Textile Antenna for the Multi-sensor

Textile Antenna for the Multi-sensor (Impulse
GPR&EMI) Subsurface Detection System
A. Oral Salman
Emrullah Bicak
Mehmet Sezgin
TUBITAK Marmara Research
Center
Information Tech. Inst., Sensor Tech.
Group, P.O. 21,
41470, Gebze, Kocaeli, Turkey
[email protected]
TUBITAK Marmara Research
Center
Information Tech. Inst., Sensor Tech.
Group, P.O. 21,
41470, Gebze, Kocaeli, Turkey
[email protected]
TUBITAK Marmara Research
Center
Information Tech. Inst., Sensor Tech.
Group, P.O. 21,
41470, Gebze, Kocaeli, Turkey
[email protected]
Abstract— The performance of a planar elliptical dipole textile
antenna for the multi-sensor (impulse GPR&EMI) subsurface
detection system is investigated. This is a new application of
textile antenna. The textile material of antenna does not create an
EMI clutter comparing solid Cu material. The antenna
characteristics of the proposed antenna in the multi-sensor head
are measured and compared with the simulated ones. A buried
target into soil, is successfully detected by the multi-sensor system
with the usage of the textile antenna in the GPR sensor.
Keywords-component; Textile antennas; subsurface detection;
multi-sensor systems; impulse GPR; impulse EMI.
I.
INTRODUCTION
Textile antennas have recently received a great interest in the
area of electronics, computer and biomedical engineering.
There are numerous studies in literature, which have been
published recently, about textile antenna designs and
applications. Some of the important ones are textile antennas
for UWB WBAN [1], Bluetooth [2] and ISM band [3, 4]
applications, dual-band [5 - 6], dual-polarized [7] and
aperture-coupled [8] textile antennas. Aerospace applications
of textiles antennas and passive microwave textile components
are announced in [9, 10]. Because textile antennas have a
flexible structure, the effects of antenna bending [11] and
crumpling [12, 13] on the antenna performance are also
investigated. In on-body applications of textile antennas, the
proximity effects of a human body are important and
investigated in [14, 15]. Because the material of a textile
antenna is fabric, the effect of moisture [16] is an important
parameter. In textile antenna design, to precisely determine
electromagnetic parameters of a textile material is crucial.
Several techniques are presented for electrical characterization
of conductive textiles [17, 18] and electromagnetic
characterization of non-conductive [19] textiles.
In this study, we propose a new application of the textile
antennas by using of textile antenna in a multi-sensor
subsurface detection system [20, 21], which is composed of a
impulse GPR and a impulse EMI sensor. In such a system,
dielectric contents of buried objects are detected only by GPR
sensor. The metallic contents can be detected by both GPR
and EMI sensors.
The conductive textile material is used as the antenna patch
material in this study. Textile material does not to create an
EMI clutter for the EMI sensor. However, solid Cu material
does. It is the advantage of textile material to the solid Cu
material. A new style of antenna feeding is also proposed by
this study. The design tips of creating textile antenna, the
measured and simulated antenna characteristics, and the
performance of the antenna in the impulse multi-sensor
subsurface detection system are given in the next chapters of
the paper.
II. PLANAR ELLIPTICAL DIPOLE ANTENNA FOR THE
IMPULSE MULTI-SENSOR SUBSURFACE DETECTION SYSTEM
A. Textile GPR Antenna
In the market, there are number of conductive textiles sold
by several companies. We chose conductive polyester woven
taffeta manufactured by Tecknit Europe Ltd. as the conductive
textile material to make antenna patches and cavity walls of
the multi-sensor head. In this product, the polyester fibers are
plated with Ni + Cu + Ni. Because the given sheet resistance1
by the manufacturer is typically 0.05  sq and the sheet
thickness is t  0.1 mm, the conductivity of the textile
material is calculated as 2  10 S/m (300 times less than Cu’s
conductivity).
The GPR sensor requires an antenna to send a short pulse
signal to target located in some environments such as soil,
wall and ice etc. An elliptical planar dipole antenna is chosen
as the antenna type for the GPR sensor. This antenna is a
linearly polarized antenna [23, 24]. The planar structure of the
antenna is important for this application because the antenna is
required to have a negligible height in the multi-sensor head
(Fig. 1). The GPR textile antenna consists of a receiver and a
5
1
The sheet resistance is a measure of electrical resistivity of thin materials
that have a uniform thickness [22]: Rs  1 /  t , where σ is conductivity (in
S/m) and t is sheet thickness (in m).
transmitter antenna, which are completely identical. The
distance between Tx and Rx antenna is 145 mm, which is
measured from the centers of two antennas. The coils of the
EMI sensor are embedded into the frame of the multi-sensor
head. The details of the EMI sensor are not given here because
it is out of scope of this article.
A detailed sketch of one of the Tx / Rx identical textile GPR
antennas. All dimensions in the figure are given in mm all
dimensions. The elliptical patches of the antennas are
conductive textiles whose properties were explained in the
previous section. The dielectric substrate of the antenna is an
FR-4 (  r  4.9 , tan   0.025 ) with 1.6 mm thickness. The
dimensions of each patch are also given in the figure. The
axial ratio, which is the ratio of major to minor axes of ellipse,
is chosen as 1.5. This value is the optimum value, which was
found for the best antenna matching and gain in [23, 24] from
the measurements. The conductive textile patches are attached
to the substrate with a two sided thin plastic adhesive tape.
The case of the multi-sensor head is plastic. Each textile
antenna is placed on a cavity. The cavity is constructed from
the same conductive textile material. In application, the
conductive textile material is fixed using two sided thin plastic
adhesive tape to the inner wall of the multi-sensor head case.
reduces the ringing effect appearing between the cavity walls
and the antenna.
For the feeding of textile antennas, textile material of the
antenna is connected electrically using a bolt; a nut and a
washer. The bolt and nut are attached to a strip line using an
intermediate conductor strip. This strip line connects coaxial
cable to the intermediate conductor strip. The advantage of
this excitation type is the rigidity of the connection between
the textile material and the coaxial cable. It is a more solid
solution than soldering coaxial cable to textile material.
Moreover, electrical connection is supplied from a wider
surface using the washer. The bolts, nuts and washers used in
antenna are made from inox, which is a specific stainless
alloy, composed of carbon steel and chromium.
B. The Measured and the Simulated Parameters of the
Textile GPR Antenna
In this section, the textile GPR antenna performance in the
multi-sensor head is discussed. The measured parameters of
the antenna are compared with the simulated results. The
measured and simulated results of the same antenna in free
space was given in [26] before.
The measured and simulated 2 SWR curves of the textile
antenna in the multi-sensor head (within the absorber layer
and cavity) are given in Fig. 2. The lower frequency of the
antenna is 645 MHz. According to the SWR curve of the
antenna, the bandwidth ratio (the ratio of the upper to the
lower operating ratio) is 7.55.
10
SWR
8
6
4
645 MHz
4.87 GHz
2
0
0
1
2
3
f (GHz)
4
5
6
Fig. 1. The detailed sketch of one of the Tx / Rx identical textile GPR
antennas. All dimensions are given in mm.
Fig. 2 The measured —– and simulated – – – SWR curves of the textile GPR
antenna in the multi-sensor head (within the absorber layer and cavity).
The conductive textile cavity is connected to the ground
electrically by a standard electric cable. An absorber layer
(Emerson&Cuming LS-24 [25]) exists between the cavity and
the antenna. The cavity and the absorber in the multi-sensor
head have several important duties. The first one is to decrease
back lobe level of the antenna, which can cause false
detections from the upper hemisphere of the multi-sensor
head. They also decrease the coupling between the receiver
and the transmitter antennas, which is a required case for a
GPR system. Another important duty of them is to protect
operator and the RF circuitry of the system from the high
power of the short pulse. The absorber layer additionally
For several frequencies, the E-plane (y-z plane) measured
and simulated normalized Ez field patterns of the textile
antenna in the multi-sensor head are shown in Fig. 3. The Hplane (x-y plane) measurements of the antenna in the multisensor head for two frequencies are shown in Fig. 4. One can
see a perfect matching between the measured and the
simulated patterns.
Note that an elliptical planar antenna is a bi-directional
antenna in free space. However, the back lobe of the antenna
is decreased up to 20 dB by placing the antenna onto the
2
All simulations in the paper are performed using CST.
absorber and the cavity. This result is expected and agrees
with the duty of absorber and cavity, which were explained in
the previous section.
The measured and simulated gains of the textile GPR
antenna in the multi-sensor head are given in Fig. 5. The
maximum gain is measured as 4.32 dBi. The textile antenna in
the head has moderate gain and broadband characteristics.
In Fig. 6, for several frequencies, the simulated current
distribution (magnitude and the direction) of the textile
antenna in the multi-sensor head can be seen. One can realize
from Fig. 6 that the current magnitude level is generally higher
along the edges and feed points of the antenna. The current is
homogeneously distributed and all parts of the conductive
textile patches contribute the radiation for the frequencies up
to 3.5 GHz. The current has also the highest level for this
region. It corresponds to the high gain region (see the gain
curves in Fig. 5 for a comparison). After this frequency, the
current is not homogeneously distributed and low-leveled.
Some parts of the patch do not contribute at the same level to
the radiation comparingly to the other parts of the patch. Thus,
the gain starts to decrease after this frequency. In the figure,
one can see the current distribution in the parts between the
patches and the rectangular border. This area is exactly the
cavity walls, which is made from the same conductive textile.
The absorbers are invisible in the figures. The current arrows
can also be seen at the side walls of the cavity. The more
frequency increases, the more current magnitude level on the
cavity decreases. The reason of this situation can be explained
by analyzing the operation of the absorber layer. The thickness
of the absorber is not enough to absorb all electromagnetic
energy at the low frequencies. Moreover, the recommended
starting operation frequency of LS-24 absorber is given as 1
GHz by the manufacturer company [25]. The current
magnitude level on the cavity is lower than the antenna
patches due to the absorber layer between the antenna and the
cavity.
6
4
Gain (dB)
2
0
-2
-4
-6
Fig. 3. The E-plane (y-z plane) measured ● and simulated —— normalized Ez
field patterns of the textile GPR antenna in the multi-sensor head for several
frequencies.
-8
-10
0
1
2
3
4
5
6
f (GHz)
Fig. 5. The measured ─●─ and simulated ─ ─ gains of the textile GPR
antenna in the multi-sensor head.
III.
Fig. 4. The H-plane (x-y plane) measured ● and simulated —— normalized Ez
field patterns of the textile GPR antenna in the multi-sensor head for two
frequencies.
THE TARGET DETECTION
In this section, the target detection by the impulse multi-sensor
subsurface detection system with the usage of the textile
antenna is discussed.
In order to show the effectiveness of the proposed antenna in
the impulse multi-sensor subsurface detection system, the
detection results of a representative target are depicted in Fig.
7. In this figure, the raw GPR data, the processed GPR images
(buried object spatial signatures)3 and the EMI detection levels
(in a. u.) of the targets are given. The target is a dielectric
cylinder, which contains small metallic ingredient. Its diameter
and height are 7 cm and 5 cm, respectively. The target is buried
in hard and dry soil (approximately  r  4  5 ,   10  10
S/m) and its burial depth into soil is 10 cm. In Fig. 7, the
horizontal axes of the GPR images represent the movement
direction index and the vertical axes represent the depth index,
both are in a. u. One can see from Fig. 7 that the GPR spatial
signature of the buried object was made visible by the proposed
textile antenna. Therefore the viability of the textile antenna for
GPR applications was also proven experimentally.
3
4
0.75 GHz
1 GHz
antenna. The textile material allows us to use the proposed
antenna without creating an EMI clutter.
The antenna characteristics were measured and compared with
the simulated ones in the multi-sensor head. In the multi-sensor
head, the antenna is inserted in a cavity, which is made also from
the same conductive textile material due to the same reason (not
an EMI clutter). There is an absorber layer between the antenna
and the cavity. The cavity and the absorber reduce the back lobe
level of the antenna; protect the operator and RF circuitry from
the high power of short pulse. The absorber layer is additionally
responsible to reduce ringing effect. After the measurements and
simulations, it is observed that the bandwidth, gain and the
operation frequency band of the textile antenna in the multisensor head were found. The gain of the antenna in the head
was moderate and has a broadband characteristic.
The feeding style of the antenna is also new. Inox bolt, nut and
washer were used for the excitation of antenna. Such kind of a
feeding is mechanically stronger than the soldering cables
directly to the textile material of antenna and it has also a wider
excitation surface.
1.5 GHz
(a)
2.5 GHz
3.5 GHz
4 GHz
Fig. 6. The simulated current distribution (magnitude and direction) of the
textile antenna in multi-sensor head for several frequencies.
IV. CONCLUSION
A planar elliptical dipole textile antenna is proposed for a
multi-sensor subsurface detection system composed of impulse
EMI&GPR sensors. This is a new application of a textile
3
To obtain the buried object spatial signatures, the background subtraction
method [20] is used.
b)
(c)
Fig. 7. (a) The raw GPR image, (b) the processed GPR image, and the EMI
detection signal level (in a.u.) of the representative target.
The simulated current distributions of the antennas in the
multi-sensor head gave us the following results. The current
magnitude level is higher along the edges and at the feeding
points of the antenna. The current is homogeneously
distributed for the frequencies up to 3.5 GHz, which
corresponds to the high gain region. The current distributions
on the textile cavity show that the current magnitude level
decreases with the increasing frequency due to the lower
performance of the absorber layer at low frequencies.
The performance of the textile antenna was also tested in
field area and the selected target was detected successfully.
The GPR images showed that the textile antenna can be used
successfully in the impulse multi-sensor subsurface detection
system.
ACKNOWLEDGMENT
A. Oral Salman thanks to Oktay Kaya, for his help in the
antenna measurements, to Nihat Kavakli, Nedret Pelitci and
Mehmet Caliskan, for their helps in the creating textile
antennas and to Orhan Baykan for his help in the field
measurements of the multi-sensor head.
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