Shielding Effectiveness Comparison of Rectangularand Cylindrical Enclosures with Rectangular and

Circular Apertures using TLM Modeling

(1)

T.R.Suresh Kumar,Muthayammal Engineering College,

Rasipuram, TamilNadu, India-637408.Email: trsureshkumar@gmail.com

Abstract-TLM method has been applied to calculate theShielding Effectiveness (SE) of rectangular and cylindricalenclosures with rectangular and circular apertures. Tounderstand the influence of the aperture topology in the SE,the dimensions of aperture are varied and SE of rectangularaperture has been compared with the circular counterpart.The resonant modes inside slotted enclosure have beencompared with the enclosure without slot and the behaviorhas been found similar to that of resonant cavity. Thismodeling will be helpful in understanding and to mitigatethe coupling of EM radiation that is coupled into theenclosures. This is accomplished by identifying the resonantmodes of enclosure cavity with apertures.

I. INTRODUCTION

The EMC regulatory requirements must be taken intoaccount during the design and the same must be satisfiedafter the product is made. During the design stage itself itshould be verified with the available numerical methods .Since experimental studies on a prototype increase costand time requirements on EMI/EMC evaluations , variousnumerical methods such as the FEM, FDTD, TLM,MOM method have been developed and applied for theanalysis and prediction of EMC/EMI problems. In thispaper TLM method is applied to model a practical EMIproblem of enclosures with apertures . As the aperture onthe enclosure become unavoidable, during the enclosuredesign, apertures must be designed such that it does notallow radiated emission through it. Apertures behavior issimilar to waveguides since both have similar structuresexcept that length of the waveguides is large compared tothat of shield thickness. To understand the apertures twopopular waveguide configurations have been considered(rectangular and circular waveguide) . SE of cylindricalenclosure with rectangular aperture has been discussed in[1]. The width and height of the rectangular aperture andradius of the circular aperture are varied and the shieldingperformance is compared. The enclosure 's resonancebehavior is similar to that of the resonant cavity withoutapertures on their walls.

II. SHIELDING EFFECTIVENESS

Electromagnetic shielding is the technique that reducesor prevents coupling of undesired radiated

978-1-4244-4819-7/09/$25.00 ©2009 IEEE

C.Venkatesh ,Surya Engineering College,

Erode, TamilNadu, India-638 107.Email: vensrilO@gmail.com

electromagnetic energy into equipment to enable it tooperate compatibly in the disturbing environment. Shieldsmay be used to contain electromagnetic fields within aregion or out of a region. A metallic enclosure with noaperture , seam or cable penetration can typically reduceradiated emissions and improve radiated immunity by 40dB or more. The relative capability of a shield to screenout undesirable electric, magnetic fields and plane wavesis called as Shielding Effectiveness (SE). It is the ratio ofthe signal received without the shield to the signalreceived with the shield [2].-

SEe = 20log10 ~nshield dBE shield

SEm

= 201oglO

1!..:nshield dBH shield

III. TLM MODELLING

TLM method is a numerical technique for solving fieldproblems using circuit equivalent. TLM algorithmimplementation involves the following [3]. Firstdiscretize the problem space and substitute it withinterconnected Nodes . When excitation is applied, pulsespropagate and get scattered according to transmission linetheory. Finally E and H are calculated from V and I atany stage. For the modeling Symmetrical CondensedNode (SCN) mesh as shown in Fig. 1 is used.

~ ......'

Fig. 1. SCN Node

1.0 Sinusoidally Modulated Gaussian Pulse

a

f:

;~' t. !~ l l i','if·\!, !1~11 1 I!\... .. ! '" ': 1

V ',~ r}, I: /l:: \1'\ ;[\i ~ f/l ; f, t~W \.. Il,\jfii\ 1\ VI l '\ Vi

r-; I, ; . ... 1;~t : \,f, I! 'tli 1,", \il \'lil'E8\., \.",J .~ It": ~ l l il ': !!\ i" . , 6.08E8\ . r • ,-.

i If 1 . ~ 8.68EB v

1~ 9.72E8

7.08E84.64E8

0.0 200.0M 4OO.0M 600.0M SOO.OM 1.00

Frequ ency in Hz

0.0 200.0M 400.0M GOO.OM 800.0M 1.OG

Frequency in Hz

Fig. 4. SE of Configuration I / Case I,----------,

rExc ita~'on(E Fi Id) _______--Y

-20

CD-c.S 40~~

l' 30

~~ 20

~ 10

~ 0:cen .10

50

60

0.0 1,0)\10" 2.0)(10· 3,0)\10" 4.0)(10" 5,Oxl 0" 6,Oxl0 "

Time (S}

~ o.

~ 0.2

i 0.0

ce

Fig .3. Sinusoidally modulated GuassianPulse

30

-0.2

. 0

·30

Fig. 2. Rectangularenclosures withrectangular(aXb) and circular(r) aperture

CD

~ 20

~~

~ 10

~13 0

ffif? -10

'6

~ -20

IV. RESULTS ANDDISCUSSION

A. SE ofIndividual Configuration

Configl : Rectangular Enclosure with Rectangular Aperture

ApertureEnclosure SE

Config. TypeType Size in cm

Fig.Case ( h=1Ocm)

No.

I a=8,5,2;b=IO 4I rectangular a-24,20,17,II

rectangular II b=1O 5

II circularI r=5.1,4,2.5 6II r=8.7,7.8,7.4,6 7I a=8,5,2; b=1O 8

III rectangularII

a=24,20,17,II9

cylindricalb=1O

I r=5.1,4,2.5 10IV circular

II r=8.7,7.8,7.4,6 11V rectangular - b-1O,7,4;a~20 12

TABLEI

DIFFERENT ENCLOSURE I APERTURE CONFIGURATIONS

It contains a hybrid junction of twelve transmission lines,which is characterized by a 12x12 scattering matrix. Thescattering matrix determines the relationship between theincident pulses and the scattered pulses.

[k vr] = S'[kViJ,[k+1 Vi] = C.[k vr

] (2)

where [k vr] and [kVi ] are the vectors of reflected and

incident pulses at the k-th time step, S is the impulsescattering matrix of the node, and C is the connection matrixdescribing the topology of the network.

For modeling a rectangular enclosure of dimensions(65 x 30 x 50 em) and cylindrical enclosure of dimensions(radius=25cm; height=50cm) was constructed. Differentconfigurations of Enclosures with varying size of apertureson the front plate as mentioned in the Table I were modeled.In all the configurations the area of rectangular aperture(a=24, 20, 17, 11, 8, 5,2cm; b=10cm) is maintained equal tothe area of circular aperture (r=8.7, 7.8, 7.4, 6, 5.1, 4,2.5cm) to enable comparison. A sample model shown inFig. 2 has been modeled with TLM Solver (MEFiSTo 3D)[4]. A sinusoidally modulated Gaussian pulse for an electricfield (E) vertically polarized with a center frequency of1 GHz of 1V is applied as excitation shown in Fig. 3 whichis placed in front of the aperture. The Gaussian pulse hasbeen used as the source of excitation because its smoothGaussian shaped spectrum can provide information from deto hundreds of gigahertz simply by adjusting the width ofthe pulse [5]. Electric field detector was placed inside theenclosure. The models were meshed with SymmetricalCondensed Node (SCN) with uniform mesh size of6mm x 6mm x 6mm. In this paper instead of considering theplane wave shielding case, E field case has been consideredsince the regions located close to the radiating sources aremost likely to have high intensity fields and the fields canhave both longitudinal and transverse components. Suchfields may be predominantly E of H field if most of theenergy is stored in the dominant component E or Hrespectively.

Fig. 5. SE of Configuration I / Case II

6.6E84.64E8

2OO.oM 400.oM 6OO.oM 8OO.oM 1.OG

Frequency in Hz

Cylindrical Enclosure Circular Aperture

~ 1~11ii~11 i II 'I

= ~ /' V ;/.\, '\ N : ;t\II4. I\ i : .~ ' /i' ;\ j.1 j \ r,1; - .: ,\ ' \ ~ !l\ I ~ i i t ....1..\ ~E8 I" ,""It

i \ .i:.: J i\ '; r' \,' ;~· iI' l· tr l"'l li I W

J ~! l' tt i 8.6AENI ' " 9.72E8

J 7.08E84.64E8

0.0 Zoo.OM 400.0M 600.0M SOO.OM 1.00

Frequency in Hz

0.0

Fig. 10. SE of Configuration IV / Case I

-20

30

40

·20

60

50

co-e 40C

~ 30

~ 20

~w ' 0

i?~ 0:c(J) . 10

! 20

III~ 10

'"R 0iIii?i5 -10iiicjj

Cylindrical Enclosure Circular Aperture

8.8 E8

-40

30

'l5 20c~ 10

1!.~ 0

~ -10w

~ -20:Q

":2 ·30<f)

30

0.0 2OO.0M 4oo.0M 600.0M 8oo.0M 1.00

Frequency in Hz

40

-50

Rectangular Enclosure Circular Aperture

40

Rectangular Enclosure Circular Aperture

Fig. 6. SE of Configuration II / Case I

co 20-e.s~ 10

".~2 · 10iIi0> -20c-;;" -30:c

<f)·40

Config2: Rectangular Enclosure with Circular Apeture

2.00SOD.OM ' .00 1.00

Frequency in Hz

0.0

Fig. I I. SE of Configuration IV / Case II

-30

60 Cylindrical Enclosure Rectangular Aperture

co~ 30~i 20

~ 10

W0>~ ·10

"~ -20

Fig. 12. SE of Configuration V

For rectangular apertures when a<b (from Fig. 4&8)the shielding effectiveness improves with the decreasingaperture area (varying a).But when a>b (from Fig. 5&9)the shielding effectiveness is almost constant for varyingaperture area (varying a). Similar SE performances havebeen observed for circular aperture case (Fig. 6&10 and7&11). From Fig. 12, it's found that varying the height ofthe aperture (b) does not change SE since a>b. So byreducing the area of the aperture, SE can be improved,when a < b and applied the field is normal to the narrowwalls of the aperture.

The SE dips found in all graphs were the resonancefrequencies due to the cavity with aperture. Resonantfrequency ofrectangular cavity TEmnp mode is

fo=(c /2) (:J\(~J\(*J (3)

.40 -+--,-_-,...-~-.--_--r-__--,_

50

40

2OO.0M 4oo.0M 6oo.0M 800.0M 1.00

Frequ ency in Hz

Cylindrical Enclosure Rectangular Aperture

0.0 200.0M 4OO.0M 6OO.0M aOO.OM 1.00

Frequency in Hz

Fig. 8. SE of Configuration III / Case I

Fig. 7. SE of Configuration II / Case II

0.0

- -a=24cm- - a=20cm

Cylindrical Enclosure Rectangular Apertur e - - a- f zcm- - a=11cmFor all b=1Oem

50

30

-30

0.0 2OO.0M 400.0M GOO.OM BOO.OM 1.OG

Frequency in Hz

60

-30

Fig. 9. SE of Configuration III / Case II

-40

40

-40 +---,~--,--_---,-_--,~-.--_---,~

o1 - a::11cm;b=10Crn I---- r=6cm

7.08E84.64E8

I a=20cm;b=l OCm I- f: 7.8cm

6.08ES4.64E8

Fig.14. SE comparison II (a=II ,b=IO;r=6 em)

Fig.I5. SE comparison III (a=20,b=1O;r=7.8 em)

0.0 2OO.0M 400.01.1 roo.OM 8OO.OM 1.OG

Frequency in Hz

0.0 200.01.4 400.0M 600.01.1 600.01.1 1.OG

FrequenOjin Hz

=4 ~. /, I

3.3 E8 \

\ \ ~E8 8.68E:.72E8

10

20

-10

-20

30

10

40

20

-20

-30

·10

Rectangular Cavity Resonant frequency (£0) in MHz(a=65cm;b=30cm; identified from

h=50cm) rectangular enclosureTEmnp £0 in MHz Rectangular CircularMode No. from aperture Aperture

equation (3) (Fig. 5&6) (Fig. 7&8)100 229 232 204

001 300 284 -101 377 332 332

200 458 464 448

010 500 512 548

OIl 583 588 588

002 600 608 656

300 688 708 724

301 750 760 -310 850 868 888

3I1 901 972 944

TABLE IIRESONANCE FREQUENCIES OF SIMPLE CAVITY AND CAVITY WITH APERTURE

Due to the presence of aperture (rectangular and circular) inenclosure resonant frequency shift happens as shown inTable II.

B. SE Comparison of Rectangular and CircularAperture

For comparing the SE performance of rectangular andcylindrical aperture, three configurations have beenconsidered as shown in Table III.

TABLE III

SE COMPARISION OF RECTANGULAR AND CIRCULAR APERTURE v. CONCLUSION

In Fig.14 SE is same for rectangular and circularaperture since area is same and their shapes are closer tothat of a square. From Fig. 13 rectangular aperture possesbetter SE since a > b and from Fig. 15 rectangular andcircular aperture are not comparable since a < b.

Cylindrical Enclosure

SERectangular Circular Area Plot

ComparisonAperture Aperture in Fig.

a (for all b-lO) em' No.incm

rincm

I 2 2.5 20 13II II 6 IIO 14III 20 7.8 200 15

In this paper, slotted enclosures have been modeled usingTLM method and the Shielding Effectiveness (SE)performance has been evaluated at lower frequencies« 1 GHz). Focus has been laid on the influence of shapeand size of aperture and the frequency characteristics of SE.From modeling, it is found that the SE improves whenaperture area is decreased and vice versa. Rectangularaperture provides more shielding effectiveness over thecircular aperture of same area when the field incident isnormal to the narrow walls of the aperture with a<b. Thepresence of aperture shifts the resonance frequencies of theenclosure. SE degradation occurs at those frequencies.

REFERENCES

[I] Boyu Zheng and Zhongxiang Shen "Shielding Effectiveness ofCylindrical Enclosures with Rectangular Apertures", 19thInternational Zurich Symposium on Electromagnetic Compatibility,pp.71O-7I4,2008.

[2] Henry W. Ott, Noise Reduct ion Techniques in Electron ic Systems,2nd Edition, Wiley Interscience Publication , 1998, pp.I64-I65.

[3] C.Christopoulos, "The Transmission-Line Modeling method(TLM)", IEEE/OPU series on Electromagnetic wave theory, IEEEPress, 1995, ppJ07-II8.

[4] Faustus Corporat ion, http://www.faustcorp.com[5] Hsing-Yi Chen and Shu-Wei Shiu, "Analysis of Radiated Fields for

the Design ofa RJ45Connector Using FDTD Method", IEEETransactions on Antennas and Propagat ion, vo1.56, no.4, pp.l041­1047, April 2008.

Frequency inHz

1- -- a.2cm:b. IOcm 1- r=2.5cm

Fig.13. SE comparison I (a=2, b=IO; r=2.5 em)

~'\

~ r\ \ r I f,.. I \ . , !\V \ t II : t3.32 I f. II \

3 32~ 1\ 1~6'oS1 M~' llr \ i'----"\I\j 1~1 ~j\ n , . ,

! \1/ 86M;VI.1 \11 II ~.72f.8E

7.QllE8 ~ ~4 ! 8 ~

0.0 200.0M 400.0M 600.0M 800.0M 1.00

70

~ 50

=

60

.1 40

t; 30

ffi 20~-5] 10Vl