Mobile Phone SAR Analysis through Experimental and Numerical Simulation

2010, 12th International Conference on Optimization of Electrical and Electronic Equipment, OPTIM 2010 Mobile Phone SAR Analysis through Experimental and Numerical Simulation Mihaela Morega 1, Andrei Marinescu 2, Alexandru Mihail Morega 1 1 University POLITEHNICA of Bucharest, Faculty of Electrical Engineering, Bucharest, Romania 2 Research, Development and Testing National Institute for Electrical Engineering (ICMET), Craiova, Romania mihaela@iem.pub.ro, amarin@icmet.ro, amm@iem.pub.ro Abstract - The opportunity for the research objectives and results presented in the paper was offered by the setup and certification of an electromagnetic compatibility laboratory, targeted on development and compliance testing of mobile communication devices, concerning their ability to provide safe working conditions to the user. The experiments are designed to assess the compliance of the emitted radiation level with the basic restrictions expressed in terms of the dosimetric estimate, i.e. the specific energy absorption rate, physical quantity widely known by its acronym, SAR. A numerical model, based on the implementation of the finite element method was designed in order to replicate, as good as possible, some experimental results. After validation, the numerical model is intended to become a flexible tool, useful for intensive dosimetric analysis, with objectives complementary to the experimental goals and possibilities. The paper introduces the experimental arrangement and the numerical model, both designed accordingly to the current testing standardization in the field. A comprehensive set of results gives data for the comparison between measurements and calculus, with the specific objective of the calibration and validation the numerical model, in conformity with the experimental setup. I. INTRODUCTION Certification of mobile communication devices, prior to placing them on the market, requires careful assessment testing, performed by accredited laboratory, specialized for the determination of the Specific energy Absorption Rate (SAR). Regarding the domestic market of mobile phones, the state is in the position to protect the safety and health of the citizens, in their position of consumers, according to EU Recommendation 1999/519/CE [1], which is also transposed into national legislation *. A specialized laboratory, fully equipped for SAR measurement in agreement to international standards [2-5], was recently opened as a high performance facility, authorized to perform conformity assessment tests regarding the electromagnetic emissions of cellular phone terminals [6] **. The conformity of the procedures and the precision of the measurements provided here, were certified * Directive of the Romanian Health Ministry no. 1193 / 2006 for the approvement of the Norms for limiting exposure of the general public to electromagnetic fields from 0 Hz, up to 300 GHz ** Laboratory for SAR Evaluation and Certification of Mobile Phone Terminals (http://www.icmet.ro/web_saremf/sar_hompage.html) at the Research, Development and Testing National Institute for Electrical Engineering (ICMET), Craiova, Romania by a successful Interlaboratory comparison with similar laboratories in Europe. The access to efficient experimental equipment designed for SAR measurement and the availability of commercial software for numerical analysis of high frequency electromagnetic radiation, does not, however, provide for an unconditional solution to the electromagnetic field (EMF) dosimetric problem. Not speaking here about the biological issues, the technical aspects only, by themselves, are complex and challenging [7]; the energy absorption in human tissue is highly dependent on several characteristics that have to be considered in the experimental and/or computational approach: * the anatomical domain characterized by shape, inner nonhomogeneous structure, relative position in relation with the radiation source, electric properties of the biological tissues (variable with the frequency, with the hydration level, variable in the living compared to nonliving state of the body and thus difficult to be determined by measurements); * the characteristics of the radiation source signal waveform and frequency band, emitted power, polarization of the electromagnetic emitted wave, continuity, antenna type and its positioning inside the hand-held device; * the physical aspects of defining the dosimetric quantities (i.e. SAR for the study presented here) the procedure of measurement and computation, the averaging techniques over space and time; * the ratification procedure - the dosimetric quantities determined on the spot (either experimentally or numerically) should be compared with reference specifications in guidelines and standards. Most of the mentioned technical aspects are already specified by international regulations: human exposure guidelines [1, 8, 9], standards of measurement equipment and standards for the conformity assessment of electric and electronic equipment [2-5]. The investigation program presented here points to the experience and know-how acquired by the research team, for setting in work a complex assembly dedicated to dosimetric analysis in the field of high frequency nonionizing electromagnetic radiation, with reference to the normative frame. The dosimetric analysis assembly combines, in a complementary manner, a sophisticated measurement equipment and a flexible numerical model. 978-1-4244-7020-4/10/$26.00 '2010 IEEE 95

II. EXPERIMENTAL SAR LABORATORY 1. Nonionizing radiation dosimetry The specific absorption of energy (dw/dm) in a lossy dielectric material exposed to high frequency EMF is currently related to the increase in temperature (T) and it is usually quantified by the Specific energy Absorption Rate, defined as (d) SAR = d d t dw d T, or SAR = C d m d t =σe2 / ρ [ W/kg], (1) where d /dt denotes the rate of a certain physical quantity, E represents the rms value of the inner electric field strength; C, σ and ρ are the physical properties of the material: specific heat capacity, electric conductivity and mass density. SAR is also the quantity measured during the tests performed for the certification of electronic wireless equipment, because it is referred to as a basic restriction in international norms for limiting human exposure to microwaves [1, 8, 9]. Two limiting levels are specified: a lower value for the exposure averaged over the entire human body and a higher value applicable for the local exposure of a particular vulnerable area of the body (for example the head). It is supposed that the restricted SAR values are averaged over time and space, as Table 1 shows. TABLE I STIPULATION ON SAR IN INTERNATIONAL STANDARDS AND GUIDELINES Averaged whole body SAR Localized SAR (head and trunk) Averaging time Europe 2 W/kg averaged 0.08 W/kg [9] over 10 g of tissue 6 min. USA * 1.6 W/kg averaged 0.08 W/kg [8] over 1 g of tissue 30 min. * in 2005 was issued a revised version of [8] which is harmonized with [9]. Since it is harmful to measure SAR directly in the human body, the standard procedures make reference to the use of phantoms [3, 4] and state specific experimental protocols aimed to ensure the best compliance to reality. The referred normative, adapted to the mobile phones fabrication in different world region are not identical; they are however harmonized. The most important aspects are the requirements on the measuring method and on the measurement accuracy provided by the test system. A special characteristic of both standards resides in the explicit and detailed provision for the measurement uncertainty; the limit of the maximum permissible uncertainty is about 3dB. 2. SAR measurement laboratory The test equipment presented in Fig.1 includes the device under test (DUT), i.e. the cell phone, the positioning and scanning system for the DUT and for the electric field probe, the calibration systems for the probe and for the tissue simulating substance, the command and data processing system. The phantom used for mobile phones testing is the Specific Anthropomorphic Mannequin (SAM) [3, 4]. Fig 1. Block diagram of the equipment Phantom enclosure and the liquids simulating the head tissue or other human body tissues are subjected to some strict requirements; when the frequency band of the mobile phone is changed the liquid should be changed accordingly, due to the working frequency influence on its dielectric properties. The robot for field probe positioning must be able to scan the whole volume subjected to exposure with a view to achieving a three-dimensional measurement of SAR with a remarkable positioning accuracy of ±0,2 mm. The miniature field probes have a special construction and very high linearity. The electric field probe is of three axes type (3D) and it can scan the phantom volume from a minimum depth of 4mm, measured from its bottom. The system available for our research is the commercial COMOSAR Satimo test bench that works with the monitoring and data processing OPENSAR software; the whole measuring system operates in an electromagnetically shielded room [6]. Fig. 2 presents the experimental setup (left) and the phone-positioning device holding the cell phone near the SAM phantom (right). The laboratory for SAR dosimetry is designed to perform measurements for all types of mobile phones existing on the market: CDMA, GSM 900/1800, WCDMA, UMTS etc. covering a frequency band from 450 to 2450 MHz. 3. SAR measurement procedure The dielectric properties of the tissue equivalent liquids are measured prior to the SAR analysis, at constant temperature. Dielectric permittivity ε and electric conductivity σ should have precise values, i.e. within ± 5% tolerance with reference to the standard [3, 4]. A performance check is made before the targeted measurements on SAR, in order to verify that the system operates according to the technical specification. This is a preliminary SAR measurement, using a setup where the generated signal comes from a dipole antenna. The components and the procedure for performance checking are the same as those used for the compliance tests. The result shall be found within the limits ±10 % of the target value, determined during the system validation check. The output power and frequency are controlled using a base station simulator. 96

III. NUMERICAL MODEL FOR DOSIMETRIC ANALYSIS Fig. 2 Experimental setup in the laboratory for SAR measurements The test proceeds further on these steps, for each position of the DUT: a radio connection is established at maximum DUT power using the base station emulator; SAR values are measured on a grid of equidistant points, on a surface situated at constant distance from the inner phantom surface; SAR values are recorded at equidistant points into a cube; peak SAR averaged values are computed and compared against the standard basic restrictions (Table 1). The product is checked in the "cheek" and "tilt" positions (defined in standards according to Fig. 3), on the left and right sides of the phantom and it is tested at the frequencies of each emission band, in the requested testing conditions. In order to minimize measurements errors, the probe tip must not be in contact with the phantom inner surface. However, the highest local SAR values are at the inner surface of the phantom and they should be evaluated; an extrapolation method is thus applied [11]. The extrapolation is based on a fourth-order polynomial least-square fit of measured data. The local SAR values are extrapolated to the liquid surface with a 1 mm step. The measurements have to be performed over a limited time due to the operating time of the DUT battery. In order to reduce the measurement duration, the distance between measurement locations should be large. Reasonable values are between 5 and 8 mm. However, an accurate assessment of the peak SAR values averaged over 10g and 1g of tissue is obtained with a very fine resolution of the three-dimensional scan. Interpolation is further used to obtain the adequate resolution. The measured and extrapolated SAR values are interpolated on a 1 mm grid with a three-dimensional thin plate spline algorithm. A 3D computational model, based on the finite elements method (FEM) was designed for the study of microwaves penetration in the human head, when exposed in the near field produced by a mobile phone device. Previous analysis of different FEM numerical models, either based on simplified geometry or built through numerical reconstruction from CT scans [11, 12], led us to the head model used in this study. The model was carefully designed to mimic the adult human head, with normal size and realistic shape, comparable to SAM, as described by the document referred in [3]. Recent literature offers a variety of studies performed with anatomical realistic numerical models [10-13]; however, the multitude of the documented versions reveals an inconvenient dispersion in the choice of physical and numerical details, that makes the comparison and the synthesis of conclusions very difficult. These circumstances support our efforts to build and evaluate our own FEM model, similar to SAM configuration, which is designed to perform numerical tests, complementary to experimental dosimetry and certification protocols. The model, implemented and processed with a commercial finite element software package, has the main quality of being available and accessible regarding our resources. Its concept is flexible and open to adjustments toward the design of different radiation sources. 1. Description of the model Human head: The computational model is inspired by the human head anatomy. A 3D reconstruction from 2D tomographic slices, that conserves the general shape of the head was previously performed with the software package 3D Slicer [14], and was further adjusted in order to comply with the Specific Anthropomorphic Mannequin (SAM) [3, 4]. The numerical results are to be compared with the measurements; the head is consequently modeled with a homogeneous inner structure, with the same physical properties as the liquid used to fill the SAM phantom in the experiment, at the working frequency of 1747.4 MHz. The electric conductivity is σ head = 1.32 S/m, the dielectric constant is ε r,head = 39.9, and ρ head = 10 3 kg/m 3 represents the mass density. Fig. 4 shows the representations of the head: (a) presents a homogeneous head, reconstructed from CT scans [14], (b) is the homogeneous head in Comsol Multiphysics, ready for FEM computation [14, 16], while (c) presents the formal SAM phantom [3, 4], used in experimental certification tests. Fig. 3 Hand held phone in the cheek (left) and tilt (right) position [3, 4]. Fig. 4 Head models: (a) homogeneous head, reconstructed from CT scans [14] (LEFT) (b) homogeneous FEM model [14, 16] (MIDLE) (c) SAM phantom [3, 4] (RIGHT) 97

The outer ear (pinna) is also considered in the FEM model. Pinnae are simulated in the SAM model by solid lowconductive half discs ( σ pinna = 0.012 S/m, ε r pinna 5), which act as ear spacers; they are centered on the external auditory canal where the reference point RE (ERP) is placed (see Fig. 4.c). Ear spacers are important to maintain the proper distance between the phone and the head and for the correct positioning of the phone, but the electric field is not commonly determined inside them. Several computational models have already considered pinna influence on the electric field distribution; it was found that peak SAR values in pinna could be slightly higher than in the head of SAM [12, 13] but SAR averaging in that small volume does not fully comply with the usual procedures [3, 4]; the issue of considering pinna tissue in peak averaged SAR evaluation is still a matter of debate. In our FEM model, the outer ears have the same shape, form, dimensions, position and properties as SAM s ear spacers; they serve exclusively for the better positioning of the hand-held phone and are not subjected to SAR estimate. Source model: The cell phone model simulates the device used in the experimental setup, as a rectangular case with the dimensions 110.9 x 46.7 x 15.6 mm, made of a low conductive material with the electric properties ε r phone = 2.5 and σ phone = 0.04 S/m. A microstrip patch antenna, fed by a rectangular waveguide, was designed to work in the microwave frequency range 0.9 2.5 GHz, and in our study it produces a continuous wave at 1.7474 GHz. The antenna is placed in the upper part of the phone-case; the radiating patch and the ground plane are printed on a circuit board with ε r pcb = 5.3. The antenna configuration (geometry and positioning) was fixed through an oriented trial and error technique, with the goal of finding a version that matches, as good as possible, the experimental results obtained with a certain cell phone model (the inner design of the cell phone was not fully accessible). The emitted power is always adjusted at the value of 1 W, regardless variations in the characteristic impedance due to the coupling between the antenna and the head. The results are thus always normalized to the source average power and they are easily scalable to other values of the power emitted by the cell phone. Both tilt and cheek positions of the phone [2, 3] are considered in this study (Fig. 3). EMF problem: We created a numerical 3D FEM model, implemented with COMSOL Multiphysics [16], RF module. The application mode harmonic propagation of electromagnetic waves, solved for E in complex numbers representation is applied for the study presented here. The general wave equation, in terms of electric field strength E 1 μ E ω 2 ε j σ ω E = 0, (2) is valid for the computational domain - head and handset - enclosed by a spherical PML (perfectly matched layer), designed to absorb the radiation as close to the head as possible, with minimal perturbation of the propagation process. In eq. (2) ω=2πf is the angular frequency, j = 1 and μ, ε, σ are the magnetic and electric properties of the subdomains (specified earlier); μ=μ 0 is the magnetic permeability of free space, same as for biological tissue. The microwaves source, placed at the feed-point of the antenna is implemented as a Port boundary condition, with the characteristics of the rectangular waveguide. 2. Numerical dosimetric estimate The currently applied standards [3] and [4] make clear specification of experimental settings and methods necessary to determine peak spatial-average SAR in human head. SAR is averaged over 1g and 10g of substance contained in a cube with the respective side length 10 mm and 21.5 mm. The cube should contain the region with maximal SAR values around the peak. The experimental averaging techniques are based on interpolation procedures, applied to several local measurements, performed at locations monitored by the positioning system of the field probe. The computation averaging techniques are highly dependent on the numerical method applied [5]. Since FDTD is widely used in high frequency EMF analysis [11, 12, 13], the recommended averaging numerical technique is based on the identification of an averaging cubic volume coherently adapted to the finite differences grid, or to the numerical implementation of the experimental procedure. Finite element modeling is just moderately expanded in nowadays high frequency electromagnetic analysis, mainly due to historical evolution. The precise identification of the averaging volume requires its prior definition as a distinct subdomain in the geometry of the model. This is possible through repetitive intervention in the model description, based on the analysis of the solution. The technique is time consuming and particularly dependent on any change in the description of the model; however, this technique was applied in the study presented here. Another approach is (same as for FDTD method) the numerical implementation of the experimental technique; this path is intended to further extend the dimension of our study. 3. The accuracy of the model Before advancing to the stage of performing numerical tests consistent with the experiment, the numerical model passed some accuracy tests. The set of numerical tests took into account two accuracy objectives, focused on getting a trustful solution: (1) how far should the PML be positioned with reference to the radiation source, (2) how small should the mesh size be chosen. Both objectives were addressed through a trial and error approach, taking the satisfactory precision of the solution, as the accuracy criterion; several post processing global quantities were analyzed: the emitted power, the power absorbed in the head, SAR averaged over 1g and 10g of tissue, the power passing through the PML, the energy balance on the domain. 98

4. Comparative evaluation of exposure conditions The numerical model was designed to simulate as close as possible the experimental conditions [17]: the head model is a digitized replica of the SAM phantom same shape, homogeneous structure, same physical properties of the simulant tissue, and both, cheek and tilt positions of the phone, comply with the standards [3, 4]; the phone under test is a commercially widely available cell phone model [18]; it is replicated in the numerical model by its size and it works at the same characteristic frequency (1747.4 MHz) and the same emitted average power 1 W; the experiment is performed at the maximum working power of the phone, which was previously determined to be 30dBm, i.e. 1 W. However, several dissimilarities have to be emphasized, because they justify the major uncertainties of the simulation: the inner structure of the cell phone is not fully known; consequently, one of the main unknowns of the numerical simulation was the precise design and positioning of the microwave antenna; we have tried several versions and the best choice is adopted here and support the results presented later; the radiation source in the experiment emitted a time division multiple access (TDMA) signal, while the numerical simulation focused only on the continuous wave (CW) signal (at the same frequency and power); the distance to the head was carefully checked but it is not sure that it was always replicated; the numerical FEM model does not account for the phantom s shell, fabricated as a 2 mm thick, nonconductive material; however, the ear spacers ensure the proper position of the phone and the minimal distance to the head. position), sliced at 4-5 mm distance from that surface. The distance could be slightly different in the two settings; however, the experimental result (Fig. 5a) and the numerical one (Fig. 5b) are in good agreement, considering the dissimilarities stressed earlier. Fig. 6 a and b evidence the results obtained in similar conditions of exposure and dosimetric estimate, while the phone device is held in the tilt position. Another type of analysis applied to SAR distribution is provided by the linear scans of the same quantity in a direction perpendicular to the section plane considered in Figs. 5 and 6; this is the direction of maximal in-depth penetration of absorbed power. Fig. 7 shows the in-depth experimental and computed SAR graphs in the already considered (a) cheek and (b) tilt positions. a. Experimental SAR distribution IV. RESULTS The comparative, experimental vs. computational study was directed toward a double objective: the validation through measurement of a FEM numerical model concept and the exploratory analysis seeking aspects of complementarity and added value, between experimental and computational approaches. The same category of data acquisition was performed, both on the experimental test bench and through the numerical simulation: (1) local SAR distribution inside a head exposed to the high frequency EMF radiation coming from a cell phone device mounted near the ear and (2) averaged peak SAR over 1g and 10g of tissue. 1. SAR distribution analysis The primary solution of both measurement and simulation is the electric field strength distribution inside the exposed head. The E-field solution is numerically post-processed in both approaches, and the target output quantity, SAR is computed (1). Fig. 5 shows the SAR spectra in a cut-plane parallel to the surface of the phone device (in the cheek b. SAR distribution on the numerical model Fig. 5 SAR spectra in plane sections through the highest exposed head region; emitted power - 1W; phone in the cheek position 99

a. Experimental SAR distribution a. The phone in the cheek position b. The phone in the tilt position Fig. 7 Measured and computed SAR scans following the in-depth direction of maximal values; emitted power - 1W b. SAR distribution on the numerical model Fig. 6 SAR spectra in plane sections through the highest exposed head region; emitted power - 1W; phone in the tilt position 2. Peak SAR averaged over 1g and 10g After locating the SAR spot, which is at the surface of the head (closest from the radiation source), the probe positioning system could scan and evaluate local E-field values, in the nodes of a 5 mm resolution spatial grid. The rest is computation interpolation between nodes and extrapolation toward the phantom shell and numerical estimate of averaged SAR values (earlier described in II.3). The FEM method provides for a fine adjustment of the geometry. A precise description of geometric shapes close to cubes is possible by choosing the exterior face of the cubes coincident with, or tangent to the head surface enclosing the SAR spot; two methods for identification of cubic adequate volumes are described in [2]. The FEM software gives the possibility to numerically integrate SAR on any previously identified subdomain. Table 2 contains the peak SAR values determined from measurements and from numerical EMF analysis, as averaged over 1g and 10g of head tissue, for the cheek and tilt positions of the cell phone held close to the head. TABLE 2 COMPARATIVE ANALYSIS BETWEEN EXPERIMENTAL AND COMPUTED PEAK AVERAGED SAR VALUES Averaged SAR cheek position tilt position [W/kg] experim. comput. experim. comput. over 1 g 1.023 1.099 0.693 0.692 over 10 g 0.580 0.545 0.404 0.361 The relative errors between measured and computed values are within ±10%. 3. Uncertainty analysis and possible error sources The precision of the measurement system and the calibration data available for its components makes possible the precise quantitative estimate of the measuring uncertainty; each conformity test is documented by a test report, which includes an estimate the uncertainties [17]. SAR values generally obtained by experimental simulation with COMOSAR system are affected by a measurement uncertainty of ±3dB. However, a typical value for the 100

simulations usually performed in our experiments and presented here is about ±1.5dB. The precision of the experimental results was already checked and confirmed through the interlaboratory comparison procedure [3, 17]. The numerical model introduced and tested in our research was designed and optimized to become a trustful tool for analysis, working together with the experimental setting. The results presented here confirm its validity in the numerical aspects and its flexibility to comply with technical aspects and normative frame. Some susceptible error sources in the design of the model are discussed further. The characteristics of the microwave source have a decisive impact on local SAR distribution and peak values. The antenna type, dimensions and position (inside the phone and relative to the head, particularly the feed point positioning and wave polarization) are of major significance. The signal waveform is less important, as long as the average power transported at the characteristic frequency is the same. CW and TDMA signals at the same average power and characteristic frequency were found to lead to similar SAR distributions. The distance between the EMF source and the surface of the body determines the in-depth penetration of the E- field and SAR distribution; when different models are compared, the proper estimate of that distance should be considered. In our case, better information on the inner design of the cell phone is necessary. The averaging technique for the peak SAR values is another source of errors; it depends on the numerical method and on the space grid. V. COMMENTS, CONCLUSIONS AND FURTHER RESEARCH The research objectives and results presented in the paper address a quantitative correspondence between different simulation tools applied in SAR dosimetry, related to the compliance testing of mobile phones. An experimental setup, which recently became functional, was certified through the interlaboratory comparison procedure. The information on the test bench and measurements was applied to the design and validation of a numerical FEM model that mimics the experimental setting. In the dosimetric evaluation of EMF penetration in tissue exposed to microwaves, the E and SAR distributions are usually determined. Standardization bodies have already agreed on the settings of experimental conditions and protocols for the certification of hand held wireless communication devices [2-5], but computational protocols are still under debate; Technical Committee TC34 of the IEEE International Committee on Electromagnetic Safety is the standardization body currently working on that task. The advantages of the numerical modeling compared with experimental measurements are obvious: rapidity in evaluation, repeatability of results and versatility of the models at lower costs. The FEM model introduced and evaluated here is intended to offer complementary information to experimental tests. Once the mathematical model is functional and validated, the experimental measurements could be restricted to a minimal set, which is necessary for the calibration of the numerical model; the computational outputs could further be diversified and multiplied without the restrictions and uncertainties that always limit an experiment. The proper combination of measurement and computation leads to an optimized testing setting, characterized by minimal uncertainties and costs. Further research will concentrate on several tasks: the equivalence among different numerical interpolation procedures and averaging techniques for the computation of the dosimetric quantities with better precision and in agreement with experimental techniques; the quantitative analysis of the uncertainties in the computational procedure; the study of sensitivities applied to the experimental and numerical models, due to the dispersion of certain physical factors, i.e. the temperature, the dielectric properties, etc.; the work on the phone model - a collection of models including different types of cell phones and a more detailed representation of the device (including the battery and the electronic printed circuit board). ACKNOWLEDGMENT The authors are grateful to Dr. Jerome Luc from SATIMO for his valuable assistance at setting into service the SAR measurement Laboratory. The authors would also like to thank to their colleagues: Dipl.-Ing. I. Dumbrava, Dipl.-Ing. G. Mihai and Dipl.-Ing. C. Vargatu from ICMET Craiova for their participation at the experimental program. The research benefited from the access to the infrastructure and resources financed by the Romanian Research Authority ANCS through the contract no.79/2007 (PNCDI_II, Capacitati Program) and the grant no.54/2006 BIOINGTEH (CNCSIS, Interdisciplinary Platform Program). 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