay jumped considerably as detected by a large Hfield current clamp. This clamp was attached to some power

cabling. The base line or noise floor was observed to be 40 dB

higher than the ambient, when measured at a 1 kHz bandwidth (see

figures 48 and 49). Figure 50 (-0.045 kHz to 0.900 MHz) shows the

same phenomena and the ADF at 560 kHz can be seen on figure 51.

Figure 52 which covers 0.9 MHz to 9.9 MHz shows electronic

switching noise that appears to change at a 10 kHz rate. The

noise is 30 dB stronger than the quiet base line as seen in

figure 53. A strong local AM radio station at 1560 kHz can be

seen through the noise. At higher frequencies the noise drops off

with little difference being seen as shown in figures 54 and 55.

The same noise was observed on another current clamp. This medium

size clamp was located in the rear bay and coupled to a cable of

a smaller physical diameter (see figures 56 and 57).

In the cockpit small diameter H-field current clamps were

attached to both sides of the instrument panel. The same noise

seen previously was observed on the left side as seen in figure

58. On the right side the signal was not as strong over the

spectrum (Figure 59). The local AM radio station mentioned before

was 10 dB higher (figure 60) than that seen in the belly bay due

to the acrylic plastic windows in the nose of the aircraft.

Generally these external signals are suppressed because of the

design of the H-field current clamps. RF currents flowing in the

wire bundles where the clamps are attached create the fields in

the toroidal cores of the clamps which are coupled by a separate

winding. This means that in the case of an external source,

impingement onto the aircraft's skin and radiation through

acrylic plastic windows on to exposed wire bundling is sufficient

to create the signals observed. Small coaxial cabling attached

throughout the airframe is not excluded from this general

problem. As long as there are conductive surfaces exposed to

external radiation, rf circulating currents will be generated if

improper termination exists.

Figures 61 and 62 show that the previously mentioned lowfrequency jamming signal is also present on some of the

aircraft's wiring. Figure 63 is a magnified view of this signal

frozen in time for detailed observation purposes. This plot was

obtained by increasing the vertical sensitivity to 2 dB per

division. These plots were acquired from a small H-field clamp

located under the left hand side of the instrument panel. In

comparison to the previous aircraft, nothing unusual was seen on

the E-field probe.

6.6 THE THREE HUEYS

These three aircraft were obtained at various times from the

flight line when available. Since the Huey UH-IH helicopter is

13
the primary aircraft for training at CAAF , more of them were

available. They were easier to acquire, but they were not

identical in the way they were equipped from a systems wiring

viewpo int

far as detectable noise was concerned. In addition, the noise

floor for each spectrum segment was generally the same except for

the third or last aircraft. The first Huey showed electronic

switching noise at around 100 kHz and was relatively weak at -80

dBm. The signal was picked up on the small H-field current clamp

located on the left side under the instrument panel (see figure

64) .

In the second Huey, noise was detected in the 3 to 4 MHz

region (Figure 65) when the engine was running. This noise was

detected on a small H-field clamp located on a cable in the right

rear bay of the aircraft. Noise, at 330 kHz at -88 dBm, was

detected on a medium H-field clamp located in the left rear bay

(see Figure 66) . In the same time frame that data were being

collected on the clamps, the internal E-field monopole detected a noisy environment of -62 dBm for frequencies from 15 to 40 MHz.

Some of the same features showed up in the quiet plots taken (see

Figures 67 and 68) . As a result, the only statement that can be

made is that the noise in the spectral region is most likely

external and the source or sources could not be identified. When

the third Huey was tested all of the noise floors were higher

with the engine and avionics running than in the first two. The

major source of switching noise detected in Huey no. 3 was

determined to be in the low frequency spectrum at around 235 kHz

as detected inside the right rear panel by a small H-field

current clamp. This noise was measured at -67 dBm plus or minus

18 dB (see Figures 69 and 70) . No other signals were found but,

due to the high ambient noise floor, others may exist.

Noise detected in the range of frequencies studied (dc to

500 MHz) can originate from various sources. The topic of noise

has been investigated for many years and several general

categories have been defined in an attempt to describe the

process based upon the source.

EMC degradation is by definition a condition which is

brought on by man-made noise even though unintentional.

Propagation of the interfering signals can occur by unknown

paths. In some cases, multiple paths can exist complicating the

task of noise signal elimination. In general, investigators [5]

have found that the amplitude of man-made noise decreases with

increasing frequency and varies considerably with geographic

location. Propagation is from power lines, ground waves and

ionospheric reflection at frequencies below the maximum usable

Each helicopter demonstrated individual as

7. CONCLUSIONS

14
frequency. Measurements of the noise indicate that peak levels

are not proportional to received bandwidths for bandwidths

greater than 10 kHz. This was often found to be the case in this

study when the starting point in the spectrum was higher than

approximately 1 MHz.

The propagation mechanism of what is believed to be unwanted

signals in the aircraft tested is still not clear. Exact source

locations were not determined for any of the signals detected due

to the constraints of the experiment. Studies of the spectrum

plots have indicated that the noise can either be periodic or

nonstationary

An example of a periodic process is that of electronic

switching noise which is characterized by its regular change in

amplitude as a function of frequency. Most of the time these

types of signals were found to exist on H-field current clamps

attached around wire bundles associated with aircraft power

circuits. Sometimes they were strong enough to be detectable by

the internal E-field monopole, which indicates a radiated nearfield situation. This radiation may take place because of poor

wiring practice due either to ground loops or nonexistent grounds

on cables where they should exist. This is an area for further

study in light of what maximum signal levels can be considered to

be tolerable. To explore the situation would probably require a comprehensive matrix approach similar to that employed by some

aircraft manufactures using EMI techniques with EMI

instrumentation.

Random or nonstationary category noise was also studied for

each aircraft tested. The internal E-field signal was observed as

a noise floor plus amplitude variations riding on it. In this way

of observing noise, with the aircraft quiet or running, we assume

that the aircraft makes a contribution to what is seen through

either a source consideration or an incident field modification

process. Table 3 is a comparison of noise floors for the various

aircraft tested and the nonstationary variation riding on that

floor with the engines and avionics running.

Several facts can be deduced from the data shown on table 3.

Over the five frequency segments covered, with specific

exceptions as noted in the discussion for periodic noise, all of

the aircraft except for one exhibited the same amount of

generated noise. Levels ran from -82 to -112 dBm for received

bandwidths of 1 kHz to 1 MHz. The only aircraft that appeared to

be noisier than the rest was the third Huey tested. With 10 dB of

attenuation switched in at the spectrum analyzer, the noise floor

was measured to be about 10 dB higher on average over the same

range of frequencies than the other aircraft. The amplitude

variations were also twice as large. A common practice used in

the location of broadband noise sources is to determine the

highest frequency detectable because, with most man-made sources,

15
the amplitude is an inverse function of the frequency. If the

geographical location of the source is approached, the near field

of higher spectral components are penetrated until the source

location is discovered.

The noise data for Huey no. 3 suggest that, even though the

source locations were not found, they were associated with the

helicopter since high frequency noise components did exist. As an

example, figure 71 (Huey no. 2, 1-100 MHz) was overlaid on figure

72 (Huey no. 3, same frequency range). The result, figure 73,

depicts graphically what table 3 indicates with the numbers. When

viewing this plot remember that 10 dB of attenuation was switched

in on Huey no. 2 beneath would not have easily been identified.

It is also interesting to note that Huey no. 3, identifies itself

by way of its serial number as the oldest of the three tested.

The data may indicate that structural degradation is occurring

and that further testing should be done.

Each Huey was found to be different from an electrical

system viewpoint so we must be careful not to make specific

conclusions at this time regarding the findings just presented.

The additional noise found and the way it manifested itself does

support and justify the need for further research and testing.

The experimental technique, although not perfected at this time,

also demonstrates the ability to detect possible EMC degradation

of sources through the reception and interpretation of noise

floor data. As a result, it suggests that more work be performed

building upon the knowledge gained from these tests.

From an EMI environmental viewpoint, a system matrix study

performed in an e 1 e c t r omagne t i c al ly quiet location as a function

of aircraft geometry and spectral frequency now appears to be a next new starting point. The results should then be directly

convertible into hardware that quality assurance could use in the

Environmental Stress Screening (ESS) program.

16
8. REFERENCES

[1] Shands.T.G. and Woody, J.A., Investigation of

Intermodulation Products Generated in Coaxial Cables and

Connectors, Rome Air Development Center, Air Force Systems

Command, Griffiss Air Force Base, N.Y. 13441, Final

Technical Report RADC - TR - 8 2 - 2 4 0 , September 1982.

[2] Shands, T.G. and Woody, J.A., Metal-Insulator-Metal

Junctions as Surface Sources of Intermodulation, Rome Air

Development Center, Griffiss Air Force Base, N.Y. 13441,

Final Technical Report RADC - TR - 8 3 - 3 1 , February 1983.

[3] Watson, A.W.D.

Improvements in the Suppression of External

Nonlinearities (Rusty Bolt Effects) Which Affect Naval Radio

Systems, IEEE International Symposium on Electromagnetic

Compatibility, Arlington, Va., August 1983.

[4] Denney, H.W., Acree, D.W. and Mantovani, J.C., EMC

Measurement Techniques for Aircraft, Rome Air Development

Center, Griffiss Air Force Base, N.Y. 13441, Final Technical

Report RADC -TR- 83 - 166 , July 1983.

[5] Reference Data for Radio Engineers, Sixth Edition, Howard W.

Sams & Co. Inc.

Indianapolis, In.

,

1979.

17
TABLE 1 Aircraft Tested

Date Received

12-0i|-86

12 - 06-86

12-06-86

12 - 07-86

12-07-86

12-08-86

12-08-86

12-09-86

Aircraft

OH-58A Kiowa

JAH-15 Cobra

UH-1H Huey

UH-60A Blackhawk

UH-1H Huey

CH-47C Chinook

UH-1H Huey

0V-1D Mohawk

Serial No.

70-15612

77-22766

72-21574

86-24507

66-17080

68-15862

66-850

68-16992

18
TABLE 2 Active Frequencies for CAAF

Radio Service Frequency for CAAF

ADF 560 KHz

OP 36.4, 138.8 & 244.8 MHz

242.1 MHz

VOR 111.2 MHz

GCA 125.4, 125.8, 128.55 MHz

133.45, 133.75, 134.1 MHz

229.6, 232.5, 234.4 MHz

237.5, 242.6, 370.3 MHz

TWR 121.9, 127.0, 127.95 MHz

241.0, 242.1, 248.20 MHz

347.5 MHz

IFF 1200 MHz

RADAR 1030, 2762, 2800 MHz

9080 MHz

19
TABLE 3. A comparison of noise floors in dBm taken with

the inside E-field monopole (helicopters running).

FREQUENCY RANGE BLACKHAWK CHINOOK COBRA MOHAWK

0-50 kHz -105 + 9 -105 + 10 - 94 + 7 -104 + 9

50 kHz - 1 MHz - 99 + 7 - 99 ± 6 -100 ± 7 - 98 + 4

1 MHz - 10 MHz - 90 + 6 - 90 ± 5.5 - 99 ± 5 - 90 ± 4

10 MHz - 100 MHz - 82 + 6 - 83 + 4.5 - 99 ± 5 - 82 + 4

100 MHz - 500 MHz - 82 + - 82 ± 4.5 -101 + 4 - 82 + 3

FREQUENCY RANGE HUEY NO. 1 HUEY NO. 2 HUEY NO. 3

0-50 kHz -104 ± 10 -112 + 10 - 93 ± 18

50 kHz - 1 MHz -103 + 7 -103 + 7.5 - 90 + 14

1 MHz - 10 MHz -103 + 7 - 90 ± 5 - 80 ± 8

10 MHz - 100 MHz -105 ± 5 - 82 + 5 - 73 ± 6

100 MHz - 500 MHz -114 ± 5 - 83 ± 4.5 - 73 + 6

NOTE: Kiowa data not included because the up-convertor was not used making

the noise figure different and therefore the data uncomparable.

20
21

1"IKu

T'e1.

['ie1dTest

(’on

i.' iratIon.
STRAIGHT THROUGH

Figure 2. Block Diagram of Electronjc System Hardware used for

Data Acquisition.

22
Vout

(p.p.)

in

volts

Vin (p.p.) in millivolts

Figure 3. Transfer Function for the Receiving Up-Converter

measured at 1000 Hz

23
K

X 3

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24

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Figure

4.

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25

Figure

5.

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Helicopter.
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26

Figure

6.

Kiowa

Helicopter.
REF

LEVEL

FREQUENCV

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27

Figure

7,

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Helicopter
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28

Figure

8.

Kiowa

Helicopter.
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29

Figure

9.

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Helicopter.
X

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30

Figure

10.

Kiowa

Helicopter.
X

ZC9

<tC9

Cum

tA

XX

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•JbZ

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lx 3

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ics

in z i AbH

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nnn

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31

Figure

11.

Kiowa

Helicopter.
REF

LEUEL

FREQUENCV

SPAN

-40DBK

25KHZ

5KHZ

NX 3

XtAA

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Kaeo

mr>HI Kin AUO

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HK<E>- I faKOO O S

0)

n z&

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32

Figure

12.

Kiowa

Helicopter.
Ni

PUNji

f:

NXX

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MZ

MAn9 Sb I U

X 3

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33

P'igure

13.

Kiowc'i

Helicopter.
X

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Oi^in

(A

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M£

xn

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34

Figure

14.

Kiowa

Helicopter.
NX 3

scaa

HKaOO

nosHI Kin AMS wH I I-IH

M'vw

H*’

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35

H A<J

Figure

15.

Kiowa

Helicopter.
NZ 2

SUZsA

HK<Z00

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AUC9

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in

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36

Figure

16.

Kiowa

Helicopter.
N

Xi

X:

P^Hi

V) \

Ni

Xi

om

Zinl u -I OCOi U i K \ b I X \ MZI

MOi

xmb ii » NX 2

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n zsn

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37

Figure

17.

Cobra

Helicopter.
XI

hHi

V> I

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Zini U •! omi

Mhi K I b i

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X 3

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38

Figure

18.

Cobra

Helicopter.
>

np:;&4is

(Sl>i-iS

H AO

39

Figure

19.

Cobra

Helicopter.
REF

LEUEL

FREQUENCV

SPAN

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5KHZ

NX 2

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tSHf> HI auts

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00

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n Zb

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40

Figure

20.

Cobra

Helicopter.
>

\HJin

nccpMS

9=»i-hiS H OO

41
xi

Ki

Zini

4X0SI

(k II

Mi

XI

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Zini U -i OMeb

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bOi

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b I iI K I ObO>

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42

Figure

22.

Cobra

Helicopter.
N

XI

Ki

Zini

«CM

Pn I Vi I

Ni

Xi

OMI

zini u -i

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oni

WAI

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Figure

23.

Coi)ra

Helicopter.
NX 3

SUZ>.fi

HK<Xi»

nosHI n*KO

AMIS mH I hhH

MS.

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M*>

no^os

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HKn-- I MSM S H

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n zf.

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snn s a 0* cc>

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nnns

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44

Figure

24.

Cobra

Helicopter.
NZ Z

Zi/iA

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Z<D»

n<T>I «p- AMSI

faS.

Ui

cn

M * * aoUieo

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AU(ior> SAhnS H AA

45
46

Figure

26.

Blackhawk

Helicopter.
~±raiwir --

ISX 3 XWA

(SMZ>Xi

SX<COO H nchHI Kt^ AUS

i-hH I AiJM

nhH

inH

U><

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Ti

04

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4X &r< U

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47

Figure

27.

Blackhawk

Helicopter.
NXX

Oh 9^

iA

UZ

uo

•JS

sO

b I UX

NX 2

KV)A

C9MZV0

cse^xco H HI KC~

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01

in

U..

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MZin I bKoi O H

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n zsh

AbH'N C9KH 2 « &r* U

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H

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48

Figure

28.

Blackhawk

Helicopter.
NXX

zs

<XCS

CLiO^

</)

>xOZ

Zin• xin oMKb

nJbZ

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in

b I b

_N

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b\sH

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Tim

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49

Blackhawk

He]

icoplo
N N

X!: —

U X 3

X- ; ; :i ^4||^|Q| zm tSXZ>Ji

L_._- -j— n caxcioo

AiTiH li H ttffH

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H

! ii 1

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S.HiJiT'

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50

Figure

30.

Blackhawk

Helicopter.
V—

[S] X 3

S[x]Z>Xi

C9K<CX H «0S HI KrAUC9

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't

01

PQ ZP<

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34.

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42.

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73, A

comparison

of

the

NOISE

floor

for

Hueys

No. 2

and

No.
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

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

SHEET (See instructions) NBSIR 88-3083 January 1988

4. TITLE AND SUBTITLE

AIRCRAFT FIELD DEGRADATION AND ELECTROMAGNETIC COMPATIBILITY

5. AUTHOR(S)

Kenneth H. Gavcey and Dennis S. Friday

6. PERFORMING ORGANIZATION (If joint or other than NBS. see In struction s)

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)

U.S. Army Aviation Systems Command (AVSCOM)

ATTN: AMSAV-QP

4300 Goodfellow Boulevard

St. Louis, Missouri 63120-1798

10. SUPPLEMENTARY NOTES

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

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

bi bliography or literature survey, mention it here)

This paper discusses the first tests undertaken to study the problem of field

degradation in army aircraft (helicopters and one fixed wing airplane) due to the

deterioration of electronic and electrical systems. The electromagnetic compatibility

(EMC) of such systems was investigated by passive measurement of the aircraft as a collection of radio frequency sources. Methods for detection of these sources were developed that Included sensitivity to both stationary and nonstationary noise that

existed

The collected data were studied to see if there existed any obvious factors

derived from the data that one could use to correct potential problems that might

affect flight safety. Emphasis was placed upon making such test methods appropriate,

inexpensive, and easily performed by army field personnel. In addition, applications

to quality control or acceptance testing, as related to the Environmental Stress

Screening (ESS) program, were examined.

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

bandwidth; data acquisition; electromagnetic compatibility (EMG) ; electromagnetic

interference (EMI); electromagnetic spectrum; field probes (or antenna); noise floor;

noise sources; spectrum analyzer

13. AVAILABILITY

[_^ Unlimited

1 1 For Official Distribution. Do Not Release to NTIS

1 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

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

14. NO. OF

PRINTED PAGES

100

15. Price

USCOMM-DC 6043-P80

*U.S. GOVERNMENT PRINTING OFFICE: 1 9 8 8-0-57 4-3 6 6/85 0 8 2