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Investigation into electromagnetic shielding efficiency of conductive knitted fabrics

Written by: Bahadur Goonesh Kumar, S Rosunee, M. Bradshaw

Dr Bahadur Goonesh Kumar, University of Mauritius; Dr S Rosunee, University of Mauritius; & Dr M. Bradshaw, De Montfort University, UK

With the rising ubiquity of a wide range of electronic devices (smart phones, tablets, laptops) and the electromagnetic (EM) waves associated with them, it is becoming increasingly important that cost effective and versatile EM shielding technologies be developed. The two major problems associated with EM waves are that they destructively interfere with radio and wireless communications and they can potentially have an adverse effect on human health. Interference owing to EM waves can have serious consequences, both material and financial, for key national electronic installations, affect communication with the outside world and result in loss of economic opportunities.

Metal sheets are considered to be the best material for electromagnetic shielding as these are highly conductive and reflect the EM waves. However, metals are expensive, heavy, sensitive to temperature changes and are not flexible. Textile materials are well known for their fineness, flexibility (ability to conform easily to three-dimensional shapes) and relatively cheap. By integrating electrical conductive yarn into the structure of the fabric, the latter becomes electrical conductive and therefore can be used as EM shielding material. In this study, different knitted structures were produced using 100 per cent cotton and electrical conductive yarns and their ability to shield against EM waves in the frequency range of 400 to 1100 MHz were investigated.

In this work, electrical conductive yarns were plated with conventional textile yarns through knitting and the knitted structures generated have been investigated for their EM shielding efficiency. The 'hybrid' knitted structures made of 100 per cent cotton yarn and electrically conductive yarn produced different surface textures and therefore, different knitting structures were used, namely 1x1 plain ribs with plaiting, 1x1 plain ribs with horizontal stripes and Fair-Isle. Single and double-bed hand knitting machines, gauge 5 and 7 dpi, were used to knit the set of fabric samples mentioned above.

1x1 plain ribs with plaiting and 1x1 plain ribs with horizontal stripes

Fair-Isle samples knitted on a single bed machines

The cotton yarn used had a linear density (count) of 36.5 tex (g/km). Several strands of the cotton yarns were used together to knit samples with good handling properties. The electrical conductive yarn has six plies and each ply has 29 filaments with a nano-coating of silver and having an electrical resistance of 4 Ohms per 100 mm and a count of 96 tex.

It was found that braiding several ends of the electrical conductive yarn together reduced the electrical resistance that improved conductivity and hence, improved the EM shielding of the knitted fabrics. Therefore, an additional set of samples was also knitted on a 5-gauge machine with two ends of the electrical conductive yarn and seven strands of the cotton yarn. Several tests were further carried out on all knitted samples. All samples had different properties in terms of weight, thickness, tightness factor, fabric density and electrical resistance that were found to be a major contributor towards creating a good shielding.

For samples knitted in 1x1 plain ribs with horizontal stripes, the number of courses for the stripes knitted with electrical conductive yarns varied from sample to sample. It was also found that by using two ends of the electrical conductive yarn instead of one, made the fabrics thicker, and increased the fabric density and the specific weight of the knitted fabric increased in comparison with those fabrics knitted with only one end of the electrical conductive yarn. In addition, the electrical resistance of the knitted fabrics also decreased by 82.3 per cent while comparing the results obtained from samples knitted with one and two ends of electrically conductive yarns. When using the Fair-Isle knitting technique, different patterns like check board, diamond, cross motif and diagonal lines were generated. All samples in this category were knitted on a single bed domestic knitting machine and the patterns were created using punch cards. From the range of knitted fabrics produced, it was found that the electrical conductive yarn could easily be used for knitting light weight, soft and flexible fabrics.

A Faraday cage was specially designed and used to measure EM shielding for all knitted fabrics. Following testing, it was found that the set of samples knitted on 1x1 plain ribs shielded better than samples knitted in stripes. For example, at power input of 30 dBm, it was found that 1x1 plain ribs samples (knitted on a 7 gauge knitting machine) provided excellent EMSE of above 95 per cent at 500, 600, 700 and 800 MHz. Since the electric polarization of EM field is perpendicular to the magnetic polarization, it was found that samples knitted in stripes shielded better after a 90? clockwise rotation, with EM shielding reaching above 93 per cent at 500, 800 and 1000 MHz, at power input of 30 dBm. In contrast, before rotation, the highest EMSE result recorded was only 1100 MHz with 84.45 per cent at same power input.

The use of two ends of electrical conductive yarn made the knitted fabric thicker, heavier and more conductive. It was expected that the knitted samples would show better shielding across all frequencies. However, these knitted fabrics provided good shielding only at specific frequencies such as 500, 800 and 1000 MHz with EMSE reaching above 92 per cent, with power input of 30 dBm.

 

For all knitted fabrics, EM attenuation may be attributed to reflection and absorption owing to the presence of electrical conductive yarns. No shielding took place when the EM waves penetrated the stripes knitted with the cotton yarns only; as studies have shown that cotton does not shield EM waves. Moreover, for samples knitted on a double bed knitting machines, it was assumed that the EM waves may be attenuated by both sets of loops within the fabric.

All samples knitted using the Fair-Isle knitting technique provided good shielding at specific frequencies with different power inputs before and after rotation. Since the structures on the reverse side of the Fair Isle samples were different, EMSE measurements were also carried out on the reverse side of each knitted sample. As the power inputs increased from 10 to 30 dBm, it was interesting to note that with fabrics knitted on a single bed, the shielding performances of all knitted fabrics in this category also increased slightly (between 0.1 up to 10 per cent). However, as compared with the EMSE results obtained with double bed knitted fabrics, a decrease in shielding was noted as the power inputs increased from 10 to 30 dBm.It was noted that layering together of two similar fabrics knitted in Fair-Isle did not produce any significant improvement in the EMSE.

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