Study of Aperture Size and its Aspect Ratio of Conductive Hybrid Yarn Woven Fabric on Electromagnetic Shielding Effectiveness

Transcription

1 Fibers and Polymers 2017, Vol.18, No.7, DOI /s ISSN (print version) ISSN (electronic version) Study of Aperture Size and its Aspect Ratio of Conductive Hybrid Yarn Woven Fabric on Electromagnetic Shielding Effectiveness Krishnasamy Jagatheesan 1 *, Alagirusamy Ramasamy 2, Apurba Das 2, and Ananjan Basu 3 1 Department of Textile Technology, PSG College of Technology, Coimbatore , India 2 Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi , India 3 Centre for Applied Research in Electronics, Indian Institute of Technology, Hauz Khas, New Delhi , India (Received January 18, 2017; Accepted March 17, 2017) Abstract: The influence of aperture aspect ratio and its size on shielding effectiveness of grid fabric made up of carbon and stainless steel (SS) has been investigated in this article. The carbon and stainless steel based hybrid yarn was prepared by direct twisting machine and a woven grid fabric was prepared from the hybrid yarn by using CCI sample loom with varying warp and weft densities. The fabric samples were analyzed for shielding effectiveness (SE) in low frequency range (50 MHz to 1.5 GHz) using coaxial transmission line holder and a vector network analyzer. It has been observed that conductive hybrid yarns placed in one direction of the fabric showed polarization effect with the resonance peak at 0.65 GHz. The grid fabric having conductive yarns in both the directions showed larger attenuation than the unidirectional (UD) grid fabric. In addition, fabric made-up of carbon filaments showed higher attenuation level than that of SS filaments due to its larger conductivity. When the both carbon and stainless steel are introduced as hybrid yarn in the fabric, a maximum SE was observed. In addition, effect of orientation of fabric layers on SE has been investigated. The increase in number of fabric layers increased the SE to a certain frequency. The carbon UD fabrics with 0 o /45 o (or) 0 o /45 o /90 o showed larger SE than the fabrics with other orientation angles. The similar behavior was observed for SS UD fabrics. The developed fabrics can be used as polarization attenuator, shielding mesh, etc. Keywords: Conductive hybrid yarns, Grid fabrics, Polarization, Vector network analyzer, Carbon grid fabrics Introduction In recent years, electromagnetic (EM) shielding grids are very popular for blocking the undesired EM radiation evolving from a circuit/system. For this purpose, grids made up of metals, metal alloys and conductive materials find large applications for the several decades. Grids for the purpose of ventilation should satisfy essential condition such as spreading hot air, well ventilation, etc. and the minimum area of apertures in the shell should be adopted in the process of engineering design [1]. In the shielding panels, honeycomb apertures are very famous that permit a very small amount of metal, equal to approximately one skin depth at lowest frequency, between the holes. This structure helps in keeping the holes closest for improving the shielding and thermal performance [2]. In practical use of enclosure, some slots or apertures for the purpose of I/O connections, cooling purposes, ventilation panels, etc., should be introduced. These apertures also allow exterior electric and magnetic fields to enter into the enclosures which behave like an efficient antenna at certain EM frequencies. This may act as a source of EM interference problems for electronic elements inside the enclosure [3]. To predict the SE of apertures, several numerical methods such as Finite Difference Time Domain (FDTD) method, the Method of Moments (MoM), the Finite Element Method (FEM) and the hybrid methods have been tried. However, it *Corresponding author: jk@txt.psgtech.ac.in involves large computing time and memory. Even some analytical approaches have been applied using an equivalent circuit technique to predict the shielding effectiveness of apertures. Wu et al. [3] studied the effect of aperture area on SE. An enclosure having aperture of same area but with different number of apertures was prepared as shown in Figure 1. The shielding effectiveness of metallic enclosures with one large aperture and three smaller apertures of same area are shown in Figure 2. It was observed from Figure 2 that case with three apertures showed better SE than the case with one aperture of the same area, hence aperture size is more important for a given aperture area. Shielding effectiveness of shape of single aperture has been analyzed by Bo et al. [1] and shown in Figure 3. It was observed from Figure 3 that for a fixed area of aperture, SE of rectangular aperture was lower than square Figure 1. Geometry of apertures having same area but with different sizes (a-b); (a) single aperture and (b) multiple apertures [3]. 1382

2 Shielding Behaviour of Hybrid Yarn Grid Fabrics Fibers and Polymers 2017, Vol.18, No SE ( S 3/2 N) 1 (3) Figure 2. SE of a metallic enclosure with one large aperture and three smaller apertures of same area [3]. Figure 3. Electrical shielding effectiveness of single aperture with different shape [1]. and circular shape apertures. Compared to square aperture, the circular aperture has little higher shielding effectiveness. Therefore this study concluded that the aperture is more preferable than the seam and hence narrow and long seam must be eliminated in the enclosures. Lu and Sha [4] calculated the shielding effectiveness of circular and square apertures by means of empirical equation as shown below. SE ( d 3 N) 1 where d is the diameter of aperture; N is the number of apertures. Theoretically, for the same number of apertures, the increase of SE is to be 7.5 db when the diameter of aperture is decreased from 8 to 6 mm while the increase of SE is to be 10.6 db when the diameter is decreased from 6 to 4 mm. For the case of square aperture, it is shown below SE ( a 3 N) 1 where a is the side length of the aperture. If the area of the aperture is considered then, (1) (2) Here, S is the area of a single aperture. The openings of metal wire grids are similar to apertures in enclosures. Being metal wire grids, less expensive and easily formable than conductive sheets, it is highly used for attenuation purpose in domestic and industrial applications. However, it cannot be used in certain places where light weight, flexibility, high strength and formability are mandatory. To fulfill such purpose, textile grids can be a good choice as they have absolutely better properties, are much lighter and can be fabricated easily. In addition, hybrid conductive materials can be incorporated in the fabric stage easily and the grids with high flexibility can be prepared easily according to the requirement especially for covering complex shaped objects. It is observed from the literature that film type shields can exhibit better and consistent shielding effectiveness throughout the frequency range whereas shielding behavior of mesh type shields would be influenced by size of the mesh and resonance peaks [5]. To fabricate an effective mesh, the influence of aperture size and shape of the grid should be investigated in detail. Based on the theory of aperture ratio, if the wavelength of EM radiation is larger than opening of the fabric, it is assumed that fabric can arrest the EM radiation [1]. However, at higher frequencies, the ratio of aperture size to wavelength increases and at one point, overall shielding effectiveness of the material decreases. Henn and Cribb [6] explored the SE of fabric having an aperture of dimension L by s for the plane wave SE as shown below, SE Aperture ( db) = log( L f) + 20log 1+ ln L D S --- L (4) where L is the maximum pore dimension of the aperture in mm, s is the minimum pore dimension in mm, and D is the thickness, or depth of the aperture in mm and f is frequency in MHz [6]. While developing a grid fabric, type of conductive material, fibre content, conductivity and permeability of fibre play a critical role. In addition, size of grid, shape and aspect ratio also play an important role. Lee et al. [7] analyzed the fabric with different aperture patterns such as square and dipole patterns. The fabric with square pattern showed higher attenuation level than dipole patterns due to its low aperture ratio. Grids made up of copper, stainless steel, etc. have been analyzed by several researchers. Roh et al. [8] prepared a grid fabric made up of copper and stainless steel and studied its shielding behavior in the frequency range of 50 MHz to 1.5 GHz. It has been observed that grid fabric with low aspect ratio (square shape) showed high SE in medium and high frequency ranges. In addition, an increase in metal content also improved the SE of grid fabrics. On the other hand, Lai et al. [9] prepared a metallized polyester filament which was used for making the grid

3 1384 Fibers and Polymers 2017, Vol.18, No.7 Krishnasamy Jagatheesan et al. fabrics. It has been observed that fabric with highly conducting silver coating exhibited highest SE than the fabric with titanium coating. Moreover, fabric with low aperture size showed higher SE than the fabric with larger aperture size. The SE value tends to be stable as the aperture size of the fabric reaches zero. The orientation of metal/pet filament inside the fabric has also significant influence on SE of fabrics. In another study, a composite yarn made from three SS filaments and cotton/ss blended roving fibers was used for preparing the shielding fabric by Cheng et al. [10]. The SE of fabric was analyzed in the frequency range of MHz. It was found that an increase in SS staple fibre content and decrease in metal mesh size showed a rising trend in SE of fabric. As well as, fabric with same metal mesh size but higher aspect ratio showed low attenuation level. It has been studied from the literature that many works have been carried out on stainless steel based fabrics for their SE [11]. However, works carried out on carbon based grid fabrics are very few. The effects of metal content, grid size, aperture ratio and shape on shielding behavior of fabrics have significant effect [10] and these variables have not been investigated for carbon fabrics. Moreover, use of hybrid yarns in the grid fabric for the purpose of EM shielding has not been reported in the literature. Thus, in this study, SE of carbon and SS based grid fabrics with different aperture sizes and ratios have been studied. The use of SS fibres in the conductive grids enhances the absorption of EM waves [12]. All the fabrics have been tested for their shielding behaviors in the frequency range of 50 MHz to 1.5 GHz according to ASTM D4935. Experimental Materials The 6k carbon fibre tow supplied by Toray, Japan was used for weaving carbon grid fabric. Similarly, stainless steel multi-filaments supplied by Baekart, Belgium were used for preparing SS grid fabric. The specifications of carbon and stainless steel filaments are listed in Table 1. In order to prepare the hybrid yarn, fine PP filament of 300 D supplied by N Pal threads, Delhi was used for covering the carbon and SS filaments. Fabrication of Conductive Hybrid Yarn The direct twisting machine supplied by M/s AGTEKS Ltd., Turkey was used for making carbon/stainless steel hybrid yarn. As shown in Figure 4, carbon and stainless filaments were fed in core and fine PP filament was used as wrapper thread for making the C/SS/PP hybrid yarn. By varying the disc speed, wraps density can be controlled. In this study, wrapping density of hybrid yarn is kept very low (30 TPM) so that PP filament will not act as insulating material at interlacing points. Table 1. Properties of carbon, SS and PP filaments Yarn type Fineness (denier) (Measured). Tenacity (gpd)* Breaking strain (%) Carbon multifilament yarn SS multifilament yarn PP multifilament yarn C/SS/PP hybrid yarn *gpd means grams per denier. Figure 4. Image of direct twisting machine. Fabrication of Conductive Grid Fabrics The conductive grid fabrics were produced using CCI sample loom and 1/1 plain structure was chosen for the study. Since plain structure has shorter yarn floating length and more compact structure than twill and satin weaves [10], the metal mesh size is smaller and very stable. Hence, 1/1 plain weave was used for this study. Different densities of warp threads were prepared using CCI warping machine and grid fabric was made with varying warp and weft thread ratios. For studying the influence of different grid openings on shielding behavior of fabric, the ratio of warp and weft threads was varied in the loom. By changing the proportion of warp/weft threads, the aspect ratio of aperture can be varied. For large opening of fabric having same aspect ratio, both the warp and weft threads were varied in same proportion. The grid fabrics with different warp and weft proportions have been listed in Table 2. The characteristics of metal composite fabric are shown in Table 3. The carbon based grid fabrics are produced by weaving carbon filaments in warp and weft directions with different thread ratios. Similarly, SS fabric was prepared by incorporating

4 Shielding Behaviour of Hybrid Yarn Grid Fabrics Fibers and Polymers 2017, Vol.18, No Table 2. The grid fabric with different proportions of warp and weft threads Composite yarn Composition Warp Weft Warp Weft Warp Weft C/PP SS/PP C:SS/PP 1/1 1/1 1/2 1/2 1/5 1/5 C/PP SS/PP C:SS/PP 1/1 1/2 1/2 1/5 1/5 1/8 C/PP SS/PP C:SS/PP 1/1 1/5 1/2 1/8 1/8 1/8 C/PP SS/PP C:SS/PP 1/1 1/ Table 3. Characteristics of metal composite fabric Composite yarn Composition Openness (cm 2 ) Metal yarns (inch) Composite yarn Composition Openness (cm 2 ) Metal yarns (inch) P1/1 P P - - P1/1 P P - - PC1/1 (1P 1C)/(1P 1C) PSS1/1 (1P 1SS)/(1P 1SS) PC1/2 (1P 1C)/(2P 1C) PSS1/2 (1P 1SS)/(2P 1SS) PC1/5 (1P 1C)/(5P 1C) PSS1/5 (1P 1SS)/(5P 1SS) PC1/8 (1P 1C)/(8P 1C) PSS1/8 (1P 1SS)/(8P 1SS) PC2/2 (2P 1C)/(2P 1C) PSS2/2 (2P 1SS)/(2P 1SS) PC2/5 (2P 1C)/(5P 1C) PSS2/5 (2P 1SS)/(5P 1SS) PC2/8 (2P 1C)/(8P 1C) PSS2/8 (2P 1SS)/(8P 1SS) PC5/5 (5P 1C)/(5P 1C) PSS5/5 (5P 1SS)/(5P 1SS) PC5/8 (5P 1C)/(8P 1C) PSS5/8 (5P 1SS)/(8P 1SS) PC8/8 (8P 1C)/(8P 1C) PSS8/8 (8P 1SS)/(8P 1SS) PCSS1/1 (1P 1CSS)/(1P 1CSS) PCSS2/2 (2P 1CSS)/(2P 1CSS) PCSS1/2 (1P 1CSS)/(2P 1CSS) PCSS2/5 (2P 1CSS)/(5P 1CSS) PCSS1/5 (1P 1CSS)/(5P 1CSS) PCSS2/8 (2P 1CSS)/(8P 1CSS) PCSS1/8 (1P 1CSS)/(8P 1CSS) PCSS5/5 (5P 1CSS)/(5P 1CSS) PCSS5/8 (5P 1CSS)/(8P 1CSS) PCSS8/8 (8P 1CSS)/(8P 1CSS) SS filaments in warp and weft directions with varying aperture ratios. For preparing the hybrid yarn based grid fabrics, C/SS doubled yarns were incorporated with different thread ratios and all the developed fabric samples were compared for shielding behavior. The schematic diagram of the fibre arrangement in the carbon grid fabric with different thread ratios is shown in Figure 5. Assessment of Electromagnetic Shielding Effectiveness of Grid Fabrics The SE of planar materials can be assessed using coaxial transmission line holder in the frequency range of 50 MHz to 1.5 GHz according to ASTM D4935. As presented in Figure 6, the reference and load specimens were fabricated having the diameter of 133 mm and placed between the coaxial holders and tightened with nylon screws. The SE values of fabric samples are assessed using scattering parameters of vector network analyzer as shown in equation (1). SE (db) = 20 log [S21] (5) Results and Discussion Influence of Conductive Yarns on SE of Grid Fabrics In this study, three different types of conductive grid fabrics were investigated namely carbon, stainless steel and carbon/ss hybrid yarn fabrics. The carbon and SS grid fabrics have similar metal content and all the fabrics have similar construction. The SE values of carbon, SS and C/SS hybrid yarn unidirectional fabrics are investigated and the results are shown in Figure 7(a). From Figure 7(a), it can be observed that carbon UD fabric shows higher SE of 22.6 db than SS grid fabric of 20 db. All the UD fabrics show polarization attenuation similar to wire grids. However, they fail to show larger attenuation of EM field. This is mainly attributed to the failure of conductive yarns to form open metal grid structures as the only weft yarns contain conductive yarns; henceforth the EM field could easily penetrate the fabric material [13]. The PCSS1/1 UD fabric having C/SS hybrid yarn does not show any improvement in SE compared to carbon UD

5 1386 Fibers and Polymers 2017, Vol.18, No.7 Krishnasamy Jagatheesan et al. Figure 5. Schematic diagrams of open-grid structures formed in the carbon woven fabric. Figure 6. Schematic diagram of vector network analyzer (a) with reference & load samples (b-c). fabric. The grid fabrics having conductive yarns in both warp and weft directions are investigated and the results are shown in Figure 7(b). From Figure 7(b), it is found that increase in frequency from 50 MHz, increases the SE of grid fabric and obtains the maximum value at 0.68 GHz. This is because of presence of conductive fibres in warp and weft at

6 Shielding Behaviour of Hybrid Yarn Grid Fabrics Fibers and Polymers 2017, Vol.18, No Figure 7. Shielding behaviors of fabrics with carbon, SS and C/SS hybrid yarns; (a) unidirectional grid fabrics and (b) woven grid fabrics. right angles which forms a better electrical conducting net and it easily intercepts EM waves and destroys their tenacity resulting in better SE [14]. Further increase in frequency, decreases the SE of grid fabrics. This may be due to skin effect of the fibre. The peak value of attenuation is observed at 0.68 GHz which shows resonance frequency of that fabric structure. The carbon grid fabric (Figure 7(b)) shows larger SE of 43 db compared to SS grid fabric of 30.1 db. This may be due to high conductivity and low skin depth of carbon fabric at high frequency. When the carbon and stainless steel blended yarn are incorporated in the grid fabric, the improvement in SE of fabric is not very significant. The C/SS hybrid yarn fabric shows the SE of 43.5 db which is not differing from SE of carbon fabric (43 db). However, a small improvement in SE is observed beyond 1.05 GHz. It can be inferred that the increase in metal content of the fabric could not improve the attenuation level whereas the position of the fibre inside the fabric has significant effect. Effect of Grid Openness and Aperture Ratio The grid openness and aperture ratio have significant effect on shielding effectiveness of grid fabrics. In addition, change in aperture size influences the incident pattern of EM wave in the fabric. Grid Openness of Carbon Fabrics in Weft Direction The effect of grid openness in weft direction on shielding Figure 8. SE of carbon fabric with varying openness in warp and weft directions; (a) warp openness (1:1), (b) warp openness (1:2), (c) warp openness (1:5), and (d) weft openness (1:8).

7 1388 Fibers and Polymers 2017, Vol.18, No.7 Krishnasamy Jagatheesan et al. behavior of fabric was investigated for carbon, SS and C/SS hybrid yarn fabrics. Figure 8 shows the SE of carbon fabrics having similar warp opening but with different weft openings. It can be seen from Figure 8(a) that, increasing trend of shielding behavior is observed for PC1/8 to PC1/1 fabrics due to smaller weft openness of the fabric. In Figure 8(b), increase in shielding level for PC2/8 to PC2/2 fabrics does not show much improvement on shielding behavior. The PC5/5 and PC5/8 fabrics show significant difference in attenuation level. This is attributed to the fact that as the dielectric area becomes larger, the conductive fibre content decreases, as a result the grid fabric gradually becomes transparent to the EM wave [15]. Hence, it can be understood from Figure 8 that an increase in weft opening decreases the overall the SE of fabrics. However, the resonance peak formed by grid structure is same for all the fabrics. Similarly, when the fabric has increased openness in warp direction it does not affect the shielding behavior initially. Figure 8(d) shows shielding behavior of fabrics having larger openness in warp direction. Despite having differences in warp openness, both PC5/8 and PC8/8 fabrics show similar SE of 21.2 db at GHz. When the warp openness is decreased, i.e. PC1/8 and PC2/8 fabrics, the SE is highest (30.3 db) at GHz compared to other two fabrics. Grid Openness of SS Fabrics in Weft Direction The SS grid fabric is also analyzed for shielding effectiveness for different weft openings. Figure 9 shows the shielding behavior of SS grid fabrics with varying openness in weft direction. From Figure 9(a), it can be observed that decrease in weft opening, (i.e.) PSS1/8 to PSS1/1 increases the shielding behavior of fabrics from 13.6 db to 29.7 db at 0.65 GHz. The increasing trend is also observed for fabrics with different warp opening (i.e.) PSS2/2, PSS2/5 and PSS2/8 fabrics (Figure 9(b)). However, if the fabric has high warp and weft openings, (i.e. PSS5/5, PSS5/8) there is not much difference in shielding behavior of fabrics (Figure 9(c)). Hence, it can be concluded that SS grid fabric with higher weft opening shows decreased SE values due to electromagnetic leakage phenomena. Likewise, fabrics with similar weft opening and different warp opening have been also analyzed for SE values and the results are shown in Figure 9(d). It has been observed from Figure 9(d) that a small increase in fabric openness in warp direction (PSS1/8 to PSS2/8) does not affect the SE of fabrics (21.5 db). However, at high opening (i.e.) PSS8/8, fabric shows a drop in SE of 12 db at 0.65 GHz. This is due to increased openness of fabric for a particular wavelength of incident wave. Grid Openness of C/SS Fabrics in Weft Direction The grid fabric made up of C/SS hybrid yarn is also investigated for shielding behavior and the results are shown in Figure 10. Similar to carbon fabric, there is an increasing Figure 9. SE of SS fabric with varying openness in warp and weft directions; (a) warp openness (1:1), (b) warp openness (1:2), (c) warp openness (1:5), and (d) weft openness (1:8).

8 Shielding Behaviour of Hybrid Yarn Grid Fabrics Fibers and Polymers 2017, Vol.18, No Figure 10. SE of C/SS hybrid yarn fabric with varying openness in warp and weft directions; (a) warp openness (1:1), (b) warp openness (1:2), (c) warp openness (1:5), and (d) weft openness (1:8). Figure 11. Shielding behavior of carbon, SS and C/SS hybrid yarn fabrics with similar aperture ratio; (a) carbon fabric, (b) SS fabric, and (c) C/SS hybrid fabric.

9 1390 Fibers and Polymers 2017, Vol.18, No.7 Krishnasamy Jagatheesan et al. trend observed for C/SS hybrid fabrics. A peak value of 43.2 db is observed for the fabric having 1:1 thread ratio in the frequency of GHz. The similar trend is found for PCSS2/2 and PCSS5/5 fabrics also. Bo et al. [1] and Wu et al. [3] stated that as the thickness/ depth of the aperture increases, the attenuation level also increases due to waveguide-beyond-cutoff effect. In the hybrid yarn grid fabric, the thickness of the fabric is highest compared to other fabrics which results in increased SE. Likewise, fabrics with similar weft openness and different warp openness are also investigated as shown in Figure 10(d). It can be observed that decrease in grid opening size has little improvement in SE. However, the improvement is less compared to the fabrics of different weft opening (Figure 7(a)). Grid Openness in Both Warp and Weft Directions of Fabric The increase in grid openness of fabric in both warp and weft directions increases the openness of fabric without changing its aperture aspect ratio. Figure 11 shows the shielding performance of grid fabric having similar aperture ratio but with different fabric openness. From Figure 11(a), it has been observed that all the carbon fabrics show a peak in the EMSE value at 0.65 GHz which is due to resonance effect. It has been observed that the increase in aperture size does not affect the resonance nature of the fabric; however, the attenuation level of the fabric is decreased. This is due to the aperture size being much higher than wavelength of incident wave as a result, more amount of waves pass through the fabric. In addition, carbon fabric with lowest aperture size shows better shielding performance (41 db) than the fabrics with other aperture sizes (PC1/2 to PC8/8). The similar trend is observed for SS grid fabrics as shown in Figure 11(b). However, the level of attenuation is less for SS based grid fabrics. To obtain similar SE of PC1/2 fabric, the SS fabric should have lowest aperture ratio (1:1). Hence, based on the required shielding level, fabrics with different aperture ratio can be chosen. This study was further continued for grid fabrics having C/SS hybrid yarns. Figure 11(c) shows the shielding behavior of C/SS fabric of various aperture sizes. An increase in grid openness decreases the attenuation level of the fabric. The C/SS fabric with 1/1 aperture ratio shows highest SE of 43dB than other fabrics at 0.64 GHz. Number of Fabric Layers on Shielding Effectiveness The effect of number of fabric layers on shielding effectiveness has been investigated for unidirectional carbon fabrics as shown in Figure 12(a). It is observed from Figure 12(a) that the increase in fabric layers from l to 3 layers of fabrics increases the SE for 50 MHz to 0.45 GHz. Beyond that no improvement in SE is found. The increase in fabric layers only improved the metal content of the fabrics and not changed the aperture size of the fabric. Hence the attenuation level of the fabric is not changed. Similarly shielding behavior of multilayer SS UD fabrics is also analyzed and the results are shown in Figure 12(b). From Figure 12(b), it is seen that increase in fabric layers increases the SE of fabric up to its resonance frequency after that not much difference in attenuation level of the fabric is found. This is due to skin effect which decreases the attenuation level of the multilayer UD fabric. Hence, it can be understood that the openness of the grid fabric decides the SE at higher frequency despite of having large metal content in the fabric. Effect of Orientation of Conductive UD Fabrics on SE The UD carbon fabrics having orientation in different directions have been investigated for SE and the results are shown in Figure 13. It has been observed from Figure 13(a) that fabrics having orientation angles of 0 o /0 o and 90 o /90 o show no difference in attenuation level (24.5 db). This is due to fibres being arranged in only one direction. When the orientation angle of fabric is changed from 0 o to 45 o or 90 o, a huge difference in attenuation level of fabrics (36.2 db/33 db) is observed. Figure 12. Number of fabric layers on shielding behavior of carbon and SS UD fabrics; (a) carbon UD fabrics and (b) SS UD fabrics.

10 Shielding Behaviour of Hybrid Yarn Grid Fabrics Fibers and Polymers 2017, Vol.18, No Figure 13. Fabric orientation angle on shielding behavior of carbon UD fabrics; (a) carbon UD fabric with two layers and (b) carbon UD fabric with 3 layers. Figure 14. Fabric orientation angle on shielding behavior of SS UD fabrics; (a) SS UD fabric with two layers and (b) SS UD fabric with 3 layers. This is due to arrangement of fibres in both directions as a result, an improvement in attenuation level of the fabric is observed. Similarly, the SE of UD fabrics having 3 layers was investigated. Figure 13(b) shows the SE of 3 layer fabrics with different orientation angles. From Figure 13(b) it is revealed that the SE fails to enhance in the frequencies ranging from 50 MHz to 3 GHz with increasing number of fabric layers at constant layer angles (0 o /0 o /0 o ) [16]. However, the multilayer fabrics display excellent attenuation level for varying layer angles. For three-layer carbon UD fabrics with 0 o /45 o /0 o and 0 o /45/ 90 o layer angles, attenuation value of db/39.1 db is nearly retained for a wide frequency range. This indicated that the grid fabric could attenuate the EM waves higher than 99 %. Likewise SS UD fabrics with different orientation angles have been investigated for shielding behavior. Figure 14 shows the SE of SS fabrics with different UD layers. From Figure 14(a), it can be observed that the fabrics having orientation angle of 0 o /45 o and 0 o /90 o show good electromagnetic shielding values (24.3 db/23.8 db) compared to 0 o /0 o and 90 o /90 o orientation angles (20.12 db/19.4 db) at 0.69 GHz. However, the SE values are decreased for fabrics at high incident frequency (1.22 GHz). This may be due to skin effect of SS fibre at high frequencies. When the number of fabric layers is increased, the SE value is also increased as shown in Figure 14(b). The SS fabrics having orientation of angle of 0 o /45 o /90 o display highest shielding effectiveness (31 db) compared to other fabrics. This is the trend observed for carbon fabrics also. It can be understood from this section that adding additional layers of fabric is an obvious way to increase SE, and the appropriate orientation angles make additional layers more effective for providing larger SE. Conclusion The conductive grid fabrics were investigated for shielding effectiveness with varying aperture size and openness in the frequency range of 50 MHz to 1.5 GHz. The use of hybrid yarns in the grid fabric was also investigated for its shielding effectiveness. It was found that carbon and C/SS hybrid yarn UD fabrics showed higher SE than SS fabrics. Higher attenuation was found at resonance frequency around 0.66 GHz for all the fabrics. Conductive yarns incorporated in both warp and weft directions showed better SE of db than UD fabrics due to attenuation of field in both

11 1392 Fibers and Polymers 2017, Vol.18, No.7 Krishnasamy Jagatheesan et al. directions. When the fabric openness was increased in weft direction, the SE of fabric was decreased. However, the resonance frequency of the fabrics remained same. When the fabric has larger openness in weft direction, a small increase in warp openness did not have any change in attenuation level. When the hybrid yarn was incorporated in the fabric, some grid structures showed improved SE. A minimum of 20 db was observed for hybrid yarn fabrics having large openness. Fabrics having same aperture ratio but increasing openness in both directions showed drastic changes in attenuation level. The increase in number of fabric layers did not improve the SE after the resonance frequency. The fabric layers having orientation angle of 0 o /45 o /90 o showed higher SE than other combinations. The developed grid fabrics can be applied for the purpose of polarization attenuators, shielding grids, etc. in the frequency range of 50 MHz to 1.5 GHz. References 1. N. Bo, S. Zhengxiang, W. Jianhua, G. Yingsan, and J. Wencai, IEEE 2007 Int. Symp. Microwave, Antenna, Propag. EMC Technol. Wirel. Commun. MAPE, 1299 (2006). 2. E. Chikando, E. Bodette, S. Connor, and B. Archambeault, IEEE, 813 (2010). 3. G. Wu, X. G. Zhang, and B. Liu, J. Electromagn. Waves Appl., 24, 1157 (2010). 4. F. Lu and F. Sha, IEEE Int. Symp. Electromagn. Compat., 739 (2002). 5. K. B. Cheng, M. L. Lee, S. Ramakrishna, and T. H. Ueng, Text. Res. J., 71, 42 (2001). 6. A. R. Henn and R. M. Cribb, IEEE, 283 (1992). 7. S.-E. Lee, K.-Y. Park, K.-S. Oh, and C.-G. Kim, Carbon, 47, 1896 (2009). 8. J.-S. Roh, Y.-S. Chi, T.-J. Kang, and S.-W. Nam, Text. Res. J., 78, 825 (2008). 9. K. Lai, R.-J. Sun, M.-Y. Chen, and A.-X. Zha, Text. Res. J., 77, 242 (2007). 10. L. Cheng, T. Zhang, M. Guo, J. Li, S. Wang, and H. Tang, J. Text. Inst., 106, 577 (2014). 11. J. Krishnasamy, A. Ramasamy, A. Das, and A. Basu, J. Electron. Mater., 45, 3087 (2016). 12. C.-I. Su and J.-T. Chern, Text. Res. J., 74, 51 (2004). 13. A. Das, J. Krishnasamy, R. Alagirusamy, and A. Basu, Fiber. Polym., 15, 169 (2014). 14. K. K. Gupta, S. M. Abbas, and A. C. Abhyankar, J. Electromagn. Waves Appl., 29, 1454 (2015). 15. F. Guan, H. Xiao, M. Shi, and F. Wang, Text. Res. J., 86, 2169 (2016). 16. Z.-C. Yu, J.-F. Zhang, C.-W. Lou, H.-L. He, A.-P. Chen, and J.-H. Lin, J. Text. Inst., 106, 1203 (2015).