Electromagnetic Absorbers Based on ChiralHoneycomb Slab

Valeriu David'<', Ionut Nica(l) and Alexandru Salceanu(l)

(1) "Gh. Asachi" Technical University of lasi, Faculty of Electrical Engineering, lasi, RomaniaEmail: valdavid@ee.tuiasi.ro

Abstract-This paper presents the electromagneticsimulations' results of some panels proposed for buildings'electromagnetic protection. These absorbers, based onhexachiral honeycom b core, were com pared with classicalSalisbury screen or Jaumann absorber, havinghomogeneous dielectric slabs. We studied and comparedthe performances of the absorbers and their behaviour atoblique incidence, or in the case of electric smalldimensions.

By means of electromagnetic simulation using CSTMicrowave Studio software, we have proposed andstudied some absorbers referring to: the influence of therelative permittivity, Cr , of the dielectric slab; thepossibility to increase the frequency range; the behaviourof the panels to oblique incidences and the effects ofscattering from the edges of the electric small Salisburyscreen (edge effects).

Keywords-Salisbury screen; Jaumann absorber; chiralhoneycomb; electromagnetic simulation

I. INTRODUCTION

In view of obtaining the electromagnetic protection ofthe electrical devices and the human beings, there aremany preoccupations in order to develop the newshields/absorbers or to increase the performance ofclassical ones [1]-[4].

The Salisbury screen is a resonant absorber obtainedby placing a resistive sheet at an odd multiple of quarterof a wavelength in front a perfect electric conductor. Theminimum thickness of the Salisbury screen is one quarterof a wavelength of the incident electromagnetic wavewhich must be absorbed, corresponding frequency, f:

H -~.~O,min - 4 f (1)

II. THE PROPOSED ABSORBERS

We proposed two types of sandwich panels, with aview to be used as electromagnetic absorbers forelectromagnetic protection of the buildings. Fig. 1 andFig. 2 show the panels fixed on the walls, indoor of thebuilding, in Salisbury screen configuration, respectivelyin Jaumann layers configuration.

Figure 1. Salisbury screen panel, fixed on the building wall

In order to decrease the absorption frequency for aSalisbury screen, or to decrease their thickness for agiven frequency, we used a chiral-honeycomb slabhaving the relative permittivity, Er , greater then 1, asdielectric between resistive sheet and perfect electricconductor sheet. The resulted minimum thickness of theabsorber is:

where Co is the light velocity in free space.

In this paper we propose some absorber panels with aview to be used for electromagnetic protection at thebuildings level, such as: hospitals; control rooms of radaror radio transmitters; offices or just houses. Because oftheir good mechanical performances [5] and their specialelectromagnetic proprieties [6], the chiral-honeycombstructures were considered to obtain the sandwich panels,used as electromagnetic absorbers inside or outside thebuildings. Thus, we considered the Salisbury screen andJaumann layers configurations, using as dielectric a chiralhoneycomb slab.

978-1-4244-4108-2/09/$25.00 ©2009 IEEE

1H min = H o.min • ~

V&r

(2)

epsilon = 1.5 -epsilon = 2 _. -epsilon = 2.5 -epsilon = 35 I epsilon = 10--

differences between the resonance frequencies in Fig. 3and Fig. 4. For considered dimensions of the unit cell, thechiral-honeycomb slab is an effectively homogeneousstructure, namely structural average cell size is muchsmaller than the wavelength, 'A, for frequencies less thanabout 5 GHz.

-35 -j-------+------+---------'~___+------=----------v----.r----_______j

We made an optimization and we proposed someSalisbury screen panels based on chiral-honeycomb slab,with a view to be used in the electromagnetic protectionin hospitals. In the case of medical diagnosis chambersbased on the microwave thermography, where it isreceived the human body electromagnetic radiation in abandwidth of about 500 MHz, centred at 4.7 GHz, or inthe case of electrotherapy with radio frequency fields,these panels eliminate unwanted reflections from theroom walls and achieve an electromagnetic protection ofthe neighbour rooms. Beside the diagnosis rooms, orelectrotherapy rooms in hospitals, these panels can beused generally in the case of narrow bandelectromagnetic sources with a known frequency.

Fig. 5 and Fig. 6 show the enlargement of theabsorption bandwidth, and the variation of the reflectivitywith the dielectric permittivity of the medium, in the caseof Jaumann absorber with two layers, each of themhaving the thickness H = 45 mm. We compared theJaumann absorber having the homogeneous dielectriclayers with that one having hexachiral-honeycomb layers.

!g -15

~ -20 -j-----------\t--";-f--\-'~-----h---------1l-t\-+-:"\-------'Y-------t+-~~----I

~- -25 +--------+It---.+---+-+------+---+--~___+-+--'-+-+---c!=~---+---+-------I----+-l-I

-40 +---------+------+------+------+-----

-5 -j----~~-----+----___+------F-------T-----.;;ff'"--T-r~-20 ........•..epsjion..~~T ._ .&. 2~I]kll- =30 .......• epsilon =3

-40 T epsilon = 4 _ ······ff·······:······························\·;j···· 1

c)=:p::llc'l- = 5

-60+---------;------;-----------;----_____1o

Supplementary, in this mode the mechanicalperformances of Salisbury screen are also increased.

Fig. 3 and Fig. 4 show the reflexivity of the absorberpanels versus frequency and the decrease of theabsorption frequency with the increase of the mediumpermittivity between the resistive sheet and the perfectelectrical conductor plane of the Salisbury screen. Inthese figures, we considered the homogeneous dielectricslab, respectively the hexachiral-honeycomb slab, madeof the same material with the same thickness, H, of 50mm.

Figure 2. Jaumann layers type panel, fixed on the building wall

-so -} ; I

2.5

2.5

1.5

Frequency I GHz

1.5Frequency I GHz

0.5

0.5

-45 +------+-----+-------+----+----------jo

-45 -j----------+------+----___+-----+----_______j

o

-40 +------+-----+-----=------+----+----------j

-30 +---------+-------+-clr--l'<--+lr-+--+---F--+----++--Nc+-t--f---I----_______j

!g -15

~ -20 -j----------+--~c_V_~"'~______;;7"=------+-~----+-_r_+_1'____+_------F-_______j

iii- -25 -j----------+-----+;---f,----!-'H-;-~-___+____=___....,;;:\--\~____f__+----F-_______j

5 I epsilon = 1 -epsilon=1.5 -epsilon =2 ---epsilon=2.5 -epsilon=30-- ---- --

-5 +---------"'IllIoo.-+------+----+------+-~~_""'l

-35 +------+----------'lII'-+---l."------1'r-----1-----+---------+,~f----_______j

-10 -j----------+"~~---+----___+------+-/''_______:_'_____r_----r-l

Figure 5. Reflectivity of Jaumann layers, with homogeneousdielectric slabs, versus frequency for various permittivity

FreqJency / G-lz

Figure 3. Reflectivity of Salisbury Screen, with homogeneousdielectric slab, versus frequency for various permittivity

-20 ~ : ....•: .•. . . . . . ..J'-< _ : r• epsilon = 1 '.A P·-'~;Ij,-'I-I = )

............• epsilon = - f·· I

'Yepsilon = 4-40 () P~-,:~;III-'n = ~ +!

-SO·f·······························;·································,,···1

-6O+-- ~ ~_ _______'____~ _____I

oFrequency JGHz

Figure 4. Reflectivity of Salisbury Screen, with hexachiral-honeycomb slab, versus frequency for various permittivity

......~

Because the effective permittivity of the chiralhoneycomb, made by cylinders and ligaments with cellwall thickness, g, is smaller then the relative permittivityof the bulk or homogeneous material, Er, there are the

Figure 6. Reflectivity of Jaumann layers, with hexachiral-honeycombslabs, versus frequency for various permittivity

151,11 ildB

151,11 indB

2.51.5Frequencyl GHz

0.5

~ . ....... ....~

"'~ ............ ....... __ ........ ..... -... -.... - -/7------:..-'\., V /~t!oo.. --........'\ <»: ~ '/1'-/

\ ,7 ~ /1\11 \\11'1 \~ I

1--= :~~:~::: :~~:: : ~;o1I-incidence angle=4501- - -- incidence angle=700

°..r--~~ ~ _ ~ _ : : _ ··1

-60+-------;-----;-----;-------;-------4

°

o-5

-10

-15-20

!g -25

~-30

~-35

-40

-45

-50

-55

-60

-65o

1:5

Frequency I GHz

Figure 10. Reflectivity of Salisbury screen for some finite values of thesquare panel's side with homogeneous dielectric having Er = 1

-30· f · · · · · · · · · · · · · _· · · · · · · ·:· · · · · · ··· · · · · · · · · · · · · · · · :· · ..• _.•.•..•..•..Y-

-40· f · · · _· · · · · · · · · _· · · · · · ···:· · · · · · · · · · · · · · · · · · · · · · · :· •........•. _•..•. _.. \\.~

Figure 9. Reflectivity of Jaumann layers, with hexachiral-honeycombslab, versus frequency for some incidence angle

IV. THE BEHAVIOR OF THE ELECTRIC SMALLABSORBER

The simulations of the proposed absorber panels weremade for large dimensions, namely the length, X, and thewidth, Y, of the panel, such as to ignore the edge effects.For a square absorber panel tip Salisbury screen, withsome finite dimensions XxX, having the thickness, H =

50 mm and dielectric constant, s, = 1 or s, = 3, thevariability of the reflexivity which frequency is shown inFig. 10, respectively Fig. 11.

-so.f _.-_._.. _.-_.._.. _.-_..-~. _.-_._.. _.. _._.. _.-_.~ .. _..._.-_._... _.-_._.\!-:

Figure 7. Reflectivity of Salisbury screen, with hexachiral-honeycomb slab, versus frequency and incidence angle

Fig. 8 and Fig. 9 show the reflexivity of the Jaumannlayers versus frequency with incidence angle asparameter in the case of parallel polarization forhomogeneous dielectric slabs, respectively for hexachiralhoneycomb slab. Both dielectric layers have thethickness, H = 45 mm, and the relative permittivity oftheir material, e, = 3.

The absorption bandwidth in the case of Jaumannabsorber with two layers is about 1.5 GHz, unlikeapproximate 0.5 GHz in the case of the Salisbury screen.

III. BEHAVIOR OF THE ABSORBER AT OBLIQUEINCIDENCE

The reflectivity of the proposed absorber shown inFig. 3, 4, 5 and 6 are done for the normal incidence of thewave to the surface of the absorber's resistive sheet. Byusing the Floquet mode-expansion technique of the CSTfrequency domain solver, we also determined the obliqueincidence performance of the absorber for both paralleland perpendicular incident polarization, over the angularrange 0°-89°.

Fig. 7 shows the reflectivity of the Salisbury screenwith thickness, H, of 50 mm, versus frequency andincidence angle, for hexachiral honeycomb dielectric.

1Frequency I GHz

Frequency I GHz

Frequency I GHz

Figure 11. Reflectivity of Salisbury screen for some finite values of thesquare panel's side with homogeneous dielectric having Er = 32.51.50.5

5-,---------,--------------,---------,----------,-------

o +--~~-t__--__+---__+_-______::;;;;;;......_.:::~&;;;;&-_..::~-5

-10 +----~f____-----J1!"-__+-~______""lo.~~--__+_--~

-15 +--------\-~~-__+--_u_~+----_+_--_____n_______1

-20 +----------t~--___+--___t\_~---__+_--~

-25 +-----Il-I---__+--------\\'++----_+_---------H-1

-30 +-----~-----+---------H+-+----__+_---------vt-1

-35 +--------f--ft----__+--------l.---+----_+_------fI

~~ :=======::I=-=-=in=Cid=en='=ce=an=gle===o=o,--f-It-----------t--------t

-50 1-incidence angle = 18°1f----+-+---__--+ ---1

-55 1- :~~:::~~: :~~:: : ~~:11-+-+-------+------fI-60 +---------I-+-------+------If----+-------+----------j

-65 +-----f--'----__+-----+----__+_----------1

-70 +---------I------+-----+-------+----------j

o

Figure 8. Reflectivity of Jaumann layers, with homogeneousdielectric slabs, versus frequency for some incidence angle

As we can see in Fig. 10 and Fig. 11, for smallelectrical dimensions of the absorber panels, namely X~A,

the reflectivity of the absorbers are different from the

on Salisbury screen, respectively Jaumann layersconfigurations, use the hexachiral honeycomb slabs asdielectric, which increase the mechanical andelectromagnetic performances of the panel. We madenumerical simulations and we compared the performanceof the absorbers having chiral honeycomb slabs withthose based on homogeneous dielectric slabs made of thesame materials. We have studied and optimized theseabsorbers referring to: relative permittivity of thematerial; behaviour of the absorber at oblique incidence;behaviour of the electric small absorber. The proposedabsorber have good performances and can be used toelectromagnetic protection of microwave based diagnosisrooms, electrotherapy rooms, indoor the hospitals and,generally, in the case of the narrow band electromagneticsources with a known frequency. It is possible to increasethe performances of the proposed absorber, namely toincrease and to adaptive modify the absorbing frequencyrange, to increase the reflexivity in the case of electricsmall absorber and also to extend theirs applications atthe big structures.

REFERENCES

[1] K. L. Ford, B. Chambers, "Application of Impedance Loading toGeometric Transition Radar Absorber Material", IEEETransactions on Electromagnetic Compatibility, vo1.49, pp. 339-345, May 2007.

[2] M. Johansson, C. L. Holloway, E. F. Kuester, "EffectiveElectromagnetic Proprieties of Honeycomb Composites, andHollow-Pyramidal and Alternating-Wedge Absorber", IEEETransactions on Antennas and Propagation, vo1.53, pp. 728-736,February 2005.

[3] B. Chambers, A. Tennant, "Influence of Switching-WaveformCharacteristics on the Performance of Single-Layer-PhaseSwitched Sceen", IEEE Transactions on ElectromagneticCompatibility, vo1.44, pp. 434-441, August 2002.

[4] R. L. Haupt, "Scattering from Small Salisbury Screen", IEEETransactions on Antenna and Propagation, vol. 54, pp. 1807-1810, June 2006.

[5] F. C. Smith, F. Scarpa, "Design of honeycomb-like compositesfor electromagnetic and structural applications", lEE Proceedingsof Science Measurement Technology, vol. 151, pp. 9-15, January2004.

[6] A. H. Sihvola, A. 1. Viitanen, I. V. Lindell and S. A. Tretyakov,Electromagnetic Wave in Chiral and Bi-Isotropic Media, ArtechHouse, 1994.

[7] V. David, E. Vremera, A. Salceanu, I. Nica, O. Baltag, "On theCharacterization of Electromagnetic Shielding Effectiveness ofMaterials", Proecedings of the 15-th IMEKO TC 4 InternationalSymposium on Novelties in Electrical Measurements andInstrumentation, Romania, pp. 73-78, 2007.

[8] E. Vremera, V. David, E. Sfartz, "Measurements in theRadiofrequency Range on Absorbing and Shielding Materials",Proceedings of the 15th IMEKO TC 4 International Symposiumon Novelties in Electrical Measurements and Instrumentation,Romania, pp. 64-68, 2007.

ACKNOWLEDGMENT

This paper has been supported by the RomanianMinistry of Education and Research under the ProjectCHlRAL-EMC 46/2006, Project BIOELECTRA5272P/2007,Project BIOMAG 5271P/2007.

1.41.1 1.2 1.3Frequency [GHz]

0.9

1 - - 'Frequency domain (diferent software) -Time domain (CST software) 1

-5 -r--;============================~

-60 -t-----,---------,-------,---------,---------,------r-------1

0.8

-10 -t-T---------------------t

-15 +------'\---------#"---------------""b------+--J

Figure 12. Comparation between reflectivity of Salisbury screen, withhexachiral-honeycomb, obtained whit two diferent softwares

-30 +--_------'l~---------~------j

-50 +--------~f---------------j

1.5

-25 +---~_____.---------'\___-+--------t

-40 +-------------rt-+--------------j

Iii'~;::::-30 +-----------'"\---------+-------------j

Frequency [GHz]

-20 +------~---~~----------j

-10~=---------------~

ideal case, namely infmite structure as is shown in Fig. 3.However, for X~10A or even for X~5A, the curves of thereflexivity versus frequency are about the same as in thecase of an infmite panel, Fig. 3.

Modifying the sheet resistance, Rs [nlsquare]) of theresistive sheet orland the thickness of the panel, it ispossible to optimize and to increase the performances ofthe electrical small Salisbury screen.

1- - 'Frequency domain (diferent software) -Time domain (CST software) 1

Figure 13. Comparation between reflectivity of Jaumann layers, withhexachiral-honeycomb, obtained whit two diferent softwares

V. RESULTS VALIDATION

We compared the obtained numerical results withanalytical solutions. Thus, using relation (1), for theproposed Salisabury screen having a thickness of 50 mm,the resonant frequency is 1.5 GHz and is the same withthat obtained from simulation in Fig. 3 (s, = 1).

For the Salisbury Screen and Jaumann layers withhexachiral-honeycomb slabs, the results obtained usingCST software were compared with those obtained usinganother software, simulating the same absorber. As isshown in Fig. 12 and Fig. 13, the simulation results,obtained with these different software, are quite identical.Also, we proposed some measurements methods todetermine the absorbers reflexivity [7], [8].

VI. CONCLUSIONS

We proposed two types of sandwich panel with aview to be used as shield/absorber for electromagneticprotection at the buildings level. These absorbers, based

Iii'~;::::-20 +----+-------d-----------+----+-------t