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Department of Electroscience
Master of Science Thesis
Niklas Aronsson & Daniel Askeroth
April 2002
A Comparative Study of
Electromagnetic Dosimetric
Simulations and Measurements
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Abstract
In this master thesis a comparative evaluation between SEMCAD simulations and
MapSAR measurements were made. SEMCAD is an electromagnetics simulation
platform derived from theFinite Difference Time Domain method, FDTD. MapSAR
is a measurements-tool for quick SAR assessment.
Specific Absorption Rate, SAR, is a quantity expressing the amount of radiated power
absorbed in a specific material. This is also the quantity in which health guidelines for
Radio Frequency, RF, devices are set. The parameter used in this thesis was the
maximum Peak SAR (i.e. not mass averaged SAR). SAR was the comparative
quantity used since it is MapSARs primary readout and it is also a quantity that
provides an easy comparison.
A simplified cellular phone was used during this thesis, allowing a variety of test
configurations and facilitating the import of the phone design into SEMCAD. The
results of this thesis indicate that changes in SAR relative the reference phone-
designs SAR was coherent between simulations and measurements (for most cases).
However, most changes in SAR are within the level of uncertainty, hindering a more
explicit analysis of the results. Another cause for concern is that the absolute readings
from the two systems differ by a factor two. In this thesis, this is compensated for by
looking at relative changes to understand the impact of change in phone design.
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Preface
This master thesis was written during the winter 2001/2002 in Lund under the
supervision of Daniel Sjberg, PhD, at the Department of Electroscience.
We would also like to thank Ramadan Plicanic, MScEE. for assistance during theDASY3-measurements. Other people deserving our gratitude are Kenneth Hkansson,
MScEE for assisting us during the tedious soldering jobs and to Andr da Silva
Frazo, MScEE, Thomas Bolin, MScAP&EE, for their revision of our thesis. For
assisting us in the design of test cases Zhinong Ying, Licentiate has earned our
appreciation.
Lund March 2002
Niklas Aronsson & Daniel Askeroth
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ABSTRACT ............................................................................................................................ 1
PREFACE...............................................................................................................................2
1 INTRODUCTION.................................................................................................................. 6
1.1 Background ................................................................................................... ....................................... 6
1.2 SAR-Specific Absorption Rate ................................................................................... ....................... 6
1.3 Depth of Penetration...................................................................................... ..................................... 7
1.3 Antennas for Cellular Phones ............................................................................ ................................ 8
1.3.1 The Patch Antenna ........................................................................... ............................................ 10
1.4 Finite Difference Time Domain (FDTD), The Maxwell Equations.............................................10
1.5 Yees Algorithm.............................................................................. ................................................... 12
1.6 Numerical Stability ............................................................................... ............................................ 12
1.7 Boundary Conditions .......................................................................................... .............................. 131.7.1 Absorbing Boundary Conditions ...................................................................................... ........... 13
1.7.2 Conducting Boundary Condition...................................................................................... ........... 13
1.7.3 Periodic Boundary Condition .......................................................................... ............................ 14
1.8 Implementation ......................................................................................... ........................................ 14
2. MEASUREMENTS AND SIMULATIONS........................................................................... 15
2.1 The Box Phone............................................................................................. ...................................... 15
2.2 Antenna Matching Procedure..................................................................................... ..................... 16
2.3 Variations in the Box Phone Configuration ................................................................................... 172.3.1 Reference Layout ............................................................... .......................................................... 17
2.3.2 Ground Positions................................................................................... ....................................... 17
2.3.3 Resistors Layouts .................................................................................... ..................................... 17
2.4 MapSAR Measurement Equipment.............................................................................. .................. 182.4.1 MapSAR Measurements ............................................................................................ .................. 18
2.5 SEMCAD Introduction ........................................................................................ ............................ 19
3 RESULTS .......................................................................................................................... 20
3.1 70-m Copper Tape Layer.......................................................................... ..................................... 20
3.2 4-m Aluminium Layer ........................................................................................... ......................... 22
3.3 Display Opening .................................................................................... ............................................ 23
3.4 Resistor Measurements ................................................................................. ................................... 24
3.5 Reliability of the Measurement Procedure .................................................................. .................. 25
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4 DISCUSSION..................................................................................................................... 26
5. REFERENCES..................................................................................................................28
APPENDIX A: SOLIDS SPECIFICATIONS.......................................................................... 29
A.1 Box Phone Specifications............................................................................ ..................................... 29
A.2 Antenna and Carrier Specifications........................... .................................................................... 29
A.3 Spherical Phantom ............................................................................................. .............................. 29
A.4 Material Data............................................................................................... ..................................... 30
APPENDIX B: A SEMCAD PRIMER .................................................................................... 31
B.1 About...................................................................................................... ............................................ 31
B.2 Model Design with SEMCAD .............................................................................................. ........... 31
B.3 Sources and Sensors .................................................................................... ..................................... 32
B.4 Grid Settings ......................................................................................... ............................................ 34
B.5 Model Visualisation................................................................................... ....................................... 37
B.6 Simulation Settings ............................................................................................... ............................ 37
B.7 Starting a Simulation ........................................................................ ............................................... 40
B.8 Post-Processing ............................................................................................ ..................................... 41
APPENDIX C MAPSAR MANUAL........................................................................................ 44
C.1 Introduction ................................................................................... ................................................... 44
C.2 Description of the Graphical User Interface................................................................................. 44
C.3 Step by Step Measurements Guide for MapSAR......................................................................... 47
APPENDIX D: MEASUREMENTS RESULTS ...................................................................... 48
D.1 Measurement Results .............................................................................................. ......................... 48
D.2 Reference Designs: Near Field-Plots ............................................................................................. 49
D.3 Ground Positions: Near Field-Plots ............................................................................................... 50
D.4 Resistors: Near Field-Plots.................................................................................. ............................ 52
D.5 Ground Positions with Display: Near Field Plots........ ................................................................. 53
APPENDIX E: NUMERICAL RESULTS OF SEMCAD SIMULATIONS................................ 54
E.1 Perfect Conducting Front Layer ......................................................................................... ........... 54
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E.2 Perfect Conducting and Non Perfect Layers Comparison..........................................................54
E.3 Resistor Simulations without Display Opening ................................................................. ........... 54
E.4 Resistor Simulations with Display Opening.................................................................................. 54
E.5 No Front Layer ............................................................................................ ..................................... 54
APPENDIX F: DASY3 MEASUREMENTS............................................................................ 55
F.1 DASY3 Results ........................................................................................ .......................................... 55
F.2 DASY3 SAR Distribution Plots......................................................................... .............................. 55
APPENDIX G: ACRONYMS................................................................................................. 58
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1 Introduction
Wire-less communication has become an important factor in everyday life for
millions of people around the world. The widespread use of these radio frequency
transmitters has made the safety concerns a hot topic and safety regulations have
accordingly been set to limit the exposition. To help manufacturers comply with thesesafety standards it is vital to quickly assess the amount of radiation at any given stage
of research and development without going through the hassle of certified testing.
This thesis evaluates two different approaches, simulating antenna transmitters as well
as using a simple small-scale workbench-measuring device.
1.1 Background
The cellular phone market has increased substantially during the last decade. Along
with market growth the possible risks related to the use of cellular phones have
become an issue. As a measure of exposure to electromagnetic radiation SAR,
Specific Absorption Rate,has been used. SAR translates to the loss in transmitted
energy that is absorbed in human tissues, usually brain tissue.
To meet with consumer demands concerning safety, standardisation institutes such as
IEEE,Institute of Electrical and Electronics Engineers, and ICNIRP,International
Commission on Non-Ionizing Radiation Protection, have set guidelines for the
maximum SAR to enable a comparison between the various cellular phone
manufacturers and models. The SAR value has now become mandatory for the
manufacturers to be included with the phone.
All new models have to meet with guidelines and legal restrictions so all phone
models have to be tested. To prevent a phone prototype from having to be partially
redesigned after failing to meet with maximum SAR regulations the ability to
simulate or assess the maximum SAR at an early stage has become essential.
SAR-simulations of a realistic cellular phone interacting with a head phantom are
very computation-intensive. Therefore simulation tools capable of meaningful
simulations such as SEMCAD have not been available until recently. The simulation
programs are derivatives of numeric methods such as MoM,Method of Moment, FEM
Finite Element Method,and FDTD, theFinite Difference Time Domain. FDTD has
become the most common method due to its ability to work with complex problems
over a wide range of frequencies.
1.2 SAR-Specific Absorption Rate
SAR is a quantity that describes the amount of absorbed radiated effect for a specific
material at a certain frequency. The quantity can be derived from either the
temperature gain or from an electric field. The method used for measurements derives
the radiated effect from the electric field since the difference in temperature is too
small to measure in the cellular phone frequency band due to the low energies
involved at these low frequencies (i.e. non-ionising radiation). SAR has become the
most frequently used quantity involved when health issues are discussed. According
to the ANSI/IEEE (American National Standard Institute/Institute of Electrical and
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Electronics Engineers) standard the maximum SAR averaged over 1 g should not
exceed 1.6 W/kg and that the whole body mass averaged SAR should not exceed 0.08
W/kg. ICNIRPs guidelines for SAR in the head average over a 10 g cube and should
not exceed 2 W/kg.
Definition:
2absP E
==
=
tTcSAR (1.1)
Where:
t
T
- Changes in heat over time [K/s]
c - Specific heat capacity [J(kg K)-1]
- Conductivity [S/m]
- Density [kg/m3]
E - Electric field strength [V/m]
Pabs - Absorbed power within the 1 or 10 g cube [W]
1.3 Depth of Penetration
Different media react differently to electromagnetic radiation. One material parameter
that has been defined to classify these differences in various materials is the depth of
penetrationorskin depth[m]. The penetration depth represents the distance atwhich the wave amplitude has been attenuated by a factor e-1 = 0.368, which translates
to an exponential decay rate e-x/.
Biological tissue is a very complex media due to that within each cell there are
variations in electrical properties. However, the cell structures in biological tissue can
be simplified by averaging its dielectric properties resulting in an approximate
homogeneous lossy media with current conducting capabilities.
The depth of penetration for a lossy material can be calculated from the relative
permittivity r, conductivity and angular frequency as:
( )
=
+
=
r
r
04
12
2
00
arctan2
1,
)(sin
1 (1.2)
A good conductor is characterised by the limit , hence the penetration depth fora good conductor is:
0
1
f= (1.3)
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Conductor (copper) Brain tissue (homogenous)
Angular Frequency [rad/s] 2900106 2900106Permittivity 0[F/m] (36)-110-9 (36)-110-9Relative Permittivity r 1 42Permeability 0[H/m] 4107 4107Conductivity [S/m] 5.8107 1.1
Attenuation Constant [m-1] 4.54105 31.0Depth of Penetration [m] 2.010-6 3.210-2Table 1.1 A comparison between conductor and a lossy material at 900 MHz
As seen in the comparison in Table 1.1, the attenuation is very large in a good
conductor, which makes it well suited to block undesired radiation. For more
information on penetration depth in lossy media see [6] and [8].
1.3 Antennas for Cellular Phones
An antenna is a transmitting and/or receiving instrument for electromagneticcommunication. The antenna can be approached from two directions, either as a
radiator in free space or as an electrical component in an electrical circuit.
Every antenna has a specific radiation pattern describing its field properties. The
radiation pattern is defined as a mathematical function or a graphical representation
of the radiation properties of the antenna as a function of space coordinates. In most
cases, the radiation pattern is determined in the far-field region and is represented as a
function of the directional coordinates. Radiation properties include power flux
density, radiation intensity, field strength, directivity, phase or polarization.[1, p.28]
Figure 1.1 Spherical Coordinates
Antenna performance parameters such as gain and directivity can be derived from the
far-field radiation pattern. These two parameters are closely related where the relative
gain parameter is defined as the ratio of the power gain in a given direction to the
power gain of a reference antenna in its referenced direction.[1, p.58] The directivity
is "the ratio of the radiation intensity in a given direction from the antenna to the
radiation intensity averaged over all directions.[1, p.39]
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SWR1
0,log20 1111
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1.3.1 The Patch Antenna
Patch antennas in cellular phones are internal antennas. The patch antenna is a thin
metal sheet etched on a dielectric carrier structure. The metal patch plane is parallel to
the ground plane, see Figure 1.4. The metal patch can take on various shapes such as
circular, rectangular or irregular.
Another important parameter is the feed point that has to be considered since it playsa significant role to the characteristic impedance. The antenna feed structure might
differ, but usually the patch is fed from a pogo pin (a spring-mounted conductor) or a
micro strip.
Figure 1.4.The Single-band Patch Antenna
Simplified half-wave patch parameters from Figure 1.4 can be calculated, however
since these parameters will be altered due to reflections from the cellular phone cover
and to changes in load parameters no effort has been made to do so. The antenna feed
placement will also alter the results, so to optimise the antenna performance the whole
phone model has to be tested. For more information concerning antennas see [4] and
[11].
1.4 Finite Difference Time Domain (FDTD), The Maxwell Equations
Electromagnetics is the study of electric charges or in this case electric fields, i.e.
spatial distributions. There are three ways to predict electromagnetic fields and their
effects on different materials; by experiment, analysis or computation. Most problems
have the complexity to make a theoretical analysis impossible and experimentation
can be prohibitively costly and time consuming. Several numerical methods are today
available for electromagnetic computational analysis likeMethods of Moments
(MoM),Finite Element Method(FEM) andFinite Difference Time Domain(FDTD).
These applications have become more and more useful with the increasing
accessibility of computer power that the last decades have brought. FDTD is wellsuited for calculating transients as well as single frequencies or continuous wave
sources.
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The Maxwell equations describe the electromagnetic field behaviour completely as
long as the data, such as the material properties and initial conditions, are known. In
equation 1.14-17 the Maxwell equations are presented in their differential form.
t
=
BE Faradays law of electromagnetic induction (1.14)
t
+=
DJH Ampres circuital law (1.15)
0= B (1.16)
= D Gausss law (1.17)
The four Maxwell equations are not independent since 1.16-17 can be derived from
1.14-15 using the equation of continuity (t
=
J ). In order to solve equations
1.14-15 the following two constitutive equations are needed, which describe thematerials behaviour.
ED = Where 0 r= (1.18)
HB = Where 0 r= (1.19)
The symbols used above are defined and known as the following quantities.
B is the magnetic flux density [Vs/m2or Tesla]
Dis the electric displacement [As/ m2]
Eis the electric field intensity [V/m]
His the magnetic field Intensity [A/m]
Jis the current density [A/m2]
The permittivity 0[As/Vm] is for free space whereas ris the relative permittivity,and is dependent on the material in question, which can also be said for the
permeability [Vs/Am]. For free space the permeability is 0and rhas materialdependencies. All relative constants are dimensionless by definition.
If the E-field is discretised in space and time with a uniform step and only field values
at these points are stored, this can be written as equation 1.20.
),,,(,,
tnzkyjxiEEn
kji = (1.20)
Using the terminology below one can substitute derivatives with an approximatedifference. Equation 1.21 shows the finite difference formula for the first derivative.
t
EE
tt
EEE
n
kji
n
kji
t
kji
t
kji
ttt
=
+
2
1
,,
1
,,
12
,,,,
12
012
lim (1.21)
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1.5 Yees Algorithm
Yee penned his algorithm in 1966 [9], since then extensive research have expandedand enhanced the original idea and applied it to a wide range of problems. The recentupswing in its popularity is undoubtedly thanks to the decreasing cost of computercalculations.
The Yee algorithm uses a numerical solution to Maxwells curl equations using adiscretised differential form. The big novelty in Yees algorithm was his griddefinition that greatly decreased the amount of memory required. The E-field isdefined on one grid and the H-field on another in the time domain. Both grids areoffset in time and space. Each field is updated with a leapfrog scheme and a finitedifference form of the curl using the values of the surrounding cells. The electric fieldis solved at one moment in time and then the magnetic field is solved at the nextmoment (half a time step) similar to the game, leapfrog. This is repeated until thesimulation is done. For a general presentation of the FDTD method and Yeesalgorithm, see [5] and [7].
Figure 1.5 The Yee Cell
1.6 Numerical Stability
To ensure a stable and converging solution the cell size and time step must be wiselychosen. Choosing a grid too fine the simulation will be penalised with an unnecessarylong computational time and in most cases not result in increased accuracy. It is clearthat the cell size should be shorter than the wavelength of interest. The cell sizesuggested by the Nyquist theorem (i.e. two samples per wavelength) will normally notsuffice due to the approximate nature of FDTD computation. A more realistic cell sizewould be somewhere between 1/10 and 1/20 of the wavelength. It is possible to use
non-uniform cells with a finer cell size (grid) near complex areas, although these areasmay have to be interpolated both in time and/or space with the rest of the grid.
For solids with penetrable materials the wavelength is shorter inside these for a givenfrequency thus requiring a smaller cell size. Cells also have to be small enough to givean accurate model of the actual geometry. Rounded objects will suffer from thestaircase effect when modelling them with rectangular cells, i.e. the problem ofbuilding circles and other non-rectangular objects with cubes.
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After the cell size has been set, next step is to determine a suitable time step t. TheCourant-Friedrich-Levy criterium sets the limit for a stable solution (Figure 1.26).
222 )(
1
)(
1
)(
1
1
zyxc
t
+
+
Courant-Friedrich-Levy criterium (1.26)
Here x, y and z are the mesh size for the smallest cell to be found globally in thegrid for each axis. The parameter c is the speed of light within the cell material. The
rule of thumb is that a propagating wave must not travel through more than one cell in
one time step. Larger time steps than the one given with the above condition will
quickly result in instability.
1.7 Boundary Conditions
Antenna models using FDTD are usually located in free space making a boundary
condition necessary to make sure that the reflected fields do not bounce back into the
computation space, since a infinite computational volume is impossible. Using an
Outer Radiation Boundary Condition(ORBC) to absorb incoming fields at the outerlimits is the only solution, otherwise the calculations have to be aborted when the
reflected wave reaches the outer computational limit. Also since the outer co-
ordinates cannot be updated using the normal finite difference equations an ORBC is
needed for simulations. Several types of boundary approximations are available to
enclose the computational space. SEMCAD offers the three types of Boundary
Conditions described in the following paragraphs [2].
1.7.1 Absorbing Boundary Conditions
Absorbing Boundary Conditions(ABC) simulates materials that absorbs outgoing
waves or free space conditions. Ideally they should allow any outgoing wave to exit
without any reflection. A popular technique that can be described as a material
absorber is thePerfectly Matched Layertechnique (PML). This has been shown to be
one of the most accurate algorithms. Other ABCs that SEMCAD has implemented
are the first and second order Mur and the Higdon operator (up to the 4thorder) that
offers better performance than PML but may be not as numerically exact.
1.7.2 Conducting Boundary Condition
The Conducting Boundary Condition(CBC) does simply what the name suggests.
The computational space is truncated with a perfectly conducting plane that can beperfect electrically (PEC) or magnetically conducting (PMC). Incoming tangential E-
or H-fields components are set to zero.
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1.7.3 Periodic Boundary Condition
Periodic Boundary Condition can simulate periodic boundaries. Values from the other
side of the grid are used for periodic computation.
1.8 Implementation
A FDTD simulation basically contains three steps. First the grid must be built and
defined and the initial conditions set. The number of cells in each dimension must be
determined as well as the size of each cell and the electrical properties of each cell.
Other specifications necessary are where and what to be monitored and for how many
time steps the computation shall proceed. After the time step has been set according to
the smallest cell size using the Courant-Friedrich-Levy criterion the fields can be
updated.
After the initial specifications are set, the second part can commence. This is the heart
of the simulation where the field is advanced step after step.
When the criteria for terminating the simulation have been fulfilled the final part of a
FDTD simulation can begin, the post processing. This is where the calculated data is
processed and then presented in any manner desired by the user.
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2. Measurements and Simulations
To make an evaluation of the MapSAR and the SEMCAD utilities possible a suitable
comparison quantity had to be selected. For this thesis SAR was chosen since it is the
best presented output from MapSAR. Different comparison scenarios were designed
to alter the near field, aiming at making the results significant. This chapter isintended to clarify the methods and scenarios used.
2.1 The Box Phone
For this master thesis a simplified cellular phone was used, referred to as the Box
phone from here on. The reason for using a simplified phone design is to minimise the
number of cells needed during simulations in SEMCAD, thereby minimising the
number of simplifications needed for the simulated model. Another concern for the
simplified model was doubts about SEMCADs ability to simulate complex models,
like a faithful representation of a real phone, and simply because it facilitates changes
in the phone.
The box phone is made of plastic materials, see Figure 2.1. For further details see
Appendix A. It transmits with an internal dual band antenna (PIFA) fitted on a plastic
carrier mounted on the antenna ground plane. Several fronts, each with different
conductive layers, were tested with the phone. Different scenarios for interaction with
these front layers will be discussed further on.
Figure 2.1 The Box Phone
The simulated Box phone differs geometrically from the actual one in two major
aspects, it has no battery and no opening around the battery to allow an on/off switch.
Other minor differences are that the simulated phone lacks assorted details such as
screws and bolts to fit the antenna carrier onto the ground plane, the PCB.
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2.2 Antenna Matching Procedure
An important step in the phone manufacturing process is tuning the antenna to the
desired resonance frequency and making sure it covers the whole frequency band with
an acceptable gain. As a visualisation of the antennas matching an S11-analysis was
made, resulting in Figure 2.2. This S11-measurement was made with the Box phone
taped to a head phantom. The Box phone antenna was designed using ie3d (a methodof momentsimulation tool) and then tuned, with copper foil and a scalpel until an
acceptable resonance frequency at 926 MHz was measured. In SEMCAD an antenna
with roughly the same dimensions as the corrected antenna displayed a simulated
resonance at 922 MHz, see Figure 2.3.
Figure 2.2 Measured Return Loss, S11 Figure 2.3 Simulated Return Loss, S11
For frequencies outside the resonance dip the plot is not reliable, as seen in Figure 2.3.
As the figure shows, strange behaviour is common for frequencies that have next to
no power in the excitation signal as the sub 800 MHz area of the plot displays. In the
area above 1 GHz the irregularities shown are mainly due to the fact that the grid used
was designed for 900 MHz.
The Box phone transmitter was set to 914.9 MHz and the simulations were done at
916 MHz. The antenna can be seen in Figure 2.4 with the GSM component at the
right and DCS to the left.
Figure 2.4The Dual Band Patch Antenna
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2.3 Variations in the Box Phone Configuration
To test SEMCADs ability to simulate complex models, different scenarios aiming to
alter the electromagnetic characteristics of the Box phone were evaluated. These
approaches involve alterations in the conductive shield and ground positions.
2.3.1 Reference Layout
Figure 2.5 shows the standard front layouts used. These will later be used as the basis
of both measurements and simulations for the schemes discussed in the following
paragraphs.
a.) No conductive shield
b.) 4 or 70-m thick conductive shieldc.) Display opening and a 2-mm gap
d.) Display opening with a conductive
display patch and a 2-mm gap
Figure 2.5 Reference Layout
2.3.2 Ground Positions
All ground positions used are accounted for in Figure 2.6. Two different materials
were used for the front layers, a 70-m thick copper-tape and a 4-m thick layer ofaluminium.
Figure 2.6 Ground Positions
2.3.3 Resistors Layouts
An interesting feature for this thesis available in the SEMCAD simulation platform is
electromagnetic models of electric circuits, such as resistors, capacitors and inductors.
To test SEMCADs ability to handle this feature two rather basic schemes using a
resistor at the centre of the phone over a 2-mm gap as well as two resistors parallel at
each side over the same gap. These layouts can be seen in Figure 2.7.
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a.) Single resistor, either 5 or 10 .b.) Two resistors parallel, each 10 or 20 .c.) As b.) with display patch
Figure 2.7 Resistor Configuration
2.4 MapSAR Measurement Equipment
SAR measurements were performed with MapSAR benchtop SAR assessment
equipment, Figure 2.8, developed by Index SAR. It consists of a spherical phantom,
an E-field probe with amplifier and a data converter. For the placement of the phone
there is also positioning-rack, allowing movements in all directions, with a partial
rotation, constructed in plastics to counteract reflections. All SAR readings measured
with the MapSAR system in this thesis are not averaged over mass. Non-averaged
readings from MapSAR are calculated from measured E-fields in the liquid.
Figure 2.8 The MapSAR Equipment
The probe movement is restricted to spherical coordinates with a fixed radius and
origin of coordinates in the centre of the spherical phantom. A plastic arm is mounted
on the sphere to render tracing of the probe movements through a drawing board. The
drawing board has an underlay with a pattern suggesting the probe-movement. Usingcoordinates from the drawing board, the MapSAR software can determine the probe
position. The software keeps track of the maximum SAR and plots a 2D graph of the
SAR where each point is displayed on top of an imported image of the phone model.
2.4.1 MapSAR Measurements
Before starting a measurement the E-field probe is positioned at the start position;
Figures 2.9-10 in the probes horizontally suspended position. The object that is to be
measured is positioned in the phone holder touching the sphere.
Figure 2.9 Figure 2.10
Start-position from a lateral view Start-position from the top view
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The measurement procedure basically consists of six steps:
i.) Placing the probe in the start position relative the phone (Figures. 2.9 & 2.10).
ii.) Taking an initial SAR-reading at the start position for comparison with (v.).
iii.) Scanning the phone surface using the mask design on the Wacom-board as a
reference.
iv.) Scanning extensively around the maximum SAR position(s) to determine the
absolute maximum.v.) Comparing the final SAR-reading at the start position to the initial (ii.) to
determine the drift i.e. changes in SAR-reading.
vi.) Noting the maximum SAR-readout.
The readings can be exported as raw data (text file) with coordinates, phone outline
and conductivity for further analysis and the 2D plot produced in the main window
can be saved as a windows bitmap file.
To minimise errors the measurements for each phone configuration were made twenty
times to avoid aberrations due to phone position and to minimise other possible
sources of uncertainty. Another possible source of uncertainty could be differences in
batteries; to be able to distinguish battery problems in the results two batteries withsimilar properties were selected and then replaced every ten measurements.
For a more extensive guide to the MapSAR benchtop SAR assessment equipment see
Appendix C.
2.5 SEMCAD Introduction
SEMCAD is an electromagnetics simulation platform using a FDTD solver. It is very
well suited for simulating antenna designs, dosimetry, electromagnetic interference
(EMI)/ electromagnetic compatibility (EMC) and optics involving complex objects.
SEMCAD (the Simulation platform for EMC, Antenna Design and Dosimetry) isdeveloped by Schmid & Partner Engineering AG (SPEAG) in conjunction with
several Swiss universities. It offers a graphical user interface for computer-aided
design called ACIS made by Spatial coupled with SPEAGs FDTD implementation
for a slick user-friendly interface. Please refer to Appendix B and the SEMCAD
Reference Manual [2] for information on how to set up and run simulations.
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3 Results
3.1 70-m Copper Tape Layer
Copper tape (70-m thick) was put on the inside of the front piece. A short piece of
conducting elastomer (conductive rubber) was used to ground the copper tape on thefront with the main board holding the carrier as well as the circuit board. The
objective here is to see how placing a ground at various positions interacts with the
copper layer. For maximum convenience the different ground positions is listed again
in Figure 3.1.
Figure 3.1 Ground Positions
Ground positions C, D and E1+E2 are the only ones showing a significant SAR
deviation compared to the case with floating copper layer according to the MapSAR
measurements with respect to the margins of error. As Figure 3.2 shows, the highest
SAR value comes with grounding at position C and the lowest values with grounding
at E1+E2. SAR is decreased by almost 40 % comparing those two values.
0.46
0.46
0.43
0.47
0.43
0.53
0.37
0.42
0.45
0.33
0.45
0.46
0.42
0 0.1 0.2 0.3 0.4 0.5 0.6
Floating
A
B1
B2
B1 + B2
C
D
E1
E2
E1 + E2
F1
F2
F1 + F2
SAR (W/Kg)
Figure 3.2 Measured SAR Values with 70-m Copper Layer for Various Ground
Positions
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The SEMCAD simulations showed consistently less than half the value measured
with the MapSAR set-up when using 93.2 mW as the phone input power in
SEMCAD. The reason for this is hard to say, and if this is an error with SEMCAD or
MapSAR is unknown. It could be a number of reasons like lack of MapSAR
calibration, old fluid in the MapSAR bowl, the fact that the SEMCAD antenna has
higher gain than the real life dito or something else. SEMCAD can show any given
number by entering the right digits to use for the input power value if the absolutenumber is important. In this thesis normalised values are compared instead.
Grid size and baselines have been more or less consistent for all SEMCAD
simulations, but minor modifications have been made in a few cases. A few baselines
have been manually added. No significant numerical difference (most likely less than
1%) has been noticed after these grid alterations suggesting that the original model
was sufficient.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Ground Position
MapSAR measurements SEMCAD simulation
Figure 3.3 Normalised MapSAR and SEMCAD Comparison for 70-m Copper Layer
Figure 3.3 shows a comparison with results obtained using MapSAR and SEMCAD.
Both series have been normalised using the SAR value for the floating front layer
from their respective run of numbers. As one can see both simulations and
measurements are fairly consistent. The major differences are with grounds at B1+B2
and E1+E2 where the SEMCAD values are considerably higher. Since the MapSAR
probe has a fixed position 5 mm behind the glass bowl whereas with SEMCAD onecan get the SAR value anywhere inside the bowl and the maximum value is usually
positioned at some depth inside the bowl it can be debated if it really is the same thing
that is being measured. Using the probe to measure an object with more than one SAR
hot spot can be difficult since it can only sample in one direction at a time. Several
maximums (like using two ground points) could very well interfere and create a
maximum at some depth inside the bowl, which could be impossible for the probe to
pick up on. The SEMCAD results in Appendix E shows that for the majority of the
simulations the maximum SAR values were at some depth inside the sphere.
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The MapSAR measurements readings are overall lower with a larger difference
relative to the worst-case scenario with ground at position C. This may be due to the
fact that locating the hottest spot with the probe can be difficult. Getting an exact
reading of that actual value is almost impossible. Unless the phone is positioned
wrong or the phone is put out of position during the measurement, finding a position
with a higher SAR-reading than the actual absolute value is impossible.
3.2 4-m Aluminium Layer
The same measurements that were done with the copper tape inside the front were
repeated with a 4-m thick aluminium layer. As Figure 3.4 shows, the ground
positions give the same relative results as they did with the copper front. Here the
ground positions that show the largest decrease are E1+E2 and D. Using the Al-front
slightly lowers SAR for all ground positions, the average decrease is 0.0279 W/Kg
calculated over all ground positions. Measurements varied with a decrease between
3% and 20% with an average decrease of approximately 12%. It should be noted that
the effect of alternating ground positions is less than with the copper front.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Ground position
Copper AluminumAluminium
Figure 3.4 A Normalised Comparison of Measurements for Al and Cu
All simulations have used an approximation of the copper layer asPerfect Electric
Conductor(PEC), which is treated as a boundary condition in SEMCAD. Using the
actual values for copper as well as aluminium during simulations showed no hint of
the SAR decrease that have been measured with MapSAR. The actual SAR difference
(if any) with an aluminium front may be lower than these measurements have shown.
And the average 12% reduction is well within the margin of error.
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As a comparison between copper and aluminium fronts, measurements with no
ground position for each case were made on a DASY3-system. For more information
regarding the DASY3 system see [3]. Just as in SEMCAD and MapSAR the
difference between copper and aluminium fronts was minimal and well within the
margin of error. The DASY3 showed a 7% higher reading for aluminium than for
copper. These measurements were performed for the left cheek position.
3.3 Display Opening
Measurements were done using the copper tape but with a rectangular opening where
a display normally would be positioned. As earlier the effects of using a ground
between the PCB and the copper front was investigated. There was some interesting
resonance with ground positions D and E1+E2 as can be seen in Figure 3.5. Both
SEMCAD and MapSAR readings shows the same behaviour but with a greater
resonance shown with MapSAR. This may be due to the small differences in the
position and size of the display opening between the actual phone and our SEMCAD
model. For the non-resonant ground positions the results were very close.
0
1
2
3
4
5
6
Floating B1+B2 D E1+E2 F1+F2
Ground Position
MapSAR (Normalised with floating) SEMCAD (Normalised with floating)
Figure 3.5 SAR Comparison with Display Opening
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3.4 Resistor Measurements
For this measurement series the copper layer was separated by a 2 mm gap. The upper
half of the copper layer still had the display opening. Resistors were soldered to the
front to join the two parts in different configurations. All resistor configurations can
be seen in Figure 3.6.
Figure 3.6 Measurement Cases Using Resistors
Case 1: A single 10 resistor.Case 2: Two 20 resistors parallel at each side.
Case 3: A single 5 resistor in the centre.Case 4: Two 10 resistors parallel at each side.Case 5: As Case 2 with floating display patch.
Case 6: No resistors.
As Figure 3.7 shows the results were somewhat similar. Simulations showed no major
numerical difference for the different cases and the measurements suggest the same
tendency with one exception. Measured values for Case 1 is somewhat higher than
simulated.
1,1
1
0,8
7
1,0
2
0,8
9
0,8
9 1,00
0,9
1
0,9
00,9
5
0,9
00,9
3 1,00
0,0000
0,2000
0,4000
0,6000
0,8000
1,0000
1,2000
Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
NormalisedSA
Measured Simulated
Figure 3.7 Simulation and Measurement Comparison with Resistors
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3.5 Reliability of the Measurement Procedure
To examine the reliability of the MapSAR measurements a statistical survey of the
results were made. The reliability of the SAR-measurements had a maximum
deviation of between 4% and 10.5% for 20 measurements including one battery
change on the same phone configuration. The standard deviation ranged between 2%
and 6%, see Appendix D.
Another factor of uncertainty is the antenna mismatch, which might result in up to
15% bias. The mismatch factor of the antenna was derived via a comparison between
the Gain parameter for SWR 2 and 3. A set of light emitting diodes show how well
matched the phone is at any given moment, all measurements showed a SWR between
2 and 3. To determine the gain more precisely than this is not possible without tests
and it is this uncertainty that the before mentioned 15% comes from.
The antenna mismatch is the result of changes in the impedance due to modifications
of the phone. The worst-case scenario adds up to a 25.5% deviation between two
single measurements.
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4 Discussion
One thing that became clear during this thesis is that some tools are better suited than
others for a given task. First of all the MapSAR system has a very small measurement
area due to the spherical shape of the phantom. The spot on the phone with the
maximum SAR should be positioned next to the probes resting position. Accreditedtesting uses a head and shoulder SAM phantom, approximating a head phantom with
a sphere instead takes some consideration. For measurement positions with a tilted
phone, some phone shapes gives two touch positions (i.e. where the phone is touching
the SAM phantom). With a ground position that moves the peak SAR to the lower
part of the phone, a MapSAR reading measuring the top of the phone will show a
reduced SAR readout. A DASY3 reading where the lower part of the phone is right
next to the cheek as well as the ear will not display the same value as the MapSAR
where the probe is at a greater distance from the more transmitting part of the phone.
The DASY3 probe will get much closer to the lower part of the phone as well and
pick up a normal value. The MapSAR system should thus only be used for
measurement around the maximum value, i.e. the area with the hotspot should be
positioned next to the probe to make up for the differences between a spherical bowl
and a head SAM where a larger area of the phone will be close to the SAM. This can
be seen as worst-case scenario measurements, finding the maximum SAR anywhere
on the phone regardless of its position and if it can be measured with a SAM
phantom.
SEMCAD simulations are ideal for visualising hotspots. Using the simulation post-
processing is a good basis for MapSAR measurements since the user can easily
determine the locations likely to show a high SAR in reality as well. However using
MapSAR to find the location of maximum SAR on a phone surface can be done fairly
quickly and to later use that position for a more thorough measurement of the absolute
value as reference. The strength of the MapSAR is the short measurement time, whichmakes it ideal for comparative tests. This is one of the biggest selling arguments of
the MapSAR system. Another benefit it has over a complex robotic set-up is that the
liquid is sealed which gives it a longer life. Also since each measurement takes 2-3
minutes at the most it is less likely that the batteries will drain during a measurement
or that the characteristics of the measured object will change due to overheating.
SEMCAD is a very useful tool when it comes to getting an idea of how complex
structures such as models of cellular phones will interact with surrounding structures.
Seemingly identical antenna geometrical dimensions in SEMCAD as the real world
original design did result in slightly different antenna characteristics. Maybe the
SEMCAD model requires a more detailed representation (like a cad file of the actual
phone). The real antenna was cut out by hand and messed up with the soldering iron,which makes it impossible to model down to a certain degree of accuracy. Another
difference is to which extent the electric properties given to solids in the SEMCAD
simulations are accurate.
While the differences in antenna performance may not be significant it may lead to
somewhat different absolute numerical results but should be of no concern for relative
comparisons between different simulation scenarios. Using SEMCAD as a serious
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development tool requires a fast computer, the faster the more realistic and rewarding
simulations are possible.
MapSAR usually reported a SAR value twice as high as the SEMCAD simulations.
This may be due to the differences in antenna gain or perhaps the fact that the
measured power from the phone had been measured without the mismatch that were
present during all measurements. Due to the fact that MapSAR is designed foraveraged measurements (the probes position was set with this in mind) it may not be
ideal to compare peak SAR. The MapSAR set up requires a corrective factor for
comparisons with DASY3 measurements due to its 4mm thick bowl whereas SAM
phantoms often use a 2mm thick shell. An uncorrected MapSAR value will thus be
lower than a DASY3 reading. With careful calibration some MapSAR measurements
could be fairly accurate, if the procedure is done carefully, and the differences
between a spherical and a SAM phantom are kept in mind. This causes some practical
issues regarding where on the test object measurements shall be done as discussed
earlier. Using a complex head phantom in SEMCAD is very feasible. It is not a very
big part of the head that interacts with the phone especially with the depth of
penetration in mind. Figure 4.1 shows how the complex shape of a SAM phantom is
approximated in a SEMCAD simulation. There is no real loss in accuracy with a morecrude head approximation, as shown in the picture to the right below, so the choice of
tissue for SAR extraction should be made with respect to whatever measurement
method one aims to simulate. With an efficient grid design using a head phantom
must not imply more cells than a spherical phantom (Figure 4.2).
Figure 4.1 Head Phantom Figure 4.2 Spherical Phantom
Both SEMCAD simulation and MapSAR simulations are fine tools for relative studiesof antennas. As seen in chapter 3 the normalised results show the same tendencies for
each test scenario with a few exceptions.
The only really useful SAR measurement is done on accredited systems like the
DASY3 but it should be possible with reference measurements from an accredited
system to predict the actual value from results using SEMCAD and/or MapSAR.
MapSAR with its spherical phantom makes it less versatile than SEMCAD and harder
to compare with results using a head phantom.
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5. References
[1.] Balanis, A. C., Antenna Theory: Analysis and Design, 2nd
edition, John Wiley & Sons
Inc. 1997[2.] SEMCAD Reference Manual. Bundled with SEMCAD and available at:
http://www.semcad.com/downloads_free/SEMCAD_RefManual.pdf
[3.] http://www.speag.com
[4.] Fujimoto, K. and James, J. R., Mobile Antenna Systems Handbook, 2nd
edition, Artech
House Publishers, 2001[5.] Kunz, K. S., and Luebbers, R. J., The Finite Difference Time Domain Method for
Electromagnetics, CRC Press 1999
[6.] Cheng, D. K., Field and Wave Electromagnetics. 2nd
edition, Addison-Wesley 1989[7.] A. Taflove. Computational electrodynamics: The Finite-Difference Time-Domain
Method. Artech House, Boston, London, 1995.
[8.] C. Gabriel, S. Gabriel and E. Corthout, The dielectric properties of biological tissues: 1-3,Phys. Med. Biol. 41, 1996. pp. 2232-2293.
[9.] K.S Yee. Numerical Solution of initial boundary problems involving Maxwells equations
in isotropic media. IEEE Trans. Antennas Propagat.,14. March 1966.[10.] Indexsar, Mapsar Sytem Manual. 2001
[11.] Stutzman, W. L. and Thiele, G. A., Antenna Theory and Design, 2nd
edition, John Wiley
& Sons Inc. 1998[12.] SEMCAD Tutorial. Bundled with SEMCAD and available at:
http://www.semcad.com/downloads_free/SEMCAD_Tutorial.pdf
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Appendix A: Solids Specifications
A.1 Box Phone Specifications
The Box phone has the following outer dimensions: 1005020 mm, see Figure A.1.All walls of the outer casing are 2 mm thick. It is a simplified model of a cellular
phone with straight edges. Material characteristics used for SEMCAD simulations are
listed in Table A.1.
FigureA.1 Cross Section of the Outer Casing
A.2 Antenna and Carrier Specifications
A dual-band (900 and 1800 MHz) Micro strip Patch Antenna was designed for the
box phone. This was fitted on a carrier made of Noryl. The carrier with mounted
antenna was screwed on to the copper ground plane (the rectangular area in Figure
A.2). Table A.1 lists the material properties.
FigureA.2 Carrier and Antenna
A.3 Spherical Phantom
A spherical phantom (see Figure A.3) with the outer diameter of 200 mm and an inner
diameter of 192 mm was used as a model of the MapSAR bowl (made of a glass-like
plastic material called Pyrex) with the same dimensions. The bowl is filled with a
complex mixture of fluids simply referred to in this thesis as the liquid.
Electromagnetic properties for the liquid as well as Pyrex are described in Table A.1.
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FigureA.3 Spherical Phantom
A.4 Material Data
3m
kg
m
S r r
Plastic 1000 0 2.9 1
Noryl 1000 0 2.7 1
Copper (PCB) 8920 5.9 107 1 1
Pyrex 2500 0.99 4.6 1
Liquid 9990 1 41.5 0.97
Table A.1. Box Phone Material Properties Used in SEMCAD
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Appendix B: A SEMCAD Primer
B.1 About
Here follows some hints for successful simulations with SEMCAD. This should be
suitable for the novice user and hopefully be useful for the more experienced users aswell. It aims to cover most aspects of simulation settings for mobile phone type
simulations. This text is written for SEMCAD 1.4 Final (build 38).
B.2 Model Design with SEMCAD
Modelling is fairly straightforward in SEMCAD. To start a new model design click on
the New Project icon (the blank paper sheet at the far left) and the user will be
prompted to input the name for this project. Everything will be saved under a folder
with this particular name in the projects folder located is in the SEMCAD installation
folder as default, this can be useful to know when making backups or migrating
projects to other computers. The next dialog box is the global model properties
(Figure B.1). SEMCAD uses dimensionless global units for its axes instead of anyreal length unit so enter the corresponding real life length in meters in the dialog box.
This can be set more or less arbitrary since one isnt limited to the global unit length
as the shortest length. Settings are always specified in model units.
Figure B.1 Global Model Properties
The tools for building solids should be more or less self-explanatory, but it can beuseful to keep reference points at corners and other locations of objects that can be of
assistance when moving or resizing objects, see Figure B.1 how to set point
coordinates. Point coordinates are also useful when building solids. Instead of
entering coordinates for a solid one can use the mouse to drag and model using points
entered earlier. Points are not attached to solids in any way they are merely a quick
way to use that particular coordinate. Unless the point coordinates as well is selected
(when performing an operation like moving or resizing) the actual coordinates used
will not be affected.
Figure B.2 The Point Coordinates Dialog Box
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By checking the box marked Relative in Figure B.2 the new coordinate will be
relative to the one last entered. If the point (10,10,10) was the last entered coordinate,
entering (-10, -10, 10) with the box Relative checked the resulting coordinate is at the
absolute position (0,0,20).
Chapter 6.2.1 of the SEMCAD reference Manual and the Tutorial do a good job
describing the tools available for making solids of all shapes and sizes. Learning bydoing should be the best approach here and anyone with some CAD experience
should feel at home in no time. Be sure to give every object an identifying name to
help future editing. Solids saved in ACIS SAT or SAB and binary STL file formats
can be imported. However it is not possible to edit imported solids in any way in
SEMCAD. Some desirable operations that are impossible are uniting, subtracting or
intersecting (gives the area common to two solids) an imported solid with an imported
or non-imported solid.
It is possible to group solids, points, sources and sensors into groups. Clicking the
new group window as shown in Figure B.3 creates a new folder in your objects list.
Grouping each solid and its point coordinates into a group or folder as the case may
be of their own makes it much easier to perform certain operations or to hide all ofthem from the model view at once. To move any object into a group highlight it and
choose cut using the right mouse button or use the standard windows short-cut
Control-X, then double click the folder representing the group you want the object to
be moved to. Then use Control-V to paste the object into its new group or select Edit |
Paste from the menu.
Figure B.3 New Group Icon
B.3 Sources and Sensors
For electromagnetic field excitation there are three sources available in SEMCAD.
These are the Plane Wave Source, Wave-guide Source and Edge Source. Only the
Edge Source (see Figure B.4 below) will be discussed and used in this thesis. The
Edge Source applies an electric field on one single edge and requires two points in
space and will appear as a line in the model. These points have to be perpendicular to
the coordinate axes. There are a few options available for specifying the properties of
the Edge Source, which will be discussed later. We will only use the Edge Source as a
Voltage Source that have internal resistance taking energy out of the grid making the
generated pulses decay faster and reaching the steady state earlier.
Figure B.4 The Edge Source
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To record the fields during the simulation and extract data for presentation some kind
of sensor has to be included. The sensors available in SEMCAD correspond to the
different types of sources used and which aspects of the simulation that is of interest.
With the Edge Source an Edge Sensor (see Figure B.5 below) is vital during antenna
simulations to extract certain interesting transient behaviours (such as S11and SWR
plots) that can be helpful when matching antennas. This sensor should be positioned
at the exact place of the source and requires two coordinates that are parallel to one ofthe grid axes as well. Using an Edge Source in phasor mode during a harmonic
simulation presents the gap impedance, voltage, current, power, SWR and S11. The
four first mentioned dimensions are complex numbers. With an Edge Source in time
mode only the voltage and current are recorded, both of which can be plotted for the
whole duration of the simulation. These can be useful to verify that the simulation has
not diverged since they both should display a periodic signal resembling the sinus
wave that excites the model.
Figure B.5 The Edge Sensor
The field sensor (see Figure B.6) records B and H-fields and features such post
processing calculation options as various SAR plots (if run in quasi-harmonic mode)
and Standing Wave as well as other information. Recording area for the field sensor
can be a box or a surface (SAR only available within a box model). Field sensors
dont lend itself very well to time domain simulations due to the extreme storage
space required.
Figure B.6 The Field Sensor
The far-field sensor (shown in Figure B.7) must be modelled as a box. The box sensor
should enclose all components interacting in a simulation. A near-to-far-field
transformation makes it possible to place the sensor nearby the radiating structures, at
a distance of only a few cells. Far-field sensors can only be run in quasi-harmonic
mode and presents the far-field patterns as well as several parameters that can be
calculated. These include the maximum radiation intensity, effective angle,
directivity, front to back ratio and the half power beam width. For more information
read section 9.2.3 of the SEMCAD reference guide.
Figure B.7 The Far-Field Sensor
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Other available sensors are port, voltage and current sensors that will not be used nor
discussed here (instead see the SEMCAD reference manual). The port sensor is
similar to the edge sensor but can be positioned as a rectangular loop instead of a
straight line as the Edge Sensor requires. Voltage and Current sensors do just what
their names suggest.
Figure B.8 Lumped Elements
It is possible to model circuit components in SEMCAD. Resistors, Inductors, and
Capacitors are available and all are inserted into the model in the same way. Just as
the Edge Source a lumped element requires a length of the minimum step size and
two points aligned with one of the grid axes. A single edge of the field is updated
according to the settings of the lumped element. How to set its characteristics will be
described later on in this text. Figure B.8 shows the lumped element icon.
B.4 Grid Settings
After the model has been successfully designed it is time to set the simulation
settings. Save the project and click the Simulations tab to move on (see Figure B.9).
SEMCAD uses an automatic grid generator that adds base lines at material
discontinuities and sources. It is designed to make most of the work independently but
can be tweaked by the user for certain needs. The single most important factor for a
successful simulation is the grid settings.
Figure B.9 The Simulations Tab
As mentioned earlier a minimum step less than one tenth of the wavelength is
recommended. For GSM (890-915MHz) with a wavelength around 30 cm a
theoretical maximum step size would be 3 cm (in free space, less for materials).
However a global max step of 13 mm is more realistic and should be sufficient for
most GSM band simulations. SEMCAD does not set these options in any way, the
user has to carefully consider and set these options.
On the top left part of the screen below the simulation folder is the default grid icon
(Figure B.10). Highlighting that icon, right clicking and choosing properties brings upthe grid settings (Figure B.11). Here are the settings for the global maximum and
minimum step size step as well as the ratio, extents and boundary conditions for all
axes. The ratio is the maximum rate at which nearby cells can grow. Say that a cell is
0.3 units in a given dimension the neighbouring cell can have a maximum width of
1.3 times 0.3 units if we use the numbers in Figure B.11.
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Figure B.10 The Grid Icon Figure B.11 Grid Settings
Under the next tab labelled Solids (Figure B.12) each modelled solid can be given
unique properties. Perfect electric (PEC) or magnetic conductors (PMC) and dielectric
are the available options (highlight each solid and choose properties to set its type).
Dielectric materials will be discretised according to the grid setting and can be given
arbitrary properties whereas PEC and PMC cells will be seen as edge conditions with
special electric and magnetic rules. Unfortunately solids require a volume so making ainfinite thin layer using a flat two dimensional rectangle and specifying it as a PEC or
PMC is impossible, it has to be modelled as a solid with some extent. Figure B.13
also shows the options to limit the maximum grid step in the area of the solid. This
step size has to be in the same range as the global step or it will simply be ignored.
Normally SEMCAD refines the step size around material discontinuities. It
automatically uses the lowest global step size at these discontinuities. To prevent
SEMCAD from doing this uncheck the Refine at boundaries box.
Figure B.12 The Solids Settings Figure B.13 Properties Dialog for Solids
One can also to some at some degree determine to which extent SEMCAD shall
automatically generate baselines. It is recommended to use the default setting with
baselines at all extremes unless it is crucial to keep the amount of cells down. Set the
base line generator as None for all solids and all baselines have to set manually.
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Continuity thresholds are useful for solids with complex shapes or solids that are not
aligned with any grid axes. The continuity thresholds set the distance in grid lines that
each cell can have and still get reconnected. This should only be used if a complex
shape like a helix antenna looks fragmented or has missing edges.
The next tab shows the base lines that have been automatically generated with respect
to the dielectric solids that make up our model (Figure B.14). Here one can set theminimal and maximum step size that can be used between the intervals (on the right
side of the given base line). Step sizes that are outside the global step sizes (as shown
in Figure B.15) will be ignored.
It is important to visualise that the model you have made will be discretised into
single cells, atoms or voxels as SEMCAD labels them from the perfect shapes of your
model. And it is these base lines that govern the size of each voxel. The result will not
contain any perfect shapes like spheres and cylinders as we have modelled but
approximations. Some complex shapes may have generated an uncalled amount of
baselines and some shapes may have too few baselines to get a faithful representation
when approximated with the rectangular voxels, so it is highly recommended that one
check the model before starting simulations. The user can add and remove base lineswithout any limitations.
When modelling thin layers or any small sized object it is wise to model them a few
cells thick. If that depth is in the same magnitude as the minimal step size a few
baselines should be added on the correct axis instead of lowering the global minimal
grid-step, which could seriously increase the computation time.
The Subgrids tab should be ignored until it has been implemented. It is unknown
when this feature will be available.
Figure B.14 Base Lines Figure B.15 Compute and View Voxels
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B.5 Model Visualisation
Now that the grid settings have been set, choose Compute Voxels (highlight and right
click the grid icon as shown in Figure B.15). It discretises our model and updates each
cells (voxel) properties. This can be seen as a transformation from our geometrically
perfect CAD model into a three-dimensional database with each cells properties.
These voxels can be visualised and it is recommended that one check the resultsbefore simulating. To do this choose View Voxels (as Figure B.15 shows) and make
sure that each part of the model has a faithful representation and that every solid
really is connected.
A certain amount of stair casing is for some of the more complex objects more or less
impossible to get rid of, or as the case most often would be a waste of simulation
time. Figure B.16 shows how a sphere can end up looking. In our case we have
decreased the step sized on the part that interferes with the telephone. A more faithful
representation of the backside of the bowl as well is probably not worth the extra time
spent calculating.
The easiest way to check the geometry is to show the voxels only and not togetherwith the original solids (uncheck Original Solids like Figure B.17 shows). Only the
voxels that are included in the slice are now shown. Using a mouse with a scroll
wheel makes it very easy to check each slice. Just highlight the box containing the
coordinate that isnt frozen and scroll up and down all along the model using the
scroll wheel. Everything should be ready for simulation if the approximated models
look acceptable and each solid has a minimum depth of a few cells.
Figure B.16 A Stair Cased Bowl Figure B.17 Voxel Viewer
B.6 Simulation Settings
Next up are the simulation settings. Highlight the light bulb icon and using the right
mouse button and choose properties (as Figure B.18). This brings up the run settings
and the global tab (Figure B.19), where one chooses if the simulation shall run inharmonic (single frequency) or transient mode (a span of frequencies with a gaussian
pulse). In harmonic mode it is recommended to set the simulation time in periods
(only possible in harmonic mode) and for transient in seconds. Ten periods is usually
long enough for simulations in harmonic mode to reach the steady state. For transient
mode the simulation time depends on the input source and should be decided based on
the layout of the input signal, more about this later.
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Figure B.18 Run Settings Figure B.19 Global Settings
Every simulation is a trade-off between simulation time and end result, which brings
us to the next settings: the absorbing boundary dialog. The PML algorithm is better
and preferred for simulations with less free space surrounding the model. We tested
both Mur and PML with 25 to 30 cm of free space around the phone in all six
directions. The PML computation took twice as long time as the one using second
order Mur though both S11graphs came out identical. Having plenty of free space
around the simulated object probably affects the result in a positive way more thanchoosing the most accurate boundary condition available.
It is safe to say that one should leave the settings unaltered for PML and Higdon
conditions unless you really know what you are doing. MUR can be set as first or
second order only. Figure B.20 shows the Absorbing Boundaries settings.
Figure B.20 Absorbing Boundaries Settings Figure B.21 Edge Source Dialog
Figure B.21 shows the properties for the Edge Source expanded. As mentioned earlier
it is recommended to run the Edge Source as a Voltage Source for these types of
simulations. The source resistance must be set for a voltage source since the Voltage
Source has an internal resistor taking energy out of the grid. With a Hard Source these
waves would have been reflected instead of taken out of the grid at the source. TheAdded Source just adds a field-strength on top of the value already calculated with the
Yee algorithm. The advantage with the Voltage Source is that the excitation signal
decays faster thus reaching steady state earlier and saves simulation time. If a
Gaussian Sine is chosen its characteristics can be set by pressing Properties as shown
at the bottom of the screenshot on labelled Figure B.21. This will bring up Figure
B.23 to your attention.
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Figure B.22 TPeak and TSigma Figure B.23 Signal Settings
The simulation duration should be at least two times TSigma, to make sure that the
simulation will reach the steady state. A suitable value for TPeak would be three
times TSigma to ensure that the start of the pulse doesnt get too steep. Figure B.22
shows how TPeak and TSigma describe the signal. These values also determine the
frequency contents of the gaussian sine. If there is a wide range of frequencies
included like one would require for simulating both GSM and DCS bands at the same
time, the power of these frequencies may be too small since one has to set a centrefrequency. This can be seen in Figure B.23 the graph in the bottom plot. Here we have
a preview of a gaussian sine with a fairly broad frequency content, with less power
around the centre frequency. If TSigma were doubled we would have seen a narrower
spectrum and more power around 900MHz. Using a centre frequency of say
1400MHz to simulate both 900 MHz and 1800 MHz yields not enough power for the
interesting frequencies according to our tests. Strange things usually happen on S11
plots for frequencies with next to none power. Using two sources each transmitting on
one band didnt get acceptable results either. Therefore we must recommend that each
band should be simulated one at a time.
Figure B.24 Lumped Elements Settings
If the model has any lumped elements they will be listed under the sensors andlumped elements tab. Highlight the lumped element of your choice and press
properties and the a box should pop up presenting the available settings (Figure B.24).
Depending on the circuit type that is checked the two input boxes adapts and shows
the correct units.
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The Sensors tab under Run Settings lists the different sensors in the current model.
Usually the default settings for each type will suffice after they have been set for
phasor or time mode. For time domain data collection the start and stop time for the
recordings can be set, as well as the number of steps so the amount of data to be saved
can be held on a manageable level. Under Phasor mode the sensor just presents what
was in the air so to speak when the simulation is done and reached the steady state
(if the simulation didnt diverge).
Figure B.25 Dielectric Materials Settings
The next tab, Materials, lists all solids. Choosing a Dielectric solids properties brings
up a dialog box like the one shown in Figure B.25. All changeable characteristics can
be set from this interface. For PEC solids only the density, heat capacity and thermal
conductivity can be set. For dielectric materials the relative permittivity and
permeability, electric and magnetic conductivity can be set as well as the parameters
mentioned earlier for PEC solids.
Be advised that parameters for 900 MHz is usually not applicable for 1800MHz as
well. Most of the human tissue parameters shown in Figure B.25 have been imported
from a parametric model by Camelia Gabriel that is included with SEMCAD. It
contains permittivity, permeability and conductivity for over 30 body tissues. These
are in a text file called Gabriel parametric model.txt and should be found in the
SEMCAD install folder by default. They are imported using the import button that is
shown at the bottom of Figure B.25. Then open the before mentioned text file and all
materials are available. This ends the simulation settings and the simulation can now
be started.
B.7 Starting a Simulation
To start a simulation, highlight the light bulb icon and right click. Choose Compute
Fields as Figure B.26 suggests. You must have computed the voxels before a
simulation is possible. When the simulation is running a box like Figure B.27 show
the status of the simulation.
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If your computer accesses the hard drive a lot during the simulation, take that as a
warning. Say for example a processor running at 500 MHz. A single clock cycle then
has a duration of 2 nanoseconds. A standard IDE hard drive running at 7200 rpm has
an average seek time somewhere around 10 milliseconds, whereas the seek time of
standard SDRAM usually is advertised as somewhere around 10 nanoseconds or
faster. Quite a few more clock cycles are lost waiting for the hard drive compared to
fetching data from RAM, so the computation time can be increased by a large factorwith heavy hard drive access. Either get more RAM or decrease the number of cells
used in the simulation.
Figure B.26 Starting a Simulation Figure B.27 Almost Done!
B.8 Post-Processing
When the simulation is done the cancel box shown at the bottom right of Figure B.27
will be labelled with close instead. Click close and you will find something like
Figure B.28, where every sensor is presented and their respective post processing
options. Please note that if you edit the model or grid settings in any way allsimulation results will be erased for that particular project.
Figure B.28 Choosing Post Processor
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The field sensor used in this simulation presents a variety of outputs as Figure B.28
shows (labelled Unnamed at the top in our case). For SAR presentations the
recommended way to present the statistics is to overlay them in the model (Figure
B.29). This is mainly because it is easier to check SAR at different positions and the
statistics box will give you the global maximum values if you feel that that is enough.
The Contour view shows a 2D graphical plot but is painfully slow if you have not
beforehand settled on which slice to display. A surface plot is in 3D and can besomewhat difficult to navigate in.
Figure B.29 Modes of Presentation
Field quantities are displayed on top of the 3D model in a rectangular plane of your
choice. There are two different SAR measures available for display. Peak SAR simply
presents the peak SAR without any averaging for mass and the IEEE-1529 uses an
algorithm for averaging over mass (for example 1 or 10 grams) according to the draft
with the same name. Both can use source power as input for normalisation.
The peak SAR post processor is useful for determining where local SAR peaks are
located and presents an easy way to see how SAR decreases as one gets further and
further away from the radiating source. Highlight Peak SAR from under the Edge
Source, press the right mouse button and choose Overlay in model. The next dialog
box lists the solids in your model and an input box for normalising the input power.
To the left of each listed solid is a box, indication if crossed that that solid will be
used for SAR computation.
Press the OK button when done and after some processing a sliced view similar to the
one available when checking voxels in Figure B.17 appears. Figure B.31 shows the
information box that accompanies the standard 3D model view. In order to see the
graphical overlay inside the solid you chose earlier switch to wire frame or facets
mode instead of shaded. The real information is just a mouse click away though; press
the statistics button shown at the top right of Figure B.31. Something resembling
Figure B.30 should appear at the left side of the screen. This presents the top SAR
value at the current slice as well as the global maximum complete with coordinates.To check the SAR value at different parts of the bowl choose an axis to slice and
scroll your way around the bowl while looking at the statistics box and the value to
the right of where it says Slice Maximum field value (current settings).
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Figure B.30 Statistical Peak SAR Information Figure B.31 Sliced SAR
For IEEE-1529 averaged SAR there are a few things to think about. One must set thetype of each solid that is to be used for SAR computation. Each body mass must be
set as body tissue or extremities tissue due to the difference in the compliance
tests for extremities as hands, wrist, ankles and the external ear. When simulating
handsfree sets or phones that is not positioned in a talk position specify extremities.
When averaging, only the SAR from the type of tissue that the chosen mass belongs
to is used for calculation. The same display possibilities that the peak SAR offers are
available for averaged SAR as well, but with all values averaged.
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Appendix C MapSAR manual
C.1 Introduction
This manual will concentrate on the basics regarding the software included with the
MapSAR bench top SAR assessment system. Only the basic functions regarding SARmeasurements are included here, for more information about calibration issues consult
the MapSAR documentation bundled with the product [10].
Figure C.1 MapSAR Graphical User Interface
C.2 Description of the Graphical User Interface
After starting the MapSAR software the programs interface appears on the top of the
desktop. A screenshot is shown in Figure C.1. For SAR measurements the centre of
attention will be at the upper left windows that displays the continuous and peak
SAR-readings. Figure C.2 shows the two displays with the continuous readings at the
top and the max SAR value below. The max SAR value is the highest encountered
SAR reading during the measurement.
Figure C.2 Continuous (top box) and Maximal (bottom box) SAR Readings
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The big window at the upper right with the grey background displays a plot of the last
100 samples (Figure C.3). This graph can be useful for determining if the probe has
been moved to fast by noting sudden jumps between samples.
Figure C.3 Plot of the Last 100 Samples
The lower right window displays the probe movement relative to the phone surface of
the imported image. A number of phone models are included in the with the MapSAR
software as well as a simple program to make simple 3D models of other phones
using pictures of the phone as aid. Figure C.4 shows an example of this presentation
with the simple rectangular box phone used for this thesis.
Figure C.4 Past Probe Movement
The window on the lower left side shows a three-dimensional plot of the SAR levels
superimposed over the simple representation of the phones design (Figure C.5). This
is a handy way to determine the position of the maximum SAR-value. Data presented
in this plot can be exported for further analysis. Included in an exported text file arethe phone outline together with the coordinates, conductivity and frequency. Each
graph can also be exported as a windows bitmap file in 2D, enabling analysis of the
near field.
Figure C.5 SAR Surface Plot
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Figure C.9 shows an example of the averaged SAR presentation. The cheek factor
correction factor is for comparison with MapSAR and other measurements methods.
Due to the relatively high thickness of the sphere used to contain the liquid for
MapSAR for comparisons with results acquired with other measurement systems with
cheek that is not as thick. 2mm is a common shell thickness for a SAM phantom
whereas the MapSAR pyrex bowl is 4mm thick. And it is to compensate for this
difference that the SAR averages may need to be multiplied with this factor. At leasttheoretically this extra thick shell should result in a lower value for MapSAR
measurements.
Figure C.10 Cheek Correction Calibration
The cheek factor needs to be calibrated and the settings
