> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
Low-Profile Dual-Band Textile Antenna with
Artificial Magnetic Conductor Plane
Sen Yan, Ping Jack Soh, Member, IEEE, Guy A. E.
Vandenbosch, Fellow, IEEE
Abstract—A dual-band textile antenna loaded with an artificial
magnetic conductor (AMC) plane is proposed for WLAN
applications. Its dual-band operation is enabled by a rectangular
patch in the 2.4 GHz band and a patch-etched slot dipole in the 5
GHz band. Since the AMC approaches a perfect magnetic
conductor (PMC) in the 5 GHz band, the slot dipole can be located
close to the ground. The proposed antenna is fully fabricated
using textiles except for a feeding connector used for testing
purposes and a via. Simulations and experiments agree well and
validate that this low profile antenna operates with a good
reflection coefficient and a high front-to-back ratio (FBR) within
the desired bands.
Index Terms—dual-band antenna, textile antenna, artificial
magnetic conductor (AMC), metamaterial.
I. INTRODUCTION
T
HE rapid development in wearable communication devices
is making high performance textile antennas more
attractive. In line with these developments, the requirements of
the antennas in terms of performance are becoming more
demanding. The main additional requirement for a textile
antenna compared to a traditional design is its ability to
minimize the interaction between antenna and human body,
despite of the fact that they are located close to each other.
Moreover, irradiation of the human body over prolonged
periods of time is controversial, since it may present a health
risk factor. The power densities available in the near field
region of an antenna are much higher (and roll-off more
rapidly) than in the far field. Although the front-to-back ratio
(FBR) and the specific absorption rate (SAR) are quantities
which are related to the near and far field of an antenna,
respectively, they are interrelated to a certain degree, since the
far field is the Fourier transform of the near field. This means
that the FBR can already provide a rough idea concerning the
SAR [1]. Another potential problem of textile antennas is
related to robustness. Since a textile antenna may curve and
deform when worn, it is crucial that it is designed in such a way
that it is not too sensitive to deformations.
Due to the high FBR required, microstrip patch antennas are
widely used for wearable devices. However, their inherent
narrowband property poses a challenge [2], especially when a
Manuscript submitted May 15, 2014.
S. Yan, and G. A. E. Vandenbosch are with the ESAT-TELEMIC Research
Div., Dept. of Electrical Eng., KU Leuven, Kasteelpark Arenberg 10, Box
2444, 3001 Leuven, Belgium (e-mail:sen.yan@esat.kuleuven.be).
Ping Jack Soh is with the Advanced Communication Engineering (ACE)
CoE, The School of Computer and Communication Engineering, Universiti
Malaysia Perlis (UniMAP), Pauh Putra Campus, 02600 Arau, Perlis, Malaysia.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
doi:
1
single antenna is needed to cover both the 2.4 and 5 GHz bands
for WBAN/WLAN applications. A dipole or monopole antenna
can be easily tuned to reach wideband operation, but their
omnidirectional patterns should be amended for on-body
application. Moreover, electric dipoles / monopoles are less
efficient when operating closely to a parallel piece of metal due
to the anti-phase image induced [2]. Conversely, magnetic
dipoles / monopoles, in practice realized by slots or apertures in
a ground plate, are also not suitable for placement near any
metal planes because of the generation of parallel plate modes
between the two metal planes, which considerably distorts the
characteristics [3].
An artificial magnetic conductor (AMC) plane is a kind of
two-dimensional metasurface. It has been widely used already
in the design of planar antennas [4,5]. One of its main
advantages is that it can be located close to a parallel electric
current forming a reflector, since the current on the AMC
corresponds to an in-phase image [5-11]. It is also possible to
suppress any parallel plate modes so that this structure may be
used as a low-profile reflector for a magnetic dipole. Such
concept has been demonstrated in literature via the design of
several AMC-based antennas for wideband or dual-band
operation [3]. The dual-band characteristic is usually realized
by exciting the higher modes of the structure, or by integrating
different resonator shapes. These methods typically reduce the
bandwidth of the AMC, or result in a more complex
optimization and fabrication process [12-15]. The small
dimensional tolerances of such a design make it unsuitable for
textile antennas.
In a WLAN application, the antenna needs to operate with a
narrow lower band (from 2.4 GHz to 2.484 GHz) and a wide
upper band (from 5.15 GHz to 5.85 GHz). The main idea of the
proposed antenna is to utilize the patch as the radiator for the
lower WBAN/WLAN band, and a slot dipole etched on the
patch for the upper WLAN band. The AMC plane replaces the
normal metallic ground. In the lower band this AMC plane
approaches a perfect electric conductor (PEC) forming the
ground for the patch antenna. In the upper band, it approaches a
perfect magnetic conductor (PMC), allowing an
efficiently-radiating magnetic dipole without exciting the
parallel plate mode in between the antenna plane and the
ground plane [3]. To our best knowledge, this is the first textile
antenna to utilize such concept. Compared with other designs,
the proposed antenna is advantageous in two aspects. Firstly,
the AMC layer is very simple. Since only a single resonance is
required, the unit cell can be designed either using square or
rectangular patches. This simplifies the fabrication using textile
materials and the manual fabrication procedure. There is more
risk for manufacturing inaccuracies if more complex shapes are
incorporated in the AMC layer. Secondly, the wide bandwidth
in the upper band enables the AMC to easily cover the whole
WLAN communication band at 5 GHz, even when a slight
frequency shift is expected when worn on the human body.
The organization of the paper is as follows. The structure and
characteristics of the AMC plane are first introduced and
analyzed prior to the evaluation of the overall integrated design
in Section III. Next, the on body performance of the antenna is
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
2
Fig. 1 Topology (a) and reflection coefficient (b) of the AMC plane.
discussed in Section IV, before drawing the conclusions.
II. CHARACTERISTICS OF AMC PLANE
Various types of AMC plane have been studied in literature,
[3-7, 10-14]. In this design, the square patch structure is chosen
due to its ease in obtaining a large bandwidth [13], besides its
fabrication simplicity when using textile materials. The
topology of a single AMC unit cell is shown in Fig. 1 (a). The
dielectric constant and loss tangent of the 1.5 mm thick felt
substrate are 1.2 and 0.044 [16], respectively. ShieldIt Super
conductive textile from LessEMF Inc. [17,18] with an
estimated conductivity of 1.18 × 105 S/m is used to form the
metallic layers. The complete AMC plane consists of a periodic
square patch structure on the top layer, a layer of substrate and a
layer of ground at the reverse side. Each square patch is 18.2
mm in size and the period is 20 mm. Another layer of substrate
is placed on top to separate the AMC plane from the antenna.
CST Microwave Studio [19] is used to simulate the structure,
using unit cell boundaries and Floquet ports to mimic the
infinite planar periodicity. Fig. 1 (b) depicts the reflection
coefficient (S11) of the AMC with a plane wave incident from
the normal direction. It can be clearly seen that the reflected
wave is nearly in anti-phase at 2.45 GHz, where it is supposed
to behave as a PEC. On the contrary, the phase of the reflected
wave is about 0o in the 5 GHz band, which exhibits a PMC-like
characteristic. The operating bandwidth of the proposed AMC
plane, defined by the phase of the reflected wave lying between
0o ± 90o, is from 5.04 GHz to 5.935 GHz. It is clear from the
calculations that this proposed structure is capable of operating
within the desired WLAN bands.
III. ANTENNA ON AMC PLANE
The overall topology of the antenna is illustrated in Fig. 2. A
coaxially-fed, rectangular patch antenna functions as a 2.45
GHz radiator. Due to the PEC-like behavior of the AMC plane
in this band, the antenna performs similarly as a conventional
microstrip patch antenna. Next, two slots in y-direction are
introduced onto the rectangular patch. Note that different slot
widths and lengths are selected to enable a wide impedance
bandwidth. A coplanar waveguide (CPW) line which is about a
quarter wavelength at 5.5 GHz is used to connect these two
Fig. 2 Topology of the proposed antenna. (a) 3-D view, (d) side view, (c)
top view, (d) fabricated prototype
slots. The CPW line is connected with the structure's ground by
a via, between the patch and the CPW strip. Since the slots
cause perturbations of the current distribution on the patch in
the lower 2.45 GHz band, the size of the patch has to be
adjusted upon the integration of the slots. The antenna is
optimized using the genetic algorithm in CST MWS with the
aim of obtaining the best S11 in both frequency bands. The final
dimensions of the proposed antenna are given in Fig. 2 (c).
The fabricated antenna prototype is shown in Fig. 2 (d). A
comparison between simulated and measured S11 is shown in
Fig. 3 (a). In the lower band, the maximum S11 is -16.2 dB
(simulated) and -8.3 dB (measured), whereas this is -11.7 dB
(simulated) and -12.5 dB (measured) in the upper band. Note
that the slot dipole enables a much wider bandwidth than a
common microstrip patch antenna. A small disagreement
between the simulated and measured curves is observed. This is
explained by the uncertainty concerning the textile substrate
properties and the mechanical inaccuracies caused by the
manual fabrication procedure with simple tools. Note that using
machine dimensioning or laser cutting does not necessary lead
to better results in practice, due to the unavoidable
deformations of the antenna during practical operation. The
most important is that the antenna performance still fulfills the
requirements for WLAN communication.
A noticeable point is that the lower band splits into two
resonances. This is related to the coupling between the large
patch and the center CPW patch. At the first resonance, the
currents on these two patches are in phase while at the second
resonance they are out of phase. More details about mode
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
3
Fig. 4. Simulated results for several ground sizes.
TABLE I
SIMULATED SAR
Fig. 3 Measured results of the antenna, (a) reflection coefficient, radiation
pattern at 2.45 GHz in x-z plane (b) and y-z plane (c), radiation pattern at 5.2
GHz in x-z plane (d) and y-z plane (e). Solid (red) line: co-polarization,
dashed (blue) line: cross-polarization.
coupling can be found in [20]. The S11 dip around 4 GHz
represents the second mode (TM20) of the patch.
The measured radiation patterns at 2.45 GHz and 5.2 GHz
are shown in Fig. 3 (b)-(e). The FBR is higher than 12 dB in
both bands. The high cross polarization in y-z plane is mainly
caused by the CPW line which is perpendicular to the radiated
slot, but that is not a concern in WLAN communication systems.
The realized gain of the antenna is about 2.5 dB at 2.45 GHz,
and between 0 to 4 dB in the higher band. The total efficiency is
above 40 % throughout the whole operating band. This value is
typical for textile antennas fabricated using this type of
materials [21].
IV. PERFORMANCE ON BODY
The fabricated antenna prototype is measured on the chest of
a male volunteer, who weighs 90 kg and is 178 cm in height.
The result of this evaluation performed in an anechoic chamber
is also shown in Fig. 3 (a). Due to the effectiveness of the
ground plane, the S11 is only slightly affected by the coupling
between the human body and the antenna. The maximum S11 is
-9.8 dB and -8.4 dB in the lower and upper band, respectively.
Thanks to the large area available on the human chest and
back, it is possible to enlarge the ground plane in order to
further reduce the backward radiation. If this additional ground
Freq.
(GHz)
Antenna with
100x100 mm2 ground
(W/kg)
Antenna with
200x200 mm2 ground
(W/kg)
2.45
5.2
5.8
0.0464
0.0232
0.0300
0.00424
0.00130
0.00168
layer is a thin conductive textile layer, this may not affect the
antenna conformability or the users' comfort. Simulations for
ground planes up to 200 × 200 mm2 show that the S11 is nearly
not affected while the back radiation is further reduced by a
value between 5 and 10 dB, see Fig. 4.
The large ground is also expected to significantly reduce the
SAR value. To analyze the contribution of the metasurface
placed between the antenna and the human body, a series of
SAR simulations was performed using a simplified human
model in CST MWS. This model is defined behind the antenna,
at 10 mm distance from the antenna ground layer in order to
emulate practical antenna to skin distances in clothing. This
distance is also larger than λ/4 at the lower operating frequency
of 2.4 GHz, ensuring a proper distance from the edges of the
combined antenna / metasurface. The model combines a 3 mm
thick layer of skin, a 7 mm thick layer of fat, and a 60 mm thick
muscle layer. This model has been validated in [22]. The input
power to the antenna for SAR calculations in this work is set at
0.5 W (rms). SAR values were calculated based on the IEEE
C95.1 standard and averaged over 10 g of biological tissue. The
SAR distributions in both bands are displayed in Fig. 5. The
calculated results are summarized in Table 1. Intuitively,
measuring SAR at the exact same position using a common
antenna will result in higher SAR with increasing frequency.
This is due to the conductivity of the human tissues, which
increases with frequency [23]. However, in this case, it is clear
from the simulated SAR values that the AMC is functioning
well as a PMC at the higher frequencies. The SAR values
observed in the 5 GHz band are lower compared to the 2.4 GHz
band. Using an enlarged 200 x 200 m2 ground plane, see Table
1, the SAR values were reduced with a factor of more than 10.
The antenna was also simulated when bent over a cylinder
along the two main axes and for different radii. For the cylinder
oriented along the x-axis, no significant S11 changes are
observed. This is due to the direction of bending, which is
parallel to the current on the patch, in this way not affecting the
current distribution considerably. Conversely, the antenna S11
changes are more profound when it is bent along the y-axis, see
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
4
Fig. 5. SAR distributions at different frequencies. (a) 2.45 GHz, (b) 5.2 GHz, (c) 5.8 GHz.
[5]
[6]
[7]
[8]
Fig. 6 Simulated reflection coefficients for bent topology.
Fig. 6. In the lower band, the resonant frequency shifts upwards
as the radius of the cylinder is decreased. Nonetheless, even
when bent the -10 dB operational bandwidth still covers the
whole 2.4 GHz WLAN band. Meanwhile, the antenna
operation in the upper band remains quasi unchanged. Only a
slight S11 degradation can be seen at 5.85 GHz (with S11 =
-9.32 dB).
[9]
[10]
[11]
[12]
V. CONCLUSION
In this paper, an AMC-integrated all-textile dual band
antenna for WLAN applications is proposed. This novel
structure combines a patch antenna and a slot dipole to enable
its dual-band property, whereas the AMC is used to effectively
suppress back radiation and, consequently, electromagnetic
coupling with the body. The simplicity of the topology used
enables its realization using flexible textile materials and a
simple manual fabrication procedure. Both simulations and
measurements performed in free space and on body indicated a
satisfactory performance in terms of bandwidth and radiation
properties for dual-band WBAN/WLAN applications.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
REFERENCES
[1]
[2]
[3]
[4]
Z. Jiang, D. E. Brocker, P. E. Sieber, D. H. Werner, "A compact,
low-profile metasurface-enabled antenna for wearable medical body-area
network devices," IEEE Trans. Antennas Propag., vol. 62, no. 8, pp.
4021-4030, Aug. 2013.
C. A. Balanis, Antenna theory: analysis and design. John Wiley & Sons,
2012.
J. Joubert, J. C. Vardaxoglou, W. G. Whittow, J. W. Odendaal, "CPW-fed
cavity-backed slot radiator loaded with an AMC reflector," IEEE Trans.
Antennas Propag., vol. 60, no. 2, pp. 735-742, Feb. 2012.
D. Sievenpiper, L. Zhang, R. F. Broas, N. G. Alexopolous, E.
Yablonovitch, "High-impedance electromagnetic surfaces with a
forbidden frequency band," IEEE Trans. Microw. Theory Tech., vol. 47,
no. 11, pp. 2059-2074, Nov. 1999.
F. Yang, Y. Rahmat-Samii, "Reflection phase characterizations of the
EBG ground plane for low profile wire antenna applications," IEEE Trans.
Antennas Propag., vol. 51, no. 10, pp. 2691-2703, Oct. 2003.
A. Vallecchi, J. R. De Luis, F. Capolino, F. De Flaviis, "Low profile fully
planar folded dipole antenna on a high impedance surface," IEEE Trans.
Antennas Propag., vol. 60, no. 1, pp. 51-62, Jan. 2012.
L. Akhoondzadeh-Asl, D. J. Kern, P. S. Hall, D. H. Werner, "Wideband
dipoles on electromagnetic bandgap ground planes," IEEE Trans.
Antennas Propag., vol. 55, no. 9, pp. 2426-2434, Sep. 2007.
S. R. Best, D. L. Hanna, "Design of a broadband dipole in close proximity
to an EBG ground plane," IEEE Antennas Propag. Mag., vol. 50, no. 6,
pp. 52-64, 2008.
D. Cure, T. M. Weller, F. A. Miranda, "Study of a low-profile 2.4-GHz
planar dipole antenna using a high-impedance surface with 1-D varactor
tuning," IEEE Trans. Antennas Propag., vol. 61, no. 2, pp. 506-515, Feb.
2013.
S. Kim, Y. J. Ren, H. Lee, A. Rida, S. Nikolaou, M. M. Tentzeris,
"Monopole antenna with inkjet-printed EBG array on paper substrate for
wearable applications," IEEE Antennas Wirel. Propag. Lett., vol. 11, pp.
663-666, 2012.
M. S. Alam, M. T. Islam, N. Misran, "Inverse triangular-shape CPW-fed
antenna loaded with EBG reflector," Electron. Lett., vol. 29, no. 2, pp.
86-88, Jan. 2013.
A. Pirhadi, M. Hakkak, F. Keshmiri, R. K. Baee, "Design of compact
dual-band high directive electromagnetic bandgap (EBG) resonator
antenna using artificial magnetic conductor," IEEE Trans. Antennas
Propag., vol. 55, no. 6, pp. 1682-1690, Jun. 2007.
O. Folayan, R. Langley, "Dual frequency band antenna combined with a
high impedance band gap surface," IET Microw. Antennas Propag., vol. 3,
no. 7, pp. 1118-1126, 2009.
N. A. Abbasi, R. J. Langley, "Multiband-integrated antenna/artificial
magnetic conductor," IET Microw. Antennas Propag., vol. 5, no. 6, pp.
711-717, 2011.
S. Zhu, R. Langley, "Dual-band wearable textile antenna on an EBG
substrate," IEEE Trans. Antennas Propag., vol. 57, no. 4, pp. 926-935,
Apr. 2009.
"Specification Sheet - Felt Sheet," RS Components Inc., 2013.
"Specification Sheet – ShieldIt Super," LessEMF Inc., 2013.
J. Lilja and P. Salonen, "On the modeling of conductive textile materials
for SoftWear Antennas," IEEE Antennas Propag. Soc. Int. Symp.,
Charleston, SC, June 2009, pp. 1-4.
"https://www.cst.com/Products/CSTMWS", Computer Simulation
Technology (CST), Microwave Studio.
[20] S. Zhang, D. A. Genov, Y. Wang. M. Liu, X. Zhang, "Plasmon-Induced
Transparency in Metamaterials," Physical Review Letters, vol. 99, no. 14,
pp. 147401, 2007.
[21] S. J. Boyes, P. J. Soh, Y.Huang, G. A. E. Vandenbosch, N. Khiabani,
"Measurement and Performance of Textile Antenna Efficiency on a
Human Body in a Reverberation Chamber," IEEE Trans. Antennas
Propag., vol. 61, no. 2, pp. 871-881, Feb. 2013.
[22] J. Gemio, J. Parron, J. Soler, "Human Body Effects on Implantable
Antennas for ISM Band Applications: Models Comparison and
Propagation Losses Study," Prog. in Electromagnetics Research, vol.
110, pp. 437-452, Nov. 2010.
[23] P. J. Soh, G.A.E. Vandenbosch, F. H. Wee, A. van den Bosch, M.
Martinez-Vazquez, D. Schreurs, "Specific Absorption Rate (SAR)
Evaluation of Biomedical Telemetry Textile Antennas," IEEE MTT-S
International Microwave Symposium (IMS), Seattle, WA, USA, June,
2013, pp. 1-3.