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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