2013 7th European Conference on Antennas and Propagation (EuCAP)
Design of a Dual-Band Antenna for Wearable Wireless Body Area Network Repeater Systems Kyeol Kwon, Jinpil Tak and Jaehoon Choi Department of Electronics and Computer Engineering Hanyang University Seoul, Republic of Korea
[email protected],
[email protected] and
[email protected] (Corresponding Author)
Abstract-A dual-band on-body antenna for a wireless body
UNIT:
area network repeater system is proposed. The designed dual
mm
�O
band antenna has the radiation directed toward the inside of the human body at MICS band and directed toward the outside at ISM band to transmit information collected from an implanted device to an external monitoring system. In addition, the return
z�x
loss property of the antenna is insensitive to human body effects
SMA Connector
by utilizing the epsilon negative zeroth-order resonance property.
(b)
(.)
Index Terms-dual band; on-body antenna; wireless body area network repeater system; insensitivity; epsilon negative zeroth-order resonance (ENG ZOR)
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y :� 2.S UNIT: z!.-x
INTRODUCTION
Recently, there has been increasing interest body area network (WBAN) systems for a
mm
wireless variety of
In
applications such as biomedical, military, and commercial
However,
these
systems
can
only
support
Figure I. Configuration of the proposed antenna: (a) top plane; (b) bottom plane; (c) middle ground plane; (d) simulation setup for the proposed antenna on a human body phantom suitable for a repeater system.
short
transmission range due to the low radiation efficiency and the ERP regulation of 25 /lW. To overcome this limitation, it is necessary to use a dual-band on-body repeater antenna to deliver weak signals from implanted devices to external devices. In addition, antenna performance is significantly affected by body tissues due to the high dielectric constant and conductivity at the microwave frequency band. Also, the input impedance and resonance frequency as well as the gain and
II.
ANTENNA DESIGN
Figures lea) and (b) show the layout of the proposed antenna for a WBAN repeater system. The proposed antenna has a dimension of 40 mm x 40 mm x 3.2 mm, and a FR-4 substrate with a relative permittivity of 4.4 is used. The antenna is comprised of a top patch to transmit signals to external devices for ISM band and a bottom patch to communicate with
radiation efficiency of an antenna can also be deteriorated when an antenna is operated on or in a body. To minimize the
the implanted devices for MICS band. By using the aperture coupled feed on the middle ground plane, the top patch is fed
human body effect, compact zeroth-order resonance (ZOR)
as shown in figure lea). Figure l(b) shows the bottom patch
antennas for implantable and wearable WBAN systems were
which is fed by CPW on the bottom ground. To consider the
proposed in [2], [3]. Additionally, to protect the human body from radio wave exposure, the antennas for WBAN must have a low specific absorption rate (SAR) [4].
actual usage, electrically small epsilon negative (ENG) ZOR structure is chosen. To realize ENG ZOR, a chip inductor with a value of 20 nH is mounted between the bottom patch and the
In this paper, we present a novel on-body antenna for a WBAN repeater system. The proposed antenna has a dual-band property that covers the medical implantable communication service (MICS) band and the industrial, scientific and medical (ISM) bands. The return loss characteristic of the antenna is insensitive to human body effects and the radiation pattern is
978-88-907018-3-2/13 ©2013 IEEE
(d)
«)
services. Especially in biomedical applications, in order to monitor a patient's health status, an implanted device needs to collect various physiological data and wirelessly transmit the information to external medical devices in real time [1].
ground plane. The gap between the bottom patch and the ground plane can be modeled as the shunt capacitance (CR), and the chip inductor can be modeled as a shunt inductor (LL)' The ZOR frequency from the above circuit description can be found using:
418
2013 7th European Conference on Antennas and Propagation (EuCAP)
MICS band
-0-
Simulated RL (wi body phantom)
___ Simula,ed
0.3
Figure 2.
ISM band
0.4
RL (\Vlo body phan,om) 0.5
2.2
Frequcncy (GHz)
II P 2.4
Figure 4.
26
Photograph of the fabricated antenna
Simulated return loss characteristics of the proposed autenna
(n)
Figure 5. Return loss measurement setup: (a) with fabricated phantom; (b) with torso phantom) proposed antenna, HFSS v.l4.0.0 by ANSYS was used. Figure 2 shows the simulated return loss characteristics of the proposed antenna in free space and on the human body
(b)
phantom. From the result of return loss, one can observe that the human body effect on the proposed antenna is minimal for both frequency bands. This phenomenon, is caused by
Figure 3. Simulated electric field distributions of the proposed autenna: (a) for the MICS baud (403.5 MHz), (b) for the ISM baud (2450 MHz)
character
(1)
where 0)0 is the ZOR frequency [5]. By choosing the proper inductance of the chip component, the bottom patch can still have a compact size (0.027Ao
x
The proposed antenna has the radiation direct toward the outside in the ISM band and less affected from the human body effect since the middle ground plane shown in figure 1(c) acts to isolate the feed system from the patch. On the other hand, the middle ground plane suppresses the off-body side radiation of the bottom patch in the MICS band. Therefore, the proposed
of
which
resonance
frequency
is
Figures 3(a) and (b) depict the simulated electric field distributions of the substrate at each resonance frequency of the proposed antenna. In figure 3(a), the electric field distribution is in-phase by the virtue of the ENG ZOR resonance emanated
antenna has suitable radiation characteristics for wearable wireless body area network repeater systems
from the bottom patch at 403.5 MHz. Besides, typical 1800 out of phase property, which is a general characteristic of a patch antenna, is observed in figure 3(b) since the resonance of the top patch occurs at 2450 MHz.
l(d), the simulation setup for the proposed
antenna on a human body phantom
ZOR,
band. In addition, the return loss property of the proposed antenna was insensitive to the existence of the human body phantom. As mentioned before, this is because of the ZOR frequency at MICS band, which is insensitive to the surrounding medium [2], and the bottom patch reduces the human body effect at ISM band.
0.00811,0 at 403.5 MHz) which
is suitable for wearable devices.
In figure
of
predominantly affected by the value of chip inductor rather than dielectric constant of medium[2] and ground effect of the middle layer. In all the measurement setups and calculations, 50 ohm load is taken into account. The measured 10 dB impedance bandwidths of the antenna were 7 MHz (400--407 MHz) at MICS band and 85 MHz (2400-2485 MHz) at ISM
(cr = 56.7 and (J = 0.94 S/m
at 403.5 MHz, and Cr = 52.7 and (J = l.95 S/m at 2450 MHz) is shown. To calculate the return loss and radiation pattern, the antenna was placed 5 mm away from the surface of the phantom. Electrical properties of the phantom were referred from the FCC guideline [6]. For design and simulation of the
Figure 4(a) shows the fabricated antenna and figures 5 and 6 show measurement setup for return loss and radiation pattern, respectively. In figures 5, there are two different kinds of body
419
2013 7th European Conference on Antennas and Propagation (EuCAP)
z
(dBI] 5.00 0.00
-10.00 ...
-15.00
-12.47
Y
-26.82 ·30.00 ·,5.00
Figure 6.
(.J
Radiation pattern measurement setup
[df);1 5.00
11"'�
MICS band
ISM band
0.00
1.71
·10.00 ·15.00
�I�------��----------���--�
·
:2-
-26J2
Vl
('jl
..J
�
.,
0::
2
�. -6- Measured Rl (w/o body phantom) • 0 • Measured RL (body phantom) ·· 0 ·· MetlSured Rl (to",o phan.om) • Simulated RL (wI body phl1ntom)
2
0.3
0.4
0.5
2.2
Frequency
Figure 7.
·30.00 ·)5.00
,I
(b)
5 o 2.4
·1
2.6
�'I "0 ·2
(GHz)
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t:::
Measured return losses of the fabricated antenna
-
.,
'0; ·3 o ·3 "0 ...
Ill.
mln
10.00
5 �
RESULTS AND DISCUSSION
...
phantoms. Figures 5(a) and (b) depict the fabricated phantom referred by FCC guide line and Torso Phantom v.5.1 by SPEAG [7], respectively. For accurate measurement of radiation patterns and return losses of the antenna, 5 mm Styrofoam spacer (relative permittivity = 1) was placed between the proposed antenna and the phantoms as shown in figure 6.
:2
90
.3
·2 1 .1
·
�:t: plane@:4035MHz 403.5 MHz ...... F pl.ne -0- .t: plane@2450 MHz �y::pl.nc 2450MH.
180
(c)
Figure 8. Measured radiation patterns of the antenna on human body phantom: (a) at 403.5 Mhz; (b) at 2450 MHz; (c) 2-D patterns
In figure 7, the measured return loss characteristics are plotted. The measured data are agreed well with the simulated results. The measured 10 dB return loss bandwidth of the antenna in the MICS and ISM bands were 7 MHz (400 MHz407 MHz) and 85 MHz (2400 MHz-2485 MHz), respectively. The insensitiveness of the return loss characteristics of the antenna is observed when the antenna is placed on various
patterns in xy and yz planes. In the figure, unexpected off-body radiation is observed owing to the reflection from a human body at MICS band. However, the major radiation is still occurred toward inside of body. When the antenna was measured in the anechoic chamber, 1.92% and 35.85% of radiation efficiency and -17.17 dBi and -4.46 dBi of peak gain were recorded at 403.5 MHz and 2450 MHz, respectively.
environments. Figure 8 shows the radiation patterns of the antenna on a fabricated human body phantom. Even though the phantom has
The specific absorption rate (SAR) is an essential factor to
much effect on the radiation characteristics, the radiated power
evaluate when the antenna is operated on or near the human body. The SAR was measured at the Radio Research Agency
can be delivered efficiently toward the body at the MICS band due to the suppression caused by the top patch. Therefore, the proposed repeater antenna is advantageous for communication with other implanted devices in the MICS band. In the ISM band, on the other hand, the maximum power was delivered outward from the body, which improves the communication efficiency between the repeater and an external devices. The peak gains were -12.47 dBi and 1.71 dBi at 403.5 MHz and 2450 MHz, respectively. Figure 8(c) shows the 2-D radiation
of Korea using ESSAY III [8]. Figure 9 shows the measured SAR distribution when the input power of 250 mW, which is a default setting for SAR measurements of mobile application devices, is delivered. Hot spot was placed at the center point of the top patch at both MICS and ISM bands. The measured I g averaged peak. SAR values were 0.977 W/kg and 7.237 W/kg at 403.5 MHz and 2450 MHz, respectively. If the maximum
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2013 7th European Conference on Antennas and Propagation (EuCAP)
Figure 9.
Measured SAR distributions: (a) 403.5 MHz; 2450 MHz
SAR of 1.6 W/kg by the ANSIIIEEE partial body SAR regulation is considered, the maximum feasible input powers are 409.416 mW at MICS and 55.271 mW at ISM bands [9]. IV.
CONCLUSION
We proposed a dual-band on-body antenna for a WBAN repeater system. The bandwidths of the proposed antenna were enough to cover the MICS band (402--405 MHz) and ISM band (2400-2485
MHz).
Also,
the
resonant
frequencies
were
insensitive to the existence of a human body phantom. In addition, the radiation pattern of the antenna is advantageous for communication with implant devices at MlCS band and external devices at ISM band. Consequently, the proposed antenna can be a good candidate for a WBAN repeater system owing to the dual-band property, the insensitiveness to human body effect, and the desirable radiation pattern. ACKNOWLEDGMENT This
work
was
supported
by
the
national
research
foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012-0005655). REFERENCES [I]
P. S. Hall. and Y. Hao, Antennas and propagation for body-centric wireless communications, Artech House, Norwood, MA, 2006.
[2]
1. Ha, K. Kwon, and 1. Choi, "Compact zeroth-order resonant antenna
for implantable biomedical service applications," Electron. Lett., vo1.47, no.23, pp. 1267-1269, November 201I.
[3]
1. Lee, S. 1. Kwak, and S. Lim, "Wrist-wearable zeroth-order resonant
antenna for wireless body area network applications," Electron. Lett., vo1.47, no.7, pp.431-433, March 2011.
[4]
U. Kim, and 1. Choi, "Design of a microstrip patch antenna with enhanced FIB for WBAN applications," Trans. IEICE, voI.E94-B, no.3, pp.I135-1141, May 2011.
421
[5]
A. Lai, T. Itoh, and C. Caloz, "Composite right/left-handed transmission line metamaterials," IEEE Microw. Mag., vol. 5, no. 3, pp. 34-50, September 2004.
[6]
D. 1. Means, and W. Kwok, Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields, Federal Communications Commission Office of Engineering & Technology, Supplement C (edition 01-01) to OET Bulletin 65 (Edition 97-01), June 2001.
[7]
http://www.speag.coml
[8]
http://emfsafety.koreasme.coml
[9]
IEEE Standard for Safety Levels with Respect to Human Exposure IEEE Standard C95. 1-1999, 1999
