Electromagnetic shielding effectiveness of copper/pet composite yarn fabrics

Indian Journal of Fibre & Textile Research Vol. 37, June 2012, pp. 133-137 Electromagnetic shielding effectiveness of copper/pet composite yarn fabrics K Rajendrakumar a & G Thilagavathi Department of Textile Technology, PSG College of Technology, Coimbatore 641 004, India Received 4 March 2011; revised received and accepted 31 May 2011 Woven fabric samples have been developed using composite yarn consisting of copper mono filament and PET filaments for evaluating electromagnetic shielding effectiveness (EMSE) against radiating electromagnetic wave spectrum over a frequency range 2.25-2.65 GHz. Coaxial transverse electromagnetic wave mode transmission method equipment has been used for testing in far field condition. It is observed that the weave and thread spacing of fabric samples significantly influence the shielding effectiveness as interpreted from the two way between subject ANOVA design. It is justified that aperture size and contact resistance of composite yarns in fabrics are the critical parameters in determining the shielding effectiveness of textile fabrics. The mechanism of fabric shielding is also discussed in detail using already established shielding theories which would help design fabrics for electromagnetic shielding. Keywords: Contact resistance, Copper monofilament, Pick density, PET filament, Shielding effectiveness, Weave 1 Introduction Since the early days of radio and telegraph communications, it has been known that a spark gap generates electromagnetic waves rich in spectral content (frequency components) and that these waves can cause interference or noise in various electronic and electrical devices such as radio receivers and telephone communications. Numerous other sources of electromagnetic emissions such as lightning, relays, dc electric motors, and fluorescent lights also generate electromagnetic waves that are rich in spectral content and can cause interference in those devices. Research works conducted also reveal that high frequency electromagnetic waves cause potential health hazards to human kind. Electromagnetic interference (EMI) has become a major problem for circuit designers, and it is likely to become even more severe in the future 1. Shielding an electromagnetic field is a complex and sometimes formidable task. The reasons are many, since the effectiveness of any strategy or technique aimed at the reduction of the electromagnetic field levels in a prescribed region depends largely upon the source(s) characteristics, the shield topology, and materials. Moreover, as it often happens, when common terms are adopted in a technical context, different definitions exist. a To whom all the correspondence should be addressed. E-mail: rakumarmt@yahoo.co.in Electromagnetic shielding effectiveness (SE) is a concise parameter generally applied to quantify shielding performance 2. Most shielding structures designed as barriers for radiated emissions are fabricated by means of standard (i.e. non-magnetic) conductive materials or by means of ferromagnetic materials which are rigid and heavy. So, researchers attempted to develop flexible and light weight barriers from polymer composites or conducting fabrics by introducing many techniques, viz. metallic coatings, conducting filler materials, inherently conducting polymers, CNTs, etc 3-16. Nonwovens, especially polypropylene nonwovens, are frequently used in technical applications mainly due to their low cost. The metallization process of PP fabrics (nonwovens) was carried out by sputtering metallic targets 17. A new range of model textile multilayer shielding materials developed with the use of component textile and textile-polymeric materials showed acceptable attenuation of electromagnetic radiation 18. Textile fabric coated with layers of electromagnetic absorbers was used as a modern support in the new generation of radar camouflage nets, providing camouflage against reconnaissance not only within the scope of the visible area, but also in nearphotographic infrared and radar scopes 19. Composite fibres developed by electroless deposition of metals like copper, nickel, cobalt and their alloys on textile

134 INDIAN J. FIBRE TEXT. RES., JUNE 2012 Table 1 Technical specifications of fabric samples Sample Weave Ends /inch substrates demonstrated their suitability in electromagnetic shielding applications 20,21. This study correlates the electromagnetic shielding theories based on rigid metallic structures with the flexible fabric assemblies made of conducting composite yarn. 2 Materials and Methods Picks /inch Weight g/m 2 Total fractional cover P1 Plain 42 32 180 0.832 P2 Plain 42 36 190 0.853 P3 Plain 42 40 200 0.875 T1 3/1 Twill 42 32 178 0.832 T2 3/1 Twill 42 36 188 0.853 T3 3/1 Twill 42 40 198 0.875 S1 4 4 irregular 42 32 176 0.832 S2 4 4 irregular 42 36 187 0.853 S3 4 4 irregular 42 40 196 0.875 2.1 Yarn and Fabrics The composite yarn having a resultant count of 500 denier was produced in a ring doubling machine by twisting copper filament of 40 micron diameter with 15 ohm/m conductivity and polyester filament yarn of 240 d/72f. Composite yarn was wound as smaller warp beam by employing miniature single end warping machine. Fabric samples in plain, 3/1 twill and irregular weave (4 4) structure with 42 ends/inch and pick densities 32, 26 and 40 picks per inch were produced in a miniature electronic rigid rapier loom and the detailed specifications are given in Table 1. 2.2 Testing for Shielding Effectiveness A shield is a metallic partition placed between two regions of space. It is used to control the propagation of electromagnetic fields from one region to the other. The space surrounding a source of radiation can be broken into two regions, namely close to the source is the near or induction field and a distance greater than the wavelength (λ) divided by 2π is the far or radiation field. The region around λ/2 π is the transition region between the near and the far fields. Shielding can be specified in terms of the reduction in magnetic and/or electric field strength caused by the shield. It is convenient to express this shielding effectiveness in units of decibels (db). Use of decibels then permits the shielding produced by various effects to be added to obtain the total shielding. Shielding effectiveness (SE) is defined as shown below: For electric fields SE = 20 log E 0 /E 1 db (1) For magnetic field SE = 20 log H 0 /H 1 db (2) EMSE (db) = 10 log 10 P 0 / P 1... (3) where decibel (db) denotes power ratio, commonly used to specify shielding effectiveness; E 0 /E 1 and H 0 /H 1 refer to the ratio of incident to transmitted electric and magnetic field components; and P 0 & P 1 denote the input and output power (watts) respectively. The total shielding effectiveness of a solid material with no apertures is equal to the sum of the absorption loss (A) plus the reflection loss (R) plus a correction factor (B) to account for multiple reflections in thin shields. Total shielding effectiveness therefore can be written as SE = A(dB) + R(dB) + B (db). (4) Shielding effectiveness varies with frequency, geometry of shield, position within the shield where the field is measured, type of field being attenuated, angle of incidence, and polarization. Shielding effectiveness depends on the design parameters like aperture size and contact resistance at seams/joints. Similar to ASTM D-4935 22, a spectrum analyzer which had coaxial transmission equipment with a rectangular cross-section was used to measure the EMSE of samples. Transmitting electromagnetic wave inside wave guide is of the form TE 10, which swept test specimen over a frequency range 2250 2650 MHz. EMSE of test samples were calculated using Eq (3). EMSE values of different fabric samples with the corresponding frequencies are given in Table 2. 3 Results and Discussion It is well documented that rigid metal assemblies are used for shielding against radiated electromagnetic emissions. Electrical conductivity and permeability of metals are the major determinants of EMSE. Metals like silver, copper, aluminum are highly conductive whereas nickel, stainless steel, mumetal are highly permeable.

RAJENDRAKUMAR & THILAGAVATHI: EMSE OF COPPER/PET COMPOSITE YARN FABRICS 135 Table 2 EMSE values of fabric samples Frequency Plain 3/1 Twill Satin GHz P1 P2 P3 T1 T2 T3 S1 S2 S3 2.25 38.3 43.98 44.28 37.2 40.15 46.28 34.09 34.93 35.53 2.27 36.9 43.14 42.77 37.03 38.33 45.25 32.42 33.41 34.02 2.29 38.6 45.69 44.73 38.42 38.99 45.95 34.2 35.05 35.64 2.31 39.7 43.22 44.59 38.67 42.21 45.9 36.24 36.24 31.14 2.33 39.3 45.34 45.85 38.99 39.11 47.14 34.57 35.72 36.18 2.35 37.2 41.91 43.94 36.16 39.24 46.17 32.42 33.41 34.11 2.37 36 41.15 43.18 35.87 37.86 46.55 31.26 32.24 32.85 2.39 37.8 46.06 45.39 37.35 37.55 46.28 32.99 33.9 34.33 2.41 38.1 46.84 45.9 37.78 38.95 46.17 33.25 34.2 34.84 2.43 36.4 46.9 46.06 35.37 37.14 41.36 31.6 32.58 33.45 2.45 35.5 44.77 45.59 34.2 36.51 37.78 30.86 31.49 32.58 2.47 36.7 47.89 47.45 35.43 37.35 39.25 31.78 32.9 33.85 2.49 36.9 48.16 49.32 36.52 37.12 42.95 31.63 32.91 33.86 2.51 35.5 45.2 49.96 35.48 36.5 43.45 30.45 31.77 32.64 2.53 35 41.18 51.22 34.76 35.25 41.78 30.09 31.11 32.01 2.55 35.9 44.73 48.36 35.64 35.92 41.97 31.07 32.02 32.86 2.57 36.5 43.22 50.39 35.34 36.84 44.46 31.67 32.53 33.44 2.59 35.6 40.34 47.2 35.53 36.32 43.61 30.71 31.64 32.25 2.61 34.1 38.0 45.44 33.94 36.55 41.87 29.31 30.11 30.85 2.63 34.3 38.15 50.93 33.97 37.3 40.07 29.49 30.34 31.02 2.65 35.4 38.35 54.05 34.48 38.55 41.65 30.88 31.45 32.19 resulting in reduced contact resistance. However, the troughs so formed in each of the curves as in Fig.1 indicate the reduced attenuation of electromagnetic waves which are attributable to the cavity resonance resulting from interior reflection of rectangular coaxial cell at multiple of half-wave lengths 23. Effects of weave pick density and their interaction were analyzed statistically by ANOVA and justified by concepts on shielding theory in the flowing paragraphs. Fig. 1 EMSE plot of fabrics differing in weave and pick denisty Use of copper filament in composite yarn is justified by its high order of electrical conductivity. Figure 1 demonstrates the pattern of EMSE of different fabric samples over a frequency range of 2.25 2.65 GHz. It is observed that EMSE of irregular fabric samples is low even at higher pick densities as compared to fabrics with 3/1 twill and plain weave. Plain weave fabrics show better EMSE which could be attributed to increased interlacements of warp and weft threads 3.1 Two way 3 3 between Subjects ANOVA Design Contribution of three levels of two independent variables namely fabric weave and pick density for a dependent variable (shielding effectiveness) was analysed by designing 3X3 between subjects ANOVA design. In this two-way design, the three effects of interest are the main effect of weave, the main effect of pick density and the unique combinations of the levels of weave and pick density known as the weave x pick density interaction. Computed F values for weave, pick density and interaction for the data presented in Table 1 are summarized in Table 3.

136 INDIAN J. FIBRE TEXT. RES., JUNE 2012 Table 3 Summary table for two-way between subjects design Source SS Df MS F F c (0.05) Weave 3080.72 2 1540.36 332.45 3.05 Pick density 1241.13 2 620.56 133.93 3.05 Interaction 546.17 4 136.54 29.47 3.05 Error 834 180 4.63 Total 4868.02 189 Computed F value for the main effect of Factor Weave as seen from Table 3 is found to be greater than the critical value (F c ) of 3.05 at 0.05 level, thus rejecting the null and concludes that weave of fabric affects the shielding performance, hence it is inferred that EMSE values are in the following order: Plain weave > twill weave > weave. Likewise, computed F value for the main effect of factor pick density is greater than the critical value at 0.05 level, thereby it is concluded that pick density of fabrics also affects the EMSE. It is found that the higher the pick density the greater is the EMSE. Similarly, the F value evaluated for the interaction effect of weave and pick density is greater than the critical value, hence it is concluded that the two independent variables (weave, pick density) combine to produce a statistically significant unique joint effect which could be attributed to the geometrical variations of fabrics caused by weave. For example, with the same pick density of 40 and a total fractional cover of 0.875 for both plain and fabrics, plain weave fabrics show higher EMSE values. The effects of weave, pick density and their interaction as analysed above are substantiated by shielding theories in the following section. 3.2 Shielding Mechanism Shielding mechanism of metallic shield is very complex involving various geometrical and design parameters like apertures and seam. Fabrics are not homogenous compared to metal sheets having spaces between warp and weft threads and discontinuities in conductivity due to the presence of fibres which are electrically non conductors. The fact that maximum linear dimension, not area, determines the amount of leakage can be visualized best by using the circuit theory approach to shielding. In this approach, the incident electromagnetic field induces current into the shield, and this current then generates an additional field. The new field cancels the original field in some regions of space, specifically the region on the opposite side of the shield from the incident field. For this cancellation to occur, the induced shield current must be allowed to flow undisturbed in the manner in which it is induced. If an aperture forces the induced current to flow in a different path, then the generated field will not completely cancel the original field, and the shielding effectiveness will be reduced. The more the current is forced to detour, the greater will be the decrease in the shielding effectiveness. Slot antenna theory states that a solid slot antenna is a most efficient radiator when its maximum linear slot dimension is equal to 1/2 wavelength, as the aperture becomes shorter, the radiation efficiency will decrease, hence the shielding effectiveness will increase at the same rate. The complement of a long narrow seam is a long skinny wire that clearly looks like an efficient antenna a dipole. This concept clearly illustrates why the length of the slot is more important than the area in determining the radiated emission. The length of the slot represents the length of the equivalent complimentary dipole. If the seam length happens to be in the order of ½ wavelength, it will become an efficient antenna. Therefore, it is necessary to guarantee electrical contact points at frequent intervals along a seam in order to reduce the length of the resulting antenna. A seam with periodic contact points can then be considered a linear array of closely spaced apertures. Along the length of the seam, there should be firm electrical contact at intervals small enough to provide the desired shielding effectiveness 24. It is evident from the above concepts that apertures, seams and contact resistance of a metal shield are the deciding factors of its shielding effectiveness. This phenomenon is also applicable to fabric structures. Therefore, it is justified that plain weave of a fabric minimizes the contact resistance due to more interlacements of warp and weft yarns and also minimizes the yarn distortion, thus exhibiting higher EMSE. Higher pick densities in fabric provides smaller apertures and minimizes the dipole effect, thereby improving the shielding effectiveness. Above theories are validated by the two way 3 3 between subjects ANOVA design. 4 Conclusion 4.1 Effects of fabric weave, pick density and their interaction on EMSE as analysed by two way between subjects ANOVA are significant at 0.05 level critical value and are validated by shielding theories.

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