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Characterization of Electromagnetic Radiation Absorber

Materials

Erik Farias da Silva, Jernimo Silva Rocha, Paulo Ribeiro Lins Junior, Shayenne Diniz da Nbregaand Marcelo Sampaio de Alencar

Department of Electrical Engineering - Federal University of Campina Grande, CampinaGrande/PB-58.109-970 - Tel/Fax: + 55 83 3101410 - Brazil

E-mails: erik.silva@gmail.com, jeronimo@dee.ufcg.edu.br, paulo_milenium@yahoo.com.br,

shayennedn@yahoo.com.br, malencar@dee.ufcg.edu.br

Abstract This paper describes the comparison ofabsorbing characteristics of ferrite, graphite andcarbon black, in the frequency band of 1.5 GHz to the3 GHz. The substance is mixed in distinct proportionswith synthetic enamel and applied on plates of EPScovered with aluminum paper (NRL Arch technique)and without covering (Insertion Between Antennastechnique). The experimental results show that thegraphite presents the largest attenuation for theelectromagnetic radiation in the in the frequency bandanalyzed.

Index Terms Absorbing media. Anechoic chamber.Material characteristic. Ferrimagnetic materials.

I. INTRODUCTION

Electronic equipments produce electromagnetic energy.

That energy can provoke degradation or at least

disturbance in the operation of nearby equipment. With

the advancement of the technology, the electronic devices

started to operate at higher frequencies, which cause more

electromagnetic interference (EMI) [1]. Research

environments, as well as anechoic chambers, for example,

are utilized for studies regarding the effects of EMI in

devices and equipments [2].

The material utilized as internal covering of thoseenvironments, called Radiation Absorber Materials

(RAM), presents absorbing characteristics that ought to be

studied for several frequencies. In this work, three types

of RAM were studied, in an attempt to observe and

compare their absorbing characteristics in the frequency

band of 1.5 GHz to 3 GHz.

The substances utilized in the study were ferrite,

graphite and carbon black. They were mixed with

synthetic enamel in distinct proportions and applied on a

plate of Expanded Polystyrene (EPS) covered with

aluminum paper and without covering. Two techniques

were utilized to analyze the absorption characteristics of

the material: the technique of NRL arch and the technique

of insertion between antennas.

II. ABSORBERMATERIALS

The Radiation Absorber Materials can be divided into

dielectric and magnetic absorbers. The dielectric absorber

presents electrical losses associated to the material

permittivity. Among the dielectric absorbers one can cite:

graphite, carbon fibers, conductivity polymers and metal

particles. The magnetic absorber depends on magneticpermeability and hysteresis characteristics. The magnetic

absorber more used is the ferrite with several formulations

and granulations [5].

In this paper, one uses barium ferrite, graphite and

carbon black as absorbing material. The ferrite is divided

into three allotropic varieties: alpha-ferrite (austenite),

with cubic structure centered in the faces (CCF), it

remains steady up to 910C. Above 910C the alpha-

ferrite transforms to gamma-ferrite, with CCF structure.

Above 1401C, the gamma-ferrite transform to delta-

ferrite with cubic structure of centered body (CCB). The

delta-ferrite is formed in the hypoeutectoid region of Fe-C

phase diagram. Due to CCB crystalline structure of thedelta-ferrite, the crystallographic planes sliding is

possibility minimal. The delta-ferrite regularly used as an

electromagnetic absorber [6].

The graphite is an allotropic form of the diamond. Its

only constituent is the Carbon. However, its atomic

structure presents more layers of Carbon atoms,

hexagonally packed. That contributes to its lamellar

character and low hardness. This characteristic added to

high thermal and electric conductivity makes the graphite

refractory.

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The carbon black (CB) is a polymeric additive used as

radiation absorber and electrical conductor. The CB

increases the electric conductivity and radiationabsorption. The CB concentration should be low (1%

generally) in order to not increase the composite viscosity

and electric conductivity [7].

III. MEASURING TECHNIQUES

The characterization of RAM is based on the

determination of transmission or reflection characteristics.

There are several characterization techniques with

reference to reflectivity from RAM. The most commonly

used are named Radar Cross Section (RCS) and NRL

Arch (Naval Research Laboratory) [4] both use an

incident signal in a target and a receiving antenna tomeasure the reflected signal. The target consists of a metal

plate with one face covered with the RAM.

For the RCS measurements, the metal plate is fixed in a

revolving support localized in front of the

transmitting/receiving antenna inside an ideal open-area

test site (OATS an outdoor area provided with a ground

screen and free from obstacles and interfering ambient

fields [3]) or anechoic chamber. The support can rotate

360 and the uncovered surface is the reference for the

measurements.

For the NRL Arch technique, the metal plate is fixed in

a static support in a distance in accordance with far field

condition. The antennas are localized in a reference arch

(manufactured in wood, commonly) used for alignment of

the two antennas, using the same angle for emission and

reception. The reflectivity is determined with reference to

uncovered metal plate and is expressed in decibels (dB).

There are several characterization techniques based in

transmitting features. The waveguide technique is

regularly used. This consists in disposing a sample of

RAM inside the waveguide for signal attenuation

measurements. The transmission coefficient is expressed

in (dB).

Another technique used is the Insertion between

Antennas [8]. In this technique, a plate covered with the

RAM is positioned between the transmitting and

receiving antennas. The plate should be transparent to the

electromagnetic waves in the used bandwidth. In this

paper, one uses the Insertion between Antennas and NRL

arch techniques.

IV. DESCRIPTION OF THE EXPERIMENT

Two types of experiment were performed, the NRL

Arch Method and the Insertion Between Antennas

Method. For both experiments, the measurements were

made using a network analyzer (Agilent 8753ET). The

frequency band ranged from 1.5 to 3GHz. The antennas

were a horn antenna, as transmitter, and a hornet antenna(with aperture of 60) as receiver.

A. NRL Arch Method

For each material tested, the reflective base used was

an EPS plate measuring 40cm on side and with 8mm of

thickness, one of the plate faces was covered by a thin

aluminum paper which has been glued to the plate. The

absorber materials were mixed with a synthetic enamel

and this mix impregnated to the plate. Twenty plates were

made, and four materials, each one with five

concentrations, were tested. Table 1 shows the absorber

materials percentage in the mix.

The network analyzer was calibrated using a reflexiveplate attached to the support; this plate was made of

aluminum paper (without the RAM). The antennas were

placed in order to maximize the received signal. That

signal was considered the reference for all the

measurements. This calibration significantly reduced the

effects of the ambient reflections.

A support was made of electromagnetic transparent

materials and was placed in front of the antennas. The

antennas stood on a wooden table. The antennas were

placed at the distance of 1.22m from each other.

B. Insertion Between Antennas Method

In this test an EPS plate was used, measuring 40cm on

each side and with 15mm of thickness. The absorber

materials were mixed with a synthetic enamel and this mix

impregnated the plate. Four plates were made, and four

materials, each one with one concentration, and tested.

Table 2 shows the RAM percentage in the mix. In this

test the two antennas were placed in front of the each

other, and the plate under test was placed between the

antennas. For all the plates, tests of reflection were made

and the results show that the reflection phenomena do not

occur.

For the calibration of the instrument, an unpainted plate

was placed between the antennas at a distance of 12cm of

the receiving antenna (where all the plates were placed

one by one) and at 1m of the transmitting antenna. The

signal measured was used as reference for the calibration

of the analyzer.

V. RESULTS ANALYSIS

The NRL arch test was inconclusive in the

determination of the EMR attenuation, because peaks and

valleys (in adjacent frequencies) were observed.

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TABLE I

NRL ARCH METHOD MATERIALSConcentration (%)

Material I II III IV V

Enamel/Ferrite 90/10 85/15 80/20 70/30 50/50

Enamel /CB/Ferrite 89/1/10 84/1/15 79/1/20 69/1/30 49/1/50

Enamel/Ferrite/Graphite 80/10/5 80/15/5 72/20/8 60/30/10 50/35/15

Enamel/Graphite 95/5 90/10 85/15 80/20 70/30

TABLE IIINSERTION BETWEEN ANTENNAS METHOD MATERIALS

Concentration (%)

Material I II III

Enamel/Ferrite 85/15 70/30 50/50Enamel/CB/Ferrite 84/1/15 69/1/30 49/1/50

Enamel/Ferrite/Graphite 80/15/5 60/30/10 50/35/15

Enamel/Graphite 90/10 70/30 50/50

Therefore, only the data related to the insertion between

antennas have been analyzed.

Table III presents the results obtained using the

insertion between antennas method. It shows the average

attenuation (in dB) for two bands of frequency, the chosen

central frequencies were 1.8 and 2.4 GHz, and the

frequency span was 300MHz.

It is possible to observe from the results that the best

absorber material under test was the Enamel/Graphite for

a percentage of 70/30, the results are shown in Fig. 1. The

largest absorption was -5.74 dB in the frequency of 1.77

GHz. Fig. 1 also indicates the best average absorption in

the analyzed frequency bandwidth (as shown in Table III).

Another large attenuation result was obtained from the

measurements with the Enamel/Graphite substrate for a

percentage of 50/50. This result indicates that there is no

direct relationship between the amount of material and the

increase in the absorption results. The third considerable

absorption was found for the Enamel/Ferrite/Graphite

substrate for a percentage of 50/35/15.

VI. CONCLUSIONS

It was observed, from the data obtained with the method

of the insertion between antennas that the mixtures based

on Enamel and Graphite present larger attenuation for the

electromagnetic radiation in the band of 1.5 to 3 GHz.

The best results were obtained for the Enamel/Graphite II

(70/30), for which the average attenuation was -3.7dB. For

further work, it is interesting to vary the width of the

absorber substrate and observe the corresponding

absorption.

Fig. 1. Enamel-Graphite (at the percentage of 70-30) attenuationversus frequency.

REFERENCES

J. L. Wallace, Broadband magnetic microwave absorbers:fundamentals limitations, IEEE Trans.

[1]Magnetics, vol.29,

[2]

Compatibility, vol. 39, no. 1, pp. 33-47,

[3]

Compatibility, vol. 38, no. 1, pp.

no. 6, pp. 4209-4214, November 1993.C. L. Holloway, R. R. Delyser, R. F. German, P. Mckenna,and M. Kanda, Comparison of electromagnetic absorberused in anechoic and semi-anechoic chambers for emissionsand immunity testing of digital devices, IEEE Trans.

ElectromagneticFebruary 1997.C. L. Holloway, and E. F. Kuester, Modeling semi-anechoic electromagnetic measurement chambers, IEEETrans. Electromagnetic79-84, February 1996.

328

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[4]Miacci, Medidas de refletividade de materiaisabsorvedores de radiao eletromagntica usando astcnicas RCS e NRL, Revista de Fsica Aplicada a

Instrumentao, vol. 16, no. 1, pp. 30-36, March 2003.

[5] J. C. Dias, F. S. Silva, M. C. Rezende, and I. M. Martin,Absorvedores de radiao eletromagntica aplicados nosetor aeronutico, Revista de Cincia e Tecnologia, Ed.Unimep, vol. 8, no. 15, pp. 33-42, June 2000.

[6] F. H. Norton,Refractories, New York: McGraw-Hill, 1949.[7] M. F. Rabelo,Aditivao de polmeros, So Paulo: ETDA,

1998.[8] E. L. Nohara, Materiais absorvedores de radiao

eletromagntica (8-12 GHz) obtidos pela combinao decompsitos avanados dieltricos e revestimentosmagnticos, Ph.D Thesis, ITA, So Jos dos Campos, 2003.

M. C. Rezende, E. V. Nohara, I. M. Martin, and M. A. S.

TABLE

INSERTION BETWEEN ANTENNAS

III

METHOD MATERIALSrage Attenuation (dB)Ave

Mater II IIIial I

Hz 1.80.3 GHz 2.40.3 GHz 1.80.3 GHz 2.40.3 GHz1.80.3 GHz 2.40.3 G

E 278 0.0220 -0.0432 0.0456namel/Ferrite -0.0224 0.0215 -0.0

E 505 0.0339 -0.0712 0.0332namel/CB/Ferrite -0.0337 0.0395 -0.0

E 88 -0.1505 -2.6529 -1.9810namel/Ferrite/Graphite -0.0251 0.0115 -0.32

Enam 98 -4.1139 -3.1995 -2.3207el/Graphite -0.0429 0.0085 -4.79

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