An Analysis Method of Effect of Linear Polarized

Electromagnetic Exposure from Mobile Phone to

Human Head with Various Incident Angles

Alfredo Bayu Satriya, Eko Setijadi Electrical Engineering Department

Institut Teknologi Sepuluh Nopember Surabaya

alfredo 12@mhs.ee.its .ac.id, ekoset@ee.its .ac.id

Abstract-This research is focused on the effect of various incident angles of the linear polarized electromagnetic exposure from mobile phone to the value of Specific Absorption Rate (SAR) in human head which is a safety standardization for mobile phone user. Method of Moment is chosen to calculate the electric field in the human head because it is the most suitable method. The incident electromagnetic is assumed as a plane wave with a frequency at 900 MHz and power density of antenna for 4.5 W/m2 • The integral used for calculation is Tensor Integral Equation (TIE). Human head is modelled as a homogeneous spheroid and is divided into cubical cells. The incident angles which are examined are 0° (horizontal polarization), 15° , 30° , 45° , 60° , 75° and 90° (vertical polarization). The results show that the incident angle does not affect the average SAR in the human head, but it affects the local and maximum SAR of the cells in human head. The difference of seven angles maximum SAR does not vary in the form of linear but in the form of damped sinusoidal. The angle which produces the least maximum SAR is 0° namely 0.1521 W/kg whereas the angle which produces the highest maximum SAR is produced by angle 15°namely 0.1748 W /kg. The highest SAR is obtained around the side of the head near the electromagnetic exposure source. None of the SAR produced by the variation of incident angle is above the limit of safety standard.

Keywords--electric field; electromagnetic incident angle; homo­geneous spheroid; Method of Moment; SAR

I . INTRODUCTION

Mobile phone is one of the telecommunication tools which is always close to human, especially when human makes a phone call, it is very close to the human brain. At that moment one does not know that electromagnetic wave which is produced by the phone could make a bad effect to one's brain. In Sweden a research shows that there is a headache because of mobile phone [ 1 ] , another research concludes that 70% of mobile phone users also get headache [2] . The use of mobile phone also can make a sleeping disorder [3 ] , and there are still some researches which show the dangers of mobile phone use.

What makes the danger of electromagnetic exposure is the heat which is produced by the electromagnetic exposure. The heat in ones head and brain could lead to hyperthermia. Hyperthermia is a condition when the temperature in the body increasing than its normal thermoregulatory because the body

absorbs heat more than it can dissipate. Hyperthermia could lead into brain damage or even dead. The limit of increasing temperature in the human brain is 4 .5°C [4] . Because of this danger of electromagnetic exposure, there are many safety standardizations made, for the example [5] and [6] . Those standardizations are made with SAR (Specific Absorption Rate) parameter. SAR is a parameter which shows the power of an electromagnetic wave which is absorbed by the human tissue.

Several works have investigated the effect of electromag­netic exposure to the human head [7-2 1 ] , mostly the results show that the value of SAR is less than safety standard, but in some cases it is found that SAR is higher, namely the electromagnetic exposure of an omnidirectional antenna [9] , a circumstance where the human is about one meter radius from base station [ 1 8] , and human uses mobile phone in a closed elevator [ 1 9] . Based on these works we need to pay more attention on the danger of electromagnetic exposure. These works [7-2 1 ] have investigated several parameters which can affect SAR, they are mobile phone's transmit power, antenna, antenna radiation pattern, frequency, polarization, the gap bet­ween mobile phone and the human head, and the human head geometry. Each of these parameters has been computed with various method, including vertical and horizontal polarization. These two polarizations resulting different value of SAR in the human head [ 16] .

Although there has been a paper about the effect of differ­ent polarization, but there has not a work which investigates the effect of electromagnetic incident angle to SAR in the human head. It becomes important because hand phone user usually holds hand phone in various angles while making a call and it makes electromagnetic also propagates in various incident angles . In addition, there is no standard for incident angle of hand phone antenna to produce the least SAR in the human head.

Before getting the value of SAR in the human head, it is necessary to know how big the electric field which reaches it. In order to get the electric field, the electromag­netic computational is used. Electromagnetic computation is a Maxwell Equation solution using numerical approximation. Numerical approximation is used because the solution of an actual problem is very complex and needs several assumptions

978-1-4799-0425-9/13/$31.00 ©2013 IEEE

for problem simplification [22] . There are various numerical methods to solve the electromagnetic problem, namely FEM, MoM, FDTD, FVTD, DWP, DWT, TLM, Monte Carlo etc . In several previous works [7-2 1 ] , MoM and FDTD are two methods mostly used in counting electric field in the human body and head. MoM is a frequency based method and it has a better accuracy than FDTD [ 1 8] .

I I . METHODS AND MODELS

The procedure of MOM requires subdivision of solution region, source equation, and integral equation

A. Human Head Model

Human head is modelled as a homogeneous spheroid like paper [ 19] and is subdivided as the procedure of MoM. The human head is modelled as a spheroid to simplify many complex details and arbitrary surface in human head, spheroid has the most same geometry with human head among other three dimensional geometry. In Table I we can see that human head consists of several tissues which have different consti­tutive parameters (permitivities and conductivities) but in this paper the human head structure is assumed homogeneous for simplification and because the thickness of each tissue is very thin. This model has a radius of 9 cm, permitivity Er = 45 .8 , conductivity (J = 0.77 and mass density p = 1030 kg/m3 . This model is subdivided into cubical cells/ blocks . The cubical cell size based on [23] must be � wavelength or less to get a good accuracy of the computation. The maximum size of cubical cell is :

f = 900 MHz

c = 3 X 108 m/s

c 3 x 108 A = 7 = 9 X 108 = 0 . 33 m

A 0 . 33 Smax = "4 = 4 = 8 . 33 cm

With maximum cell size 9 cm, the possible modelling of the human head consists of 32 cubical cells (N = 32) of which the size is 4 .5 cm, but in this paper, we select a model which consists of 1 60 cells 1 60 as presented in Fig. 1 .

B. Electric Field Polarization Equation

Nowadays, there are three kinds of antenna which is often used in mobile phone namely helix antenna, planar inverted­F antenna (PIFA) and Planar Monopole Antenna (PMA) [25] . Both PIFA and PMA could produce electromagnetic wave with linear polarization which is used for mobile communication. In order not to limit the analysis for certain antenna design, this paper simulate the source electromagnetic without antenna design but by formulating the electromagnetic pattern. The incident electromagnetic wave is assumed as uniform plane wave and its electric field is assumed linear polarized. When the incident electric field propagates along z-axes towards z­positive, the electric field is defined as:

E = (xax + yayejO )e-jkz ( 1 )

" (a)

• . . . 1-+-++-+--+-++--1 . . � z

• (c)

" (b)

(d) Fig. l. Problem Modelling: (a)Human head and mobile phone,(b) Antenna feeder point, Homogeneous spheroid consists of 1 60 cells: (c) 2-D (d) 3-D

TABLE I. PROPERTIES OF HUMAN HEAD AT 900 MHz

Tissues Radius (cm) tr a p (kg/m3 ) Skin 9.00 39.5 0.69 1 0 1 0 Fat 8.90 10.0 0 . 17 920

Bone 8.76 17 .4 0 . 1 9 1 8 10

Brain 8.35 44.1 0.89 1 040

Suppose E has an incident angle (8) like in Fig .2, and ax = a cos 8 and ay = a sin 8 , equation ( 1 ) becomes :

E = (xax + yayejO ) e-jkz = (xa cos 8 + ya sin 8 )e-jkz

E = (x cos 8 + y sin 8)ae-jkz (2)

where 0 is phase difference (0 = 0 and 7r for linear polari­zation) . The value of a as an incident electric field can be calculated by :

S = E x H (3)

(4)

so a = Eo = 4 1 . 1 9 Vm- 1

C. Tensor Integral Equation

Tensor Integral Equation (TIE) is an equation which is proposed by Livesay and Chen to count the electric field in the human body from the exposure of the electromagnetic wave. Suppose an electromagnetic wave with incident electric field (Ei (r» propagates into an arbitrary biological body, there will be a part of the electric field which will propagate inside the biological body (E(r» and there is another part which will be scattered (ES (r» , the relation between these three fields is defined as :

(5)

y

�----���-----.x Gy

Fig. 2. Linear polarized electromagnetic wave towards z-positive with incident angle

the current density in arbitrary biological body is defined as:

Jeq (r) = [a (r) + jW (E (r) - Eo ) ]E(r) = T(r)E(r) (6)

with the scattered electric field from arbitrary electric field can be expressed as :

where

ES (r) = PV Iv Jeq (r' ) . G(r, r' ) dV' Jeq (r) - --

3jWEo (7)

G(r, r' ) = -jw/.lo [l + �; ] 1j; (r, r' ) (8) o

1j; (r r' ) = exp(-jko l r - r' l ) (9) ,

47f l r - r' l ko = W (/.lO EO ) 1/2 ( 1 0)

After some derivations [23 ] , we get the Tensor Integral Equa­tion which is defined as :

Ei (r) = [ 1 + T (r) ]E(r) - PV r T(r' )E(r' ) · G(r, r' ) dV' 3JWEo Jv ( 1 1 )

D. Method of Moment

Method of Moment (MOM) is a numerical technique which is designed to solve an integral equation. MOM has become popular since the research made by Richmond in 1 965 and Harrington in 1 967. Generally, MOM has four steps, namely : l . Derivation of the appropriate integral equation, 2. Discretization of the integral equation into matrix equation, 3. Evaluation of matrix elements, 4. Obtaining the desired parameters . [22]

The derivation, discretizaton and evaluation of the electric field in arbitrary biological body in generally has done in [23] . The discretization of (7) is :

( 1 2)

where

( 1 3)

N = number of cell

Green function matrix element evaluation of (7) is defined as:

for m i= n

Gmn = -jw/.lo k06.vnT (rn ) exp( -jO:mn ) X p X q 41ra�n

x [ (O:�n - 1 - jO:mn ) 6pq + cos e:;;,n

cos e;;qn (3 - O:�n + 3jO:mn ) ]

where

m, n = 1 , 2 , . . . N p, q = 1 , 2 , 3

for m = n

where

E. SAR

( 14)

( 15)

( 1 6)

( 17)

( 1 9)

Specific Absorption Rate (SAR) is a parameter that des­cribes the amount of electromagnetic power absorbed by the human tissue while being exposured by elctromagnetic waves. SAR calculation is described as:

where

SAR = a(r) I E I 2 2p(r)

l E I = J E2 + E2 + E2 x y z

(20)

(2 1 )

There are many organizations including FCC, IEC, CENE­LEC, ANSI, ICNIRP, and IEEE which have specified their safety standard for maximum SAR in the human tissue, but they all have the same limit, namely l .6 W/kg in each gram of human tissue and 2 W /kg in each ten grams of human tissue.

III . RESULTS

The results show that incident angle of electromagnetic does not affect the average SAR of the human head model. With the change of range 1 5 ° , from the degree of 0 till 90, the average SAR is remain 0.02 1 6 W/kg. This value of average SAR is much less than the maximum limit of the safety standard. While the average SAR shows steady value, the maximum SAR of the human head does not. The obtained maximum SAR of simulation in the human head is never same as the incident angle varies from OOto 90° . For getting the simultaneous effect of the incident angle, the interpolation of the maximum SAR values is presented in Fig. 3 and maximum SAR values of each angle are presented in Table II . The change

0 . 1 8 r--r--r--,---,----,----,---.------.--,-----,

0 . 1 75

en 0 . 1 7

i � 0 . 1 65 E E · x � 0 . 1 6

0 . 1 55

0 . 1 5 '--'--'---'---'---"---'----'----'---'---' - 1 0 o 1 0 20 30 40 50 60 70 80 90

E l e ct r o m a g n e t i c I n c i d e nt An g l e

Fig. 3 . Interpolation result of incident angle effect to the maximum SAR in human head

TABLE II. MAXIMUM ANGLE OF VARIOUS INCIDENT ANGLE

Incident Angle Maximum SAR (W/kg)

0 0 . 1 521

1 5 0 . 1748

30 0 . 1 59 1

45 0 . 1726

60 0 . 1654

75 0 . 1685

90 0 . 1705

of the maximum SAR value is not linear, the interpolation of SAR produces a graphic which is like a damped sinusoidal, where the angle OO or horizontal polarization produces the least maximum SAR that is 0. 1 52 1 W/kg and the angle 15 produces the highest maximum SAR that i s 0. 1748 W /kg. The different maximum SAR value between these seven angles can be affected from the local SAR of each cell. The distribution of electric field in each cell changes when the incident angle is changed. This can be seen in Fig. 4. While being exposured from electromagnetic, there are parts of the human head which get more electric field than the others, this different distribution of the electric field in each cell are squared in order to get the SAR distribution, so the differences in SAR of cells are quite significant. This significant differences can be seen from the visualization of SAR in the center of human head which are refer to Fig . 5 and presented in Fig.6 and 7 (only two angles are presented) . There are parts of the head which have a relatively high SAR, about 0. 17 W Ikg (shown in white color) but there are also parts which are not affected by the electromagnetic exposure since there are no SAR in that part (shown in black color) . However, these seven values of maximum SAR are still below the safety standard.

IV. C ONCLUSION

The incident angle does not affect the average SAR in the human head, but it affects the local SAR of the cells. The incident angle affects the maximum SAR in human head since the SAR distribution also changes. The maximum SAR does not vary in the form of linear but in the form of damped

20

� 1 5 � u Q) u:::

u ·5 1 0 u (]) [jJ

5

-- a n g l e D -- a n g l e 30 -- a n g I 8 60 -- a n g l e 90

°0'--�--L-�--8L--�

1 0�-L---'--�---'

1 8

Di stance from The Antenna i n I-axi s (cm)

Fig. 4. Electric field in human head due to electromagnetic exposure with various incident angle

... r

(a) (b) . . . · ... z . .

r • x

(c) . . . . .

•

Fig. 5 . Cells in the center of human head model, (a) upper view, (b) front view, (c) side view

sinusoidal . The angle which produces the least maximum SAR is 0° or the polarization whereas the angle which produces the highest maximum SAR is 1 5° . The distribution SAR in the human head is not prevalent, the highest SAR is obtained around the side of the head near the electromagnetic exposure. None of the SAR produced by the variation of incident angle is above the limit of safety standard.

REFERENCES

[1] G. Oftedal, J. Wilen, M. Sandstrm, and K. H. Mild, "Symptoms expe­rienced in connection with mobile phone use," Occupational Med. , vol. 50, no. 1 , pp. 19 , 2000.

[2] Szyjkowska A. , Bortkiewicz A. , Szymczak w., Makowiec-Dabrowska T. , "Subjective symptoms related to mobile phone use-a pilot study",

[ 1 0] G. M. J. Van Leeuwen, J. J. W. Lagendijk, B. 1. A. M. Van Leersum, A. P. M. Zwamborn, S. N. Hornsleth, and A. N. T. Kotte, "Calculation of change in brain temperatures due to exposure to a mobile phone",

(a) Phys. Med. BioI. , vol. 44, pp. 23672379, 1999.

0,144 0.108 0.072 0.036 0.18

Fig. 6 . SAR visualization of 0° incident angle in the center of human head from (a) upper view, (b) front view, (c) side view

(a)

0.144 0.108 0.072 0.036 0 .18

W/kg

o Fig. 7. SAR visualization of 90° incident angle in the center of human head from (a) upper view, (b) front view, (c) side view

PubMed.gov, US National Library of Medicine National Institutes of Health.2005 .

[3] S . Thome, A. Hrenstam and M. Hagberg, "Mobile phone use and stress, sleep disturbances, and symptoms of depression among young adults - a prospective cohort study", BMC Public Health vol. 1 1 , 3 1 January 201 1 .

[4] A . C . Guyton and J . E . Hall, Textbook of Medical Physiology. Philadel­phia, PA: Saunders, 1996.

[5] "Evaluating Compliance with FCC Guidelines for Human Exposure to Radio Frequency Electromagnetic Fields," Federal Communications Commission, Washington, DC, OET Bull. 65, 1997.

[6] "Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300 Ghz)," Health Phys. , vol. 74, pp. 494522, 1998.

[7] D. M. Sullivan, D. T. Borup, O. P. Gandhi, "Use of the finite-difference time-domain method in calculating EM absorption in human tissues," IEEE Transactions On Biomedical Engineering, Vol. 34, No. 2, February 1987.

[8] O. P. Gandhi and J.-Y Chen, "Electromagnetic absorption in the human head from experimental 6-GHz handheld transceivers," IEEE Transac­tions On Electromagnetic Compatibility, Vol. 37, No. 4, November 1995.

[9] K W. Kim and Y R-Samii, "Handset antennas and humans at Ka­band: The Importance of Directional Antennas," IEEE Transactions On

Antennas And Propagation, Vol. 46, No. 6, June 1998

[ I I ] J.Wang and O. Fujiwara, "FDTD Computation of temperature rise in the human head for portable telephones," IEEE Trans. Microwave Theory Tech. , vol. 47, pp. 15281 534, Aug. 1 999.

[ 1 2] P. Bernardi, M. Cavagnaro, S . Pisa, and E. Piuzzi, "Specific Absorption Rate and temperature increases in the head of a cellular-phone user," IEEE Trans. Microwave Theory Tech. , vol. 48, pp. 1 1 1 8 1 126, July 2000 .

[ 1 3] X. K. Kang, L. W. Li, M. S. Leong, and P. S. Kooi, "A Method of Moments study of SAR inside spheroidal human head and current distribution along handset wire antennas," J. Of Electromagnetic Waves and Appl. , vol. 1 5 , No. 1 , 6175 , 200 1 .

[ 14] C.-M. Chen, J.-H. Tarng and J-M. Huang, "Electromagnetic absorption in human brains by using fast wavelet transform methods," Proceedings

of APMC200I , Taipei, Taiwan, R.O.C. [ I S] O. P. Gandhi, Q.-X. Li, and G. Kang, "Temperature rise for the human

head for cellular telephones and for peak SARs prescribed in safety guidelines," IEEE Trans. Microwave Theory Tech. , vol. 49, pp. 160716 13 , Sept. 200 1 .

[ 1 6] A . Hirata, M . Morita, and T. Shiozawa, "Temperature increase in the human head due to a dipole antenna at microwave frequencies," IEEE Trans. Electromagn. Compat. , vol. 45, pp. 1 09 1 17 , Feb. 2003 .

[ 17] A. Hirata and T. Shiozawa, "Correlation of maximum temperature increase and peak sar in the human head due to handset antennas," IEEE Transactions On Microwave Theory And Techniques, Vol. 5 1 , No . 7 , July 2003 .

[ 1 8] Md. F. Ali, S. Mukherjee, S. Ray, "SAR analysis in human head model exposed to mobile base-station antenna for GSM-900 band," Loughborough Antennas and Propagation Conference. 2009 .

[ 19] T. A. J. Mary, S. J. Priyadarshini, C. S. Ravichandran, R. George, "Analysis of SAR on human head modeling in metallic enclosures," 20 10 .

[20] S . Bri, S . Kassimi, M. Habibi, Ahmed Mamouni, "Specific Absorption Rate (SAR) distribution in the human head at Global System Mobil (GSM) frequencies," European Journal of Scientific Research ISSN 1450-21 6X Vol.49 No.4 (201 1 ) , pp. 590-600.

[2 1 ] L. Zhao and K-L. Wu, "A hybrid NFMlMoM full-wave analysis of layered prolate head model exposed to handset antenna," Progress In Electromagnetics Research, Vol. 123 , 205-225, 2012 .

[22] M. N. O. Sadiku, Numerical Technique in Electromagnetics with Matlab 3rd ed. Texas, USA : CRC Press, 2009.

[23] Livesay, D. E., and Chen K-M., "Electromagnetic field induced inside arbitrarily shaped biological bodies," IEEE Transactions On Microwave Theory And Techniques Vol. MTT-22, No. 12, 1974.

[24] Ulaby, F. T. , Fundamentals of Applied Electromagnetics Media Ed., USA : Prentice-Hall, Inc. , 200 1 .

[25] Rowell, C. and Lam, E. Y, "Mobile Antenna Design," IEEE Antennas and Propagation Magazine Vol. 54, No. 4, August 2012.