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  • Bioelectromagnetics 26:125^137 (2005)

    A Numerical and Experimental Comparisonof Human Head Phantoms for ComplianceTesting of MobileTelephone Equipment

    Andreas Christ,1* Nicolas Chavannes,2 Neviana Nikoloski,1 Hans-Ulrich Gerber,2

    Katja Pokovic ,2 and Niels Kuster1

    1Foundation for Research on InformationTechnologies in Society (ITIS),Zu rich, Switzerland

    2Schmid & Partner EngineeringAG, Zu rich, Switzerland

    A new human head phantom has been proposed by CENELEC/IEEE, based on a large scaleanthropometric survey. This phantom is compared to a homogeneousGenericHead Phantomand threehigh resolution anatomical head models with respect to specific absorption rate (SAR) assessment.The head phantoms are exposed to the radiation of a generic mobile phone (GMP) with differentantenna types and a commercial mobile phone. The phones are placed in the standardized testingpositions and operate at 900 and 1800 MHz. The average peak SAR is evaluated using both experi-mental (DASY3near field scanner) and numerical (FDTD simulations) techniques. The numerical andexperimental results compare well and confirm that the applied SAR assessment methods constitute aconservative approach. Bioelectromagnetics 26:125137, 2005. 2005 Wiley-Liss, Inc.

    Key words: dosimetry; FDTD methods; measurement techniques

    INTRODUCTION

    In the past, numerous human head models havebeen proposed for use in the testing of mobile telecom-munications equipment (MTE) for compliance withsafety standards. Head phantoms both for the numericaland for the experimental assessment of the specificabsorption rate (SAR) have been suggested by differentresearch groups. Since the possible health risks ofmobile phones have become an issue of public concern,the mobile phone industry as well as several govern-mental regulatory bodies have been emphasizing theneed for a standardized procedure for the compliancetesting of MTE which meets the highest requirementswith respect to accuracy and repeatability. It is ob-vious that a human head model which is well definedin terms of shape and dielectric parameters representsthe cornerstone of any testing procedure, for example[IEEE, 2003; IEC, 2004]. Furthermore, the assessedexposure should be higher than the maximum humanexposure occurring under normal operational condi-tions. Since the maximum exposure is not known, basicrequirements for phantoms used in the compliancetesting procedures of handheld devices have been out-lined in Kuster et al. [1997] and were incorporated byIEEE [2003].

    . The peak spatial average SAR shall be a conserva-tive estimate of the actual value expected to occur in

    the heads of a significant majority of persons duringnormal usage of wireless handsets.

    . The test results shall not unnecessarily overestimatethe peak SAR expected in actual users in order toprevent unnecessary inhibition of the advancementof new mobile communications technologies.

    . The phantom shall enable high repeatability, allowstable and repeatable device positioning for peakSAR measurements and be effective for verify-ing repeatability and reproducibility among inter-laboratory comparisons.

    . The phantom shall be practical for routine com-pliance testing.

    . The phantom shall satisfy these criteria for con-temporary and future handset designs and be

    2005Wiley-Liss, Inc.

    Grant sponsor: Schmid and Partner Engineering AG (SPEAG),Switzerland; Grant sponsor: Swiss Commission for Technologyand Innovation (KTI); Grant sponsor: Mobile ManufacturersForum (MMF), Belgium.

    *Correspondence to: Andreas Christ, Foundation for Research onInformation Technologies in Society (ITIS), Zeughausstr. 43,CH-8004 Zurich. E-mail: [email protected]

    Received for review 2 September 2003; Final revision received 14October 2004

    DOI 10.1002/bem.20088Published online in Wiley InterScience (www.interscience.wiley.com).

  • unbiased with respect to any particular handset de-sign or shape, i.e., handset designs which producelower assessed SAR values should correspondinglyresult in reduced exposure in real-world situations,and vice-versa.

    The objective of this study was to provide sup-porting scientific evidence that SAM meets the abovementioned requirements when compared to the expo-sure of actual human anatomies, including children.In order to substantiate the numerical results, the SARin the homogeneous liquid filled phantoms was alsodetermined experimentally.

    PROPOSED STANDARD PHANTOMS

    The Generic Twin Phantom [Kuster et al., 1997]and the Specific Anthropomorphic Mannequin (SAM)[IEEE, 2002] have been proposed and widely used forcompliance testing. Both phantoms are based on aseries of studies about the dependence of the absorptionupon internal anatomy, head size and shape (adultvs. child), tissue parameters, phone, accessories, etc.[Kuster and Balzano, 1992; Hombach et al., 1996;Meier, 1996;Meier et al., 1997;Kuster, 1998; Schonbornet al., 1998; Drossos et al., 2000; Kuster, 2001]. Thefindings of these studies led to the conclusion that aconservative approach is achieved if the phantomsatisfies the following requirements.

    . The H-field generated by the handset at the interfaceof the shell and the head tissue-simulating liquid(HSL) is higher or equal to the H-field generated atthe skin of any user during intended operation of thephone. In other words, the separation between phoneand tissue-simulating liquid should be equal orsmaller than in the real world.

    . HSL leads to higher absorption than any combina-tion of head tissues [Drossos et al., 2000].

    Specific Anthropomorphic Mannequin

    In order to maximize consumer trust in com-pliance testing procedures, the Standardization Co-ordinating Committee 34 of IEEE (SCC34-SC2)recognized the necessity to base the phantom on alarger survey of human heads [Gordon et al., 1989].Consequently, it developed and proposed the SAM.Theshape of SAM (rightmost in Fig. 1) corresponds to the90th percentile of data collected regarding the adultmale head. The material of the shell was selected aslossless and as thin as technically feasible, i.e., 2 mm.At the ear reference point, the thickness is increasedto 6 mm, the average thickness of the compressed ear.The ear itself is shaped such that it permits the accurateand repeatable positioning of the phone under test. Thedielectric parameters of the tissue-simulating liquidfor SAM were derived from worst case considera-tions [Drossos et al., 2000]. They are er 41.5 and s0.97 S/m at 900MHz and er 40.0 and s 1.40 S/m at1800 MHz.

    The SAM phantom has also been adopted byCENELEC [CENELEC, 2001a] as well as ARIB[ARIB, 2002] and IEC [IEC, 2004] for the compliancetesting of handheld phones. Lacking better criteria,CTIA has further proposed the use of SAM for radiatedpower andRF receiver performance tests [CTIA, 2001].

    Generic Twin Phantom

    Prior to the introduction of the SAM phantom bythe standardization bodies, the Generic Twin Phantom(second from the right in Fig. 1) was formerly used forMTE compliance testing by different research groups,manufacturers, and test houses. Although the model isobsolete, it has been included since a large number ofphones was authorized based on measurements withthis phantom [Federal Communications Commission,2003].

    For the development of theGenericTwinPhantom,the head dimensions of 52 adult volunteers (male and

    Fig. 1. Anatomicalandhomogeneousphantomsat same scale. (From left to right: 3-year-old child(3YC),HR-EF-1, adultmale, genericphantom,SAM).

    126 Christ et al.

  • female) were measured in an area of 160 150 mm2covering the region around the cheek, the ear, and thetemporal bone. For the measurements, a planar devicewas used which slightly compressed the ear similar toduring usage of a mobile phone. The shape of thephantom was designed such that the distance betweenthe handset surface and the inner surface of the shellwould always be smaller than for 90% of the peopleinvestigated. The physical phantom shell is construct-ed of Ureol (er 3.7) with a thickness of 2.7 mm. It isfilled with brain tissue-simulating liquid (er 41.0,s 0.85 S/m at 900MHz and er 40.1, s 1.71 S/m at1800 MHz). The ear of the Generic Twin Phantom isrepresented by a lossless spacer with a radius of 15 mmand a thickness of 2mmpositioned at the location of theopening of the auditory canal. This leads to an overallshell thickness of 4.7 mm in the ear region. A detaileddescription of the Generic Twin Phantom can be foundin Kuster et al. [1997].

    HIGH RESOLUTION ANATOMICALHEAD MODELS

    Numerical Representation

    The spatial peak SAR obtained with the twohomogeneous phantoms was compared with the spatial

    peak SAR obtained with three high-resolution anato-mical human models. For all these models, at least skintissue, muscle, fat, bone, CSF, gray and white brainmatter, blood, cartilage, vitreous humor, lens and eyesclera were distinguished. The tissue parameters asprovided by Gabriel et al. [1996] were assigned to allanatomical head models independent from their age(Table 1). For the comparison of the different head sizesbased on gender and age, Figure 1 shows the modelstogether with the two standard phantoms at the samescale.

    All anatomical models have been derived fromimages (MRI or photographs) of slices through thebody. Since the models need to be rotated into thetesting positions and high grid resolution is required inthe ear region, a pre-discretized representation usingcubical cells (voxels) would lead to decreased accuracydue to multiple discretization. Therefore, a particulardata format has been developed which preserves the de-tails of the source images and keeps the computationalexpenses for storage and manipulation (e.g., rotation orscaling) low. First, all boundaries of the tissue regionsof all images are identified by a biologist and markedwith t