Evaluation of Different Types of Human Head Shape and Ear effects for Mobile Phone Exposure on EM

Absorption

Mohammad Rashed Iqbal Faruque1, 2, Mohammad Tariqul Islam1

1Institute of Space Science (ANGKASA), 2Dept. of Electrical, Electronic and Systems Engineering,

Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM, Bangi,

Selangor, Malaysia. E-mail: rashedgen@yahoo.com

Norbahiah Misran1, 2, Hafizah Zainool Abidin1, 2, Nurul Hafizah Mohd Hanafi1, 2

1Institute of Space Science (ANGKASA), 2Dept. of Electrical, Electronic and Systems Engineering,

Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM, Bangi,

Selangor, Malaysia.

Abstract— In this paper, the effects of electromagnetic (EM) absorption on human ear and head shapes has been investigated. Different types of human head models such as spherical, cubical, and realistic human head models were used in this paper. Also, different types of ear shapes were considered with the cubical, spherical, and head models whereas a low-loss thin ear, or a lossy realistically shaped ear, was used with the realistic head models. Commercially available software CST Microwave Studio based on the finite-difference time-domain (FDTD) method was used for the numerical simulations. The SAR circulation depends appreciably on the size and shape of the ear, despite of the head model. The head shapes are fewer affected than the ear shape. Comparison studies of SAR bring in the realistic human head for homogeneous and inhomogeneous cases are also investigated. Results, it can be seen that the local maximum SAR motivated in the homogeneous human head model is larger than that induced in the inhomogeneous human head model. Also it is demonstrated that, the maximum local SARs in the head tissue with the low-loss thin ear are larger than those with the lossy realistically shaped ear.

Keywords- ear sizes, finite difference time domain (FDTD) method, head model, mobile phone, specific absorption rate (SAR).

I. INTRODUCTION

The exposure of the human head to the near field of a mobile phone has been evaluated by measuring the SAR in a human head phantom, or by calculating it using a human-head numerical model. With the speedy and ever more extensive use of mobile phones, public anxiety regarding the possible health hazards has been growing, that brings an increased obligation on electromagnetic (EM) dosimetry for mobile phones. The basic parameter in the EM dosimetry is defined in terms of the SAR, or the absorbed power in unit mass of tissue [1]. The SAR is generally evaluated using either phantom measurement or computer simulation. The finite-difference time-domain (FDTD) method is currently the most widely accepted means for the SAR computations [2]. In Europe the basic limit of SAR set for the general public is 2 W/kg

averaged over a volume equivalent to 10 gm and a period of 6 min. [3]. The ANSI/IEEE standard [4] defines a stricter limit for an uncontrolled environment of 1.6 W/kg averaged over a volume of 1 gm and a period of 30 min.

Earlier studies have exposed that the SAR circulation for a head bared to the near field of a cellular phone is affected by various factors [4-7]. In particular, a number of previous studies have analyzed the effects of the shape of the ear and the overall shape of the head [5-8].

To investigate the possible range of variations of the induced field strengths in the various tissues requires an widespread effort, since the local field strengths strongly depend on a large number of parameters, such as: operational frequency and antenna input power; position of the device with respect to the head; design of the device; the outer shape of the head; the distribution of the different tissues within the head, and the electric properties of these tissues.

The electric parameters of a human body vary with levels of physical and metabolic activity, health, and age. The variations in all these properties lead to a stretch in the analyzed absorption distribution. Currently the most often used system for testing handheld cellular telecommunications equipment is the measurement setup using a homogeneous anatomically-shaped phantom filled with a liquid simulating brain tissue [9-14]. The rationale behind this approach comes from the energy absorption mechanism in the close near-field of antennas [15-16], which concludes that the most determining parameters for volume-averaged values are the time-averaged antenna input power, operational frequency, design of the device, and its position with respect to the head, and to a much lesser extent, on the physical properties of the head.

In this paper the authors focus on the effects of the factors of ear shape, and head properties, such as size, shape, and electrical properties of tissues in order to calculate the SAR induced in human head models exposed to a cellular

2011 IEEE 10th Malaysia International Conference on Communications (MICC) 2nd – 5th October 2011 | Sutera Harbour Resort, Kota Kinabalu, Sabah, Malaysia

978-1-4577-0978-4/11/$26.00 ©2011 IEEE 39

phone. To find the effects of ear size and shape, head size or shape on electromagnetic absorption characteristics, the realistic head model, spherical head model, and a cubical head model are scaled and then SAR distributions for these models, when exposed to a cellular telephone model have been investigating using the FDTD method.

II. METHOD AND TECHNIQUE

Commercially available software CST Microwave Studio based on the finite-difference time-domain (FDTD) method was used for the numerical simulations. In electrodynamics by far the most flexible way to investigate effects which depends on multiple parameters is with computer simulations, since geometry and domain properties can be easily varied. Many numerical techniques exist for the analysis of complex near-field scattering problems, whereby the FDTD technique [2] has proven to be the most efficient method for studying absorption in strongly inhomogeneous bodies. FDTD is currently used by various groups to study the absorption in the human head from mobile phones [6-9] and to test novel antenna designs [10-11].

The results of local maximum SAR induced in the two homogeneous human head models will be presented. Then the FDTD method is again used to calculate the local maximum SAR induced in the realistic human-head model, which is simulated as a homogeneous model with six electrical property sets, counting the head tissue, simulating the bone tissue, the skin tissue, the eye tissue, the blood tissue, the muscle tissue, the brain tissue, and simulated as an inhomogeneous model with six tissues, at 1800 MHz. A parallel Compared study of the SAR induced in the realistic human head model for homogeneous and inhomogeneous models is investigated and also different types of ear shapes such as thick, thin and flat shapes is considered in this paper.

III. HEAD MODELS

The authors used three types of head models, such as Cubical, Spherical and Realistic head model. Accurate phantoms of homogeneous human heads can be generated on the basis of magnetic resonance imaging (MRI). The translation of the three-dimensional (3-D) data sets of relaxation times into the tissue distribution is a difficult task and generally requires a person trained either in medicine or biology, and who is able to distinguish both transitional and marginal regions. MRI produced in different laboratories, by different scientists and from different test subjects predictably contains differing discretizations.

In these studies different shapes, different dielectric constant, different conductivity, and different mass density of homogeneous phantoms in different tissues based on MRI data, sets of three different adults are used, and they are also simulated to inhomogeneous phantoms and their results are compared.

The first phantom in realistic head model was taken with the highest resolution. Its voxel size is 1 mm in all three Cartesian dimensions. Realistic head model has the largest volume. The brain region was segmented very carefully. For the entire head, 13 tissue types were simulated. However, the lower part of the head was assigned to only one tissue type.

The second head phantom is considered as cubical, and has nearly the same voxel size. It was developed for the training of medical students and distinguishes among 120 tissue types. For the EM analysis this large number is needed to be reduced to 13 different tissues for which electric parameters are available [3].

The third head phantom is considered as spherical that has taken and has a voxel size of about 12 mm3. The discretization is relatively crude. In the original MRI model the skin was not identified. In the computer model the skin was added as an outer layer with a thickness of one voxel. The brain region of head phantom Spherical is homogeneous and assigned only one tissue type comparing the data of different publications reveals a spread in the values given for the electric parameters of different types of tissue. Table-1 shows the values of different types of head phantom models used for this study.

TABLE 1 THREE DIFFERENT HEAD MODELS FOR SIMULATIONS

IV. CELLULAR PHONE MODELS

A standard PIFA antenna was used as the simplest type of cellular phone. The PIFA has a λ/2 wavelength casing balun and a small return loss (-20 dB) under the flat phantom, the details of which are described in the standards documents [11-14]. The physical cellular phone models have two typical shapes as shown in Fig. 1(a) and 1(b). They are constructed of metal plates, a λ/2 wavelength monopole antenna element, and a semi-rigid coaxial cable with subminiature version (SMA) connectors, considered in this paper. The SMA connector at the bottom is connected to a coaxial cable that leads to a signal generator. The SMA connector is connected to an additional SMA connector. The top of the additional SMA connector is aligned with the top plate of the phone box, and the jack of the additional SMA connector is plugged into the antenna wire element.

Realistic head model

Spherical head model

Cubical head model

Head Volume 4.44 dm3 3.35 dm3 4.26 dm3

Tissues 13 120 12 Computational 175 × 230 159 × 208 159 ×206

Space ×226 ×201 ×249

Voxel Size ( 1 mm )3 ( 1.075

mm)3 (1.875 mm)2

× 3 mm

40

V. RESULTS & DISCUSSION

The results of maximum local SARs induced in three homogeneous human-head models versus the distance between the cellular phone and the human-head model are shown in Fig. 2. From Fig. 2, it is clear that the spherical head model has the maximum value of the local maximum SAR, while the realistic human head model has the minimum value

(a)

SMA connector

Feeding point

4.8 mm 13.5mm

46.5mm

Speaker

24.2mm

18mm

24.2mm

82.5mm

Φ=2mm

(b)

82.5mm Φ=2mm 7.5mm

4.8 mm

120mm

24.2mm

Speaker

35mm 15.5mm

SMA connector

Feeding point

13.5mm

17.5mm

15.5mm

28.5mm

38.5mm

53.5mm

Fig. 1 Mobile phone models with (a) thick shape and (b) thin shape Fig. 3 compared the maximum local SAR induced in

the real human head model for homogeneous and inhomogeneous cases at 1800 MHz

1800 MHz

0

0 .0 1

0 .0 2

0 .0 3

0 .0 4

0 .0 5

0 .0 6

5 10 15 2 0Distance between the

phone model and the head model (mm)

Loc

al M

axim

um S

AR

(W/k

g)

Ho m o g e ne o usm o de l (B o ne )Ho m o g e ne o usm o de l (S kin)Ho m o g e ne o usm o de l(B lo o d)Ho m o g e ne o usm o de l (Eye )Ho m o g e ne o usm o de l (M us c le )Ho m o g e ne o usm o de l (B ra in)Inho m o g e ne o usm o de l

1800 MHz

0

0 .0 1

0 .0 2

0 .0 3

0 .0 4

0 .0 5

0 .0 6

5 10 15 2 0

Dis tanc e be twe e n the pho ne mo de l and the he ad mo de l

(mm)

C ub ic a lhe a dmo d e l

S p e ric a lhe a dmo d e l

R e a l is t iche a dmo d e l

Fig. 2 SAR induced in four homogeneous phantom-muscles head models with relative dielectric constant εr = 53.5, conductivity σ = 1.34 S/m, and mass density ρ = 1.04 g/cm3 at 1800 MHz

Loc

al M

axim

um S

AR

[W/K

g]

41

of the local maximum SAR. The results of the local maximum SAR motivated in the cubical and spherical head models are closer and they are between those obtained by the realistic head models. The average difference of the local maximum SAR induced in these three head models is approximately within 9% at 1800 MHz respectively.

This observation emphasizes that the shape of the human head plays a minor role in calculating the SAR induced in the human-head models. These yield that the conclusion made in ref [4] that the maximum local SAR is scarcely affected by the shape of the human head exposed to a cellular phone. In the following study, the realistic human-head model is simulated six times as a homogeneous model with six electrical property sets, such as the bone tissues, the skin tissues, the blood tissues, the eye tissues, the muscle, and brain tissues, and one time as an inhomogeneous model with six tissues taken into account to calculate the SAR results at 1800 MHz respectively.

A parallel compared results of maximum local SAR induced in the realistic human-head model for homogeneous and inhomogeneous cases at 1800 MHz are shown in Fig 3. It is found that local maximum SAR induced in homogeneous models has larger values than those induced in inhomogeneous models. It should be noted that the electrical property of a human head significantly affect the result of the SAR induced in homogeneous or inhomogeneous head models. Today, correct calculation or measurement of SAR distribution in a human head while using a cellular phone has become an important issue.

The SARs for the spherical head model with different ear shapes was considered. Two electromagnetic sources, namely the PIFA antenna and the thick physical cellular phone model were used in our simulation.

Fig. 4 shows the SAR distributions measured along the axis immediately above the antenna and on the evaluation plane, which is the measurable axis (plane) closest to the internal bottom without the boundary effect [14] For the PIFA antenna, the flat-ear model has a broad single SAR peak around the antenna feeding point, whereas the other ear models have sharp twin peaks around the sharp corners of the ear well. There is no significant difference in the ear effects between the PIFA [Fig. 4(a)] and the phone model [Fig. 4(b)].

There is asymmetry in the measured SAR distribution for the flat-ear model with the dipole antenna [Fig. 4(a)], possibly owing to an error in positioning the dipole antenna and the imperfect symmetry of the dipole elements. There is also asymmetry in the calculated SAR distribution, possibly owing to a half-cell shift of the feed point from the center of the head model.

In [11, 13], point out that the SAR distribution depends on the shape of the ear. Ghandhi and Kang [15] numerically explored those possible underestimations of the maximum local SARs might result from the use of such an ear and eyes whereas also Burkhardt and Kuster [12] reported that using the low-loss pressed (thin) ear results in a higher maximum SAR. Kand et al [10], numerically and experimentally studied that the effects of the ear, and reported that the difference in the peak SAR between low-loss and lossy ears was negligible, although they used only simple head & source model, i. e., cubical head models and dipole antenna. In our simulation results seems to same as Kanda’s result but different things they used simple head and source models, i. e., cubical models and dipole antenna but in our simulations also used simple head and spherical head models and PIFA antenna. The magnetic field

Fig. 4 Calculated SAR distributions along center axis of reference plane 4.5 mm above the bottom of internal surface of spherical head model: (a) PIFA antenna. (b) Cellular-phone.

(a)

0

2

4

6

8

10

12

-40 -30 -20 -10 0 10 20 30 40

Assessment axis (mm)

Loca

l SA

R [W

/kg]

Flat Thick Thin

(b)

0

1

2

34

5

6

7

-40 -30 -20 -10 0 10 20 30 40

Assessment axis (mm)

Loca

l SA

R [

W/k

g]

Flat Thick Thin

42

dominates the SAR coupling method in the near field of the cellular phone. The current flow along the boundary between liquid and phantom shell increases around the corners of the ear where the direction of the current speedily changes. The particulars of the physical background of this effect have not been clarified. Therefore, further investigation on this effect, as well as the impact of this effect on anatomical human heads, is necessary. To maximum SAR s values varied different types of ear models result in small differences for the cubical head models with a dipole, as reported by Kanda et al. [10]. This might be the result of different phone–head separation distances: the ear width, the ear thickness also ear sizes.

VI. CONCLUSION

The effects of head shape and ear size on SAR using the spherical and cubical head model with various ear shapes is numerically investigated. Results show that the SAR distribution depends on the ear shape. The effect of the ear shape for the realistic head models with a low-loss thin ear, or a lossy realistically shaped ear, is similar to that for the cubical or spherical head model. It is also apparent that the maximum local SARs in the head tissue with the low-loss thin ear are larger than those with the lossy realistically shaped ear. The effect of head size is at the low end of the SAR range for effects of ear shape and phone position.

REFERENCES

[1] “Radiofrequency Electromagnetic Fields; Properties, Quantities and Units, Biophysical Interaction, and Measurements,” National Council on Radiation Protection and Measurements, Bethesda, MD, 67 (1981). [2] M. T. Islam, M. R. I. Faruque, and N. Misran, “Design analysis of ferrite sheet attachment for SAR reduction in human head,” Progress In Electromagnetics Research, PIER 98, pp. 191-205, 2009. [3] M. R. I. Faruque, M. T. Islam, and N. Misran, “Effect of human head shapes for mobile phone exposure on electromagnetic absorption,” Informacije MIDEM, Vol. 40, no. 3, pp. 232-237, 2010. [4] Q. Balzano, O. Garay, and T. Manning, "Electromagnetic energy exposure of the telephones", IEEE Trans. Veh. Tech., vol. 44, pp.390 – 403, 1995 [5] O. Gandhi and G. Lazzi, “The role of a lossy ear in alteri- ng the SAR distributions due to wireless telephones,” Proc.XXVI General Assembly of URSI, 13-21, Vol. 634, Toronto, Canada, Aug. 1999. [6] M. R. I. Faruque, M. T. Islam, and N. Misran, “Analysis of electromagnetic absorption in mobile phones using metamaterials,” Electromagnetics, Vol. 31, Issue 3, pp. 215 – 232, 2011. [7] M. R. I. Faruque, M. T. Islam, N. Misran, “Electromagne- tic (EM) Absorption Reduction in a Muscle Cube with Metamaterial Attachment,” Medical Engineering & Physics (Elsevier), vol. 33, Issue. 5, pp. 646-652, June 2011.

[8] S. Watanabe, H. Wakayanagi, T. Hamada, M. Taki, Y. Yamanaka, and H. Shirai,: in Proc. XXVI General Assembly of URSI, Toronto, Canada, Aug. 13–21, (1999), p. 847. [9] M. R. I. Faruque, N. Misran, R. Nordin, M. T. Islam, B. Yatim, “Estimation of specific absorption rate (SAR) and temperature increases in the human head of portable tele- phones,” Scientific Research and Essays (SRE), vol. 6, no. 6, pp. 1209-1215, 18 March, 2011. [10] M. Kanda, Q. Balzano, P. Russo, A. Faraone, and G. Bit- Babik, “Effects of ear-connection modeling on the electromagnetic-energy absorption in a human-head phantom exposed to a dipole antenna field at 835 MHz,” IEEE Trans. Electromagn. Compat., Vol. 44, Issue 1, pp. 4–10, 2002. [11] O. P. Gandhi, G. Lazzi, and C. M. Furse, “Electromagnetic absorption in the human head and neck for mobile telephones at 835 and 1900 MHz,” IEEE Trans. Microw. Theory Tech., Vol. 44, Issue 10, pp. 1884-1897, 1996. [12] M. Burkhardt and N. Kuster, “A modeling of the ear for compliance testing of handheld MTE with SAR safety limits at 900/1800 MHz,” IEEE Trans. Microw. Theory Tech., Vol. 48, Issue 11, pp. 1927–1934, 2000. [13] S.Watanabe, M. Taki, T. Nojima, and O. Fujiwara,

“Characteristics of the SAR distributions in a head exposed to electromagnetic fields radiated by a hand-held portable radio,” IEEE Trans. Microw. Theory Tech., Vol. 44, Issue10, pp. 1874–1883, 1996.

[14] M. R. I. Faruque, M. T. Islam, N. Misran, “Design of metamaterial attachment for SAR reduction in human head,” Applied Computational electromagnetic society Journal (ACES Journal), Vol. 25, no. 12, pp. 1097-1107, December 2010. [15] O. P. Gandhi and G. Kang, “Some present problems and a proposed telephones at 835 and 1900 MHz,” Phys. Med. Biol., vol. 47, pp. 1501-1518, 2002: [16] M. R. I. Faruque, M. T. Islam, and N. Misran,“Evaluation of specific absorption rate (SAR) reduction for PIFA anten -na using metamaterials,” Frequenz Journal, Vol. 64,Issue no. 7/8, pp. 144-149, 2010.

43