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Postprint
This is the accepted version of a paper presented at 2015 International Conference on Electromagnetics
in Advanced Applications (ICEAA).
Citation for the original published paper:
Ängskog, P., Ödman, T., Bäckström, M., Vallhagen, B. (2015)
Shielding Effectiveness Study of Two Fabrics with MicrowaveProperties Before and After High
Power Irradiation.
In: Proceedings of the 2015 International Conference on Electromagnetics in Advanced Applications
(ICEAA)ICEAA '15 - 17th Edition IEEE conference proceedings
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-175152
________________________________________________________________________________________
1
Electromagnetic Engineering, KTH Royal Institute of technology, Stockholm, Sweden,
e-mail: pangskog@kth.se
2
Department of Electronics Mathematics and Natural Sciences, University of Gävle, Gävle, Sweden.
3
SAAB Electronic Defence Systems, SAAB AB, Göteborg, Sweden.
4
SAAB Aeronautics, SAAB AB, Linköping, Sweden.
Shielding Effectiveness Study of Two Fabrics with Microwave
Properties Before and After High Power Irradiation
P. Ängskog(1,2), T. Ödman(3), M. Bäckström(1,4) and B. Vallhagen(4)
Abstract − Over the past decade several applications for
fabrics with electromagnetic properties have emerged, most of
them relating to garments, including jackets with built-in
antennas and workwear with increased radar visibility. Beside
these have surfaced two protective applications, namely to
protect transports of confidential equipment from discovery
and identification; and to protect sensitive apparatus from
damage by high power electromagnetic irradiation e.g. in field
operations. In this paper results are presented from
measurement of shielding effectiveness before and after high
power radiation for two types of fabrics under consideration
for the latter applications. Shielding effectiveness
measurements have been conducted between 1 and 18 GHz
while the high power irradiation was given with 28 kV/m at
1300
MHz.
1 INTRODUCTION
Conductive fabrics have a wide range of
applications. The fabric may have reflecting
properties and thus act as a reflector of signals, or it
may have absorbing properties and thus attenuate the
signal passing through the material [1].
Electromagnetic interference (EMI) shields [2], [3],
antennas [4], and wearable monitoring devices [5] are
some examples of products that can be found on the
market.
By integrating a reflective fabric in garments the
radar cross section (RCS) increases and hence the
radar visibility [6]. Areas where an increased RCS is
desired are professional clothing for road workers
and fishermen and rescue suits for people working in
the off-shore industry. A study of microwave
properties of two kinds of fabric, in the shipborne
radar frequency range, 2-18 GHz, has been conducted
in [1].
When transporting confidential goods a common
problem is the rugged heavy duty packaging required
preventing the goods from identification and damage.
Civilian as well as military radars exhibit high
electromagnetic field strengths while scanning.
Another rising issue is High Power Microwave
(HPM) radiators, a kind of electromagnetic radiation
weapon designed to disrupt or destroy electronic
equipment. Here a significant improvement can be
achieved by replacing metal containers with light-
weight, easy-to-use fabric-based packaging materials
that maintains the shielding while facilitating
handling.
Another application is to protect medical equipment
from electromagnetic radiation, especially when
using this type of equipment in the field; e.g. military
field hospitals in base camps where high power
transmitters are abundant. This protection may be
achieved preparing equipment specific covers of EMI
shielding fabric or as specially sewn tent sections
lined with the same EMI shielding fabric. As a side
effect this lining will help avoiding compromising
emanations from equipment localized inside the
compartment.
To be eligible, it is essential that these fabrics are
robust, inexpensive, light-weight, easy to handle, and
last but not least important; these materials must
withstand HPM irradiation without structural
breakdown, something that has been observed e.g. in
coated window glass [7].
Little is known about what happens when
conductive fabrics are exposed to strong
electromagnetic fields. This is examined in the
present work by comparing shielding effectiveness
(SE) measurement results for two types of fabric,
both in two different qualities. A comparison of SE
before and after subjecting the samples to HPM
irradiation is presented.
2 METHODS
Two different methods to determine the shielding
properties of the fabrics were employed. The first
method was a traditional comparative measurement
with a plane wave under normal incidence in a semi-
anechoic chamber (SAC). In the second method a
nested reverberation chamber (RC) was used to
measure the isotropic transmission cross section of
the test object with a mode stirred incident field [7],
[8] thus achieving a result representing all incident
angles.
The samples were tested in the SAC with a plane
wave at normal incidence to get a qualifying
reference for the subsequent tests using the RC.
2.1 Semi-anechoic Chamber
The plane wave shielding effectiveness is
determined from a traditional comparative
measurement, “hatch on/hatch off”, measured at
normal incidence using two polari
wave shielding effectiveness of
SE
,
, is given by:
,
,
,
where
,
denotes the po
w
reference case and
,
d
received when the fabric sample is
m
panel.
The transmitting and receivin
p
ositioned at a 300 mm distance f
r
the respective side. The size of th
e
300 by 300 mm.
2.2 Mode Stirred Reverberation
Inside the RC a smaller c
a
‘Akilles’) (size 1.53 x 0.93 x 0.69
square aperture sized 300 mm b
y
Mode stirrers are located inside
b
cavity to generate different field
p
the boundary conditions.
Shielding properties of the fa
b
measured at isotropic conditions
isotropic environment yields an a
v
transmission of plane waves, in
c
sample (ideally) from all direction
s
all polarizations. In this case the
s
are expressed in terms of the iso
t
cross section,
, of the fabric (
w
indicate that the cross section has
isotropic conditions). At plane w
a
transmission cross section,
, of
a
following definition: [8]
, ,
·
In (2)
is the power tran
s
aperture. The parameters and
d
incidence of the plane wave and
i
is the power density of the incident
f
In an isotro
p
ic environment one
g
Fig. 1. The nested cavity (a.k.a. A
k
mode-stirred reverberation chambe
r
zations. The plane
the test aperture,
(1)
w
er received in the
d
enotes the power
m
ounted on the test
g antennas were
r
om the sample on
e
test aperture was
Chamber
a
vity (nicknamed
m
3
) is nested via a
y
300 mm, Fig. 1.
b
oth chamber and
p
atterns by altering
b
ric samples were
in the RC. An
v
erage value of the
c
ident on the test
s
and with (ideally)
s
hielding properties
t
ropic transmission
w
here the brackets
been measured at
a
ve conditions the
a
n aperture has the
, ,
. (2)
s
mitted through the
d
enote the angle of
i
ts polarizatio
n
. S
inc
f
ield.
g
ets:
P
σ
·
S
where
is achieved by aver
a
and polarizations and
,
i
p
ower density in the reverb
concept of scalar power density
[9].
The measured field inside t
h
considered uncorrelated wit
h
p
ositions of the mode stirre
r
measured using an average of
all positions, per frequency, as
The transmission cross s
e
absolute result, given in square
p
roperties of the structure. As
i
outcome of a measurement of
isotropic external environment)
the average shielding effec
t
overmoded cavity (denoted by
aperture by:
,
In (4) V is the cavity volum
e
Q the cavity quality factor.
sc
S
density of the field inside th
e
denoted by the brackets, is t
a
field points over the entire i
n
cavity.
2.3 High Power Microwave
I
The high power irradiation
FMV Microwave Test Facility
in Linköping, Sweden, see Fig.
At the facility the samples
field strength 28 kV/m at 130
0
having a pulse repetition freq
u
10 seconds.
3 RESULTS
The four fabrics tested, we
with steel weft and two no
n
k
illes) inside the
r
.
Fig. 2. The Swedish microwav
e
Saab, Linköping, Sweden.
S
,R
(3)
a
ging
over all angles
i
s the so called scalar
eration chamber. The
was introduced by Hill
h
e cavity can thus be
h
respect to different
r
and hence the
SE
the measured field for
described above.
e
ction consis
t
s of an
meters, of the shielding
i
s shown in e.g. [8] the
(or
in case of an
can be used to calculate
t
iveness
of an
index cav) backing the
2·
··
. (4)
e
, λ the wavelength and
cav
c
,
is the scalar power
e
cavity. The average,
a
ken over uncorrelated
n
ternal volume of the
I
rradiation
was conducted at the
(MTF) located at Saab
2.
were subjected to the
0
MHz with 5 µs pulses
u
ency of 390 Hz during
re two woven fabrics
n
-woven fabrics with
e
test facility, MTF at
polypyrrole (Ppy) fibers. The woven fabrics have in
an earlier measurement shown highly reflective
properties [1] while the non-woven showed mainly
absorbing characteristics.
When comparing the SAC measurements in Fig. 3
with the results from the RC measurements in Fig. 4
it is clear that the RC measurement replicates the
SAC measurement very well. The main exception is
that the normal incidence in the SAC shows high
frequency dependent variations that are “averaged”
out in the RC which shows a much smother behavior.
Another observation is that the reflective fabrics
have a substantially higher (20-40 dB) SE than the
absorbing fabrics independent of fabric thickness.
From these two measurements we can conclude that
mode stirred camber measurements are reliable in the
characterization of SE.
When studying Fig. 5 and Fig. 6 it is apparent that
HPM-irradiation does not negatively affect the
fabrics; neither the woven, reflective, nor the non-
woven, absorbing type.
4 CONCLUSIONS
Four fabrics were tested, two from Kings Metal and
two from EEONYX, both in one thick and one thin
quality.
The fabrics from Kings Metal are woven with steel
weft and polyethylene warp in the proportions 40/60
and 30/70 respectively. These fabrics have earlier
proven to reflect the signal well [1].
The fabrics from EEONYX of non-woven design
with fibers of Ppy have in the same previous
measurements shown absorbing characteristics.
Both materials proved to be unaffected by HPM
irradiation which is a prerequisite to function in the
proposed applications to replace heavy-weight sheet
metal containers or to protect sensitive electronic
equipment in field applications.
References
[1] T. Ödman, M. Lindén and C. Larsson,
”Reflection/Transmission study of two fabrics with
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[2] S. Kim, S. Jang, S. Byun, J. Lee, J. Joo, S. Jeong
and K. Park, ”Electrical properties and EMI
shielding characteristics of polypyrrole–nylon 6
composite fabrics,” J. Appl. Polym. Sci., vol. 87, nr
12, pp. 1969-1974, 21 March 2003.
Fig. 3. SE measurement results from the semi-
anechoic chamber with vertical polarization.
Fig. 4.
measurement results, from the nested
reverberation chamber.
4·
⁄
, [7].
Fig. 5. Thick EEONYX fabric. There is no noticeable
change in shielding effectiveness before and after the
HPM irradiation.
Fig. 6. Thin King's Metal fabric. There is no
noticeable change in shielding effectiveness before
and after the HPM irradiation.
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[8] M. Bäckström , T. Nilsson and B. Vallhagen,
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[9] D. A. Hill, M. T. Ma, A. R. Ondrejka, B. F. Riddle,
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