# A Novel Cellular Handset Design for an Enhanced Antenna Performance and a Reduced SAR in the Human Head

Authors: Salah I. Al-Mously; Marai M. Abousetta
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

This paper presents a novel cellular handset design with a bottom-mounted
short loaded-whip antenna. This new handset design is modeled and simulated
using a finite difference time-domain (FDTD)-based platformSEMCAD. The proposed handset is based on a current commercially
available bar-phone type with a curvature shape, keypad positioned above the
screen, and top-mounted antenna. The specific absorption rates (SARs) are
determined computationally in the specific anthropomorphic mannequin
(SAM) and anatomically correct model of a human head when exposed
to the EM-field radiation of the proposed cellular handset and the handset
with top-mounted antenna. The two cellular handsets are simulated to operate at
both GSM standards, 900 MHz as well as 1800 MHz, having different
antenna dimensions and intput power of 0.6 W and 0.125 W,
respectively. The proposed human hand holding the two handset models is
a semirealistic hand model consists of three tissues: skin, muscle, and bone.
The simulations are conducted with handset positions based on the IEEE
standard 1528-2003. The results show that the proposed handset has a
significant improvement of antenna efficiency when it is hand-held close to head,
as compared with the handset of top-mounted antenna. Also, the results show that
a significant reduction of the induced SAR in the human head-tissues can be
achieved with the proposed handset.


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## Body

## 1. Introduction

Due to enormous increase in the number of
cellular handset users around the world, many questions are raised about the
possible hazard effect of the cellular handset electromagnetic field (EMF)
radiation. Thereby, health concerns regarding the use of a cellular handset
near the human head have been growing and took a lot of attention by
researchers.The
interaction of the cellular handset with the human head has been investigated
by many published papers with considering; first, the effect of the human head
on the handset antenna performance, including the feed-point impedance, gain,
and efficiency [1–4], second, the impact of the antenna EM radiation on the
user’s head due to the absorbed power, which is measured by predicting the
induced specific absorption rate (SAR) in head tissues [5, 6].The
protocol and procedures for the measurement of the peak spatial-average SAR
induced inside a simplified head model of the cellular handset users are
specified by IEEE Standard-1528 [7] and IEC 62209-1 [8]. Both standards
specified the specific anthropomorphic mannequin (SAM) as a simplified physical
model (phantom) of the human head. This SAM has also been adopted by many
committees, associations, and commissions [9–11]. The SAM has been developed by
the IEEE Standards Coordinating Committee 34, Subcommittee 2, Working Group 1
(SCC34/SC2/WG1) as a lossless plastic shell, filled with a homogeneous liquid,
and a thin lossless ear spacer, whereas (SCC34/SC2/WG2) has suggested the same
SAM but with different plastic shell parameters [5].Anatomically
correct models of a nonhomogeneous human head at different ages were used to
evaluate the performance of the handset on a human-head phantom [5, 12, 13]. In
this paper, a nonhomogeneous high-resolution numerical correct model of a European
female head [14], available with SPEAGE-Schmidt & Partner Engineering AG [15], is used.Handset
models with a keypad positioned above the screen are available commercially.
Linux released a more comfortable of such a handset with a top-mounted external
antenna and a curvature shape [16]. This new design ensures that much of the
handset rests in the palm of the hand, thus, improving support and control. In
addition to the improved grip, the thumb rests in a comfortable position
directly above the buttons of the keypad. The improved angle for the thumb
makes it unnecessary to shift the handset around in the hand while typing text
[16].In this
paper, the proposed handset design with a bottom-mounted antenna is based on
the handset model in [16]. An FDTD-based platform SEMCAD [15] is used
for simulation. The Antenna performance is evaluated for both handset models in
free space, hand-held, and hand-held close to head. A semirealistic hand model
consists of three tissues is designed to simulate the human hand. The induced
SAR’s in head models are evaluated at GSM standards, 900 MHz and 1800 MHz, with
antenna  intput power of 0.6 W and 0.125 W, respectively. Handset positions, cheek and tilt (15°), with respect to head are adopted according to
IEEE standard 1528 [7].


## 2. Cellular Handset Design and FDTD Simulation

## 2.1. Handset Structure

The handset
model in [16] (will be referred later as model no. 1) is simulated using an
FDTD-based platform SEMCAD (Simulation Platform for Electromagnetic
Compatibility, Antenna Design and Dosimetry) ver. 12 JUNGFRAU [15]. The
proposed handset with a bottom-mounted antenna (will be referred later as model
no. 2) is also designed and simulated, where most handset components, such as PCB,
LCD, Battery, and keypad, are considered in the design simulation. These
components are not located identically in both handset models due to different
antenna positions. Both models are simulated to operate at 900 MHz as well as 1800 MHz.Figure1(a)
shows the physical model of the handset released by Linux [16], whereas Figure
1(b)
exhibits the proposed physical model with bottom-mounted antenna. Figure
2
shows the numerical equivalent of both physical models used for the FDTD
simulation. The maximum dimensions of both handsets are set to 45×16×130 mm
with a PCB symmetrically embedded inside the housing. The acoustic output position
is set according to IEEE standard 1528 [7].
Figure 3  shows the numerical
components structure of the handset models. The dielectric parameters of handset
materials given in [6] are used.The physical model of (a) the handset
released by Linux, and (b) the proposed handset with bottom-mounted antenna.
(a)(b)The CAD representation of both handset models.
(a)(b)Numerical components structure of (a) the
handset model no. 1, and (b) the proposed handset model no. 2.
(a)(b)


## 2.2. Antenna Design and Specifications

Instead
of using a helical antenna, a short-whip antenna top loaded with a small
cylinder [17] is suggested for both designs of models as depicted in
Figure 4 .
Table 1 shows the physical and electrical antenna specifications that optimized
at both GSM standards for both handset models.Table 1
The
proposed antenna dimensions and specifications for both handset design models
at different frequencies.
Model   no. 1FrequencyMatching   lumped elementL1D1L2D2Impedance in ohm900 MHz29.65 nH19 mm1 mm2 mm6 mm46.4+j0.01800 MHzNo matching needed18 mm1 mm2 mm6 mm47.3−j0.016Model   no. 2FrequencyMatching
lumped elementL1D1L2D2Impedance in ohm900 MHz25.24 nH23 mm1 mm2 mm6 mm42.2+j0.01800 MHzNo matching needed22 mm1 mm2 mm6 mm47.9−j0.001Figure 4
The proposed loaded short-whip antenna with
dimensions.


## 3. Grid Generation and Simulation Factors Setting

## 3.1. Cellular Handset in Free-Space

To align the simulated handset components to the FDTD grid accurately, a minimum spatial
resolution of0.1×0.1×0.1mm3 and maximum spatial resolution of
5×5×5mm3 in the x, y, and z
directions are chosen with grading
ratio of 1.2. For the handset model no. 1, the mesh cells amounts are 4.58979
Mcells and 3.95494 Mcells, at 900 MHz and 1800 MHz, respectively, whereas for the
model no. 2, the mesh cells amounts are 6.82675 Mcells and 4.89154 Mcells, at 900 MHz and 1800 MHz, respectively.


## 3.2. Cellular Handset in Hand

A semirealistic
human-hand model consists of three tissues: skin, muscle, and bone, are
designed usingSEMCAD [15] to simulate both handset models in hand, as
shown in Figure 5. The FDTD grid has a minimum spatial resolution of
0.5×0.5×0.5mm3 and maximum spatial resolution of 10×10×10mm3 in the x, y, and z directions, with grading ratio of 1.2. For the hand-held of
model no. 1, the mesh cells amounts are 4.58979 Mcells and 3.95494 Mcells, at
900 MHz and 1800 MHz, respectively, whereas for the hand-held of model
no. 2, the mesh
cells amounts are 6.82675 Mcells and 4.89154 Mcells, at 900 MHz and 1800 MHz,
respectively.The CAD representation of the proposed semirealistic
hand model holding the proposed handset; (a) all hand tissues, (b) hand-bones
only.
(a)(b)


## 3.3. Cellular Handset in Hand Close to Head

As
defined in IEEE standard 1528-2003 [7], two handset positions are considered in
presence of human-head, cheek and tilt (15°). The head
is simulated using both, homogeneous and nonhomogeneous phantoms.The
homogeneous head model is a SAM phantom available with [15] and consists of two
dielectric materials, shell and liquid. The material parameters are defined in
[7, 8], with shell and ear spacer defined in [5], at 900 MHz and 1800 MHz.The nonhomogeneous
head phantom is a high-resolution European 40-year female head (HR-EFH), derived
from MRI scan [15], and is imported to the SEMCAD platform. This CAD
phantom consists of 121   different slices, with slice thicknesses of 1 mm (ear
region) and 3 mm, and a transverse spatial resolution of
0.2 mm. The following
different 25 tissues are recognized: air,
blood vessel, bones, brain/grey matter, brain/white matter,
cerebellum, cerebrospinal fluid, ear (cartilage), eye-cornea, eye-lens, eye-
vitreous body, fat, jaw, mastoid cells (bones), mid-brain, muscles, nasal
cavity, parotid gland, spin, skull, spinal cord, spine, thalamus, tongue, and ventricles.Head and hand tissues properties are set
according to the material properties data-base in [15] and to that given in
[18], where both are based on [19].The FDTD-grid for each handset in hand close to
head has a minimum spatial resolution of0.5×0.5×0.5mm3 and maximum
resolution of 10×10×10mm3 in the x, y, and z directions with
grading ratio of 1.2. The absorbing boundary conditions (ABCs) are set as a
perfectly matched layer (PML) mode with a very high-strength thickness [15].Table2 lists the amounts of mesh cells according
to FDTD-grid setting for both handset models in hand close SAM and HR-EFH, at
900 MHz and 1800 MHz.Table 2
The generated FDTD-grid properties of both handset models in
hand close to head, SAM and HR-EFH.
HR-EFH-Head phantomFrequencyHandsetMesh cells amount/CheekMesh cells amount/Tilt900 MHzModel no. 1276∗253∗300 = 20.9484 Mcells284∗241∗310 = 21.2176 Mcells900 MHzModel no. 2290∗251∗281 = 20.4540 Mcells282∗239∗305 = 20.5564 Mcells1800 MHzModel no. 1268∗244∗288 = 18.8329 Mcells276∗233∗302 = 19.4210 Mcells1800 MHzModel no. 2289∗242∗277 = 19.3728 Mcells274∗231∗296 = 18.7350 McellsSAM-Head phantomFrequencyHandsetMesh cells amount/CheekMesh cells amount/Tilt900 MHzModel no. 1208∗135∗234 = 6.57072 Mcells208∗131∗252 = 6.86650 Mcells900 MHzModel no. 2230∗137∗217 = 6.83767 Mcells230∗131∗223 = 6.71899 Mcells1800 MHzModel no. 1200∗127∗230 = 5.84200 Mcells200∗123∗236 = 5.80560 Mcells1800 MHzModel no. 2222∗129∗209 = 5.98534 Mcells219∗120∗214 = 5.62392 McellsThe simulations (in all cases) assume a steady-state
voltage at the 900 and 1800 MHz, with a feed point of a 50-Ohm voltage source
of 1-mm gap. A transient excitation of 12 periods is set as guarantee to achieving
a steady state. The absorbing boundary conditions (ABCs) are set as a perfectly matched
layer (PML) mode with a very high strength thickness [15].In case of the handset close to head (both SAM
and HR-EFH), the acoustic output referenced to earpiece is set according to
IEEE standard 1528 [7]. Due to different antenna positions in both handset
models, the distances between the antennas feed points and the nearest tissue
voxel are different too. For the handset
model no. 1 the acoustic output position is set at the origin, whereas for the
handset model no. 2   the acoustic output position is set at (x=−15, y=0 and z=−104mm). Figures 6(a)  and 6(b)  show
both handset models close to head (SAM) at cheek position indicating the
coordinate system, whereas Figure 6(c) shows the handset model no. 2 in hand close
to head.Coordinate system; (a) the handset model no. 1 referenced
as seen from the right side of the SAM, atcheek position, (b) the handset
model no. 2 referenced as seen from the right side of the SAM, at cheek position, and (c) the handset model no. 2 in hand close to SAM at cheek position.
(a)(b)(c)


## 4. EM Interaction between the Handset Anatenna and Human Head

The EM interaction between the handset antenna and
human head is evaluated by; first, evaluating the effect of human head and hand on the handset antenna
performance through computing the
antenna parameters, including input return loss, gain, radiation efficiency,
and total efficiency, second, evaluating the impact of antenna EM radiation on the
head through computing the induced SAR and power absorption.


## 4.1. Antenna Performance

Table3
demonstrates the antenna parameters including; input return loss, gain,
radiation efficiency, and total efficiency, for both handset models in all
cases at 900 MHz. Table 4 lists the antenna parameters at 1800 MHz.
Figure 7 shows
the radiation beam pattern in (V/m) for both handset models in hand close to
HR-EFH at cheek position and for both 900 and 1800 MHz frequencies,
whereas Figure 8  shows the radiation beam pattern at tilt position.Table 3
Computational
results of the antenna performance parameters of both handset models at 900 MHz
in all cases.
Frequency900 MHz|S11| in (dB)Gain (dBi)Radiation efficiencyTotal efficiencyHandset modelModelModelModelModelModelModelModelModelno. 1no. 2no. 1no. 2no. 1no. 2no. 1no. 2Handset in free-space−28.4−21.51.721.885.76%86.9%85.63%86.33%Handset in hand only−12.9−15.41.23−0.648.4%41.0%45.9%39.7%Handset in hand close to SAM
(Cheek position)−15.4−17.5−5.98−5.867.3%11.7%7.1%11.5%Handset in hand close to SAM (Tilt position)−17.3−18.2−2.75−2.518.8%21.5%18.5%21.2%Handset in hand close to HR-EFH (Cheek position)−13.8−24.2−5.5−3.512.8%17.3%12.3%17.2%Handset in hand close to HR-EFH (Tilt position)−17−19−2.8−2.125.0%25.6%24.5%25.3%Table 4
Computational
results of the antenna performance parameters of both handset models at 1800 MHz in all cases.
Frequency1800 MHz|S11| in (dB)Gain (dBi)Radiation efficiencyTotal efficiencyHandset modelModelModelModelModelModelModelModelModelno. 1no. 2no. 1no. 2no. 1no. 2no. 1no. 2Handset in free-space−31.2−33.43.93.895.3%95.8%95.2%95.7%Handset in hand only−19−17.22.90.8667.1%50.0%66.2%49.0%Handset in hand close to SAM
(Cheek position)−17.8−22.80.2−0.1522.3%30.1%22%30.0%Handset in hand close to SAM (Tilt position)−15.4−210.670.8626.1%36.7%25.0%36.4%Handset in hand close to HR-EFH (Cheek position)−17.1−211.30.225.0%33.4%24.5%33.1%Handset in hand close to HR-EFH (Tilt position)−16.5−19.20.60.4727.2%39.2%26.6%38.7%The three-dimensional radiation pattern in
(V/m) of both handset models in hand close to HR-EFH atcheek position
and operating at different frequencies.
(a)
Handset model no. 1 atcheek position
and operating at 900 MHz(b)
Handset model no. 2 atcheek position
and operating at 900 MHz(c)
Handset model no. 1 atcheek position
and operating at 1800 MHz(d)
Handset model no. 2 atcheek position
and operating at 1800 MHzThe three-dimensional radiation pattern in
(V/m) of both handset models in hand close to HR-EFH attilt position
and operating at different frequencies.
(a)
Handset model no. 1 attilt position
and operating at 900 MHz(b)
Handset model no. 2 attilt position
and operating at 900 MHz(c)
Handset model no. 1 attilt position
and operating at 1800 MHz(d)
Handset model no. 2 attilt position
and operating at 1800 MHz


## 4.2. SAR and Power Loss Computation in Head

The impact of the electromagnetic (EM) wave
irradiation on the  living body is measured by evaluating the SAR which is
defined as the amount of EM energy absorption in the unit mass as follow [20]:(1)SAR=σEρ|E|2, where σE (S/M) is the
conductivity, E (V/m) is the the induced electric field vector, and ρ (kg/m3)
is the material density. Using SEMCAD platform, an algorithm based on
SCC34/SC2/WG2 computational dosimetry, IEEE-1529 [21], the spatial peak SAR can
be computed over any required mass.The spatial-peak SAR should be evaluated in a
cubical volume of the body tissues that is within 5% of the required mass [15].
The averaged peak-SAR (Spatial-peak SAR [IEEE-1529]) can be specified over a
cube of 1g and 10g mass, and normalized to a certain source power. Referred to the
IEEE standard C95.1b-2004 [22] (for low-power devices, uncontrolled
environment), the antenna input power is
set to 0.6 W at 900 MHz and 0.125 W at 1800 MHz, respectively, in all cases.Table5 lists the computed peak SAR averaged over
1g and 10g, and the absorbed power in tissues, for both handset models at both
positions and at 900 MHz. Table 6 lists the computed parameters at 1800 MHz.Table 5
The computed averaged peak-SAR (over
1g and 10g) and power absorption in tissues, radiated power, total loss, total
isotropic sensitivity, and computation error for both handset models in hand close
to head at different positions and at 900 MHz.
900 MHz-Cheek900 MHz-TiltSAMHR-EFH (Adult)SAMHR-EFH (Adult)Handset modelModel no. 1Model no. 2Model no. 1Model no. 2Model no. 1Model no. 2Model no. 1Model no. 2Input power (mW)600600600600600600600600Peak-SAR1g(W/Kg) in head4.233.342.992.721.861.344.171.09Peak-SAR10g(W/Kg) in head3.022.382.552.271.290.981.400.92Peak-SAR1g(W/Kg) in hand1.442.701.692.932.023.542.183.57Peak-SAR10g(W/Kg) in hand0.821.250.891.311.181.651.191.65Radiated power (mW)42.6069.0074.00103.50127.20110.10147.00152.00Absorbed power in head
(mW)335.20241.40312.00218.50206.80126.60206.00122.00Absorption rate in head
(%)55.8740.2352.0036.4234.4721.1034.3320.33Absorbed power in hand
(mW)92.27161.5094.30167.90133.20224.80130.00207.00Total loss (mW)103.26107.20109.0099.83106.20115.50108.70110.00Total isotropic sensitivity
(dBm)−94.5−96.6−97−98.4−99.3−98.7−99.9−100.1Computation error (%)4.43.51.81.74.82.90.81.6Table 6
The computed averaged peak-SAR (over
1g and 10g) and power absorption in tissues, radiated power, total loss, total
isotropic sensitivity, and computation error for both handset models in hand close
to head at different positions and at 1800 MHz.
1800 MHz-Cheek1800 MHz-TiltSAMHR-EFH (Adult)SAMHR-EFH (Adult)Handset modelModel no. 1Model no. 2Model no. 1Model no. 2Model no. 1Model no. 2Model no.  1Model no. 2Input power (mW)125125125125125125125125Peak-SAR1g(W/Kg)
in head1.380.471.810.281.290.141.930.15Peak-SAR10g(W/Kg)
in head0.870.301.130.180.820.080.970.10Peak-SAR1g(W/Kg)
in hand0.731.220.731.250.801.470.801.43Peak-SAR10g(W/Kg)
in hand0.420.640.420.660.450.710.460.73Radiated power (mW)27.5037.4530.6541.4031.6745.5533.3448.40Absorbed power in head
(mW)59.5025.4661.0021.5051.1812.3954.2012.16Absorption rate in head
(%)47.6020.3748.8017.2040.949.9143.369.73Absorbed power in hand
(mW)24.4051.2824.9752.3029.0055.2029.5555.57Total loss (mW)7.667.217.437.807.667.847.197.65Total isotropic sensitivity
(dBm)−98.8−100.8−100−101.3−100.1−101.6−100.3−101.9Computation error (%)4.43.81.41.54.43.20.61.0Figure9 shows the sliced-distribution of the
averaged peak  SAR1g  in the HR-EFH phantom exposed to EM radiation of both model no. 1
and model no. 2  antennas at cheek position and at different frequencies,
whereas Figure 10  shows the sliced-distribution of the averaged peak  SAR1g  in
the HR-EFH phantom exposed to EM radiation at tilt position.Figure 9
Sliced-distribution of the averaged peak SAR1g
in the yz-plane of the HR-EFH
phantom in cases of handset models at cheek position. The antenna input powers are 0.6 W
and 0.125 W for the frequencies 900 MHz and 1800 MHz, respectively. (a) Model no. 1 at 900 MHz,
(b) Model  no. 2 at
900 MHz, (c) Model no. 1  at 1800 MHz,
(d) Model no. 2  at 1800 MHz.Figure 10
Sliced-distribution of the averaged peak SAR1g
in the yz-plane
of the HR-EFH
phantom in cases of handset models at tilt position. The antenna input powers are 0.6 W and
0.125 W for the frequencies 900 MHz and 1800 MHz, respectively. (a) Model no. 1  at 900
MHz, (b) Model no. 2  at 900 MHz,
(c) Model no. 1  at 1800 MHz,
(d) Model no. 2  at 1800 MHz.


## 5. Total Isotropic Sensitivity

The total isotropic sensitivity (TIS) [15] is a
measure of the handset receiving performance. The TIS and TRP (total radiated
power) together determine effectiveness of the handset as a piece of radio
equipment, in particular the maximum range at which the handset can operate
from the base station with some given level of performance [23]. The computed
TIS for both handset models at 900 MHz and 1800 MHz are given in Tables 5 and
6.


## 6. Computation Error

The computation error is defined as [24] (2)Computation error=|Pin−(Prad+Pabs+PLoss)|/Pin,PLoss=Pd+Pc,where Pin is the input power, Prad is the
radiation power, Pabs is the absorbed power in tissues,
and PLoss is the total power loss. PLoss includes the dielectric loss (Pd) and the metallic ohmic loss
(Pc).


## 7. Discussion

The
results in Tables3 and 4 reveal that presence of a head close to the handheld set
of model no. 1 degrades the handset performance, significantly reducing the handset
total efficiency to about (8%–28%) of
the total efficiency of the handset in free space. Adopting a bottom-mounted
antenna, model no. 2, the total efficiency of the handset model no. 1 can be improved
by (3.3%–45.5%),
whereas the gain is reduced by (0.19–2.15
dBi). The antennas of both handset models were matched well for all the cases.Since the proposed handset model has an antenna
in a low-noise area of the handset and well separated from the potentially
noisy components, it has the potential to achieve better TIS. According to the results
obtained in Tables5 and 6, the
different cases of the handset model no. 2  in hand close to head do show better
TIS values, as compared with model no. 1, due to the improved total efficiency.Moreover, Tables5 and 6 show that the averaged
peak-SAR1g induced in head close to hand-held of model no. 1 can be
reduced by (28%–92.2%)
using the proposed handset model no. 2, and the power absorbed in head can also be
reduced by (27.9%–77.5%). The
computation errors are less than 2% for all cases in presence of HR-EFH, whereas
for the cases of SAM presence they are (1.4%–4.4%).The differences in the induced SAR and
absorption power values in both SAM and HR-EFH phantoms are due to their
different masses, volumes, and densities distribution. According to simulation results,
HR-EFH mass is approximately 4.71 kg and the volume is approximately4118cm3,
while the SAM mass is approximately 6.024 kg (considering a homogeneous density
of 1000kg/m3) and the volume is approximately 6043cm3.The
proposed human-hand model mass is approximately 0.248 kg and its volume is
approximately186cm3.All computations are performed on a 2.0-GHz
Intel centrino Laptop machine (Dell, inspiron-630 m)
with 2 GB memory (dual-channel technology), and operating under MS
Windows-vista. The runtime and memory requirements depend on the simulation
space. Less memory and runtime are required for the handset simulation in free
space, whereas, more memory and runtime are required for the handset in hand
close to head. The machine-memory is enough to achieve all simulations with the
mesh cells amounts listed in Table2. The runtimes are about 1–10 hours.


## 8. Conclusion

A cellular handset with a keypad over the
screen and a bottom-mounted antenna has been proposed and numerically modeled,
with the most handset components, using an FDTD-basedSEMCAD platform.
The proposed handset model is based on the commercially available model with a
top-mounted external antenna. Both homogeneous and nonhomogeneous head phantoms
have been used with a semirealistic hand design to simulate the handset in hand
close to head. The simulation results showed a significant improvement in the
antenna performance with the proposed handset model in hand close to head, as
compared with the handset of top-mounted antenna. Also, using this proposed handset,
a significant reduction in the induced SAR and power absorbed in head has been
achieved.



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*Source: 642572-2008-05-18.xml*