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Some present problems and proposed experimental phantom for SAR compliance testing of cellular telephones at 835 and 1900 MHz

DOI:10.1088/0031-9155/47/9/306
Authors:
Om P Gandhi at University of Utah
Om P Gandhi
Gang Kang
Gang Kang
Gang Kang
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Abstract and Figures

This paper compares the maximum allowable powers of some typical cellular telephones at 835 and 1900 MHz for compliance with the limits of specific absorption rates (SAR) given in ANSI/IEEE, ICNIRP and the proposed modification of ANSI/IEEE safety guidelines. It is shown that the present ANSI/IEEE guideline is the most conservative with the ICNIRP guidelines allowing a maximum radiated powerthat is 2.5-3 times higher, and the proposed IEEE modification of treating pinna as an extremity tissue the least conservative allowing even higher radiated powers by up to 50%. The paper also expands the previously reported study of energy deposition in models of adults versus children to two different and distinct anatomically-based models of the adult head, namely the Utah model and the 'Visible Man' model, each of which is increased or reduced by the voxel size to obtain additional head models larger or smaller in all dimensions by 11.1% or -9.1%, respectively. The peak 1 g body-tissue SAR calculated using the widely accepted FDTD method for smaller models is up to 56% higher at 1900 MHz and up to 20% higher at 835 MHz compared to the larger models, with the average models giving intermediate SARs. Also given in the paper is a comparison of the peak 1 g and 10 g SARs for two different anatomically-based models with 6 mm thick smooth plastic ear models used for SAR compliance testing. The SARs obtained with the insulating plastic ear models are up to two or more times smaller than realistic anatomic models. We propose a 2 mm thin shell phantom with lossy ear that should give SARs within +/- 15% of those of anatomic models.
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SOME PRESENT PROBLEMS AND A PROPOSED EXPERIMENTAL
PHANTOM FOR SAR COMPLIANCE TESTING OF CELLULAR
TELEPHONES AT 835 AND 1900 MHz
Om P. Gandhi
(1)
and Gang Kang
(2)
(1)
Department of Electrical and Computer Engineering, University of Utah, 3280 MEB, 50 S Central
Campus Drive, Salt Lake City, UT 84112, U.S.A. E-mail: gandhi@ece.utah.edu
(2)
As (1) above, but E-mail: gkang@ece.utah.edu
ABSTRACT
We compare the peak 1- and 10-g SARs for two different anatomically-based models with the corresponding 6 mm
thick plastic ear models recommended for SAR compliance testing both in U.S. and Europe. The SARs obtained with
the insulating plastic ear models are two or more times smaller than realistic anatomic models. To alleviate this
problem, we propose a 2 mm thin shell phantom with lossy ear that gives SARs within ±15% of those of anatomic
models. Also, the SARs for smaller (-9.1%) models are higher than for larger (+11.1%) scaled versions of the two
models.
I. INTRODUCTION
We have previously reported that the peak 1-g SARs for scaled models of children are higher than the corresponding
value for full-scale models of the human head [1]. In the same paper, we have also shown that there is a deeper
penetration of absorbed energy in smaller models of the head for electromagnetic fields of mobile telephones both at 835
and 1900 MHz. The explanation for both of these observations are the scaled thinner ear and skull and smaller overall
dimensions of the brain for children as compared to that for the adult. To examine this issue further, we have taken two
different and distinct anatomically-based heterogeneous models of the human head and neck and have scaled them up or
down for the voxel size by 11.11% and -9.1% i.e. by approximately ± 10% along each of the three axes, respectively.
These variations are certainly within the range of the head sizes encountered for men and women [2]. Three different
scaled models for each of the adult heads are thus considered for SAR distributions. Used for the SAR calculations is
the finite-difference time-domain (FDTD) method which is extremely popular and has been highly tested for dosimetry
of cellular telephones (see e.g. refs. 1, 3). Using the six models thus created, the following issues are examined.
1. Effect of the head size on the peak 1-g SAR both at 835 and 1900 MHz. For these calculations, a number of
handset sizes and monopole or helix antennas are considered.
2. Comparison of the peak 1- and 10-g SARs with those obtained using the corresponding 6 mm thick plastic ear
models. This study is done since plastic ear models are recommended for SAR compliance testing both in
Europe and the U.S. [4, 5].
3. To alleviate the problem of lower SARs with the 6 mm thick plastic spacer (in the shape of pinna) phantoms,
we propose a smoothened ear model of the human head.
II. TWO MODELS OF THE HEAD
For the present studies, we have used two different anatomically-based models of the human head and neck. Model 1 --
the so-called Utah model was obtained from the magnetic resonance imaging (MRI) scans of a male volunteer. This
model described in detail in [1, 3] has a pixel resolution of 2 x 2 mm for the cross sections and 3 mm spacing between
the various slices or cross sections. As described in these papers, this model has been segmented into 31 tissues of
which 15 tissues are involved for the model of the head and neck considered for the present calculations. To create a 2 x
2 x 2 mm resolution model, we subdivided each of the 2 x 2 x 3 mm resolution voxels of the Utah model into 12
smaller cells of 1 mm resolution along each of the three axes and then combined 2 x 2 x 2 smaller cells into new voxels
of dimension 2 mm along each of the axis. The tissue classifications for the new voxels of dimension 2 x 2 x 2 mm
was decided by the majority tissue in the subvolume of the voxel i.e. the tissue for five out of eight cells forming the 2
x 2 x 2 mm resolution model. A second tissue-classified model used for the present calculations is based on the scans
of the "Visible Man" model, which was kindly provided by John Ziriax and Patrick Mason of Brooks AFB, Texas.
This model with 1 mm resolution is classified into 24 tissues. As for the Utah model, here too, we combined 2 x 2 x
2 cells to form the 2 x 2 x 2 mm resolution model of the head and neck.
A visualization of the two heterogeneous models used for the calculations is given in Fig. 1.
(a) The Utah model (b) The "Visible Man" model
Fig. 1. A visualization of the two anatomically-based 30°-tilted models of the head used for SAR calculations.
III. VARIATION OF SAR WITH HEAD SIZE
A thrust of this study is to evaluate coupling of electromagnetic fields from cellular telephones to the various sizes of the
human head. Toward this end, we have created two additional sizes of the head for each of the two models, described in
Section II by considering the cell size from 2 mm to 2.222 mm for the larger models and down to 1.818 mm,
respectively. This results in scaling the dimensions of the two head models up by 11.11% and down by 9.1% i.e. by
approximately ± 10% along each of the three axes, respectively. The peak 1-g SAR values determined for a variety of
antennas (monopoles and helices), various handset dimensions and larger, average, or smaller scaled versions of the two
anatomic models are given in Table 1. Because of the possible revision of the IEEE Safety Standard to focus on the
peak 1-g SAR for body tissues instead of all tissue, the data given in Table 1 give only the peak 1-g SARs for body
tissues and the brain. The salient features of the results are as follows:
1. The peak 1-g SARs for both the body tissue and the brain increases monotonically with the reducing head size
(and pinna thickness) for both of the head models and for all handset dimensions and the antennas i.e.
monopoles as well as helices.
2. The peak 1-g SARs for body tissues for smaller models may be up to 50 to 60 percent higher at 1900 MHz and
10-20% higher at 835 MHz as compared to that for the larger head models. This is understandable since the
shielding effect of the pinna is larger at the higher frequency of 1900 MHz.
3. Similar to the results previously reported for head models of adult and the children [1], there is a deeper
penetration of absorbed energy for the smaller head models as compared to that for the larger head models [6].
IV. COMPARISON WITH SAR FOR PLASTIC EAR MODELS
A 6 mm plastic spacer homogeneous model with brain-simulant properties is presently used by industry and has also
been proposed to obtain a "conservative" determination of the peak 1-g SAR of body tissues for SAR compliance
testing of cellular telephones [5]. An identical 6 mm "plastic ear" homogenous model has also been proposed to
determine peak 10-g all-tissue SAR for compliance testing against the ICNIRP Guidelines [4]. For our calculations,
smooth ear models corresponding to the two anatomic models of Fig. 1a, b is used. For example, the model thus
developed for the Utah model of the human head shown in Fig. 1a is given in Fig. 2. A dielectric constant
ε
r
= 2.56
is assumed for the plastic in the shape of the smoothened ear and the 2 mm thick container, while homogeneous
dielectric properties representative of the head tissues (
ε
r
= 41.5,
σ
= 0.9 S/m at 835 MHz and
ε
r
= 40.0,
σ
= 1.4
S/m at 1900 MHz) are assumed for the rest of the model [5].
Fig. 2. A proposed 6 mm thick smooth lossy ear phantom to obtain peak 1- and 10-g SARs within ±10-15% of those
obtained for anatomically-realistic models. This model with the lossy ear replaced by a 6 mm thick plastic ear
(
ε
r
= 2.56) gives SARs that are a factor of 2 or more lower as given in Tables 2, 3.
In Tables 2 and 3, we compare the peak 1-g all tissue and body tissue SARs and peak 10-g any tissue SARs calculated
for the average size versions of both Utah and "Visible Man" models, with the corresponding 6 mm thick smooth
plastic ear average models of homogeneous brain-simulant dielectric properties suggested in [4, 5]. The calculated
SARs are normalized to the maximum possible radiated power of 125 mW for the PCS telephones at 1900 MHz and
600 mW for the analog AMPS mode of 835 MHz telephones. As seen from Tables 2 and 3, the plastic ear model gives
peak 1- and 10-g SARs that are lower by factors of two or more than the all-tissue SARs required for compliance testing
against the present ANSI/IEEE and ICNIRP safety guidelines, respectively. This is due to a physical separation of 6
mm and absence in the plastic ear model of the high SAR region e.g. the pinna. For an anatomic model, on the other
hand, the lossy ear acts as a coupler conducting EM fields into the head resulting in higher SARs.
V. A PROPOSED EXPERIMENTAL PHANTOM FOR SAR COMPLIANCE TESTING
To alleviate the problem of underestimation of SARs with the 6 mm thick plastic spacer (in the shape of pinna)
phantoms, we propose a smoothened ear model of the human head such as that shown in Fig. 2. Since the SAR in the
pinna region is substantial, the proposed phantom will use homogeneous lossy dielectric properties (
ε
r
= 41.5,
σ
=
0.9 S/m at 835 MHz and
ε
r
= 40.0,
σ
= 1.4 S/m at 1900 MHz) everywhere including the volume occupied by the
smoothened pinna. For a 2 mm plastic shell of
ε
r
= 2.56 or
ε
r
= 4.0 assumed to contain the fluid for the desired
dielectric properties, the calculated peak 10-g all-tissue SARs are within ±15% of the SARs obtained with realistic
anatomic model of the head. A similar agreement for peak 1-g SARs within ±10% is also obtained for this thin shell
lossy pinna phantom with the SARs for anatomic models at 1900 MHz [6], but the SARs are still considerably smaller
at 835 MHz. At this lower frequency, it may be possible to use a higher conductivity fluid to get SARs to agree better
with those of anatomically-realistic models.
Table 1. Comparison of the peak 1-g SARs for the various sizes of the two models of the head.
Handset
Dimensions
mm
Antenna Model Tissue
Peak 1-g SAR (W/kg)
Larger
Head Model
Average
Head Model
Smaller
Head Model
1900 MHz, 125 mW Radiated Powe
r
835 MHz 600 mW Radiated Power
22x42x122
22 x42x122
22x42x82
22x42x122
22x42x122
Helix 20 mm length
Helix 20 mm length
Helix 20 mm length
Monopole 40 mm length
Monopole 40 mm length
Utah
Visible man
Utah
Visible man
Utah
Body tissues
Brain
Body tissues
Brain
Body tissues
Brain
Body tissues
Brain
Body tissues
Brain
0.96
0.22
1.16
0.09
0.89
0.22
0.95
0.13
0.76
0.25
1.32
0.31
1.22
0.13
1.23
0.33
1.00
0.14
1.02
0.33
1.45
0.45
1.61
0.18
1.39
0.48
1.21
0.25
1.13
0.45
22x42x122
22 x42x122
22x42x122
22x42x122
22x42x102
Helix 20 mm length
Helix 20 mm length
Monopole 80 mm length
Monopole 80 mm length
Monopole 80 mm length
Utah
Visible man
Utah
Visible man
Utah
Body tissues
Brain
Body tissues
Brain
Body tissues
Brain
Body tissues
Brain
Body tissues
Brain
3.84
0.83
3.88
0.37
2.92
1.04
3.01
0.64
2.65
1.08
3.91
1.03
4.29
0.49
3.20
1.24
3.29
0.80
2.81
1.28
4.20
1.20
4.36
0.56
3.47
1.38
3.43
0.97
2.86
1.43
REFERENCES
[1] 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. Microwave Theory and Techniques, vol. 44, pp. 1884-1897,
1996.
[2] C. Lentner, Geigy Scientific Tables, vol. 3, Basel, Switzerland: CIBA-GEIGY, 1984.
[3] G. Lazzi and O.P. Gandhi, "Realistically-tilted and truncated anatomically-based models of the human head for
dosimetry of mobile telephones, IEEE Trans. on Electromag. Compat., vol. 39, pp. 51-61, 1997.
[4] CENELEC European Standard EN50361, "Basic standard for the measurement of specific absorption rate related to
human exposure to electromagnetic fields from mobile phones (300 MHz-3 GHz)," CENELEC European
Committee for Electrotechnical Standardization, 2001.
[5] IEEE Standards Coordinating Committee 34, IEEE Recommended Practice for Determining the Spatial-Peak
Specific Absorption Rate (SAR) in the Human Body Due to Wireless Communication Devices, Draft standard,
2001.
[6] O.P. Gandhi and G. Kang, "Some present problems and a proposed experimental phantom for SAR compliance
testing of cellular telephones at 835 and 1900 MHz," Physics in Medicine and Biology, vol. 47(8), pp. 1501-1518,
May 7, 2002.
Table 2. Comparison of the peak 1-g SARs for all tissues (ANSI/IEEE Guidelines) and body tissues for two
anatomically-based models with the corresponding 6 mm thick smooth plastic ear models.
Peak 1-g SAR (W/kg)
Model
Monopole, 40 mm length
Monopole, 40 mm length
Monopole, 40 mm length
Helix, 20 mm length
Helix, 20 mm length
Helix, 20 mm length
Monopole, 80 mm length
Monopole, 80 mm length
Helix, 20 mm length
Helix, 20 mm length
Antenna
Handset
Dimensions
mm
22 x42 x122
22 x42 x82
22 x42 x82
22 x42 x122
22 x42 x82
22 x42 x82
22 x42 x122
22 x42 x122
22 x42 x122
22 x42 x122
1900 MHz, 125 mW Radiated Power
835 MHz, 600 mW Radiated Power
Utah
Utah
Visible man
Utah
Utah
Visible man
Utah
Visible man
Utah
Visible man
1.02
1.00
0.95
1.32
1.23
1.17
3.20
3.29
3.91
4.29
1.00
0.98
0.80
1.26
1.30
0.97
2.73
2.80
3.66
3.65
1-g body tissues,
anatomically-
based model
6 mm thick plastic
ear, homogeneous
model
1-g all tissues,
anatomically-
based model
2.40
2.24
2.55
3.05
2.96
3.37
9.09
3.43
13.20
4.46
Table 3. Comparison of the peak 10-g SARs for all tissues (ICNIRP Guidelines) for two anatomically-based models
with the corresponding 6 mm thick smooth plastic ear models.
Peak 10-g SAR (W/kg)
Model
Monopole, 40 mm length
Monopole, 40 mm length
Monopole, 40 mm length
Helix, 20 mm length
Helix, 20 mm length
Helix, 20 mm length
Monopole, 80 mm length
Monopole, 80 mm length
Helix, 20 mm length
Helix, 20 mm length
Antenna
Handset
Dimensions
mm
22 x42 x122
22 x42 x82
22 x42 x82
22 x42 x122
22 x42 x82
22 x42 x82
22 x42 x122
22 x42 x102
22 x42 x122
22 x42 x102
1900 MHz, 125 mW Radiated Power
835 MHz, 600 mW Radiated Power
Utah
Utah
Visible man
Utah
Utah
Visible man
Utah
Utah
Utah
Utah
0.62
0.61
0.56
0.77
0.76
0.67
1.96
2.28
2.64
3.27
Anatomically-
based model
1.14
1.09
1.03
1.44
1.37
1.28
4.03
3.58
5.53
5.02
6 mm thick plastic
ear, homogeneous
model
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Article
This paper discusses techniques for the use of accurate models of wireless communication devices for the determination of SAR compliance. We discuss how computer-aided design and manufacturing files can be used to provide accurate models of both the internal and external components of these types of devices. Results are compared from simulations involving two detailed representations of a commercially available mobile telephone, one of which has been constructed using primitive shapes (e.g., boxes and cylinders), and the other which has been derived from a computer-aided design file. As these simulations require a very high resolution, the use of parallel processing has been applied to the finite-difference time-domain method, to enable solution on an IBM SP-2 parallel supercomputer. As these resources are expensive and not widely available, an expanding-grid finite-difference time-domain code has been developed. This has allowed the use of these high-resolution simulations to be carried out using everyday workstation technology which is widely available. It should be stressed that the numerical simulation techniques discussed in this paper are particularly useful as they allow the compliance testing of a device before it is ever constructed. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 24–29, 1999.
Validation of the finite-difference time-domain method for near-field bioelectromagnetic simulations
Article
Validations of the accuracy of the FDTD method for near-field simulations are critical at this time to assess the accuracy of the FDTD method for the simulation of personal communication devices. Excellent comparisons between the FDTD method and analytical or measured results are shown for a dipole antenna next to a layered half-space, a layered box, and a sphere, and also an infinitesimal dipole near a sphere. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 16: 341–345, 1997.
Differences in Energy Absorption Between Heads of Adults and Children in the Near Field of Sources
Article
This paper was motivated by a recent article in which the levels of electromagnetic energy absorbed in the heads of mobile phone users were compared for children and adults at the frequencies of 835 MHz and 1,900 MHz. Significant differences were found, in particular substantially greater absorption in children's heads at 835 MHz. These findings contradict other studies in which no significant changes had been postulated. The clarification of this issue is crucial to the mobile communications industry since current SAR evaluations as required by the FCC are only performed with phantoms based on the heads of adults. In order to investigate the differences in absorption between adults and children due to their differing anatomies, simulations have been performed using head phantoms based on MRI scans of an adult (voxel size 2 x 2 x 1 mm3) and two children (voxel size 2 x 2 x 1.1 mm3) of the ages of 3 and 7 y. Ten different tissue types were distinguished. The differences in absorption were investigated for the frequencies of 900 MHz and 1,800 MHz using 0.45 lambda dipoles instead of actual mobile phones. These well-defined sources simplified the investigation and facilitated the comparison to previously published data obtained from several numerical and experimental studies on phantoms based on adults. All simulations were performed using a commercial code based on the finite integration technique. The results revealed no significant differences in the absorption of electromagnetic radiation in the near field of sources between adults and children. The same conclusion holds when children are approximated as scaled adults.
Comparison of numerical and experimental methods for determination of SAR and radiation patterns of handheld wireless telephones
Article
  • Feb 1999
Some recent developments in both the numerical and experimental methods for determination of SARs and radiation patterns of handheld wireless telephones are described, with emphasis on comparison of results using the two methods. For numerical calculations, it was possible to use the Pro-Engineer CAD Files of cellular telephones for a realistic description of the device. Also, we used the expanding grid formulation of the finite-difference time-domain (FDTD) method for finer-resolution representation of the coupled region, including the antenna, and an increasingly coarser representation of the more-distant, less-coupled region. Together with the truncation of the model of the head, this procedure led to a saving of computer memory needed for SAR calculations by a factor of over 20. Automated SAR and radiation pattern measurement systems were used to validate both the calculated 1-g SARs and radiation patterns for several telephones, including some research test samples, using a variety of antennas. Even though widely different peak 1-g SARs were obtained, ranging from 0.13 to 5.41 W/kg, agreement between the calculated and the measured data for these telephones, five each at 835 and 1900 MHz, was excellent and generally within +/-20% (+/-1 dB). An important observation was that for a maximum radiated power of 600 mW at 800/900 MHz, which may be used for telephones using AMPS technology, the peak 1-g SARs can be higher than 1.6 W/kg unless antennas are carefully designed and placed further away from the head.
Changes in the dielectric properties of rat tissue as a function of age at microwave frequencies
Article
  • Jul 2001
The dielectric properties of ten rat tissues at six different ages were measured at 37 degrees C in the frequency range of 130 MHz to 10 GHz using an open-ended coaxial probe and a computer controlled network analyser. The results show a general decrease of the dielectric properties with age. The trend is more apparent for brain, skull and skin tissues and less noticeable for abdominal tissues. The variation in the dielectric properties with age is due to the changes in the water content and the organic composition of tissues. The percentage decrease in the dielectric properties of certain tissues in the 30 to 70 day old rats at cellular phone frequencies have been tabulated. These data provide an important input in the provision of rigorous dosimetry in lifetime-exposure animal experiments. The results provide some insight into possible differences in the assessment of exposure for children and adults.
SAR calculations in an anatomically realistic model of the head for mobile communication transceivers at 900 MHz and 1.8 GHz
Article
  • Nov 1994
A new mathematical model of the head has been constructed from a set of serial MRI slices from one subject. Finite-difference time-domain (FDTD) calculations of the specific energy absorption rate (SAR) have been performed on this model with a 2 mm resolution for a generic mobile communication transceiver represented by a quarter-wavelength monopole on a metal box. The antenna was mounted either at the centre or corner of the top face of the box. The frequencies considered were 900 MHz and 1.8 GHz. Three irradiation geometries were considered, a vertical handset in front of the eye and vertical and horizontal orientations at the side of the ear. The effect of a hand grasping the handset was considered. The head model was scaled to represent the head of an infant and a subset of calculations was performed to verify that the SAR deposited in the infant head did not exceed that in the adult. Results are also presented for a half-wavelength dipole. The maximum SAR values produced by the generic transceiver for the horizontal orientation at the side of the head which is the most typical position, averaged over 10 g of tissue at 900 MHz and 1.8 GHz, are 2.1 and 3.0 W kg(-1) per W of radiated power. The corresponding values over 1 g of tissue are 2.3 and 4.8 W kg(-1) per W at 900 MHz and 1.8 GHz. However, if one were to consider all possible operational conditions, the placement of the transceiver in front of the eye will give 3.1 and 4.6 W kg(-1) per W averaged over 10 g of tissue and 4.7 and 7.7 W kg(-1) per W over 1 g of tissue at 900 MHz and 1.8 GHz, respectively.
Temperature rise for the human head for cellular telephones and forpeak SARs prescribed in safety guidelines
Conference Paper
The bioheat equation is solved for an anatomically-based model of the human head with resolution of 3×3 mm to study the thermal implications of exposure to EM fields typical of cellular telephones both at 835 and 1900 MHz. Up to 4.5°C temperature elevation may be caused for some locations of the pinna by a cellular telephone warmed by electronic circuitry to temperatures as high as 39°C, with temperature increases for the internal tissues such as the brain and the eye that are no more than 0.1-0.2°C higher than the basal values. Another objective was to study the thermal implications of the SAR limits for the occupational exposures of 8 W/kg for any 1-g, or 10 W/kg for any 10-g of tissue suggested in the commonly used safety guidelines. Such SARs would lead to temperature elevations for the electromagnetically exposed parts of the brain up to 0.5°C, with 10 W/kg for any 10-g of tissue resulting in somewhat higher temperatures for larger volumes
A planar diversity antenna for handheld PCS devices
Article
A polarization diversity antenna (PDA) is described, and its performance is compared to that of a monopole antenna at frequencies near 900 MHz. Numerical modeling of each antenna, using the finite-difference time-domain (FDTD) technique, incorporates a cellular telephone handset in the vertical orientation, the user's head and hand, and the mobile communications environment. Results indicate that the two modes of the PDA are sufficiently uncorrelated for diversity operation and that, overall, the values of the mean effective gain (MEG), efficiency, and averaged specific absorption rate (SAR) in the head are better for the PDA than for the monopole antenna. However, in terms of the MEG, the PDA is more sensitive than the monopole antenna to the presence of the user's body. For the PDA, most of the power absorbed in the user's body is deposited in the hand, whereas for the monopole antenna, most of the absorbed power is deposited in the head. For both antennas, the MEG depends on the environment (urban or suburban)