NASA / TM-2000-210626

RF Loading Effects of Aircraft Seats in an

Electromagnetic Reverberating

Environment

Truong X. Nguyen

Langley Research Center, Hampton, Virginia

December 2000

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NASA / TM-2000-210626

RF Loading Effects of Aircraft Seats in an

Electromagnetic Reverberating

Environment

Truong X. Nguyen

Langley Research Center, Hampton, Virginia

National Aeronautics and

Space Administration

Langley Research Center

Hampton, Virginia 23681-2199

December 2000
Available from:

NASA Center for AeroSpace Information (CASI)

7121 Standard Drive

Hanover, MD 21076-1320

(301) 621-0390

National Technical Information Service (NTIS)

5285 Port Royal Road

Springfield, VA 22161-2171

(703) 605-6000
RF LOADING EFFECTS OF AIRCRAFT SEATS IN AN

ELECTROMAGNETIC REVERBERATING ENVIRONMENT

Truong Nguyen, NASA Langley Research Center, Hampton, VA

Abstract

Loading effects of aircraft seats in an

electromagnetic reverberating environment are

investigated. The effects are determined by

comparing the reverberation chamber' s

insertion losses with and without the seats. The

average per-seat absorption cross-sections are

derived for coach and first class seats, and the

results are compared for several seat

configurations. An example is given for how

the seat absorption cross-sections can be used

to estimate the loading effects on the RF

environment in an aircraft passenger cabin.

Introduction

There has been significant interest in

the effects on aircraft digital avionics due to

radio frequency (RF) electromagnetic fields.

These fields may come from sources external to

the airplane such as high power radars, or

possibly from carry-on personal electronic

devices (PEDs) inside the plane.

Electromagnetic fields can induce currents on

circuits causing undesirable effects, which may

include system shutdown in digital equipment.

In order to characterize the electromagnetic

coupling from RF sources on the internal

electromagnetic environment, it is desirable to

understand the effects of different loading

sources in the aircraft cavity. These sources

may be due to RF absorption or leakage from

aircraft seats, human bodies, electrical wires,

windows, and others. This paper addresses the

effects caused by aircraft seats, which are of

interest since there are large numbers of them

on commercial aircraft.

Recent NASA Langley Research

Center's acquisition and conversion of a 20

year old Boeing 757 airplane to a research

vehicle briefly made available a number of

aircraft seats. Since actual testing inside the

aircraft was impractical for this purpose,

evaluation in a reverberation chamber (also

called mode-stirred chamber) becomes the next

best option. A high Q cavity like a

reverberation chamber is expected to be much

different from a low Q environment like in an

aircraft. However, as it will be shown later, a

parameter called seat absorption cross-section,

that is independent of the cavity's Q, can be

extracted and used in predicting the effects of

the seats in an actual aircraft environment.

Approach

Description

The twenty year old seats used in this

experiment included ten first class and eight

coach class seats. The first class seats come in

pairs, where as the coach class seats come in

two rows of three and one row of two chairs.

To address the effects of aircraft seats,

testing on an actual airframe would be ideal but

expensive. Rather, it is advantageous to utilize

a mode-stirred chamber instead, since

electromagnetic fields in a mode-stirred

chamber behave very similar statistically to

those within an aircraft body [1].

The mode-stirred method has been

suggested as a thorough and cost effective

method for testing avionics products for HIRF

compliance. It is also very effective for

measuring RF absorption and total radiated

power. Readers unfamiliar with the modestirred chamber testing techniques are

recommended to review reference [2,3].
Using a reverberation chamber, the

chamber's average gain is first measured

without the seats to establish a baseline

representing all of the losses in the chamber not

associated with the aircraft seats. The average

gain is then measured again but with the seats

in the chamber. The differences between the

two measurements are the effects associated

only with the presence of the seats. The results

are then used to derive the seats' absorption

cross-section (O-a), which will be shown to be

independent of the Q of reverberation

environment. (o- a )can later be used to estimate

the loading effects due to the seats on the Q of

another reverberating environment.

Theory

The inverse of the quality factor Q of a

reverberation chamber is the sum of the inverse

of the Q of all loss factors in the chamber [4]

including the seats. Or,

1 1 1

- + {1}

Q NTotal ¢.) Total seats ?_0 QN seats

where QN seats is associated with loss due to the

seats only. r3T°tal _0 is the total chamber's quality

factor without the seats, and QTotal is the total N seats

chamber's quality factor with the additional

presence of the aircraft seats. Both QTotal N seats and

Q Total 0 can be readily calculated from measuring

the chamber gain with and without the seats.

From [4],

_ 2TcV

QN seats NZ @a), {2}

where N is the number of seats; (O-a) is the

absorption cross section per seat assuming the

losses for all the seats of the same kind are the

same; V is chamber's volume; and Z is the

wavelength.

Substituting {2} into {1} and solve for

(O-a), we get:

(o" a) = --N----_- 2/vV [1QtGl ntotal 1]•

h N seats _,0

{3}

The chamber quality factor Q can be

expressed as

(16_r2Vl((P,ec) I

16_:2V G >,

where r/l, r/2

P input

(a)

The

{4}

transmit and receive antennas

efficiency,

the average power at the receive

antenna (Watts),

power input to the chamber at

transmit antenna' s terminal

(Watts), and

average chamber gain, or = (Gc).

%,,

procedure for measuring the

average chamber gain is described in [2]. In

short, the average chamber gain is the ratio of

the average power coupled out of the chamber

via an efficient receive antenna to the power

delivered into the chamber. The antenna

efficiency rh,r/2 are typically about 0.75 for

log periodic antennas and 0.95 for dual ridge

horn antenna [5]. Ideally, they should be unity

for 100% efficiency. For simplicity, they are

assumed to be unity for this purpose since they

are difficult to measure. In addition, we also

assume that the antennas are well matched, and

no mismatch corrections are necessary. The

power transmitted into and coupled out of the

chamber can simply be measured at the antenna

terminals.

Substitute {4 } into {3 },

(O,,.a) -- 771772 _2 [( i

81vN GN;eats)

{5}
whe;e<Co>and aretheave;age

chamber gains with and without the seats,

respectively.

Test Methods

To use equation {5 }, one needs to know

the average chamber gain with and without the

seats, (G N ,_,> and (G o} respectively.

Chamber gain is measured by taking the ratio

of the received power against the transmitted

power. Typically, only a small amount of

power at the desired frequency is injected into a

reverberation chamber through the transmit

antenna. The receive antenna measures the

power at the receive antenna over a complete

stirrer revolution. The receive power is then

normalized to (or divided by) the input power

to yield chamber gain for that frequency. If the

maximum value of receive power is used in the

computation, the ratio is called the maximum

chamber gain. If the average receive power is

used, the result is called the average chamber

gain (G).

The chamber gain measurement was

performed using two different methods: "modetune" for frequencies below 1 GHz and "modestir" for frequencies above 1 GHz.

In theory, there is little difference

between the two methods, except with how the

input power is measured and maintained

constant as the paddle rotates. In the modestirred method, the forward power is

maintained constant as the stirrer rotates. In

mode-tuned method, the net input power

(forward power minus reflected power) is

maintained constant as the stirrer rotates.

In practice, the mode-tuned method is

much more difficult to implement since both

the forward power and reflected power are

measured, and the power is adjusted to

maintain constant net power for every paddle

positions. This is necessary at lower

frequencies (near the chamber start frequency,

or about 100 MHz) as the forward and reflected

powers fluctuate significantly with the paddle

rotations.

In contrast, at higher frequencies

(typically above 1 GHz), both the forward and

reflected power levels remain relatively

constant with paddle rotations. In addition, the

reflected power is low given efficient and

matched transmit antenna. The mode-stirred

method is typically used in this range of

frequency since only the forward power is

monitored, and therefore easier to perform.

Test Set Up

The seats were tested in six

configurations: 3, 6, 8 coach class seats, and 2,

6, 10 first class seats. They were arranged

approximately half a meter off the floor in the

middle of the 14.3m x 7.0m x 2.9m (47 x 23 x

9.5 feet) mode-stirred test chamber as shown in

figure 1. For better repeatability, they were

positioned at least a third to a half of a

wavelength away from any conducting surface

as according to the guidelines in [2]. However,

this set up was not entirely realistic as many of

the seats were close to the airframe body in an

actual aircraft.

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Figure 1: Ten first class seats in a

reverberation chamber

A pair of log period antennas were used

for frequencies below 1 GHz. Above 1 GHz up

to 8 GHz, a pair of dual-ridge-horns antennas

were used. The antennas were positioned in

such a way as to prevent direct coupling
between antennas,and so as not to point

directly attheseats.

Analysis and Results

Both the maximum chamber gain data

(from peak spectrum analyzer's reading) and

the average chamber gain data (from the

average spectrum analyzer's readings) were

collected. However, only the average chamber

gain data are presented in this paper since the

analysis on maximum chamber gain requires

further theoretical developments.

In figures 2-3, the average chamber gain

curves, along with their best fit curves, are

shown compared with empty chamber data to

demonstrate the effects of the seats in a

reverberation chamber. Average empty

chamber gain data is shown with no offset.

Others are shown with 10, 20 and 30 dB offsets

for better clarity and to prevent curves from

riding on top of one another. For coach class

seats, figure 2 shows average chamber gains for

no seat (with no offset), with 3 seats (-10 dB

offset), with 6 seats (-20 dB offset), and with 8

seats (-30db offset). For first class seats, figure

3 shows chamber gains for no seat (with no

offset), with 2 seats (-10dB offset), with 6 seats

(-20dB offset), and with 10 seats (-30db offset).

In addition, the best-fit curve for empty

chamber data is plotted with the same offsets (-

10, -20 and -30 dB) to illustrate the effects of

increasing the number of seats.

In figures 2 and 3, the trends show

chamber loss compared to an empty chamber

increases as the number of seats increases.

Even though the effects of the seats on the

average chamber gain are small in dB value,

they may have a significant impact on the Q of

the aircraft due to the large number of seats

installed in a typical aircraft. Figures 2 and 3

also show that the losses are very broadband, so

they can be easily represented with just few

points across the frequency span.

If one were to divide the total seat loss

by the number of seats, the results would be a

value that may be mistakenly identified as the

loss per seat. This value tends to decrease as

the number of seats in the chamber increases,

therefore is not useful in practice. This

phenomenon also indicates that the effects on

the chamber gain decrease with the chamber

gain, or chamber quality factor Q.

-10

-20

-30

-40

E

._ -5o

(3

< -60

-70

-80

\

Flefe_

I I I

0 2000 4000 6000 8000

Freq (MHz)

Figure 2. Average Chamber Gain with 3,6,8

Coach Class Seats and best-fit curves

-10

-20

_- -30

r_ -40

e_

E

-50

(3

< -60

-70

-80

nc

Chamber Trendlines

I I I

2000 4000 6000

Freq (MHz)

8000

Figure 3. Average Chamber Gain with

2,6,10 First Class Seats and best-fit curves

Instead, equation 5 describes the

absorbing cross section parameter that is valid

with any chamber Q. This value depends only

on the QN ,eat,, which is independent of
chamberQ as shownin equation1. However,

its effect on the Q dependson the Q value of

the environment.The lower the cavity Q, the

lesstheeffecttheseatswill haveonit.

As an illustration, if the Q of the

environment is low, the seat loss may not

contributemuchto the overallcavity gainsince

othertypesof chamberlosses(like lossesdue

to wall, windows, etc.) dominatesthe cavity

gain value. Since Q is directly relatedto the

cavity gain, the presenceof the seatsdoesnot

affect the Q significantly. If the Q is high,

however,theRF losscausedby the seatis now

a major contributor to the overall loss.

Therefore, adding or removing seats

significantlyaffectsthecavity Q.

Equation5 canbeapplieddirectlyto the

averagechambergaindatain figures2 and3to

provide absorbing cross section value.

However,the result would be extremelynoisy

since both the empty chamber data and

chamberwith seatsdata areinherently noisy.

The differencebetweentwo noisy setsof data

would be even noisier. Since the magnitude

difference between the two sets of data is

already small, the noise in the calculation

resultswouldbelargerthantheresultitself.

Sinceaverageseatlossis expectedto be

slowly varying with frequency, various

techniquescanbe usedto reducenoisein the

results.Our approachis to best fit eachgain

curve with an analytical function, then apply

equation5 to the smoothedfunctionsto obtain

theaverageabsorbingcrosssection.

The resultsareshownin figure 4 and5

for comparing (era) for various seat

configurations.The averagesof the resultsare

also shown as references.Both figures show

that the resultsagreewell, with the first class

seatresultsshowingbetteragreement.

This approachalso appearsto be valid

for cavitylossesdueto othersources,including

humanbody RF absorption,wiring, etc.These

losses can be measuredin a reverberation

chamber,andthe resultscanbe usedto predict

the effects in an actual aircraft environment.

Onejust has to ensurethe frequencyis such

that both the laboratory and aircraft

environments are overmoded [2] and are

statisticallysimilar,typically implying thereare

at leastseveralhundredcavitymodesin eachof

the cavities. The use of this approachfor

undermoded cavities has not been

demonstrated.

0.10

0.08 t

0.06

E

I_ 0.04

0.02

0.00

_. vs. Frequency

(Coach Class Seats)

Average

_-_3 seats

0 2000 4000 6000

Freq (MHz)

Figure 4. Coach Class Seat's Absorbing

Cross-Section (o-a /

0.10

8000

0.08

0.06

E

b

0.04

0.02

o.oo

% vs. Frequency

(First Class Seats)

Average

6 seats

2 seats

10 seats

2000 4000 6000

Freq (MHz)

Figure 5. First Class Seat's Absorbing

Cross-Section (o- a )

8000

Application to Aircraft Environment

This section illustrates how (o-a) is

used to estimate the RF loading effects of

aircraft seats in the aircraft passenger cabin. It

is necessary to know the Q of the cabin, either

empty or with the seats. Since there is currently

no such published data available for a Boeing
757, we useQ datafor anemptyBoeing707-

720B [6] with no seats.An assumptionmadeis

that the Q's for both airplanesare about the

same.Further,it is assumedthatthecavitygain

can be scaledwith the volume ratio between

the two airplanes.The assumptionsappeared

reasonablesinceboth airplaneswere built by

the samemanufacturer,and the volumeswere

on the sameorder.In fact, rough estimatesof

the internal volumes are about 573 m3 for a

B757 and464 m 3 for B707. Also, we assume

there are 200 coach class seats in a Boeing 757

for this calculation.

In [6], several techniques were used for

measuring the cavity Q of a passenger cabin,

and the results do not agree with each other

very well. The reasons have not been resolved.

For this illustration, however, we use the

highest envelope of Q data for the highest

cavity gain. This tends to exaggerate the effects

of the seats slightly. The assumed values for Q

are shown in table 1. (o-a) takes an

approximate value of 0.04 at all frequencies for

simplicity, which is reasonable according to

figure 4. Applying equation 5 and solving for

(a ...... )/(ao) in dB, one will arrive at the table 1

below:

Table 1: Estimated reduction in average

cavity gain in a B757 with 200 seats

Freq

(MHz)

100

Q (o-a) .....

(dB)

50 0.04 -1.26

110 0.04

137 0.04

237 0.04

350 0.04

375 0.04

850 0.04

1250 0.04

1800 0.04

1975 0.04

225 -1.23

300 -1.16

600 -1.02

925 -0.98

1000 -0.97

1800 -1.1 9

3200 -1.01

5000 -0.94

5800 -0.89

From table 1, the presence of the 200

seats reduced the average cavity gain by about

1.25 dB at 100 MHz to about 0.9 dB at 5.8

GHz. The loss is not significant to have a major

impact on the RF environment, and is barely

observable.

The assumption is the environment

behaves similarly to a mode-stirred chamber

statistically, and that there is no direct transmit

and receive antenna coupling. In the case of RF

coupling from a personal electronic device onto

a wire, depending on the distance between

them, direct coupling effects may dominate the

mode-stirred effects, then this approximation

does not apply.

Conclusion

Absorption cross-sections of a set of

aircraft seats in a mode-stirred environment

were derived from measured cavity gains. They

are independent of the quality factor Q of the

chamber in which the parameters were

measured and are relatively constant with

frequency. They can be used to predict the

effects of the seats on the electromagnetic

reverberating environments in an aircraft. The

same approach can be used in characterizing

absorbing bodies in aircraft and other

reverberating environments.

Acknowledgement

The author would like to thank Mr.

Reuben Williams and Mr. Kenneth Dudley,

both of NASA Langley, and Ms. Sandra

Koppen, Lockheed Martin, for their supports in

acquiring the seats and in software

development. Special thanks to Mr. Michael

Hatfield, NSWC, for providing technical

directions leading to the understanding of

chamber Q.

References

[1] Hatfield, M.O.; Freyer, G.J.; Johnson,

D.M.; Farthing, C.L.; "Demonstration

test of the electromagnetic reverberation

characteristics of a transport size
aircraft," NSWC Dahlgren Division,

Report No. NSWCDD/TR-93/339, 1993.

[2]. Crawford, M.L. and Koepke, G.H.,

"Design, Evaluation and Use of a

Reverberation Chamber for Performing

Electromagnetic Susceptibility/

Vulnerability Measurements," NBS Tech.

Note 1092, 1986.

[3] "Environmental Conditions and Test

Procedures for Airborne Equipment,"

RTCA/DO-160D, July 29, 1997.

[4] Hill, D.A; Ma M.T..; Ondrejka

A.R.;Riddle B.F.; Crawford M.L.; Johnk

R.T., "Aperture Excitation of Electrically

Large, Lossy Cavities," IEEE Trans. On

Electromagnetic Compatibility, Vol 36,

No. 3, August 1994.

[5] Ladbury, J.; Koepke, G.; and Camell, D.,

"Evaluation of the NASA Langley

Research Center Mode-Stirred Chamber

Facility," NIST Technical Note 1508,

1999

Hatfield, M.O.; Johnson, D.M.; Capt.

Loughry, T.A.; Ondrejka, A.R; Johnk,

R.T; Freyer, G.J; Slocum, M.B. "Phase II

Demonstration test of the electromagnetic

reverberation characteristics of a

transport size aircraft," NSWC Dahlgren

Division, Report No. NSWCDD/TR97/84, 1997.

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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

RF Loading Effects of Aircraft Seats in an Electromagnetic Reverberating

Environment WU 522-14-21-02

6. AUTHOR(S)

Truong X. Nguyen

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

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13. ABSTRACT (Maximum 200 words)

Loading effects of aircraft seats in an electromagnetic reverberating environment are investigated. The effects

are determined by comparing the reverberation chamber's insertion losses with and without the seats. The

average per-seat absorption cross-sections are derived for coach and first class seats, and the results are

compared for several seat configurations. An example is given for how the seat absorption cross-sections can be

used to estimate the loading effects on the RF environment in an aircraft passenger cabin.

14. SUBJECT TERMS

Reverberation; Mode-stirred; Aircraft seat; Absorption-cross-section;

Immunity; Susceptibility; HIRF; EMC; Quality factor; RF absorption

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