Proceeding of the 2011 IEEE Students' Technology Symposium 14-16 January, 2011, lIT Kharagpur

REDUCTION OF EMI FOR OBLIQUE INCIDENCE OF EM WAVES - A CASE STUDY

K Brahmananda Reddy M.Tech, ECE department GITAM UNIVERSITY Visakhapatnam, India brahma.gitam(a{gmaii.com

NS Srikanth Prasad B.E, ECE department GIT AM UNIVERSITY Visakhapatnam, India srikanthjavvaj i@gmail.com

Abstract-Reduction of Electromagnetic Interference (EM I), i.e.,

shielding effectiveness of different shields against the angle of

incidence with conductors and conductive polymers using plane­wave theory are carried out. The plane wave shielding

effectiveness of new combination of these materials is evaluated

as a function of angle of incidence for the single shield. With

recent advancements in synthesizing stable highly conductive

polymers, this light-weight, mechanically strong materials appear to be viable alternatives to metals for EMI shielding. The analysis

is done at a particular frequency for single shield of different

materials.

Keywords- Electromagnetic Interference; Shielding effectiveness; Oblique Incidence.

I. INTRODUCTION

Electromagnetic Interference (EMI) is the disruption of operation of an electronic device due to either electromagnetic conduction or radiation emitted from an external source. The source may be artificial or natural, that carries rapidly changing electrical currents. The disturbance may interrupt, obstruct, degrade or limit the effective performance of the device. So shielding prevents the coupling of undesired radiated electro­magnetic energy into equipment. Electromagnetic shields are to be designed and developed to minimize the electromagnetic interference and to improve compatibility of the circuits. Various types of electromagnetic shields like single, double and multi layered conductors, conductive polymers sandwiched between conductive layers etc. are proposed whose design concentrates on optimizing performance on shielding effectiveness.

Most of the difficult shielding problems occur in mobile systems in which many transmitters, receivers, and other sensitive equipment must be mounted closely together, and weight is minimized. Predicting the shielding effectiveness of any enclosure such as equipment package or screening the room is difficult. Therefore any theoretical treatment for problems of this nature is necessarily approximate only. A number of light-weight polymers, intrinsically nonconductive but made conductive upon doping, have been studied in recent years [1-3]. These materials have several potential applications, such as electromagnetic interference (EMI) shielding, microwave absorbers, gas sensors, display units, junction devices, etc. [4,5]. The properties like conductivity variation

Ande Srikanth B.E, ECE department GIT AM UNIVERSITY

Visakhapatnam, India srikanth ande2007@yahoo.com

K Ajit Kishan B.E, ECE department GITAM UNIVERSITY Visakhapatnam, India ajitkishan@gmail.com

over wide temperature range, light weight and high mechanical strength of the polymers make them attractive in high­frequency shielding applications. In previous investigations, thin films of two new polymeric materials, namely polyacetylene and poly-p-phenylene-benzobis-thiazole (PBT), have been doped with iodine either electrochemically or by ion implantation, and their conductivity are measured by using the cavity perturbation technique [2,3]. Conductive polymers are useful as shielding materials in applications involving high data-rate electronics (e.g., supercomputers) and in aerospace applications where weight is a constraint.

The measured values of polyacetylene [1,2] are included in Table 1 for comparison. The conductivity of the material is given by (J" = 27if0808"wherefo is the resonant frequency of the cavity [3] and 80 is the permittivity of the free space. The conductivity is found to increase with the dopant levels, with the Polymer E (Table 1, Mnemonic E) doped electrochemically with 80% by weight of iodine, yielding the highest conductivity. Therefore it can be used over a wide frequency range.

Table 1: Measured complex dielectric constant Ei,==li�li" of Conductive Polymer Films Doped with Iodine various levels. The frequency of measurement isfo= 9.375 GHz, i.e., the centre frequency of the X·band.

Mnemonic Material Doping 6,.=:£'-&"

Ion implantation

A PST to a tluence of

3-j838 10'6 ions/cm2 Ion implantation

S PST of a tluence of

3-j1158 1017 ions/cm2

Polyacetylene Electrochemical;

C 4.5% h by

5-j607 Cis- (CHloo�s)x

weight

Polyacetylene Electrochemical;

D Trans-(CHloNs)x 4.5% hby 5-j909

weight

Polyacetylene Electrochemical;

E Cis- (CH 108)x 80% h by weight 5- j4.E5

TS11COMSOP144 978-1-4244-8943-5/11/$26.00 ©2011 IEEE 119

An extension of plane-wave transmission line theory of shielding analysis of different shields with conductors and conductive polymers is presented in this paper. The plane wave shielding behavior of the single shield constructed with polymer and materials like copper and aluminum for oblique incidence is analyzed in the paper. The analysis is done using the material polyacetylene, mentioned with mnemonic E in Table I. This material is represented as Polymer E.

II. SHIELDING THEORY

Two basic mechanisms, reflection loss and absorption loss, are responsible for a major part of the shielding. Therefore shielding theory is based on the transmission behavior through metals and reflection from the surface of the metal as in Figure I.

OUlsidc of cnclosurc

Incidcnl

\\lave

\vavc Hx E y Inlcrnal

RcllcClCd

wavc

Insidc or cnclosurc

Ey

Hx

llcnualcd lransmincd

Figure 1: Representation of shielding mechanisms for plane-waves.

A. Angle of Incidence The reflection and transmISSIOn of the electromagnetic

waves for single layered dielectric is considered for a general incident angle as shown in Figure I. The dielectric is assumed to be homogeneous and isotropic with electrical constants e,f.1 and 0". The z-axis is the direction of stratification. (J1 denotes the incident angle.

Considering for the single interface of thickness I, Ei, Hi be the incident electric and magnetic fields, E" Hr be the reflected electric and magnetic fields due to the impedance mismatch between the two media and EI, HI be the transmitted electric and magnetic fields strengths respectively as shown in the figure 1. Electric field components Ey = Ez = 0 for Transverse Electric wave, and magnetic field components Hy = Hz = 0 for Transverse Magnetic wave respectively.

The impedance of the shield material is [6] given by:

TJ = j2nfIJ. 1 (11 +2njfE) ... ( )

where f.1 is the permeability of the metal, 6 is the permittivity

TSllCOMSOP144

Proceeding of the 2011 IEEE Students' Technology Symposium 14-16 January, 2011, lIT Kharagpur

of the material, 0" is the conductivity of the metal and f is the frequency of operation. The impedance of the shield is varied according to the polarization as follows [7]:

Transverse Electric Polarization ... (2)

Transverse Magnetic Polarization Using Snell's law, we obtain cos (J2 as [7]:

cos (J2 = [1 - G�)2 sin2(J1 f/2 ... (3)

where (J2 is the angle of refraction in the shield, and the wave number k2 is :

k2 = W[,u2 (62 + (O"djw))] 1/2 ... (4)

B. Single Shield The structure of the single shield is as shown in figure 1. The shield can be interpret as a transmission line, then the input impedance normal to, and output fields of an infinite planar sheet of thickness I are :

Z = z(l) cosh yl + 1J sinh yl (5) Y] 1J cosh yl + z(l) sinh yl ...

H(l) = 1J cosh yl +1JZ(l) sinh yl H(O) ... (6)

E/l\ = Z(l) E/O\ (7) \ '/ Z(l) cosh yl + 1J sinh yl \ ''/ ... where Z(l) is the impedance looking to the right of the plane x = I. Since the field components across the plane are continuous

Ei + Er = Et; Hi + Hr = Ht ... (8) The reflected wave travels back to the source impedance for a sheet with matched impedance at the plane x = O. Thus

Ei = TJHi; Er = -TJHr; Et = -Z(l)Ht ... (9) When equation (9) is substituted in (8), the following expressions are obtained for reflection coefficients:

_ Hr _ ZrZ(l) qH - Hi - Zj+Z(l) ... (10)

and the corresponding transmission coefficients:

P - Ht - � - 1 + (11) H - Hi - Zj+Z(l) - qH '"

When two mismatched are considered as in a planar sheet, the net transmission coefficients across the two boundaries [8,9] :

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By definition, the reflection loss is

R = -20109101pl dB . . . (13) When successive re-reflections are considered, the transmission coefficients across the sheet are:

T - E(l) -Z(I) H(l) -

Z(l)T H . . . (14) E - Ei - Zw Hi - Zw where E(O), H(O) and E(I), H(I) are the actual values at interfaces 0 and I respectively, with reflection taken into account; Zw is the impedance of the incident wave. With the use of equations (11) to (13) and (15), the transmission coefficient can be expressed as:

where

4ZwT/ _ (zw-T/)(z(l)-T/) (16) P H = (Zw+T/)(Z(l)+T/)' q H - (Zw+T/)(Z(l)+T/)'" When Z(l) = Zw, then TE = T H, or T = P (1- q e-2Y1) -le-yl ... (17)

By definition, the total shielding effectiveness is

1(1- qe-2Yl )eYl l S = -20109101TI = 2010910

Ipi

= A + R + B . . . (18) where A,R,B are the absorption loss, reflection loss and the correction term due to the successive re-reflection and this expression is the complete formula for shielding effectiveness of a single shield.

III. RESULTS

In this paper copper, aluminum and conductive polymer are considered as materials of single shield. The angle of incidence of both Transverse Electric waves and Transverse Magnetic waves is considered for analyzing shielding effectiveness of all single shields. The shielding effectiveness is calculated against angle of incidence using Eq.(19). Figure 2 to 4 depict the polarization and angle dependencies of the shielding effectiveness of single shields of copper, aluminum and conductive polymer with total thickness of 10 mils and 40 mils.

TSIICOMSOP144

Proceeding of the 2011 IEEE Students' Technology Symposium 14-16 January, 2011, lIT Kharagpur

1800

� 1400

.� o 1200 � � 10)) 0; :E til 800

for 40mils IhiC�

At a frequency of mMHz

for 10mils thickness

for TE waves ----_ .. --_ ... ---_ .. farTM waves

60Cl � for TE wa"IIes

�------------��--= -= --=- -= --=-= --= --� --� --=-�- -

farTM waves 400

0 10 20 30 40 50 60 70 III 90 Angle of incidence (Theta)

Figure 2: Variation of the shielding effectiveness with Angle of incidence of single shield of Aluminum with 10 mils & 40 mils thickness.

!!l2000 c � � m c � 1500 � � c � t5 1(0)

for 40mds thickness

At a frequency of3lJMHz

for lOmlls thickness

for TE waves -- ------_ ... -_. for Th1 waves

for TE waves --- ... ---_ ... -_ ... --farTM waves 5OO0L-�,0��2O��30�-���5O��60=-�70��1Il=-�90

AnQle of incidence (Theta)

Figure 3: Variation of the shielding effectiveness with Angle of incidence of single shield of Copper with 10 mils & 40 mils thickness.

110

100 Al a frequency o(3llMHz i ! / 90

!!l .; eo

� 70 .� U

for TE waves," . for �mils Ihickn

.:: __________________ �::�'::--�,// � 60 -"" ... �" � � ro r-�--���� __ ___ 6i 40 for 10mils thickness

20

100 10 20 30 40 50 60 70 III 9J

Angle of incidence (Theta)

Figure 4: Variation of the shielding effectiveness with Angle of incidence of single shield of Polymer E with 10 mils & 40 mils thickness.

It is interesting to note that the shielding effectiveness is practically independent of the angle of incidence (less than 2 dB change) for angles from normal incidence to about 30° in both cases i.e., perpendicular and parallel polarizations. The total variation in shielding effectiveness with respect to angle of incidence from 0° to 89° is represented in Table 2.

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Table 2: The total variation of Shielding effectiveness for single shields of materials Aluminum, Copper and Polymer-E for different layer thicknesses of 10 mils and 40 mils at a frequency of 300MHz for an oblique incidence of 00 to 890 for different polarizations.

Total variation in shielding effectiveness in dB for

Material different polarizations

Considered Perpendicular Parallel

Thickness Thickness Thickness Thickness

fO mils 40 mils fOmils 40 mils

Aluminum 37 37 33 34

Copper 36 34 35 34

Polymer E 35 45 33 33

IV. CONCLUSIONS

The shielding effectiveness of single shields constructed with conductive polymer, metals such as copper and aluminum is evaluated using transmission line theory. Better shielding is possible for perpendicular polarization than for parallel polarization. Shielding effectiveness increases with the angle of incidence for perpendicular polarization and decreases

for parallel polarization.

When we compare metals with polyacetylene on a basis of the Conductivity/Weight ratio CW, it can be seen that single shields constructed with the conductive polymer have reached the same level as many metals. Therefore single shields with conductive polymers appear to be a viable alternative to metals in lightweight shielding applications. From the above analysis

TSllCOMSOP144

Proceeding of the 2011 IEEE Students' Technology Symposium 14-16 January, 2011, lIT Kharagpur

an electromagnetic shield can easily be designed to achieve required shielding effectiveness against angle of incidence of an electromagnetic wave.

ACKNOWLEDGMENT

We extend our thanks to Dr. P.V.Y Jayasree, M.E., Ph.D, Associate Professor, Dept. of Electronics and Communication Engineering, GITAM Institute of Technology of GITAM University for all the support and encouragement rendered in this work.

REFERENCES

[I] Naarman, H., "Synthesis of new conductive electronic polymers," Proc. Int. Congo Synthetic Metals, Kyoto, Japan, June 1986.

[2] Ehinger, K., S. Summerfield, W. Bauhofer, and S. Roth, "DC and microwave conductivity of iodine-doped polyacetylene," 1. Phys. C: Solid State Phys., Vol. 17,3753-3762,1984.

[3] Naishadham, K. and P. K. Kadaba, "Measurement of the microwave conductivity of a polymeric material with potential applications in absorbers and shielding," IEEE Trans. Microwave Theory Technol., Vol. 39, 1158-1164, July 1991.

[4] William, J., "Shielding tests for cables and small enclosures in the 1- to 10-GHz range," IEEE Trans. Electromagnetic Compatibility, Vol. 12, 106-112, February 1970.

[5] Konefal, T., J. F. Dawson, and A. C. Marvin, "Improved aperture model for shielding prediction," IEEE International Symposium on Electromagnetic compatibility, 187-192,2003.

[6] Paul, R. c., Introduction to Electromagnetic Compatibility, John Wiley Interscience, New York, 1992.

[7] Yasumitsu, M. and T. Koki, "Electromagnetic absorption and shield properties of lossy composite multilayers," IEEE Trans. Electromagnetic Compatibility, Vol. 32, 370-374, August 1990.

[8] Schulz, R. 8., V. C. Plantz, and D. R. Brush, "Shielding theory and practice," IEEE Trans. Electromagnetic Compatibility., Vol. 30, 187-201, August 1988.

[9] Kiang, J.-F., "Shielding effectiveness of single and double plates with slits," IEEE Transactions on Electromagnetic Compatibility, Vol. 39, No. 3, 260-264, August 1997.

[ 10] Jayasree, P. V. Y., et aI., "Analysis of Shielding effectiveness of Single, Double and Laminated Shields for Oblique Incidence of EM Waves", Progress In Electromagnetics Research B, Vol. 22, 187-202,2010.

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