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  • Electrically-Small Antenna Performance

    Enhancement for Near-Field Detuning Environments

    Christian W. Hearn

    Dissertation submitted to the Faculty of

    The Virginia Polytechnic Institute and State University

    in partial fulfillment of the requirements for the degree of

    Doctor of Philosophy

    in

    Electrical Engineering

    William A. Davis, Chair

    Warren L. Stutzman

    Majid Manteghi

    Timothy Pratt

    Werner Kohler

    October 25, 2012

    Blacksburg, Virginia

    Keywords: Multi-resonant small antenna, spherical mode decomposition

  • Electrically-Small Antenna Performance Enhancement for Near-Field Detuning Environments

    Christian W. Hearn

    ABSTRACT

    Bandwidth enhancement of low-profile omnidirectional, electrically-small antennas has evolved from the

    design and construction of AM transmitter towers eighty years ago to current market demand for battery-

    powered personal communication devices. Electrically-small antenna theory developed with well-known

    approximations for characterizing radiation properties of antenna structures that are fractions of the

    radiansphere. Current state-of-the-art wideband small antennas near ka≈1 have achieved multiple-octave

    impedance bandwidths when utilizing volume-efficient designs.

    Significant advances in both the power and miniaturization of microelectronics have created a second

    possible approach to enhance bandwidth. Frequency agility, via switch tuning of reconfigurable

    structures, offers the possibility of the direct integration of high-speed electronics to the antenna structure.

    The potential result would provide a means to translate a narrow instantaneous bandwidth across a wider

    operating bandwidth.

    One objective of the research was to create a direct comparison of the passive- multi-resonant and

    active-reconfigurable approaches to enhance bandwidth. Typically, volume-efficient, wideband antennas

    are unattractive candidates for low-profile applications and conversely, active electronics integrated

    directly antenna elements continue to introduce problematic loss mechanisms at the proof-of-concept

    level.

    The dissertation presents an analysis method for wide bandwidth self-resonant antennas that exist in

    the 0.5≤ka≤1.0 range. The combined approach utilizes the quality factor extracted directly from

    impedance response data in addition to near-and-far field modal analyses. Examples from several classes

    of antennas investigated are presented with practical boundary conditions. The resultant radiation

    properties of these antenna-finite ground plane systems are characterized by an appreciable percentage of

    radiated power outside the lowest-order mode.

    Volume-efficient structures and non-omnidirectional radiation characteristics are generally not viable

    for portable devices. Several examples of passive structures, representing different antenna classes are

    investigated. A PIN diode, switch-tuned low-profile antenna prototype was also developed for the

    comparison which demonstrated excessive loss in the physical prototype.

    Lastly, a passive, low-profile multi-resonant antenna element with monopole radiation is

    introduced. The structure is an extension of the planar inverted-F antenna with the addition of a

    capacitance-coupled parasitic to enhance reliable operation in unknown environments.

  • iii

    ACKNOWLEDGEMENTS

    I would like to thank Dr. William A. Davis who served as my committee chairman, advisor, and mentor

    for the past four years. Not only am I indebted to him for the opportunity to return as a Ph.D. student in

    the Virginia Tech Antenna Group, but he has also served as an inspiration for his expertise in multiple

    technical areas.

    The award of an engineering Ph.D. to a non-traditional student at mid-career may be considered a

    testament to the student’s patience and persistence. In this particular case, obstinance is a more accurate

    description. However, the ultimate achievement would not have been possible without support, guidance

    and mentorship from an impressive roster of professional role models along the way. During the course of

    my technical career, it has been a distinct honor and privilege to have studied under the directions of Mr.

    John D. Gordon (retired physicist-NSWC-Carderock, UERD), Dr. William D. Stanley (Eminent Professor

    Emeritus-ODU), and Mr. Thomas G. Campbell (IEEE Fellow-Advanced Program Director, Applied EM,

    Inc.).

    I am grateful for the support provided by Dr. C.J. Reddy and senior staff at Applied EM,

    Incorporated. The collaborative relationship between the Virginia Tech Antenna Group, Applied EM Inc.,

    and ZAI, facilitated by Dr. Kevin Baugh, was a lifeline for a significant portion of the dissertation

    research.

    Both my heartfelt gratitude and love belong to my wife Kimberly. Her support, empathy and patience

    during this academic odyssey kept our family on course with eyes on the finish line. Finally, I am grateful

    to my entire family, including my father, my dear aunt, my in-laws and the extended family, for their

    unwavering trust as I climbed higher and higher up the academic mountain. Thank you.

  • iv

    TABLE OF CONTENTS

    1 CHAPTER - Introduction .......................................................................................................................... 1

    1.1 Overview of Dissertation 2

    1.2 References 4

    2 CHAPTER - Background ........................................................................................................................... 5

    2.1 Introduction 5

    2.2 Finite Ground Plane Detuning 8

    2.3 Low Profile Antenna Solutions - Small versus Low-Profile 10

    2.3.1 Planar Inverted-F Antenna 11

    2.3.2 Microstrip Antenna 13

    2.4 Active Reconfigurable Narrowband Structures 17

    2.5 Wideband Electrically-Small Antennas 19

    2.6 Summary 21

    2.7 References 21

    3 CHAPTER - Approach/Tools .................................................................................................................. 23

    3.1 Introduction 23

    3.2 Analysis Tools - Q and Spherical Mode Decomposition 24

    3.2.1 Quality Factor 24

    3.2.2 Spherical Mode Decomposition 26

    3.3 Examples of Approach 28

    3.3.1 Electrically-Small Antenna Ground Plane Interaction - Modal Analysis 29

    3.3.2 Far-field versus Near-field Spherical Mode Decomposition 30

    3.4 Low-Profile Example – Microstrip Patch Antenna 32

    3.5 Summary 36

    3.6 References 36

    4 CHAPTER - Narrowband Reconfigurable, Active Antenna Systems ..................................................... 38

    4.1 Introduction 38

    4.2 Short Monopole Placement in Handheld Device Configuration 39

    4.3 Planar Inverted-F Antenna 42

    4.3.1 Height Reduction Discussion 47

  • v

    4.3.2 Miniaturization - Single Resonant Antennas 55

    4.4 Switch-Tuned PIFA 60

    4.5 Conclusions 65

    4.6 References 65

    5 CHAPTER - Wideband ESA Models ....................................................................................................... 67

    5.1 Introduction 67

    5.2 Multi-resonant Spiral 68

    5.3 Ravipati Antenna 72

    5.4 Wideband Compact J-Pole Antenna 78

    5.5 Multi-Resonant Patch - U-Slot Antenna 84

    5.5.1 Sensitivity to Finite Ground Plane 88

    5.5.2 Discussion - U-Slot Microstrip Patch Antenna 89

    5.6 Conclusions 89

    5.7 References 90

    6 CHAPTER - Hardware Technologies ...................................................................................................... 91

    6.1 Introduction 91

    6.2 High Efficiency Power Amplifiers for Portable Applications 92

    6.3 Integrated Inverse Class-F Amplifier Design with low-profile PIFA 92

    6.4 Hardware Evaluation 96

    6.5 Summary 100

    6.6 References 101

    7 CHAPTER - Multi-resonant Low-Profile Example................................................................................ 102

    7.1 Introduction 102

    7.2 The Antenna Structure 102

    7.3 Summary 113

    7.4 References 113

    8 Chapter - Conclusions ......................................................................................................................... 115

    8.1 Summary 115

    8.2 Contributions 117

    8.3 Future work 118

    8.4 References 118

  • vi

    LIST OF FIGURES

    FIG. 2.1. SCHEMATIC DIAGRAM OF A VERTICALLY-POLARIZED OMNIDIRECTIONAL ANTENNA ENCLOSED BY A

    SPHERE RADIUS Ρ=A, CENTERED ABOUT THE ORIGIN. ............................................................................................ 5

    FIG. 2.2. CAPACITOR AND INDUCTOR OCCUPYING EQUAL CYLINDRICAL VOLUMES. .................................................................. 6

    FIG. 2.3. COMPLEX IMPEDANCE VERSUS DIAMETER OF GROUND PLANE FOR 1

  • vii

    FIG. 4.3. NORMALIZED IMPEDANCE (A) AND VSWR (B) RESPONSES FOR CENTER-MOUNT AND EDGE-MOUNTED

    MONOPOLES ON A HANDHELD DEVICE SHOWN IN FIG. 4.2. ................................................................................. 41

    FIG.

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