EM modeling of antennas and RF components for wireless communication systems

281 Pages·2006·7.48 MB·English

by  Frank Gustrau

Frank Gustrau · Dirk Manteuffel
EM Modeling of Antennas
and RF Components
for Wireless Communication
Systems
With215Figures
123
ProfessorDr.FrankGustrau
FHDortmund–UniversityofAppliedSciences
Sonnenstraße96
44139Dortmund
Germany
E-Mail:[email protected]
Dr.DirkManteuffel
IMSTGmbH
Carl-Friedrich-Gauß-Straße2
47475Kamp-Lintfort
Germany
E-Mail:dirk [email protected]
LibraryofCongressControlNumber:2006923241
ISBN-10 3-540-28614-4 SpringerBerlinHeidelbergNewYork
ISBN-13 978-3-540-28614-1 SpringerBerlinHeidelbergNewYork
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Preface
Thisbookaddressesnumericalfull-wavemethodsfortheanalysisanddesignof
antennasandmicrowavestructures.Inthelastdecadesthesenumericalmeth-
ods that are used to calculate approximate solutions of Maxwell’s equations
haveevolvedfrompureacademicdisciplinestopowerfulanduser-friendlyen-
gineering software tools. Meanwhile numerous commercial software packages
existthatarewidelyusedintheRFengineeringcommunity.Developmentsin
thesoftwareaccompaniedbyprogressincomputertechnologyallownowadays
many practical problems to be solved on standard PC systems.
The text is written with different groups of readers in mind. First it ad-
dresses RF engineers who embark on numerical modeling using commercial
fieldsolvers.Seconditaddressesgraduatestudentstakingpracticalcoursesin
electromagnetic field simulation. Both groups need a basic understanding of
the mathematical background and both groups need a practical introduction
tocommercialEMmodelingsoftwaretomaketheirfirstsimulationexperience
a success.
There are numerous books on the market that deal with mathematical
details of different numerical methods for the calculation of electromagnetic
fields. For engineers who want to know what these methods can do for them
and how to use the software tools, these theoretical books are too far away
from daily work. On the other hand, there are tutorials and manuals that
provide information on how to use specific software packages. These tutorials
focusonthehandlingandspecialfeaturesofthesoftwareandmethodsapplied
therein. However, the theoretical background and broader scope is generally
neglected. This book tries to fill the gap by providing a practical and com-
prehensive introduction for engineers involved in the numerical analysis and
design of antennas and radio frequency components for wireless communi-
cation systems. The reader will find a down-to-earth approach to full-wave
high-frequency simulation tools and how to use these tools efficiently in ap-
plied industrial engineering.
The book begins with an introduction in Chapter 1 that approaches the
subjectoffieldsimulationbygeneralconsiderationsaboutmodelingstrategies
VIII Preface
as well as capabilities and limitations of the software tools. Part I covers the
basic electromagnetic theory and definitions of the most important antenna
and circuit parameters. Chapter 2 starts with Maxwell’s equations in differ-
ential and integral form as well as boundary conditions for the electric and
magnetic fields. Next the propagation of waves in lossless and lossy media is
discussed.Inthiscontextuniformwavesareusedtoillustratedifferenttypesof
polarization. The general concept of energy conservation is transferred to the
electromagnetic field by defining electric and magnetic energy density as well
aspowerflowandpowerloss.Thechaptermovesontoadefinitionoftheelec-
tromagneticpotentialsandendswithashortreviewofwavesontransmission
lines. Altogether, this chapter serves as a basis for the understanding of the
numerical methods presented in Chapter 4. Since there are time-domain and
frequency-domain methods, the time-dependent and phasor representation of
the fields are described in parallel throughout this chapter.
The electromagnetic field theory provides a complete set of equations and
quantitiestosolveanyelectrodynamicproblem.Unfortunately,thequantities
defined so far are difficult to handle. Therefore, a set of more easily accessi-
blequantitieshasbeendefinedtocharacterizetheelectromagneticfeaturesof
antennas and circuits. Chapter 3 reviews these technically important radio-
frequencymeasures.Thechapterstartswiththeintroductionofthescattering
parameters that link circuit-based quantities like impedance and voltage to
field-based quantities like electric and magnetic field strength. Using the ex-
ample of a dipole the antenna parameters are introduced: radiation pattern,
gain,efficiency,directivity,nearfield,farfieldandbandwidth.Finally,antenna
arrays and the concept of beam steering are discussed.
Part II is concerned with the mathematical basis of numerical meth-
ods as well as instructions for their practical use. Commercial software is
generally based on one of the following methods: Finite-Difference Time-
Domain method (FDTD), Finite Element Method (FEM) or Method of Mo-
ments (MoM). Consequently, Chapter 4 focuses on these methods and illus-
trates their underlying theoretical concepts and mathematical implementa-
tions. Each of the methods could fill a book in itself, and there are numerous
excellent textbooks on the market. Therefore, the chapter concentrates on
the aspects that are essential when applying rather than programming the
numerical methods. Other - currently less popular - methods are mentioned
briefly to show the reader that additional tools exist for special applications.
Chapters 5 and 6 represent the core of the book. Chapter 5 discusses the
steps usually undertaken when creating an antenna or RF component sim-
ulation model with up-to-date full-wave engineering software. The process
of modeling can be divided into three parts: pre-processing (setting up the
model),solving(generatinganapproximatesolutionofMaxwell’sequationsby
applying one of the discussed numerical methods) and post-processing (eval-
uating the results and calculation of additional quantities from the solution
of the solver). Concerning pre-processing the following topics are addressed:
creatinggeometryusingCADcapabilities,importingCADdata,definingma-
Preface IX
terial properties, applying boundary conditions and sources and setting up
solver parameters. The process of meshing (subdivision of the computational
domain into smaller elements) can be regarded as part of the pre-processing
or − if adaptive mesh refinement is considered − meshing represents an es-
sentialpartofthesolutionprocessitself.Dependingonthenumericalmethod
used,thesolverdeterminestheelectricandmagneticfieldswithinthecompu-
tationaldomainorthesurfacecurrentdensitiesontheinterfacesbetweendif-
ferent materials. During the post-processing, additional quantities like circuit
and scattering parameters and farfield pattern are calculated from the fields
or current densities. At the end of the chapter, advantages and disadvantages
of the numerical methods for different types of problems are presented in a
list to provide a summary that can be used as a checklist later on.
Chapter 6 illustrates the application of the different numerical methods
on a variety of canonical examples. The examples address a broad field of
applications:coaxiallinediscontinuities,apertureantenna,dipoles,microstrip
antennas,microstripfilterandcavity.Thesesimplestructurescanbeusedfor
validation purposes when starting with new software. After studying these
two core chapters, the reader should have a clear vision on how to choose
the right numerical software for a particular problem and how to devise an
efficient simulation model.
Part III is concerned with practical applications to demonstrate the use
of field simulation software in industrial projects. Chapter 7 starts with the
design process of a planar GPS receive antenna. The different parts of the
model are tuned separately and are afterwards combined to a final design.
Second, the radiation characteristics of a GSM base station antenna are an-
alyzed including exposure of a realistic human body model. The modeling
processoriginatesfromasimplepuremetaldesignwhichisvalidatedbyman-
ufacturersdatasheetsandanalternativesimulationmodel.Third,thedesign
of mobile phone antennas and the influence of the user on the antenna are
investigated. Finally, an Ultra-Wideband (UWB) antenna is described and
integrated in a realistic scenario.
Throughout this book, the following EM software tools have been used:
• Microwave Studio from CST GmbH, Darmstadt, Germany
• HFSS from Ansoft Corporation, Pittsburgh, USA
• ADS/Momentum from Agilent Technologies, Inc., Palo Alto, USA
• EMPIRE from IMST GmbH, Kamp-Lintfort, Germany
• 4NEC2 based on NEC2 from Lawrence Livermore National Laboratory,
USA
• Concept II from the Technical University Hamburg-Harburg, Germany
Theauthorswouldliketothankthecompaniesfortheirsupportandfortheir
kind permission to use the software in the scope of this book.
X Preface
We are also grateful to Springer Publishers for advice and cooperation.
Furthermore, we would like to thank those colleagues and students who were
associated with the writing of this book.
Dortmund, Kamp-Lintfort Frank Gustrau
Spring 2006 Dirk Manteuffel
Contents
1 Introduction............................................... 1
Part I Basics
2 Electromagnetic Fields .................................... 5
2.1 Maxwell’s Equations ..................................... 5
2.2 Material Equations ...................................... 6
2.3 Boundary Conditions .................................... 8
2.4 Waves in Free Space ..................................... 9
2.5 Polarization ............................................ 11
2.6 Waves in Lossy Media.................................... 12
2.7 Energy Conservation..................................... 14
2.8 Electromagnetic Potentials ............................... 15
2.9 Green’s Function ........................................ 17
2.10 Waves on Transmission Lines ............................. 18
3 Radiofrequency and Antenna Theory ...................... 23
3.1 Scattering Parameters.................................... 23
3.2 Resonators and Antennas................................. 27
3.3 Antenna Parameters ..................................... 28
3.3.1 Antenna Radiation ................................ 28
3.3.2 Nearfield and Farfield .............................. 30
3.3.3 Radiation Pattern ................................. 31
3.3.4 Directivity, Efficiency and Gain ..................... 33
3.3.5 Matching and Bandwidth .......................... 35
3.4 Antenna Arrays ......................................... 37
3.4.1 Farfield Approximation ............................ 37
3.4.2 Beam Steering .................................... 40
XII Contents
Part II Numerical Methods
4 Numerical Methods........................................ 45
4.1 General Aspects of Numerical Modeling Software ............ 46
4.2 Finite-Difference Time-Domain (FDTD).................... 48
4.2.1 Fundamentals..................................... 48
4.2.2 Discretization in Space and Time.................... 49
4.2.3 Numerical Stability and Dispersion .................. 54
4.2.4 Boundary Conditions .............................. 56
4.2.5 Excitations ....................................... 56
4.2.6 Circuit Parameters and Frequency-Domain Quantities . 57
4.2.7 Nearfield-to-Farfield Transformation ................. 60
4.2.8 Prediction Filter for Structures with High Q Value .... 63
4.3 Method of Moments (MoM) .............................. 66
4.3.1 Fundamentals..................................... 66
4.3.2 Thin Wire Approximation .......................... 68
4.3.3 Metal Surfaces .................................... 70
4.3.4 Treatment of Dielectric Material .................... 71
4.3.5 Method of Moments for Layered Structures ........... 71
4.4 Finite-Element Method (FEM)............................ 73
4.4.1 General Remarks.................................. 73
4.4.2 Variational Expression ............................. 73
4.4.3 Finite Elements ................................... 73
4.4.4 Shape Functions .................................. 74
4.4.5 Element Matrix ................................... 76
4.4.6 Matrix Compilation ............................... 76
4.4.7 Linear Systems.................................... 77
4.5 Other Methods.......................................... 78
5 Creating an Efficient Simulation Model .................... 81
5.1 General Structure of Numerical Modeling Software .......... 81
5.1.1 Preprocessor...................................... 82
5.1.2 Solver............................................ 83
5.1.3 Postprocessor ..................................... 83
5.2 Geometry .............................................. 84
5.2.1 Interactive Construction via Graphical User Interface
(GUI)............................................ 84
5.2.2 Object Definition via Macro Language ............... 86
5.2.3 Import of CAD (Computer Aided Design) Data ....... 86
5.2.4 Summary......................................... 88
5.3 Material Properties ...................................... 89
5.4 Ports .................................................. 90
5.4.1 Waveguide Ports .................................. 91
5.4.2 Plane Wave Excitation............................. 91
Contents XIII
5.4.3 Lumped Ports .................................... 93
5.5 Lumped Circuit Elements ................................ 94
5.6 Boundary Conditions .................................... 94
5.6.1 Electric and Magnetic Walls ........................ 94
5.6.2 Absorbing Boundary Conditions..................... 96
5.7 Meshing................................................ 97
5.7.1 Overview......................................... 97
5.8 Simulation Control Parameters............................ 99
5.9 Estimation of Required Resources .........................100
5.10 Postprocessing ..........................................101
5.11 Selection of Numerical Method ............................102
5.12 Summary...............................................103
6 Canonical Examples .......................................107
6.1 Introduction ............................................107
6.2 Coaxial Line Discontinuity................................108
6.2.1 Theoretical Background ...........................108
6.2.2 Analysis with FDTD Software ......................109
6.2.3 FDTD Convergence Study..........................115
6.2.4 Analysis with FEM Software........................117
6.2.5 FEM Convergence Study ...........................120
6.3 Coaxial Line Transition with Quarter Wavelength Transformer 123
6.3.1 Theoretical Background ............................123
6.3.2 Analysis with FDTD Software ......................124
6.3.3 Analysis with FEM Software........................126
6.4 Microstrip Filter ........................................128
6.4.1 Theoretical Background ............................128
6.4.2 Model Definition ..................................130
6.4.3 Analysis with FDTD Software ......................131
6.4.4 Analysis with FEM Software........................135
6.4.5 Analysis with MoM Software .......................137
6.5 Feedpoint Impedance of a Dipole Antenna ..................139
6.5.1 MoM Model Definition.............................139
6.5.2 MoM Convergence Study of Input Impedance and
Resonance Frequency ..............................142
6.5.3 Transmission Between Distant Antennas .............143
6.5.4 Analysis with FDTD...............................144
6.5.5 Analysis with FEM Software........................146
6.6 Aperture Antenna .......................................148
6.6.1 Rectangular Waveguide ............................148
6.6.2 Analysis with FDTD Software ......................149
6.6.3 Analysis with FEM Software........................155
6.7 Planar Inverted-F Antenna (PIFA) ........................157
6.7.1 General Remarks..................................157
6.7.2 Model Definition and FDTD-Based Antenna Design ...158