Abstract—If the wireless power transfer system is located to a

person; the magnetic fields produce by the wireless power

transfer system will vary in the region occupied by the human

body. According the Institute of Electrical and Electronic

Engineers (IEEE) and International Commission on

Non-Ionizing Radiation Protection (ICNIRP), the predicated or

the measured values have to be spatially averaged in an area

representing the dimension of the human body and compared

with the adopted reference level of exposure. Although spatial

averaging is better approximation compared to point

measurement, the standard of how to perform such an

assessment does not exist. This paper describes the method for

spatial averaging magnetic field. We analyzed the difference of

spatial averaging value (1-and 2-dimensional templates consists

of different number of position) and compared it with the

standard uncertainty for measurement drift. The proposed

methods are given to present the effectiveness of the choice of

measurement position.

Index Terms—Wireless power transfer system, magnetic

field, exposure, human, spatial average, reference level,

uncertainty.

I. INTRODUCTION

As wireless power transfer technology advances, a variety

of applications such as mobile consumer electronics,

automotive, biomedical devise and industrial system are

developed. Although the wireless power transfer

technologies provide the public with convenience and safety

which wired power connection couldn’t provide, there is

another potential danger arising from the system s form of

electromagnetic field(EMF). Just like the EMI is one of the

most import criteria of electronic system, EMF is a

significant measure of safety for human body and also for

other electric devices.

Exposure guidelines and standards[1]-[4] from regarding

the protection of human from electromagnetic fields adopt

basic restrictions of exposure to such fields. The reference

levels are provide due to difficulties involved in assessing the

basic restriction for a particular exposure condition. The field

value for single point is required for comparions with the

reference value. However, When the sources of

electromagnetic fields(EMFs) is close to the body, the EMFs

will vary depending on the position at which it is measure. To

address this, exposure guidelines and standards recommend

that the reference levels must be compared with the root

Manuscript received October 25, 2012; revised November 29, 2012. This

work was supported in part by the KCC(Korea Communicaitons

Commission), Koea, under the R&D program supervised by the KCA(Korea Communication Agency)” (KCA-2012-08921-01304)

The authors are with the Electronics and Telecommunications Research

Institute, Daejeon, Korea (e-mail: sehong@etri.re.kr, choihd@etri.re.kr,

jungick@etri.re.kr, smkim@etri.re.kr).

mean square(RMS) fields quantities spatially averaged over a

specific area representing the dimensions of the human body

in the absence of a person. Although spatial averaging is a

better approximation compared to point measurements, a

harmonized standard of how to perform such an assessment

does not exist.

The purpose of this paper is to propose spatial averaging

methods and to assessment for strength and distribution of

magnetic field from wireless power transfer system.

II. MEASUREMENT AND ANALYSIS

We performed measurements of the magnetic field

strength generated by an wireless power transfer system of

desktop computer. The wireless power transfer system of

desktop computer shows is Fig 1. The wireless power transfer

system of desktop computer is used the coupled magnetic

resonance method, the resonance frequency and transmitting

power is 1.71 MHz and 85 W, respectively.

Taking into account that the exposure standards in use

require the reference levels to be compared with the

maximum expected RMS values spatially averaged over a

volume representing the human body, many spatial averaging

techniques have been proposed[5]-[8], but a harmonized

standard of how to perform such an assessment does not exist.

The magnetic field strength was examined in constricted

volumes corresponding approximately to the dimension of

the human body. The plane with width of 40 cm and the

height of 180 cm was chosen to be dimension (4x18, 10 cm

grid step).The measurement distance and spacing is 50 cm

and 10 cm, respectively. We used EHP-200 instrument

(Narda STS, Germany). The measurements were repeated 3

times at each position.

The Fig. 2 respresent the measured distribtuons of

magnetic field strength. The magnetic field strength of

wireless power transfer system of desktop computer are

0.012 A/m (minimum), 0.402 A/m (maximum), and the total

spatially averaged value(𝐻𝑎𝑙𝑙 ) is 0.213 A/m, which don’t

exceeds the reference level, 0.43 A/m of ICNIRP’s

guideline[1].

The spatial averaging measurement takes considerable

evaluation time. Therefore, we suggest the improved

measurement method to reduce the evaluation time for the

spatial averaging measurement. The reduction of the number

of measurement positions was investigated by creating 1- and

2- dimensional templates consisting of 3, 6, 9, 18 and 27

positions (see Fig. 3).

The spatially averaged value of different templates

(𝐻𝑡𝑒𝑚𝑝𝑙𝑎𝑡𝑒 ) were comparing averaged value (𝐻𝑎𝑙𝑙 ) from all

90 measured positions. The mean value ( 𝐷𝑠𝑚 ) of this

differenence value were calculated

Seon-Eui Hong, Hyung-Do Choi, Jeong-Ik Mom, and Seong-Min Kim

Evaluation Method of Electromagnetic Field Exposure

Levels from Wireless Power Transfer System

International Journal of Computer and Electrical Engineering, Vol. 5, No. 3, June 2013

334DOI: 10.7763/IJCEE.2013.V5.726

1.8

m

0.4 m

10 cm10 cm10cm

10cm

10 cm

10 cm

Fig. 1. Wireless power transfer system of desktop computer and measurement

position for whole body

Fig. 2. Magnetic field strength distribution for wireless power transfer system of Desktop computer

TABLE I: TEMPLATE SCHEMES

Template

Horizontal axis

Interval

(cm)

vertical axis

Interval

(cm)

Total

points

Surface a

b

20

20

20

30

27

18

Line*

d

e

f

-

20

30

50

9

6

3

* Line is center line of whole body

We compared these differences (𝐷𝑠𝑚 ) with the standard

uncertainty for measurement drift. That is, we tried to find the

maximum difference value, which is less than the standard

uncertainty for repeated measurement. The standard

uncertainties by repeated measurement were in 0.74

dB(student t-distribution; 𝑡0.05 = 4.30, degree freedom 2).

So we selected the value of 0.74 dB as the criterion for

measurement point reduction. And another selection criterion

for measurement position is the least number of measurement

points.

We selected the measurement position as the suitable

spatial averaging for human exposure measurement from

wireless power transfer system of desktop computer. The

selected the measurement positions are shown Fig. 3e.

III. CONCLUSION

To evaluate exposure compliance with reference levels,

the spatial averaging process is applied. The spatial averaging

measurement takes considerable evaluation time. We suggest

the improved measurement method to reduce the evaluation

time for the spatial averaging measurement. If the difference

spatial averaging value between reduced measurement

position and measurement position for whole body dense is

less than the standard uncertainty for repeated measurement,

the reduced measurement position is considered sufficient to

evaluate human exposure to EMF from wireless power

transfer system of desktop computer.

TABLE II: THE SPATIALLY AVERAGED VALUE AND THE DIFFERENCE

BETWEEN AVERAGES MEAN VALUE FOR DIFFERENT TEMPLATES AND

AVERAGED VALUE FROM ALL 90 MEASURED POSITIONS

Template Spatially averaged Value (Htemplate )

[A/m]

Dsm

(dB)

Surface a

b

0.189

0.181

0.45

0.83

line

d

e

f

0.197

0.188

0.205

0.30

0.60

0.23

1.8

m

0.4 m

0.2 m

0.2 m

0.2 m

0.2 m

1.8

m

0.4 m

0.2 m

0.3 m

0.3 m

0.2 m

(a) (b)

1.8

m

0.2 m

0.2 m

1.8

m

0.3 m

0.3 m

0.3 m

0.8 m

1.3 m

(c) (d) (e)

Fig. 3. Template schemes

REFERENCES

[1] A. Ahlbom, U. Bergqvist et al.,“Guidelines for limiting exposure to

time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz),” Health Phys., vol. 74, no. 4, pp. 494-522, Apr. 1998..

[2] A. Ahlbom, U. Bergqvist et al., “For limiting exposure to

time-varying electric, magnetic, and electromagnetic fields (1Hz - 100 kHz),” Health Phys., vol. 99, no. 6, pp. 818-836, 2010.

[3] IEEE Standard for Safety Levels With Respect to Human Exposure to

Radio Frequency Electromagnetic fields, 3 kHz to 300 GHz, IEEE Std. C95.1-2005, 2006..

[4] Technical Requirements for the human Protection against

Electromagnetic waves, KCC Notice 2012-2. [5] Electronic Communications Committee (ECC), ECC Recommendation

(02)04 (revised electronic Communications Committee (ECC), ECC

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Mag

ne

tic

fie

ld s

ten

gth

[A

/m]

Vertial plane height [cm]

International Journal of Computer and Electrical Engineering, Vol. 5, No. 3, June 2013

335

Recommendation (02)04 (Revised Bratislava 2003, Helsinki 2007):

Measuring Non-Ionizing Electromagnetic Radiation (9 kHz–300 GHz),

2007. [6] Limits of Human Exposure to Radiofrequency Electromagnetic Fields

in the Frequency Range From 3 kHz to300 GHz, Canada, Safety Code

6, 1999. [7] IEEE Recommended Practice for Measurements and Computations of

Radio Frequency Electromagnetic Fields With respect to Human

Exposure to Such Fields, 100 kHz- 300 GHz, IEEE Std. C95.3 2002, 2002.

[8] IEC 62369-1, “Evaluation of human exposure to electromagnetic fields

from short range devices (SRDs) in various applications over the frequency range 0 GHz to 300 GHz -Part 1: Fields produced by devices

used in electronic article surveillance, radio frequency identification

and similar systems,” 2008.

Seon-Eui Hong was born in Daejon, Korea, in 1995. She received the B.S and MS degrees in radio science and

engineering from chungnam National University, Daejeon,

in 1997 and 1999, respectively. Since 1999, she has been with the Electronics and Telecommunications Research

Institute, Daejeon, Korea, Where is current a Senior

Member of the radio technology group. Her current research interests include evaluation of human exposure

levels from Wireless power transfer system and the usage equipment in

workplace.

Hyung-Do Choi received the M.S. and Ph.D. degrees

in material sciences from Korea University, Seoul, in

1989 and 1996, respectively. Since 1997, he has been with the Electronics and Telecommunications Research

Institute, Daejeon, Korea, where he is currently a

Senior Member of the radio technology group. His current research interests include electromagnetic

compatibility countermeasures, electromagnetic wave

absorber design, and absorbing and shielding materials.

Jung Ick Moon was born in Daegu, South of Korea, in 1975. He received the B.S. degree in electrical

engineering from Yeugnam Univ. in 1996, and the M.S.

degree and Ph.D. degree in electrical and electronics from KAIST, South of Korea in 2000, 2004,

respectively. Since 2004, he has worked as a

Researcher in Electronics and Telecommunications Research Institute (ETRI), South of Korea. His main

interests in research are broadband antenna design, antenna measurement

system and wireless power transmission.

Sung Min Kim was born in Daegu, South of Korea, in

1973. He received the B.S. and M.S. degree in

electronics engineering from Kyungpook National Univ. in 1997, 2009, respectively. Since 2002, he has worked

as a Researcher in Electronics and Telecommunications

Research Institute (ETRI), South of Korea. His main interests in research are RF circuit design and wireless

power transmission.

International Journal of Computer and Electrical Engineering, Vol. 5, No. 3, June 2013

336