ITE Trans. on MTA Vol. 4, No. 3, pp. 269-276 (2016)

Copyright © 2016 by ITE Transactions on Media Technology and Applications (MTA)

The Specific Absorption Rate Evaluation of 1.2 GHz Band Wireless Camera by a Thermographic Method Tetsuya Yoshida †, Kazuyuki Saito †† (member), Koichi Ito ††, Masaharu Takahashi †† Abstract Until now, the 700 MHz band has been used for the field pickup unit (FPU), which is used for the live broadcasting of events, such as marathons and long-distance relay races. However, the frequency band is slated to migrate to 1.2 and 2.3 GHz bands based on an action plan for radio spectrum reallocation developed by the Ministry of Internal Affairs and Communications, Japan. With the frequency migration of the FPU, the size of transmitting antennas can be downsized. Therefore, 1.2 and 2.3 GHz band antennas can also be mounted on wireless cameras for professional-use. In this study, we measured the specific absorption rate (SAR) on the body of the operator exposed to electromagnetic waves radiated from the transmission antenna of the wireless camera via a 1.2 GHz band. We also calculated the SAR to confirm the validity of the measurement method, and we compared the measured results with the calculated ones. As the result, SAR distributions between the measured and the calculated results were nearly identical. It is thus possible to evaluate the SAR using the method suggested in this paper. Keywords: wireless camera, 1.2 GHz band, specific absorption rate, thermographic method, finite-difference time-domain method.

energy absorbed in the human body has a thermal effect

1. Introduction

in human tissue. As a wireless camera is used on the operator's shoulder, the thermal effect of the microwave

Currently, a 700 MHz band (770 – 806 MHz) is used 1) ,

which is used to

energy coming from the camera should be taken into

transmit video and audio in mobile signal relays, such as

consideration. Up until now, the specific absorption rate

marathons and long-distance relay races. However, the

(SAR) at the human head with mobile phones has been

frequency band is slated to migrate to a 1.2 GHz band

evaluated 4); however, little dosimetry has been found

(1.24 – 1.3 GHz) and a 2.3 GHz band (2.33 – 2.37 GHz)

with the wireless camera. The frequency band and the

by March 31, 2019, based on the action plan for radio

shape of the transmitting antenna mounted on the

spectrum reallocation developed by the Ministry of

wireless camera are different from those of the mobile

for the field pickup unit (FPU)

Internal Affairs and Communications,

Japan 2).

phone. Additionally, the output power of the wireless

After the migration, as the wavelength of the

camera is higher than that of the mobile phone. A total

frequency band will be shorter than that of the 700 MHz

of 25 W are required at a 1.2 GHz band and 40 W at a

band, it will be possible to downsize the transmitting

2.3 GHz band in order to secure the area equal to an

antennas and to save the power of the signal

FPU at a 700 MHz band, 5 W 5). For these reasons,

transmission. Therefore, in addition to FPU for mobile

considering the future use of the wireless camera, it is

relays, the 1.2 and 2.3 GHz band antennas will also be

necessary to evaluate its effect further. In addition, the

mounted on wireless cameras for professional-use. In

same situations have been investigated only via

fact, wireless cameras have already been used abroad.

numerical simulations 6) ; no experiments have been

For example, in the United Kingdom (UK), wireless

completed. Numerical simulations generally require a

GHz 3).

fairly large amount of computational resources (memory

These will also be used in Japan. However, microwave

and computational time) to calculate one particular

cameras are used for 2.5 GHz, 3.5 GHz, and 7.2

condition among many possible situations. Therefore, a Received November 30, 2015; Revised March 15, 2016; Accepted May 19, 2016 † Graduate School of Engineering, Chiba University

fast and easy experimental procedure with validated accuracy and repeatability is of particular interest when

(Chiba, Japan)

considering these various situations.

†† Center for Frontier Medical Engineering, Chiba University

In this paper, assuming that a 1.2 GHz band wireless

(Chiba, Japan)

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ITE Trans. on MTA Vol. 4, No. 3 (2016)

camera was being used, we measured the amount of electromagnetic (EM) exposure using a tissue-equivalent semi-hard phantom. A phantom is a model consisting of material that is electrically equivalent to tissue. As an evaluation index of the EM wave exposure when using the wireless device, the SAR [W/kg] was employed. It is defined by the following equation (1). (1) Fig. 1 Wireless camera model.

where σ, ρ, and E are the electrical conductivity [S/m], the density of the biological tissue [kg/m 3], and the electric field (r.m.s) [V/m], respectively. Three different indexes are defined by a whole-body average SAR, a local peak SAR, and a local average SAR. A whole-body average SAR and a local average SAR must be averaged over a defined period of time. In wireless devices at a high frequency band, the absorption predominantly affects parts of the body that are close to the device. Therefore, the most critical value is the local peak SAR. In the SAR measurement method, there are two primary ways of measuring the electric field: the probe method 7) and the thermographic method 8). The method used for the SAR measurement of the mobile phone is Fig. 2 Transmitting antenna model.

the former. However, in this paper, we used the thermographic method to measure the SAR distribution of the surface of the human head because this method

in the camera model, such as shown in as Fig. 2. The

can reproduce various situation of variously-shaped

transmitting antenna was not connected to the wireless

electromagnetic wave emission apparatuses.

camera electrically. The coaxial cable went through the

Moreover, we calculated the SAR to confirm the

cylinder inside; the top and middle cylinders were

measurement method's validity. Then, we compared the

connected to the inner conductor and outer conductor,

measured results with the calculated ones and

respectively, as shown in Fig. 2. The wireless camera model used in the measurement

confirmed the validity of the measured results.

and calculation are, respectively, shown in Figs. 3 and 4.

2. Material and method

The thickness of the metallic plate of the calculation

2.1 Wireless camera model

model was 2 mm. However, we placed a copper foil seal

In this study, we used the wireless camera model with

on the wireless camera surface, assuming a metal object

a transmitting antenna for a 1.2 GHz band, which is

in the measurement model. The transmitting antenna

shown in Fig. 1. We constructed this model with

was fed with a coaxial cable. In the measurement,

reference to the camera used in the broadcasting

coaxial feeding was carried out at the feeding point, as

Japan 9).

The wireless camera was modeled

shown in Fig. 2. However, in the calculation, we fed the

using a metallic plate, and the inside of the camera

transmitting antenna via gap feeding between the

model was hollow.

cylindrical elements, which is shown in Fig. 4, to save

industry in

computational resources (such as memory and

We employed the half-wavelength cylindrical dipole antenna 6) shown in Fig. 2 as a transmitting antenna for

computational time).

the 1.2 GHz band. The diameter of the cylindrical

2.2 Upper human body model

element is 12 mm, the distance between each cylindrical

We made the upper human body model for the

element is 3 mm, and the space between the cylindrical

measurement and calculation. As we expected that

element and balun is 10 mm. We put transmitting

electromagnetic waves have the greatest effect on the

antenna on the wireless camera, and the balun was put

head of the operator, we employed the electrical 270

Paper » The Specific Absorption Rate Evaluation of 1.2 GHz Band Wireless Camera by a Thermographic Method

Fig. 3 Wireless camera model (measurement).

Fig. 5 Upper-half body phantom.

Table 3 Composition of the phantom.

Fig. 4 Wireless camera model (calculation).

Table 1 Electrical properties of the human model. (@ 1.27 GHz)

Table 2 Thermal properties of the human model 10).

Fig. 6 Human model (calculation).

properties of the human brain

10) .

The electrical

properties of the human model are shown in Table 1.

calculation model and the measurement model were

Moreover, Table 2 shows the thermal properties

exactly the same and are shown in Fig. 6. In the measurement and calculation of this paper, the

explained in Sec. 2.4. The tissue-equivalent phantom of the upper human

models were not grounded. We consider this model to be

body, which was used in this study, is shown in Fig. 5.

valid because the ground electric current does not flow

Various types of phantoms exist, such as solid, liquid,

through the earth and into the body of the user with the

semi-hard, and gel. In this study, as we considered the

video camera. We consider this to be the same as the

SAR distribution of the model surface, we created the

real-world situation of someone using an actual wireless

semi-hard phantom of the human body (upper-half body

camera.

phantom) and used the thermographic method.

2.3 Experimental setup and method

Compositions of the phantom that correspond to the

As stated previously, in this study, we measured the

electrical properties of the brain are listed in Table

3 10).

SAR using the thermographic method.

The errors of the electric properties between the

The thermographic method was used to measure a

fabricated phantom and reference values of the brain

rise in temperature when a high-power EM wave was

were less than 10 %. Thus, we confirmed the validity of

radiated to the solid phantom, which is electrically

the phantom for this study.

equivalent to tissue, for a short period of time. Then, the

Figure 6 shows the human model used in the

temperature rise of the phantom was converted to the

calculation. We made the model by scanning the upper-

SAR using the following equation (2).

half body phantom. Therefore, the sizes of the 271

ITE Trans. on MTA Vol. 4, No. 3 (2016)

with an electromagnetic shield to prevent the influence

(2)

of the EM wave. where, c is the specific heat of the phantom [J/kg K], ∆T

The microwave signal from the transmitting 1.27 GHz

is the temperature rise [K], and ∆t is the time of the

band antenna was generated by the generator (8753E

radiating EM wave [s].

Hewlett Packard, Palo Alto, California), amplified by the

The experimental setup of the SAR measurement

power amplifier (AS0825-85 Milmega, Isle of Wight,

using the thermographic method is shown in Fig. 7.

U.K.), and input into the transmitting antenna.

Assuming that a 1.2 GHz band wireless camera was

Radiation power from the transmitting antenna, which

being used, we put a wireless camera model with a

was measured using a power meter (NRT Rohde &

transmitting antenna on the right shoulder of the upper-

Schwarz, Munich, Germany), was 79 W. The value of

half body model. The distance between the transmitting

this 79 W has nothing to do with the radiation power of

antenna and the surface of the phantom was 50 mm, as

the real wireless camera. When we use the

shown in Fig. 8. The thermographic camera was covered

thermographic method, we must raise the temperature of the measurement subject surface in a short time. Therefore, as a result of having maximized the radiation power from an amplifier, it became this value. The EM wave was exposed for two minutes, and then we measured the temperature distribution of the phantom surface using a thermographic camera (TVS-200 NEC Avio Infrared Technologies Corporation Ltd., Tokyo, Japan). Then, we set the emissivity of the thermographic camera to 0.84. The temperature rise due to the radiated EM wave was converted to the SAR from eq. (2). 2.4 Calculation model and method For the calculation method, we employed the finitedifference time-domain (FDTD) method. We used a program developed in our laboratory for the calculation of the FDTD method. The calculation conditions are listed in Table 4. In this calculation, we employed the non-uniform mesh model to recreate the details of the transmitting antenna. The minimum cell size was 0.20 × 0.20 × 0.20 mm near the feeding point, which is shown in Fig. 9. We calculated the electrical field distribution and converted it to the SAR with equation (1). The calculation model used in this study is shown in Fig. 9. The sizes, shapes, and positions of the wireless

Fig. 7 Experimental setup.

camera and transmitting antenna were the same as in the measurement setup.

3. Results 3.1 SAR distributions on surface of human body The measured and calculated SAR distributions on the

Table 4 Calculation conditions.

Fig. 8 The distance between transmitting antenna and surface of the phantom.

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Paper » The Specific Absorption Rate Evaluation of 1.2 GHz Band Wireless Camera by a Thermographic Method

Fig. 9 Calculation model.

Fig. 11 SAR distributions of xz plane.

Fig. 10 SAR distributions of yz plane.

Fig. 12 Observation lines.

surface of the human model in view of the yz and xz planes are shown in Figs. 10 and 11. All distributions were normalized with the maximum value of the

compared the SAR values on the head surface at two

calculated result. These results show that the SAR

lines between the measurement and calculation

distributions between the measurement and calculation

quantitatively. As shown in Fig. 12, the lines pass

were fairly consistent, and the elevation of the SAR was

through the right occipital, which indicates a high SAR

observed at the right occipital of the phantom (around y

area. Figure 13 shows that the SAR values of the

= 0 mm and z = 350 mm in Fig. 10, around x = 120 mm

measured and calculated results on the head surface at

and z = 350 mm in Fig. 11). This region is close to the

the observation lines. These figures show that the SAR

transmitting antenna on the wireless camera.

values on both of the lines are nearly identical between

3.2 SAR profiles on the observation line

the measured and calculated results. However, the peak

To assess the validity of our measurement, we

value is different. We believe that the cause of this is the 273

ITE Trans. on MTA Vol. 4, No. 3 (2016)

Fig. 14 Differences of the observation point.

result. We consider that this is due to the position of the thermographic camera in the experimental setup of the SAR measurement. Upon measuring the whole upper body phantom from the just side, we defined the line which tied up both shoulders as the position of the just side of the upper body phantom, as shown in Fig. 14(a). Therefore, the camera looks from slightly behind the head. In addition, in the measurement by the thermographic camera, because of the short distance between the phantom and the camera, the position and the difference of the angle cause a gap of the observation side (Fig. 14(a)). In contrast, because we calculated the SAR distribution looking from the infinity of the x-axis

Fig. 13 SAR profiles on the observation lines.

direction of the calculation model, there is no difference difference in the SAR calculation method between the

of the calculation result by the difference of the

measurement and the calculation. In the calculation, all

observation position such as the measurement (Fig.

the

absorbed

14(b)). Therefore, it is considered that some differences

electromagnetic wave by the upper-half body phantom is

between the measurement result and the calculation

converted to the SAR. By contrast, in the measurement,

result occur.

electromagnetic

energy

of

the

the temperature rise value is converted to the SAR.

3.3 Discussion of SAR peak point

Even if the temperature rises locally, it is difficult to

We calculated the electric current distribution and

capture the sudden difference of the temperature

electric field distribution on the surface of the wireless

because of the thermal diffusion to the material of the

camera to discuss the cause of SAR elevation around the

circumference. Therefore, the peak SAR value of the

head of the operator. Figure 15 shows the observation

measurement is lower than the calculation value.

plane, which is the surface of the wireless camera model

Additionally, we believe that another cause is that the

on the side of the human body. The electric current

more locally SAR value was calculated by cell size being

distribution on the observation plane is shown in Fig.

subdivided in the calculation model near the

16, and the electric field (r.m.s) distribution on the same

transmitting antenna. As previously stated, we believe

plane is shown in Fig. 17(a). In Figs. 16 and 17, the

that the comparison of the rise range and the rise value

maximum levels of the electric current and electric field

of the SAR are more important in confirming the

are the value on the transmitting antenna and are 0 dB

validity of the measurement result than the comparison

points. Figure 16 shows an electric current that occurred

of the peak value. Therefore, the validity of this

via an EM wave emitted from the transmitting antenna

measured result was confirmed, and we found that it

near the right ear (around y = –30 mm and z = 350 mm

was possible to measure the SAR distribution and the

in Fig. 16). With this result, it is possible to say that the

SAR elevation areas using this measurement method.

wireless camera body was the ground plane for the

As shown in Fig. 13, there is a roughly 20 mm gap in

transmitting antenna. Comparing Figs. 16 and 17(a), the

the graph of the measurement result and the calculation

peak points of the electric field distribution and the 274

Paper » The Specific Absorption Rate Evaluation of 1.2 GHz Band Wireless Camera by a Thermographic Method

electric current distribution on the surface of the wireless camera model were almost at the same position. Moreover, Fig. 17(b) shows the electric field distribution on the same plane without the camera model. Comparing Figs. 17(a) and (b), both electric fields near the right ear (around y = –30 mm and z = 350 mm in Fig. 17(a)) were different whether or not the camera model was there. The electric field near the right ear of Fig. 17(a) is lower, about 6.4 dB, than Fig. 17(b). In this calculation model, the human body model and the

Fig. 15 Observation plane of calculation model.

wireless camera model were not connected, but the space between both models was very close to 1mm. Therefore, we ascertained that the SAR value near the right ear was elevated because of the influence of the electric field which occurred via the electric current on the surface of the camera model. However, the absolute value of this electric field was lower, by about 36.5 dB, than the electric field that occurred through the transmitting antenna was.

4. Conclusion In this paper, assuming that a 1.2 GHz band wireless Fig. 16 The electric current distribution on the wireless camera

camera was used, we evaluated the amount of

model.

electromagnetic exposure of operators when using the wireless camera. The measured and the calculated SAR distributions using the tissue-equivalent semi-hard phantom were nearly identical. Therefore, we found that it was possible to measure the SAR distribution and the SAR elevation areas using this measurement method. Regarding the results of the calculation of electric current and electric field distribution on the wireless camera model, a slight electric current was shown to be near the right ear, and the electric field occurred near this electric current. Therefore, it was ascertained that not only was the microwave energy emitted from the transmitting antenna but also the electric current on the surface of the wireless camera model was related to the SAR elevation.

References 1) "Portable OFDM digital transmission system for television program contribution," ARIB STD-B33, ver. 1.1 (Nov. 2005) 2) Frequency reorganization action plan by Ministry of Internal Affairs and Communications (Oct. 2013. revised ver) 3) Example of wireless camera: Sony HDC1500, http://broadcastrf. com/sony-hdc1500 4) Osamu Fujiwara, Takahiro Joukou, Jianqing Wang: "Dosimetry analysis and safety evaluation of realistic head models for portable telephones," IEICE Transactions on Communications, J83-B, 5, pp.720-725 (May 2000) Fig. 17 The electric field distributions.

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ITE Trans. on MTA Vol. 4, No. 3 (2016)

Koichi Ito

5) Naohiko Iai, Hidekazu Murata, Hidenori Ishida, Masanobu Iwamoto, Toshiharu Morizumi, Masayoshi Onishi, Toshihiro Ishida, Kazuyuki Saito, Junichi Ota, Masanori Hattori, Minoru Okada, Hiroyuki Takesue, Hitoshi Yanagisawa, Shinichi Nishizawa, Tomomi Hukazawa, Yukihiro Koike, Tsukuru Kai: "Research and development trend of the broadcast technology (broadcasting format / radio, light transmitting technology / broadcast work-site operations)," ITE Technical Report, 69, 7, pp.733-751 (2015) 6) Naoto Kogo and Tetsuomi Ikeda: "Analysis of radiation pattern and SAR considering operation of wireless camera in 1.2 and 2.3 GHz Bands," ITE Technical Report, 37, 23, pp.29-32 (June 2013) 7) V. Hombach, K. Meier, M. Burkhardt, E. Kühn and N. Kuster, "The dependence of EM energy absorption upon human head modeling at 900 MHz," IEEE Trans. Microwave Theo. Tech., 44, 10, pp.18651873 (Oct. 1996) 8) A.W. Guy, "Analysis of electromagnetic fields induced in biological tissues by the thermographic studies on equivalent phantom models," IEEE Trans. Microwave Theory Tech., 34, 6, pp.671-680 (June 1986) 9) Specifications and figure of wireless camera: Sony HDW-650, http://www.sony.jp/products/catalog/SPC_HDW-650.pdf 10) Koichi Ito and Katsumi Furuya and Yoshinobu Okano and Lira Hamada: "Development and the characteristics of a biological tissue-equivalent phantom for microwaves," IEICE, J81-B-II, 12, pp.1126-1135 (Dec. 1998) (in Japanese)

received the B.S. and M.S. degrees from Chiba University, Japan, and the D.E. degree from the Tokyo Institute of Technology, Japan. From 1976 to 1979, he was a Research Associate at the Tokyo Institute of Technology. From 1979 to 1989, he was a Research Associate at Chiba University. From 1989 to 1997, he was an Associate Professor at Chiba University, and from 1997 to 2016, he was a Professor at Chiba University. He is currently a Visiting Professor at the Center for Frontier Medical Engineering, Chiba University. From 2005 to 2009, he was Deputy Vice-President for Research, Chiba University. From 2009 to 2015, he served as Director of the Center for Frontier Medical Engineering, Chiba University. In 1989, 1994, and 1998, he visited the University of Rennes I, France, as an Invited Professor. He has been appointed as Adjunct Professor to the University of Indonesia since 2010. His main research interests include small antennas for mobile communications, research on evaluation of the interaction between electromagnetic fields and the human body by use of phantoms, microwave antennas for medical applications, and antenna systems for body-centric wireless communications. Professor Ito is a Life Fellow of the IEEE, Fellow of the IEICE and a member of the Japanese Society for Thermal Medicine. He served as Chair of the Technical Committee on Human Phantoms for Electromagnetics, IEICE, from 1998 to 2006, Chair of the Technical Committee on Antennas and Propagation, IEICE, from 2009 to 2011, Chair of the IEEE AP-S Japan Chapter from 2001 to 2002, General Chair of iWAT2008, an AdCom member for the IEEE AP-S from 2007 to 2009, an Associate Editor for the IEEE Transactions on AP from 2004 to 2010, a Distinguished Lecturer for the IEEE AP-S from 2007 to 2011, General Chair of ISAP2012 and a member of the Board of Directors, BEMS, from 2010 to 2013. He currently serves as a Councilor to the Asian Society of Hyperthermic Oncology. He has been elected as a delegate to the European Association on Antennas and Propagation (EurAAP) since 2012 and Chair of Commission K, Japan National Committee of URSI since 2015.

Tetsuya Yoshida received the B.E. and M.E. degrees in medical system engineering from Chiba University, Chiba, Japan, in 2014 and 2016, respectively. He currently works for Sony Energy Devices Corporation. His research interests include interaction between electromagnetic wave and human body. He received the ITE Best Presentation Award in 2014. Kazuyuki Saito

received the B.E., M.E. and D.E. degrees all in electronic engineering from Chiba University, Chiba, Japan, in 1996, 1998 and 2001, respectively. He is currently an Associate Professor with the Center for Frontier Medical Engineering, Chiba University. His main interest is in the area of medical applications of the microwaves including the microwave hyperthermia. He received the IEICE AP-S Freshman Award, the Award for Young Scientist of URSI General Assembly, the IEEE AP-S Japan Chapter Young Engineer Award, the Young Researchers' Award of IEICE, the International Symposium on Antennas and Propagation (ISAP) Paper Award, and Young Investigator Award of the Japanese Society for Thermal Medicine in 1997, 1999, 2000, 2004, 2005, and 2012 respectively. Dr. Saito is a member of IEICE, IEEE, JSTM (Japanese Society for Thermal Medicine) and JSES (Japan Society for Endoscopic Surgery).

Masaharu Takahashi received the B.E. degree in electrical engineering from Tohoku University, Miyagi, Japan, in 1989, and the M.E. and D.E. degrees in electrical engineering from the Tokyo Institute of Technology, Tokyo, Japan, in 1991 and 1994, respectively. From 1994 to 1996, he was a Research Associate, and from 1996 to 2000, an Assistant Professor with the Musashi Institute of Technology, Tokyo, Japan. From 2000 to 2004, he was an Associate Professor with the Tokyo University of Agriculture and Technology, Tokyo, Japan. He is currently an Associate Professor with the Center for Frontier Medical Engineering, Chiba University, Chiba, Japan. His main interests are electrically small antennas, planar array antennas, and EM compatibility. He was the recipient of the 1994 IEEE Antennas and Propagation Society (IEEE AP-S) Tokyo Chapter Young Engineer Award.

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