design of absorber-lined chambers for emc measurements using a geometrical optics approach

IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. EMC-26, NO. 3, AUGUST 1984

Design of Absorber-Lined Chambers for EMCMeasurements Using a Geometrical

Optics ApproachSHANTNU R. MISHRA, MEMBER, IEEE, TOMAS J. F. PAVLASEK, SENIOR MEMBER, IEEE

Abstract-Absorber-lined chambers (ALC's) have been found usefulfor EMC measurements over an extremely wide frequency range. How-ever, only limited information about the field structure inside ALC's canbe efficiently determined by measurements, while the characteristics of therough absorbing surface do not render their analysis amenable to formalmethods. The computation of field structure as a function of variousparameters must thus resort to empirical modeling.A simple computational technique presented here predicts fields inside

ALC's to a good approximation. Use is made of the Geometrical Theoryof. Optics (GO). The absorber material is empirically modeled by itsreflectivity as a function of frequency and angle of incidence.To establish the validity of the technique, computed results are com-

pared to measured data. The methodology is then extended to computefields inside a variety of ALC configurations. A study of the effect ofshape, size, and absorber characteristics on these fields is presented todemonstrate the utility of the technique as a tool for ALC design purposes.Key Words-Anechoic chambers, internal field analysis, various configu-rations, design model.Index Code-Fl8d, F18e, A3.

I. INTRODUCTIONTHE VIRTUALLY exponential growth of urban ambient

electromagnetic (EM) field levels [1], [2], coupled withan increased proliferation of electronic equipment, both analogand digital, has been a cause of concern for some time now. Asa result, regulatory authorities, standards organizations, andthe electronics and computer-related industry have become in-volved with the general problem of EMC (electromagnetic com-patibility) at local, national, and international levels [31-[5].In order that EMC measurements (EM emission and suscepti-bility) may become an integral part of electronic component,subassembly, and system design, as well as production, eco-nomic means of measurement over an extremely wide rangeof frequencies (dc-GHz range) must be sought.

Because of the practical limitations of 'open-field' or'free-space' measurement techniques [6], [7], it os often preferablethat measurements be made in enclosed controlled environ-ments. At lower frequencies, TEM cells [8], parallel-plate lines[9], and other "known-field" simulation techniques [101, [11]have been extensively studied. However, there is an inherentupper frequency limit for such devices [12]. At higher fre-quencies, shielded enclosures lined with absorber material,

Manuscript received February 7, 1983; revised April 13, 1983. This workwas supported by The Department of Communications, Canada, and by theNatural Sciences and Engineering Research Council, Canada.

S. R. Mishra is with the Division of Electrical Engineering, NationalResearch Council of Canada, Ottawa, Ontario, Canada.

T. J. F. Pavlasek is with the Department of Electrical Engineering, McGillUniversity, Montreal, Quebec, Canada.

i.e., absorber-lined chambers (ALC's), have been proposedand extensively studied [13], [14]. Such chambers employabsorber material of a quality inferior to that needed for antennaand EM scattering measurements, thus rendering them eco-nomically practical. The trade-off is, of course, in reducedfield uniformity which, however, may not be a serious draw-back for EMC measurements [15], [16].

The design of ALC's, however, is still based on limitedmeasured data and experience [17] -[19]. The reason for thispractice is that calculation of field structure within an ALC isnot easy. Little is known about absorber material behavior asa function of frequency and its scattering characteristics fordifferent incidence angles, especially in the extended low-fre-quency range where the degraded material performance is be-low that normally intended in chambers for antenna-patternmeasurements. Because of its rough shape and absorbingproperties, the material is not amenable to analytical model-ing. To date, insufficient information about the properties ofabsorbers has made it possible to define, even empirically,suitable scattering coefficients usable in a geometrical theoryof diffraction type of computation. However, it is possible touse such limited information as is available to predict approxi-mately the fields inside ALC's.

This paper presents a simple computational technique forpredicting fields inside ALC's. Use is made of the geometricaltheory of optics [20] . TheE-field at any point is approximatedby computing the phasor sum of the direct ray from thesource and one reflected ray component from each of the wallsof the chamber. The reflection coefficient of the absorber ma-terial is approximated as a function of angle of incidence. Acomparison of computed fields with measured results [17],[21] shows that the technique provides a good first-orderapproximation. The paper then describes how this techniquemay be used as a tool to approach optimized design of ALC'swithin the constraints of size limitations, simple shapes, anduse of inexpensive absorber material.

II. MODELING OF THE ABSORBER MATERIAL

The foam pyramidal or contoured material generally usedfor ALC lining is characterized by its normal reflectivity RN(expressed in negative decibels). The normal reflectivity is afunction of depth of material. Fig. 1 shows the relationshipbetween the normal reflectivity and depth for typical material.Reflectivity at angles of incidence other than normal incidenceis higher. Fig. 2 is a plot of reflectivity at different angles for

0018-9375/84/0800-0 111$01.00 © 1984 IEEE

III


SOURCE IsPLfCEO ON THECENTRRL RXI5OF IHE CHAMBER .,

,

t_.

x

SIDE WRLL

IU

.el02 t05 lX .2 .5 2 S 2 ,

RBSORBER THICKNESS (log scole)

Fig. 1. Reflectivity at normal incidence as a function of thickness for foampyramidal type absorber material.

so soxX

C 40 40

_ /°

w 210 2~~~~~~~~~~~0

oj

90 80 70 60 50 40 30 20 10 0PNGLE OF INCIDENCE (IN DEGRE)

Fig. 2. Reflectivity Re as a function of angle of incidence for materials withdifferent normal reflectivities (RN). (-50 dB, -40 dB, -30 dB, -20dB, - 10 dB; solid lines Re = RN cosO).

materials of different thickness (normal reflectivity). Super-imposed on these data is a cosine function, and it can be seenthat the reflectivity Ro, as a function of angle of incidence,may be expressed empirically as

R0 =RN cos 0.

This has been used as a first approximation to the behavior ofabsorber material.

No precise information is available [25] to date which en-ables the phase of the reflection coefficient to be defined, evenfor normal incidence. Since absorber material may be con-sidered as a conductive material, a phase change of 1800 maybe attributed to R0 as a first approximation.

III. COMPUTATIONAL TECHNIQUEThe computation technique employed is based on geo-

metrical theory of optics (GO) or 'ray' theory. This has pre-viously been used as a basis for determining shape and 'quiet'areas within ALC's and anechoic chambers in a qualitative

- L

FLOOR

Fig. 3. Outline of ALC model and coordinate system.

fashion, and thus as a design aid [19], [22]. Some computa-tions using such a technique have been reported but only fortwo-dimensional structures [23], [24], and these did not takeinto account the complexity of material behavior [25]. Thediscussions presented in this paper describe how, for thepresent problem, the GO technique can be extended to three-dimensional structures and, above all, how absorber materialproperties can be incorporated effectively in an approximatefashion.

The particular case discussed is that of an ALC with rec-tangular cross section, as shown in Fig. 3. This figure alsoshows the coordinate system used. For computational pur-poses, any of the six walls can be 'covered' by different typesof absorber material or by completely reflecting material. Thenet field at any point (x, y, z) is computed as the sum of sevencomponents, a direct one from the source and first reflectioncomponents from each of the six walls, Only primary reflec-tion terms are used and the higher order ones are neglected inthe computations presented. However, these can be incorpor-ated in the computations if greater accuracy is desired.A modular computer program was developed for the pur-

pose. Fig. 4 is a block diagram of the program structure show-ing various modules. This modular nature of the program al-lows it to be adapted to calculate fields for different geometries,source structures, and material characteristics. Differentmodules (routines) can be substituted to take into accountmany possible situations.

The computer program has been designed with the intentionof incorporating future modifications as better material model-ing becomes possible and eventual use of GTD (geometricaltheory of diffraction), once scattering coefficients (presumablyin empirical form, as a first step) can be defined.

VI. COMPARISON OF COMPUTED AND MEASUREDDATA

Fig. 5(a), (b), (c), and (d) shows the fields inside a cube-shaped ALC for each edge 4X long, fully lined with absorbermaterial characterized by normal reflectivities of -7.5, -10,-12.5, and -15 dB, respectively, and excited by a half-wavedipole near one end wall. This situation corresponds to a cubicchamber of 1.2 m to a side, at 1 GHz. The measured data [21]for a chamber of similar dimensions and lined with Emersonand Cumming EC-CV-4 material are presented in Fig. 5(e).It is to be noted that the properties of this material in the

112

-_- z

MISHRA AND PAVLASEK: ABSORBER-LINED CHAMBERS FOR EMC MEASUREMENTS

PROGRRM STRUCTURE

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REFLECTED ANGULARCOMPONENT -DEPENOENCE

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Fig. 4. Block diagram of the computational program.

downward extended frequency range are not known and are

implied only by the extrapolation of manufacturer supplieddata. However, a comparsion to Fig. 5(a)-(e) indicates thatcomputed results using a reflectivity value of -12.5 dB cor-

respond best to the measured data. For the same reflectivityvalue (-12.5 dB), fields were calculated in a partly lined cham-ber with one wall completely reflecting, and are shown in Fig.6(a). The corresponding measured field distribution is shownin Fig. 6(b). As can be seen, there is good agreement betweenthe computed and measured results. Similar comparisons were

made at different frequencies and for various configurationsfor which measured data are also available.

This comparative study indicates on the one hand that thecomputational technique is a good first-order approximationfor predicting fields inside ALC's. On the other hand, thiscomparison also serves as a means for obtaining a rough esti-mate of the properties of the absorber material in the extendedfrequency regions where such data are not available. Thus, forexample, the computed data at a frequency of 600 MHz corre-

sponds best to an absorber reflectivity value of EC-CV-4, ofabout -5 dB.

Once the validity of the computational technique is verifiedby comparison with measured data, results can be computed

for a wide variety of cases for which measured results are notavailable or may not be conveniently measured. Thus a para-metric study of fields can be carried out in order to evolve an

optimized ALC design. The results of such a parametric studyare presented next.

V. SOME COMPUTED RESULTS AND APPLICATIONOF THE TECHNIQUE TO ALC DESIGN

The computational technique provides a valuable tool instudying the effects of various parameters on the fields insideALC's. Fields were computed by changing different param-eters in order to study the following:

a) effect of lining the chamber with materials of differentthicknesses (reflectivity value) for a constant room size;

b) effect of changing the room size while retaining the same

lining material;c) effect of lining different walls with different materials,

thus determining the relative contribution due to reflec-tion from different walls;

d) change in room shape, height: length: width ratios; ande) effect of changing the characteristics and the location

of the source within the chamber.

113


,~~~~~~~~L

4

0~~~~

(c) (d)

0

4~~~~~~

_ 1 2-2-1 0 1 2(a) (b)

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6o~~~~~~~L

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0 1 2 -2 -1 2 1

(c)-25Ban (d)-Sd.()Maue:(2cm3AClndwt

4 at p o

0(2

-3

-2 -

(e)

Fig. 5. Isophot (contours of equal amplitude) map of E-field in a cube-shaped fully lined ALC, dimension 4X cubed. Contours are on a relativefield strength linear scale.Computed for lining material with reflectivity: (a) -7.5 dB, (b) -10 dB,

(c) -12.5 B, and (d) -15 dB. (e) Measured: (120 CM)3 ALC lined withEC-CV-4, at 1-GHz horizontal plane, half-wave dipole source in all cases.

14


C-)

-60.0 -40.0 -20.0 0.0 20.0 40.0 60.0

AMPLITUDE CONTOURS LINEAR SCRLE(a)

- y SOURCE

-30 0 30

AMPLITUDE CONTOURS LINEAR SCALE

-J

EDz-

L}

LLJ-JLa

(b)

Fig. 6. Comparison of measured and computed fields in a partly lined ALC(All walls lined except side wall parallel to E-field polarization which isrelecting). Isophots are drawn on a linear scale. (a) Computed (4 X3chamber (120 cm)3 at 1-GHz lined with - 12.5-dB material, side wallreflecting. (b) Measured fields in (120 cm)3 ALC at 1-GHz EC-CV-4material, side wall reflecting.

Some sample results are shown here to highlight importantconsiderations in the design of ALC's. These results are in theform of maps showing contours of equal amplitude (isophots)of E-fields in horizontal or vertical planes containing the centralaxis of the rectangular cross-section ALC as shown in Fig. 3.

Fig. 7(a)-(e) shows the effect of lining a cube-shaped cham-ber (dimension 4A to a side) with materials characterized by-10 to -50 dB reflectivity when illuminated by a half-wavedipole near one end wall. With materials having low reflectivity(good absorbers), the field of the dipole is very similar to itsfree-space field; whereas with increased reflectivity (poorer ab-sorption), increased perturbation of the field is evident. Asseen in Fig. 7(c) and (d), even with perturbation the fielduniformity in the central region is not greatly disturbed andthus such a chamber may be adequate for EMC measurements.Fig. 8 shows the 'effect of using a totally reflecting side (floor,or side wall, or wall opposite to the source). The case shown isthat for the cube of the same size as shown in Fig. 7. Fig. 8(a)shows the field when this chamber is fully lined by -20-dBmaterial, whereas'Fig. 8(b), (c), and (d) shows the effect ofhaving the floor (wall perpendicular to source polarization),side wall (wall parallel to source polarization), and wall op-posite the source replaced by completely reflecting material. Itis to be noted that, in the case of Fig. 8(b), the high reflectivityof the floor does not perturb the field in a major way and the

chamber would thus be usable. However, in the case of Fig.8(c) and (d), there is substantial degradation. This is influencedin the measured case by two factors;-the polarization of thefield and the radiation pattern of the source. The calculatedresults available at this time, however, only take into accountthe second factor.

Fig. 9(a)-(i) forms a study showing the effect of changingthe shape of an ALC from a cube to a rectangular box shape,In all cases, the chamber has a length of L = 4X and is fullylined with -I 5-dB reflectivity material. The illuminating sourceis a vertically polarized half-wave dipole near one end wall.When viewed along the rows this set of figures shows the ef-fect of changing width (4X, 3X, and 2X) for a chamber ofheight 3X (Fig. 9(a), (b), and (c)), 2X (Fig. 9(d), (e), and (f))and IX (Fig. 9(g), (h), and (i)). When viewed along the col-umns, these figures show the effect of changing height (3X,2X, and 1 X) in the case of a chamber having a width of 4X (Fig.9(a), (d), and (g)), 3X (Fig. 9(b), (e), and (h)), and 2X (Fig.9(c), (f), and (i)). From these figures it is apparent that, to acertain extent, the reduction in height of a chamber does notcause serious degradation in the central region of the chamberfor a vertically polarized source; however, best field uni-formities result in approximately square cross-sectional cham-bers, especially when they are much longer compared to theother dimensions.

115


r

z

-j

oL

*9

1 ,/--2 -I a I

(b)

K

-2

-3

-i

LJ

! i

z

V10

(a) (b)

z

-j

Ln0

(c) (d)

(e)Fig. 7. Isophot maps (amplitude contours 2-dB intervals) ofE-field in a fully

lined ALC (4X cubed), H-plane, half-wave dipole source, material liningvariable. (a) -50 dB, (b) -40 dB, (c) -30 dB, (d) -20 dB, (e) -10 dB.

Fig. 10 is the analysis of an oblong ALC of constant physi-cal size for a 2:1 frequency range. It shows the fields when a

vertically polarized half-wave dipole source is used and thewall is replaced by a completely reflecting metallic ground.The change in frequency is analogous to a change in the elec-trical dimensions of the chamber along with the correspondingchange in the reflectivity of the material. In each case, fieldsare shown along the horizontal and vertical cross-sectionalplane passing through the central optical axis of the chamber.The location of the source is in the center of the chamber;thus, these figures represent an evaluation of the usefulness ofALC's for EM emission measurements.A cost-effective approach in the design of ALC's is to line

only selected areas with absorber material [26]. As indicatedin the earlier discussion, the wall that is responsible for con-

tributing most to the standing-wave ratio in the chamber is thewall opposite the source. Fig. 11 shows the results of a studyattempting to assess the use of a better absorber for this wall.A chamber 3X X 3X X 6X is chosen for the purpose. In Fig.1 l(a), (b), (c), and (d) all walls are lined with a -45 dB ma-

5~~~~L

z

-2 -1 a 2 -12 -11 o l 2

(c) (d)

Fig. 8. Effect of using a totally reflecting side on fields in an ALC. Isophotsshown at 2-dB intervals. ALC size (4X). H-plane half-wave dipole source.Material reflectivity -20 dB (a) fully lined, (b) floor (side perpendicular toE-polarization) reflecting, (c) side wall parallel to E-polarization reflect-ing, and (d) wall opposite source completely reflecting.

terial except for the wall opposite the source, the reflectivityof which is varied from -45 dB (good absorbing) to 0 dB (re-flecting). The resultant fields are shown. In Fig. 11(e), (f),(g), and (h) the same computation is repeated, but now the-45-dB material is replaced by -15-dB material. From ob-serving these, it may be noticed that, if all walls are poor,

improving only one wall has validity, but only to a certainextent, i.e., over-improvement of one single wall does notnecessarily produce additional beneficial results. Conversely,making a strategic location poorly lined as compared to therest of the chamber does result in dramatic degradation.

VI. CONCLUSIONSThe paper presents the use of a simple computational tech-

nique for computing fields inside ALC's. The validity of thetechnique is verified by comparing the computed results withmeasured data to show that a good first approximation is pos-sible. However, it may be stressed that application of the tech-nique may not be carried beyond its limitations. The limita-tions are: modeling of the absorber is approximate; the GOtechnique may not be applicable when the chamber dimen-sions are smaller than of the order of a wavelength.

However, within these limitations the technique can be,and has been, successfully used as a design tool. Approximatefields and the effect of changing the parameters on the fieldscan be studied. The use of the technique in optimizing the de-sign of ALC's with respect to size, shape, lining materials, andthus costs, have been indicated.

(a)

116


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117

H=3 W=2 L=40.0

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-J

:Z -2 0

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H=2 W=4 L=4

-2.0 -1.0 0.0 1.0 2.0

H= 1 W=4 L=4

-2.0 -1.0 0.0 1.0 2.0

r POSTION

H=2 W=3 L=4

H=ul W=3 L=4

T POSITION

H=2 W=2 L=4

H= I W=3 L=4

-1.0 -0.5 0.0 0.5 L.

T POSITtONFig. 9. Study of effect on the fields of changing the ALC shapes. Isophots

(2-dB intervals), fully lined ALC, material reflectivity-15 dB, half-wavedipole source, H-plane. (Chamber dimensions, as indicated, normalizedwith respect to wavelength.)

r -1.0

:s -3.0o- .

0.

a -32.o.s

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0.0

w

-j

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ow

-1.0

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1=1.5 W=1.5 L=3I

311 m 7

F

H=2 W=2 L=4 1=2 W=2 L=4

. u /J iJ .{_ 8_ i

FULLT LINED FLOOR REFLECTING SIDE WALL REFLECTING

Fig. 10. Analysis ofan oblong ALC. Vertically polarized dipole source in thecenter of the room. Isophot at 2-dB intervals. Column 1-H-plane sectionfully lined. Column 2-E-plane section fully lined. Column 3-H-planesection with the wall reflecting. Row 1: -25 dB, Row 2: -20 db, Row 3:- 15-dB lining. (Chamber dimensions, as indicated, normalized withrespect to wavelength.) If a horizontally polarized dipole were used, thenthe column 3 figures represent an E-plane section when the floor isreflecting.

0

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0

1.5

3

4.5

1.5 a 1.'5 1.5 6 1.5FRONT WALL -15 dB FRONT WALL 0 dB

(e) (f) (g) (h)

Fig. 11. Study of effect of special absorber coverage in strategic locations. Isophot maps (2-dB intervals) of E-field in the H-plane in rectangular box shapedALC ofdimensionsH = 3X, W = 3X, L = 6X. (a)-(d) all walls lined with -45-dB material except wall opposite source which has (a) -45-dB, (b) -30-dB,(c) -15-dB, (d) 0-dB reflectivity. (e)-(h) all walls lined with -15-dB material except wall opposite source which has (e) -45-dB, (f) -30-dB, (g) -15-dB, (h)0-dB reflectivity.

118

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simulation and measurement," Can. Electrical Eng. J., vol. 6, no. 2,pp. 3-8, Apr. 1981.

[2] J. R. Herman, "Electromagnetic ambients and man-made noise," DonWhite Consultants, Inc., Gainesville, VA, 1979.

[3] M. N. Yazar, "Civilian EMC standards and regulations," IEEE Trans.Electromagn. Compat., vol. EMC 21, no. 1, pp. 2-5, Feb. 1979.

[4] H. K. Mertel, "International and national radio interference regula-tions," Don White Consultants, Inc., Gaithersburg, MD, EMC Ency-clopedia Series, vol. 1, 1978.

[5] Recommended program for Electromagnetic Interference/Electro-magnetic Compatibility Standards Writing Activities in Canada,Canadian Standards Association Steering Committee on Electromag-netic Interference/Compatibility, Jan. 12, 1981.

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[7] M. L. Crawford, "Comparison and selection of techniques for measur-ing EM radiated emissions and susceptibility of large equipment," inProc. 3rd Symp. and Technical Exhibition onEMC (Rotterdam, TheNetherlands), May 1-3, 1979, pp. 115-122.

[8] M. L. Crawford, "Generation of standard EM fields using TEMtransmission cells," IEEE Trans. Electromagn. Compat., vol. EMC-16, pp. 189-195, Nov. 1974.

[9] G. Meyer, "Application of a broad band measuring line in fieldimmunity testing," in Proc. 2nd. Int. Symp. and Technical Exhibi-tion on EMC (Zurich, Switzerland), 1977, pp. 241-246.

[10] M. L. Crawford, "Techniques for the measurement of electromagneticradiation and susceptibility of electronic equipment," in Proc. Ist Int.Symp. and Technical Exhibition on EMC (Montreaux, Switzerland),1975, pp. 38-44.

[11] E. E. Donaldson et al. "Field measurements made in an enclosure,"Proc. IEEE, vol. 66, no. 4, Apr. 1978.

[12] G. Meyer, "The TEM measuring line-A critical overview," in Proc.EMC-81, 4th Symp. and Technical Exhibition on EMC, (Zurich,Switzerland), Mar. 10-12, 1981, pp. 407-412.

[13] S. R. Mishra, "Analysis and design of enclosures for electromagneticsusceptibility measurements over a wide frequency range (20 MHz-30GHz)," Ph.D. dissertation, McGill University, Montreal, Canada, May1982.

[14] S. R. Mishra, and T. J. F. Pavlasek, "Amplitude and phase structures

of fields in 'degraded' anechoic enclosures for EM interferencemeasurements," in Proc. IEEE URSI Meet. (Quebec, Canada), June2-6, 1980, p. 228.

[15] T. J. F. Pavlasek, "Research and development studies for anechoicEMC test facilities stage 1," Department of Communications (DOC),Canada, Final Rep., Serial no. OSU-00148, Mar. 31, 1979.

[16] D. R. J. White and M. Mardiguian, "Errors in EMC compliance testingand their control, "EMC Technology andInterference ControlNews,vol. 1, no. 4, pp. 14-22, Oct. 1982.

[17] S. R. Mishra, T. J. F. Pavlasek, and N. Yazar, "Design criteria for costeffective broad-band absorber lined chambers for EMS measurements,"IEEE Trans. Electromagn. Compat., pp. 12-19, Feb. 1982.

[18] L. H. Hemming, "Anechoic materials for conducting EMC tests inshielded enclosures," in Interference Technology Engineers' MasterDirectory and Design Guide (ITEM), 1982, pp. 152-157.

[19] F. J. Nichols and L. H. Hemming, "Recommendations and designguides for the selection and use ofRF shielded anechoic chambers in the30-1000 MHz frequency range," in Proc. IEEEEMC Symp. (BoulderCO), Aug. 18-20, 1981.

[20] S. R. Mishra and T. J. F. Pavlasek, "EM fields inside ALC's (absorberlined chambers): A computational approach," in Proc. 1982 IEEE Int.Symp. on Electromagnetic Compatibility (Santa Clara, CA), Sept.8-10, 1982, pp. 232-236.

[21] T. J. F. Pavlasek, "Research and development studies for anechoicEMC test facilities phase II," DOC, Canada, Final Rep., Serial no.OSU79-00143, Mar. 31, 1980.

[22] B. F. Lawrence, "A new generation of anechoic chambers," in Proc.4th Symp. and Technical Exhibition on Electromagnetic Compati-bility, EMC-81 (Zurich, Switzerland), Mar. 10-12, 1981, pp. 384-394.

[23] L. W. Pearson, et al. "Phenomenology of the intentional multipathreflections in a conically tapered anechoic chamber-A geometricaloptics model," presented at 1978 Conf. on Antenna Applications,Monticello, IL, 1978.

[24] G. Benham, "A new approach to anechoic chambers," in Proc. EMC-81, 4th Symp. and Technical Exhibition on EMC (Zurich, Switzer-land), Mar. 10-12, 1981.

[25] S. R. Mishra and T. J. F. Pavlesek, "Experimental study of microwaveabsorber material behavior in the near-field," in Proc. IEEE EMCSymp. (Boulder CO), Aug. 18-20, 1981, pp. 204-207.

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119

design of absorber-lined chambers for emc measurements using a geometrical optics approach

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