Investigation into electromagnetic shielding efficiency of conductive knitted fabrics
Written by: <Authors><Author><Id>5211</Id><Name>Bahadur Goonesh Kumar</Name><FriendlyName>Bahadur-Goonesh-Kumar</FriendlyName></Author><Author><Id>5212</Id><Name>S Rosunee</Name><FriendlyName>S-Rosunee</FriendlyName></Author><Author><Id>5213</Id><Name>M. Bradshaw</Name><FriendlyName>M-Bradshaw</FriendlyName></Author></Authors>
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.