A PLANAR AND INTEGRATED RECTENNA FOR WIRELESS POWER RECEPTION
by
VINAY RAMACHANDRA GOWDA
Presented to the Faculty of the Graduate School of
The University of Texas at Arlington in Partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE IN ELECTRICAL ENGINEERING
THE UNIVERSITY OF TEXAS AT ARLINGTON
May 2011
Copyright © by VINAY RAMACHANDRA GOWDA 2011
All Rights Reserved
ACKNOWLEDGEMENTS
I would like to offer special thanks to my advisor Dr. Mingyu Lu for his continuous help, guidance and support throughout my thesis. His patience, experience, and knowledge have been invaluable throughout my research and I am truly grateful for this. I would like to thank him for giving me a opportunity to work in the Wave Scattering Research Centre which I truly loved working in.
I would like to express my gratitude to Dr. Jonathan Bredow and Dr. Saibun Tjuatja for their encouragement and for reading the thesis.
I would also like to thank Dr. W. Alan Davis and Dr. William E. Dillon, my graduate advisors for their advice and guidance throughout my Master of Science (M.S) degree.
Also, I received academic guidance from Shaoshu Sha, Suman Kumar Gunnala and
Vinay Vikram Magadi for which I will be truly obliged.
I would also like to thank Dr. Huiqing Zhai for his help in HFSS simulaitions.
I am indebted to my family, my Mother Radha C, my Father Ramachandra Gowda and my adorable sister Bindu Ramachandra for their unwavering love and support throughout my entire life.
April 15, 2011
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ABSTRACT
A PLANAR AND INTEGRATED RECTENNA
FOR WIRELESS POWER
RECEPTION
Vinay Ramachandra Gowda (M.S)
The University of Texas at Arlington, 2011
Supervising Professor: Mingyu Lu
In this thesis, a rectenna ("rectifier + antenna") for wireless power reception is designed and experimentally verified. The rectenna consists of two major components: one is a microstrip patch antenna and the other is a half-wave rectifier circuit. The microstrip antenna collects wireless power, and then, the received radio-frequency power is rectified to DC by the rectifier.
The microstrip antenna and the rectifier circuit are simulated, fabricated, and tested separately.
Before they are integrated, matching network is designed in between them to match the first harmonic. A few integrated rectenna’s are built around 2.4 GHz ISM band. The rectenna’s are entirely built over printed circuit boards, hence are planar and compact. Measurement results demonstrate 65% of power efficiency for the rectenna’s.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS ...... iii
ABSTRACT ...... iv
LIST OF ILLUSTRATIONS...... viii
LIST OF TABLES ...... xi
Chapter Page
1. INTRODUCTION……………………………………..………..…...... 1
1.1 History of Wireless Power ...... 1
1.2 Application of Wireless Power ...... 2
1.3 Overview of Thesis ...... 4
2. FUNDAMENTALS OF ANTENNNA ...... 6
2.1 Definition of an Antenna ...... 6
2.2 Antenna Parameters ...... 6
2.2.1 Directivity ...... 6
2.2.2 Gain ...... 7
2.2.3 Input Impedance ...... 8
2.2.4 Antenna Efficiency ...... 8
2.2.5 Beamwidth ...... 10
2.3 Frii’s Transmission Equation ...... 10
2.4 Microstrip ...... 11
2.4.1 Introduction ...... 11
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2.4.2 Microstrip Patch Antenna ...... 12
2.5 Feeding Methods ...... 14
2.5.1 Microstrip Line Feed ...... 14
2.5.2 Coaxial Feed ...... 15
2.5.3 Aperture-Coupled Feed ...... 16
2.5.4 Proximity-Coupled Feed ...... 17
2.6 Rectangular Patch ...... 18
2.6.1 Transmission Line Model ...... 19
2.6.2 Cavity Model ...... 20
2.7 Design Procedure ...... 21
2.7.1 Specification of the Design ...... 21
2.7.2 Parameters of the Patch Antenna ...... 22
2.8 Fabrication Procedure for the Antenna ...... 23
2.9 Results ...... 25
2.9.1 Verification of the Frii’s Transmission Equation ...... 29
3. INTEGRATED RECTENNA ...... 30
3.1 Rectifier ...... 30
3.1.1 Simulation ...... 30
3.1.2 Input Impedance ...... 33
3.2 Integrated Rectenna ...... 37
3.2.1 Impedance Matching ...... 38
3.2.2 Calculations for the Integrated Antenna...... 42
3.2.3 Efficiency of the Rectenna ...... 45
4. CONCLUSION AND FUTURE WORK ...... 46
vi
REFERENCES ...... 47
BIOGRAPHICAL INFORMATION ...... 51
vii
LIST OF ILLUSTRATIONS
Figure Page
1.1 Wireless Toothbrush ...... 3
1.2 Splashpower mat ...... 3
1.3 Christmas tree lit without wires ...... 4
1.4 Block Diagram of Wireless Power Transmission System ...... 5
2.1 Thevenin Equivalent of an Antenna ...... 8
2.2 Antenna reference terminals ...... 9
2.3 Reflection, conduction and dielectric losses ...... 9
2.4 Two dimensional representation of Beamwidth ...... 10
2.5 Microstrip Layer Structure ...... 12
2.6 Representative shapes of Microstrip patch elements ...... 13
2.7 Microstrip Feed for Patch Antenna ...... 15
2.8 Coaxial Feed ...... 16
2.9 Aperture-Couple Feed ...... 17
2.10 Proximity-Coupled Feed ...... 18
2.11 Top View of Patch Antenna ...... 19
2.12 Side/Horizontal view of Patch Antenna ...... 20
2.13 Charge Distribution and Current Density creation On a Microstrip Antenna ...... 21
2.14 Fabricated Antenna ...... 25
2.15 HFSS simulation picture of Patch Antenna ...... 26
viii
2.16 Simulated S11 of Patch Antenna ...... 27
2.17 Measured S11 of Patch Antenna ...... 27
2.18 Gain of Patch Antenna (Simulated) ...... 28
2.19 Current Distributions on Patch Antenna ...... 28
3.1 Schematic Diagram of the Rectifier ...... 31
3.2 Circuit Diagram of the Rectifier ...... 31
3.3 Simulated Waveform for Capacitance Value of C = 10pF ...... 32
3.4 Simulated Waveform for Capacitance Value of C = 15pF ...... 32
3.5 Simulated Waveform for Capacitance Value of C = 30pF ...... 33
3.6 Schematic Diagram for finding Zin using ADS ...... 33
3.7 Simulated Imaginary part of Input Impedance ...... 34
3.8 Simulated Real part of Input Impedance ...... 34
3.9 Fabricated Board of the Rectifier ...... 35
3.10 Input Signal to the Rectifier ...... 35
3.11 Output of the Rectifier for an Input of 10 dBm ...... 36
3.12 Output of the Rectifier for an Input of 20 dBm ...... 36
3.13 Smith chart representing the Impedance ...... 37
3.14 Simple Block Diagram of the Rectenna ...... 38
3.15 Smith chart representing the Whole Rectenna ...... 39
3.16 Impedance of the Antenna using Network Analyzer ...... 40
3.17 Impedance of the Rectifier using Network Analyzer ...... 41
3.18 Rectenna with the Connector and adaptor ...... 41
3.19 Rectenna with an LED as load ...... 42
3.20 Integrated Rectenna on a Single Board ...... 43
3.21 Output of the Rectenna ...... 44
ix
3.22 Integrated Rectenna with Inset ...... 44
x
LIST OF TABLES
Table Page
2.1 Verification of Frii’s Transmission Equation ...... 29
3.1 Before Integration of Patch and Rectifier ...... 45
3.2 After Integration of Patch and Rectifier ...... 45
xi
CHAPTER 1
INTRODUCTION
1.1 History of Wireless Power
The discussion of wireless power transmission as an alternative to transmission line power distribution started in the late 19 th century. Both Heinrich Hertz and Nicolai Tesla theorized the possibility of wireless power transmission. Tesla demonstrated it in 1899 [31].
Despite the novelty of Tesla’s demonstration and his personal efforts to commercialize wireless power transmission, he soon ran out of funding because it was much less expensive to lay copper than to build the equipment necessary to transmit power through radio waves. William
C. Brown contributed much to the modern development of microwave power transmission which for many reasons dominates research and development of wireless transmission today. In the early 1960s brown invented the rectenna which directly converts microwaves to DC current. He demonstrated its ability in 1964 by powering a helicopter from the solely through microwaves.
“In 1982, Brown (Raytheon) and James F. Trimer (NASA) announced the development of a thin-film plastic rectenna using printed-circuit technology that weighed only one-tenth as much as any previous rectenna” [31]. No commercial development past the prototype stage has been funded. Despite these advances wireless power transmission has not been adopted for commercial use except for the sole exception of pacemakers and electric toothbrush rechargers. However, research is ongoing because of the many promising applications suited for wireless power transmission.
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An extensive amount of work is done in the field of wireless powering, inductive powering for short ranges, RFID tags and low power sensors. This is accomplished by receiving incident wave with an antenna and then rectifying the RF voltage. Generally, the operation range of these rectenna’s is in the near field region. The main concentration of this thesis is the working of the rectenna in the far field region.
1.2 Application of Wireless Power
There are many applications which make use of wireless power. Some of the applications are wireless toothbrush, wireless lit Christmas tree, Alticor’s espring water purifier, power mat etc.
1.3.1 Wireless Toothbrush.
Wireless tooth brush consists of two inductive coils and they work on the principle of inductive coupling. Inductive coupling uses magnetic fields that are a natural part of current's movement through wire. Any time electrical current moves through a wire, it creates a circular magnetic field around the wire. Bending the wire into a coil amplifies the magnetic field. The more loops the coil makes, the bigger the field will be.
If you place a second coil of wire in the magnetic field you've created, the field can induce a current in the wire. This is essentially how a transformer works, and its how an electric toothbrush recharges. It takes three basic steps:
1. Current from the wall outlet flows through a coil inside the charger, creating a magnetic
field. In a transformer, this coil is called the primary winding.
2. When you place your toothbrush in the charger, the magnetic field induces a current in
another coil, or secondary winding , which connects to the battery.
3. This current recharges the battery.
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The Figure 1.2 shows the wireless tooth brush and Figure 1.3 shows the splash power mat.
Also, the splash power mat also works on the same phenomenon of inductive coupling.
Devices which have to be charged are kept on the splashpower mat for recharging.
Figure 1.1 Wireless Toothbrush Figure 1.2 Splashpower mat
1.3.2 Christmas Tree without Electrical Wiring
The wireless Christmas tree does not make use of wires and uses radio frequencies to transmit energy from a power source to the LED bulbs on the tree. The working principle is a transmitter can be placed anywhere in the lamp, for example, plugged into the wall and sits on a table. The transmitter in the lamp sends out a continuous, low RF signal. Anything with either AA or AAA batteries set within its range and equipped receiver will be continuously charged. The figure 1.4 shows the Christmas tree lei without any wires. 3
Figure 1.3 Christmas tree lit without wires.
1.3 Overview of Thesis
The primary components of a wireless power transmission system which is been used in the thesis consist of a transmitting antenna and a Rectenna. The transmitting antenna is been fed by a RF source along with an amplifier to increase the input power to the transmitting antenna. The transmitting antenna used in the thesis is a horn antenna which has a moderate gain (7db @ 2.4 GHz), low SWR (Standing wave ratio), broad bandwidth and simple construction and adjustment. The microwave signal is transmitted by the transmitting antenna in the free space transmission channel. This signal is then been received by the rectenna which converts the RF signal into a DC signal by a rectenna. A rectenna is a passive element which consists of antenna ad rectifying circuit. The antenna used in the rectenna maybe a dipole,
Yagi- Uda, microstrip or parabolic disc antenna. The patch antenna achieves the highest efficiency amongst all the other types of antennas. Schottky barrier diodes are generally used in the rectifying circuit due to the faster recovery time and good RF characteristics at high frequency of operations. The figure 1.1 gives a block diagram of a wireless power transmission unit.
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Free Space
RF Power Microstrip Rectifier Source Amplifier patch Circuit antenna
Figure 1.4 Block Diagram of Wireless Power Transmission System.
The issues related to efficiency, rectification and integration of the rectenna are addressed in the thesis as follows.
Chapter 2 basically deals with the explanation of the basic concepts of the antenna like directivity, impedance, Frii’s transmission formula, gain etc.
Chapter 3 deals with the design and fabrication of the microstrip patch antenna. The design of the antenna is done in Ansoft HFSS (High Frequency Signal Simulator). The antenna is then fabricated on Rogers PCB board and tested.
Chapter 4 describes the design and fabrication of the rectifier. The rectifier design is done in
ADS. The middle part of this chapter introduces with the integration of the antenna and the rectifier by impedance matching. The integrated rectenna will be a small and compact one.
Chapter 5 presents a discussion on what is the future work related to this topic and the conclusion.
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CHAPTER 2
FUNDAMENTALS OF AN ANTENNA
2.1 Definition of an Antenna
The IEEE definition of an antenna would be “The part of a transmitting or receiving system that is designed to radiate or receive electromagnetic waves”. A simple definition would be – any conducting element capable of sensing electromagnetic waves and is used for transmitting or receiving them. An antenna can also be defined as an electrical device which couples radio waves in free space to an electrical current used by a radio receiver or transmitter. In reception, the antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage that the radio receiver can amplify. Alternatively, a radio transmitter will produce a large radio frequency current that may be applied to the terminals of the same antenna in order to convert it into an electromagnetic wave (radio wave) radiated into free space . [2].
2.2 Antenna Parameters
There are several parameters that affect the performance of an antenna which can be controlled during the design process. The parameters are explained as follows:
2.2.1 Directivity
The directivity of an antenna is defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power that is been radiated by the antenna divided by 4 π. If the
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direction is not specified, the direction of maximum radiation is implied. The directivity of a nonisotropic source is equal to the ratio of its radiation intensity in a given direction to the radiation intensity of an isotropic source. [2]