EMC’09/Kyoto
21Q1-1
Effect of Human Body on Near-Field Resonant
Coupling Wireless Power Transmission System
Qiaowei Yuan1, Qiang Chen, Kunio Sawaya
Department of Electrical and Communication Engineering
Graduate School of Engineering of Tohoku University
Sendai, Japan
1
qwyuan@ecei.tohoku.ac.jp
Abstract—A practical wireless power transmission system
consisting of a larger rectangular wire loop and a small square
wire loop with a parasitic square helical coil is proposed for
efficient wireless power transmission in an indoor environment.
The effects of the non-resonant object such as the conducting box
and the human body on the power transmission efficiency and
the resonant frequency are investigated numerically. The results
show that the power transmission efficiency is reduced
significantly when a non-resonant object is very close to the
receiving element. However this reduction can be negligible when
the relative distance between the receiving element and the nonresonant is larger than 0.5m or 0.03Ȝ at the resonant frequency
of 19.22MHz.
Key words: Wireless, Power transfer, Near field, Resonant
coupling, Efficiency, Human body, SAR.
I. INTRODUCTION
Wireless power transmission(WPT) technology has been
focused on again for its widely applications to charge
ubiquitous electronic devices such as MP3, mobile phone,
household robots and so on without cord [1]-[5]. The authors
in [5] have experimentally demonstrated that the efficient
power transmission can be achieved by using two strongly
coupled helical coils [1], showing the potentiality to charge
electronic devices. Comparing with the near magnetic field
induction method [2],[3], the near-field resonant coupling
method can transmit the power to longer distance and
comparing with the far field radiation method [6]-[10], it is
more efficient without wasting a vast energy due to omnidirectional antenna. One can use directive electromagnetic
beam such as lasers, but this is not very practical and could be
even dangerous. It requires an uninterrupted line of sight
between the source and the device as well as a sophisticated
tracking mechanism when the device is mobile terminal.
In [5], the power transmission efficiency of two dielectric
disks and two capacitive loaded conducting-wire loops are
calculated by two parameters of the resonance width and the
coupling coefficient which are introduced in the coupledmode theory (CMT), showing the applicability of the
transmitting systems even in the presence of extraneous
environmental objects. But the effects of the material of the
non-resonant objects and the distance between the WPT
system and the non-resonant objects have not been
investigated sufficiently.
In [11], a practical wireless transmitting system consisting
of a large rectangular wire loop and a small square wire loop
with a parasitic square helical coil was proposed by the
present authors. The power transmission efficiency was
defined as the ratio of the receiving power at the receiving
element and the driving power at the transmitting element and
calculated by full-wave electromagnetic analysis. The power
transmission efficiency at the presence of a human body or a
conducting box and the volume average specific absorption
rate (SAR) by human body have been investigated in [11], but
the results are very limited.
In this paper, the effects of the material of the non-resonant
object and the relative distance between the WPT system and
the non-resonant object on power transmission efficiency are
further investigated in detail. Based on the perspective of the
safety of the human being under the WPT system, the average
SAR is also investigated.
II. A PRACTICAL WIRELESS POWER TRANSMISSION
SYSTEM
A schematic model of proposed WPT system is shown in
Fig. 1. This WPT system is assumed to be worked at the
operating frequency of 20 MHz and applied to a room with 2
m height and 6 m width. A rectangular loop S with side length
of Sa = 6 m and Sb = 2 m which is linked to the driving circuit
is used as the transmitting element. The loop S resonates in
one-wavelength at the operating frequency of 20 MHz.
Because the operating frequency should be officially decided
in the future, the shape and size of loop antenna has to be
designed to meet to the official WPT operating frequency and
the size of a practical room, it is not a difficult work. A
square loop D with side length of Da = 0.3 m connected to a
load is supposed to be mounted on a mobile receiving terminal
to charge the terminal. Similarly, the size of receiving element
can be adjusted to the size of practical mobile terminal. The
resonant frequency tuning and matching circuit of the
receiving element are realized by using a parasitic square
helical coil C. The side length, the pitch and the number of
turns of the helical coil C are 0.3 m, 0.02 m and 5,
respectively. The radii of all wires including loop S, loop D
and helical coil C are 2 mm. All wires are supposed to be
made of cooper with the conductivity of 5.8×107S/m.
Copyright © 2009 IEICE
25
EMC’09/Kyoto
21Q1-1
(3a) at off-resonant frequency of 19MHz
Fig. 1. A model of WPT system consisting of large rectangular loop and a
small rectangular loop with a parasitic helical coil
III. PRINCIPLE OF NEAR-FIELD RESONANT COUPLING
WPT SYSTEM
Resonant coupling is the fact that two same-frequency
resonant objects tend to couple, while interacting weakly with
other off-resonant elements. The operating frequency of WPT
system can be found by investigating the frequency
characteristics of the input impedance of the transmitting
element and receiving element. Fig. 2 shows the input
impedance of transmitting element and receiving element
calculated by the method of moments (MoM), indicating that
both the transmitting and receiving elements resonate at the
frequency of about 19.22MHz. Therefore, it can be predicted
that the proposed WPT will transfer the power efficiently at
this frequency.
100
(3a) at the resonant frequency of 19.22MHz
Fig. 3. Distributions of Poynting vector
IV. EFFECT OF NON-RESONANT OBJECT ON POWER
TRANSMISSION EFFICIENCY AND AVERAGE SAR OF HUMAN
BODY
500
R(Transmitting)
0
0
X [:]
R [:]
50
R(Receiving)
-50
X(Transmitting)
X(Receiving)
-100
5
10
15
20
Frequency [MHz]
-500
25
Fig. 2. Frequency characteristics of input impedance of the transmitting
element S and receiving element D
In order to investigate the mechanism of the proposed nearfield coupling WPT system, the distribution of Poynting
vector in xz-plane at y=0.05 m at the resonant frequency of
19.22MHz is compared with that at the off-resonant frequency
of 19MHz in Fig. 3. It can be seen that a strong energy
coupling between the transmitting element and receiving
element occurs when two elements resonate at the frequency
of 19.22MHz.
Fig. 4. WPT sysytem at the presence of human body
Human body and conducting box are used as the nonresonant objects in this numerical simulation. For the
simplicity, the human body is modeled as a rectangular
dielectric box with the size of 0.5m × 0.2m× 1.7m as shown
in Fig. 4. The distance between the non-resonant object and
the receiving element in xy-plane is denoted by d as shown in
Fig. 5. The muscle-type dielectric material with relative
Copyright © 2009 IEICE
26
EMC’09/Kyoto
21Q1-1
absorption of the human body.
50
Transmitting efficiency [%]
dielectric constant of 107.2 and conductivity of 0.67S/m and
the skull-type material with relative dielectric constant of 36.6
and conductivity of 0.092S/m are used as the material to
represent the human body approximately. The conducting box
has the same size as the human body. The load of receiving
element is a resistance of 1:.
d=0.3m
40
w/o object
30
conductive box
20
muscle-type
skull-type
10
0
18.5
19
19.5
Frequency [MHz]
20
Fig. 6. Transmission efficeincy with/without object
50
Copyright © 2009 IEICE
27
Transmitting efficiency [%]
conductive box
30
20
muscle-type
10
skull-type
0
18.5
19
19.5
Frequency [MHz]
20
Resonant Frequency of WPT system[MHz]
Fig. 7. Transmission efficeincy with/without object
19.5
19
muscle-type
skull-type
conductive box
18.5
0.2
0.4
d [m]
0.6
0.8
Fig. 8. Resonant frequency of WPT as a function of d
100
50
d=0.3m
500
R with muscle-type
R w/o muscle-type
0
0
X [:]
The power transmission efficiencies for three different
kinds of objects are shown in Fig. 6 and Fig. 7. In Fig. 6, the
center of the receiving element is located at (0, 0, 0.5 m)
while the bottom center of the human body is located at (0, 0.3
m, 0). The distance d in xy-plane is 0.3 m. In Fig. 7, the
location of the receiving element is the same as that in Fig. 6,
but the object is moved along y-direction by 0.2 m, it means
that d is 0.5 m. For the case of d = 0.3 m, the resonant
frequency of WPT system shifts down to 19MHz with about
40% reduction of the power transmission efficiency by the
presence of the muscle-type human body or the skull-type
human body. However, the shift of the resonant frequency of
WPT system and the reduction of the transmission efficiency
are not so much for the case at the presence of the conducting
box, and also for the case of d=0.5 m even with the human
body as shown in Fig. 7.
Fig. 8 shows the resonant frequency of WPT system as a
function of distance d. Fig. 9 shows the input impedance of
the receiving element with and without the muscle-type
human body. From Fig. 8, it can be observed that the resonant
frequency of the WPT system shifts down when the human
body comes close to the receiving element. This shift of the
resonant frequency of the WPT system is almost the same to
the shift of the resonant frequency of the receiving element as
shown in Fig. 9. Fig. 9 shows that the resonant frequency of
the receiving element moves down to 19MHz due to the
presence of the muscle-type human body. Fig. 10 and 11 show
the average SAR of the human body for the cases of muscletype body and skull-type body, respectively. The excited
voltage is 1v for SAR calculations. It is found that the average
SAR increases when the human body comes close to the
receiving element as shown in Figs. 10-11. However, it is very
interesting that the average SAR at the resonant frequency is
much smaller than that at other off-resonant frequency when d
is greater than 0.5 m, confirming that the resonant coupling
WPT has a very weak interaction with other off-resonant
environmental objects and the resonant system may reduce the
R [:]
Fig. 5. Disitance d between receiving elemment and non-resonant object in
xy-plane
w/o object
d=0.5m
40
X with muscle-type
-50
X w/o muscle-type
-100
17
18
19
Frequency [MHz]
-500
20
Fig. 9. Impedance of receiving element with/without human body
EMC’09/Kyoto
21Q1-1
d=0.3m
Average SAR [W/kg]
power transfer by strong resonant coupling has been
confirmed by calculating the distribution of Poynting vector in
near field. The effect of the object such as human body or
conducting box has also been investigated in detail, showing
that the reduction of the power transmission efficiency due to
the object is very serious when the object is very close to the
receiving element. The reduction caused by the human body is
much larger than that by the conducting box. However, this
reduction of the power transmission efficiency can be
alleviated significantly when the object is separated from the
receiving element by a distance of more than 0.5m (or 0.03Ȝ at
19.22MHz).
2
(u10-5)
Muscle-type
d=0.4m
1.5
d=0.5m
1
d=0.6m
d=0.8m
d=0.7m
0.5
0
18.5
19
19.5
Frequency [MHz]
20
Fig. 10. Average SAR of muscle-type human body
Average SAR [W/kg]
(u10-5)
d=0.3m
3
d=0.4m
2
d=0.5m
d=0.6m
d=0.7m
d=0.8m
ACKNOWLEDGMENTS
This research was financial supported by telecom
engineering center of Japan.
Skull-type
REFERENCES
1
0
18.5
19
19.5
Frequency [MHz]
Fig. 11. Average SAR of skull-type human body
20
Finally, the power transmission efficiency as function of d
is shown in Fig. 12, indicating that the effect of the nonresonant object such as human body and conducting box on
the transmission efficiency can be negligible when the
distance between the receiving element and the non-resonant
object is separated over than 0.5m.
Transmitting Efficiency [%]
50
40
30
conductive box
20
muscle-type
10
Skull-type
0
0.2
0.4
d [m]
0.6
[1] Kurs, Aristeidis Karakis, Robert Moffatt, J. D. Joannopoulos, Peter Fisher,
Marin Soljacic, ”Wireless Power transmission via Strongly Coupled Magnetic
Resonances,” SCIENCE, Vol.317, pp. 83-86, 6 July 2007.
[2] J.Murakami, F. Sato, T. Watanabe, H.Matsuki, S. Kikuchi, K. Harakaiwa
and T. Satoh, ”Consideration on Cordless Power Station – Contactless Power
Transmission System,” IEEE Trans on magnetics, Vol. 32, pp.
[3] K. Hatanaka, F. Sato, H. Matsuki, S. Kikuchi, J. Murakami, M. Kawase, T.
Satoh, ”Power Transmission of a Desk With a Cord-Free Power
Supply, ”IEEE Trans on magnetics, Vol. 38, pp. 3329-3331, No. 5, Sept.
2002.
[4] T. Sekitani, M. Takamiya, Y. Noguchi, S. Nakano, Y. Kato, T. Sakurai
and T. Someya, ”A large-area wireless power transmission sheet using printed
organic transistors and plastic MEMS switches,” nature materials, Vol. 6, pp.
413-417, June 2007.
[5] Aristeidis Karalis, J.D. Joannopoulos, and Marin Soljacic, ”Efficient
wireless non-radiative mid-range energy transfer,” Annels Physics, 323(2008),
pp.34-48.
[6] US0,645,576 (1900-03-20) Nicola Tesla, System of Transmission of
Electrical Energy.
[7] US0,649,621 (1900-05-15) Nicola Tesla, Apparatus for Transmission of
Electrical Energy.
[8] US0,787,412 (1905-04-18) Nicola Tesla, Art of Transmitting Electrical
Energy through the Natural Mediums
[9] H. Matsumoto, ”Research on Solar Power Satellites and Microwave
Power Transmission in Japan,” IEEE Microwave Magazine, Vol.3, No.4,
pp.36-45, December 2002.
[10] C. T. Rodenbeck, K. Chang, ”A Limitation on the Small-Scale
Demonstration of Retrodirective Microwave Power Transmission from the
Solar Power Satellite”, IEEE Antennas and Propagation Magazine, Vol.47,
No.4, pp.67-72, August 2005.
[11] Qiaowei YUAN, Qiang CHEN, Kunio SAWAYA, “Transmission
Efficiency of Evanescent Resonant Coupling Wireless Power Transfer System
with Consideration of Human Body Effect,” IEICE Technical Report,
AP2008-91, p.95-99, September, 2008, Japan.
0.8
Fig. 12. Transmission efficeincy as a funtion of d at the presence of
non-resonant object
V. CONCLUSIONS
The resonant frequency of the proposed WPT system
consisting of a larger rectangular wire loop and a small square
wire loop with a parasitic square helical coil has been
determined by making the transmitting element and receiving
element resonating at the desired frequency. The efficient
Copyright © 2009 IEICE
28