A PLANAR AND INTEGRATED FOR 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

iii

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 + ") for wireless power reception is designed and experimentally verified. The rectenna consists of two major components: one is a and the other is a half-wave rectifier circuit. The collects wireless power, and then, the received -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 ...... 6

2.2.2 Gain ...... 7

2.2.3 Input Impedance ...... 8

2.2.4 ...... 8

2.2.5 Beamwidth ...... 10

2.3 Frii’s Transmission Equation ...... 10

2.4 Microstrip ...... 11

2.4.1 Introduction ...... 11

v

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 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 ...... 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 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 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 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.

1

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.

2

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 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 which has a moderate gain (7db @ 2.4 GHz), low SWR ( 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.

4

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.

5

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 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 current that may be applied to the terminals of the same antenna in order to convert it into an electromagnetic 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

6

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]

If the direction is not specified, the direction of maximum radiation intensity is considered and the directivity is expressed as

4 D = directivity (dimensionless)

Do = maximum directivity

U = radiation intensity (W/ unit solid angle)

Umax = maximum radiation intensity (W/ unit solid angle)

Uo = radiation intensity of isotropic source (W/ unit solid angle)

Prad = total radiation power (W)

2.2.2 Gain

Gain of an antenna in a given direction is defined as the ratio of the intensity in a given direction to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. The radiation intensity corresponding to the isotropically radiated power is equal to the power accepted (input) by the antenna divided by 4 π.” In equation form this can be expressed as

Gain = 4 π , 4π

U = radiation intensity in a direction contained in E and E field component θ θ � Pin = total input (accepted) power

7

2.2.3 Input Impedance

Input impedance is defined as the impedance presented by an antenna at its terminals or the ratio of the voltage to current at a pair of terminals or the ratio of the appropriate components of the electric to magnetic fields at a point. The ratio of the voltage to current at these terminals with no load attached, the input impedance of an antenna is defined at Z A = RA + j X A. [2].

Figure 3.1 gives a thevenin equivalent of an antenna.

Figure 2.1 Thevenin equivalent of an Antenna.

ZA = antenna impedance at the terminal a-b (ohms)

RA = antenna resistance at terminals a-b (ohms)

XA = antenna reactance at terminals a-b (ohms)

2.2.4 Antenna Efficiency

Associated with an antenna are a number of efficiencies and can be defined using Figure 3.2.

The total antenna efficiency e0 is used to take into account losses at the input terminals and within the structure of the antenna. Such losses may be due, referring to Figure 3.3, to

1. Reflections because of the mismatch between the transmission line and the antenna

2. I2R losses (conduction and dielectric)

8

Figure 2.2 Antenna reference terminals.

Figure 2.3 Reflections, conduction and dielectric losses.

In general, the overall efficiency can be written as

e0 = e r * e c * e d

where e0 = total efficiency (dimensionless)

2 er = reflection(mismatch) efficiency = (1 − |п| ) (dimensionless) ec = conduction efficiency (dimensionless) ed = dielectric efficiency (dimensionless)

п = voltage reflection coefficient at the input terminals of the antenna

[п = (Zin − Z0)/ (Zin + Z0) where Zin = antenna input impedance,

Z0 = of the transmission line]

VSWR = voltage = 1 + | п| / 1 − |п|. [2].

9

2.2.5 Beamwidth

The beamwidth of a pattern is defined as the angular separation between two identical points on opposite side of the pattern maximum. In antenna measurements there are different beamwidths. One of the most widely used is the half power beamwidth (HPBW). It is defined by

IEEE as: “In a plane containing the direction of the maximum of a beam, the angle between the two directions in which the radiation intensity is one-half value of the beam.” Another important beamwidth is the angular separation between the first nulls of the pattern, and it is referred to as the First-Null Beamwidth (FNBW).

Figure 2.4 Two dimensional representation of Beamwidth. [2]

2.3 Frii’s Transmission Equation

The Frii’s transmission equation relates the power received to the power transmitted between two antennas separated by a distance R > 2D 2 /λ, where D is the largest dimensions of either antenna. Given two antennas, the ratio of power available at the output of the receiving antenna, Pr, to power input to the transmitting antenna, Pt, is given by

10

4 where Gt and Gr are the of the transmitting and receiving antennas, respectively, λ is the , and R is the distance. [3]. The antenna gains are with respect to isotropic (and not in ), and the wavelength and distance units must be the same. This simple form applies only under the following ideal conditions:

• The antennas are in unobstructed free space, with no multipath.

• Pr is understood to be the available power at the receive antenna terminals. There is

loss introduced by both the cable running to the antenna and the connectors.

Furthermore, the power at the output of the antenna will only be fully delivered into the

transmission line if the antenna and transmission line are conjugate matched.

• Pt is understood to be the power delivered to the transmit antenna. There is loss

introduced by both the cable running to the antenna and the connectors. Furthermore,

the power at the input of the antenna will only be fully delivered into free space if the

antenna and transmission line are conjugate matched.

• The antennas are correctly aligned and polarized.

• The bandwidth is narrow enough that a single value for the wavelength can be

assumed.

2.4 Microstrip

2.4.1. Introduction

At microwave frequency, microstrip is often used as a transmission line because of its very good performance in transferring energy and microwave signals. One of microstrip line's significant advantages is that it does not generate as much parasitic capacitances and inductances as lumped elements do. Furthermore, compared with another kind of transmission line - stripline, microstrip is much easier and cheaper to fabricate and easy to connect surface 11

mounted components. As regards to our project, the operating frequency of the system is about

2400 MHz; microstrip is the preferable transmission line components. Figure 2.5 shows the typical structure of a microstrip.

Figure 2.5 Microstrip Layers Structure.

Microstrip line is constructed with three layers with different material separately: a thin metal layer (radiating patch) is printed on a substrate layer which is usually made of materials like

Fiberglas, polystyrene or Teflon, and the dielectric constant (εr ) varies from 3.2 to 12. In this work, we have used Rogers RT/duroid 5880 as the substrate with a dielectric constant of

εr = 2.2. The downwards side of the substrate is the plane which is also a layer of metal which is a good conductor (Copper). The characteristic impedance of a microstrip is closely associated with the width of the conductor, the thickness and the material of the substrate; lower-impedance microstrip lines are comparably wider and vice-versa. For a fixed width microstrip line, the transmission line impedance does not change with the length. [22].

2.4.2. Microstrip Patch Antenna

It is not very easy to give an exact definition of a microstrip patch antenna because of its flexibility in construction. While the most commonly used microstrip patch antenna has similar structure as a microstrip line. On one side of a thin dielectric substrate layer, is an extremely thin layer of conductor, which forms the radiating element, and on the downwards

12

side is the , which is also made of metal material. Thin substrates with higher dielectric constants are desirable for element size. Thin substrates with higher dielectric constants are preferred for microwave circuits as they require tightly bound fields to minimize the undesired radiation and coupling. Often microstrip antennas are referred to as patch antennas. The radiating patch may be square, rectangular, thin strip (dipole), circular, elliptical, triangular, or any other configuration. These along with the others are illustrated in the

Figure 2.6. Square, rectangular and circular are the most commonly used because of the ease of analysis and fabrication, and their attractive radiation characteristics, especially low cross- radiation.

Figure 2.6 Representative shapes of Microstrip patch elements. [2]

The operating frequency of microstrip antennas usually ranges from 1GHz to

15GHz. When we compare the other kinds of antenna with microstrip antennas have the following advantages:

a. Lower profile, light in weight, easily compacted with circuit and conformability to mounting

13

hosts, lower the cost of fabrication. b. Easily to get different polarization. c. Can work in dual frequency or multi-frequency.

The main disadvantages are: a. Narrow bandwidth b. Higher losses in conductor and dielectric material results in lower efficiency. c. Lower transmitting power for single patch antenna.

2.5 Feeding Methods

There are many configurations that can be used to feed microstrip antennas. The most popular among the feeding techniques are the microstrip line, coaxial probe, aperture coupling and proximity coupling.

2.5.1 Microstrip Line Feed

This type of feed is a natural choice as this involves excitation of the antenna by a

microstrip line on the same substrate. In this mechanism the microstrip line is usually

designed to have a characteristic impedance of 50 ohms. This is the simplest form of feed

and has the advantage that the feed also can be fabricated along with the antenna

itself without this being a separate process and results in a planar antenna structure

rather than is the case with coaxial feed. The major disadvantage of this type of feed is the

mismatch between the antenna and the 50 ohm . This is because the edge

resistance of a patch is reasonably high but decreases as we move into the patch toward

its center. Since this value is greater than 50, an impedance matching circuit has to be

used between the patch edge and the feed line. The matching circuit, besides giving

rise to spurious radiation, cannot be accommodated in arrays because of the non-

availability of physical space on the substrate. Also the microstrip line blocks radiation

from the portion of the patch with which it is in contact resulting in reduced radiation. 14

This is a serious limitation when the width of the patch is small as in the case of antennas

designed at millimeter wave frequencies. The microstrip-line feed is easy to fabricate,

simple to match by controlling the inset position and rather simple to model. As the

substrate thickness increases, surface waves and spurious feed radiations increase, which

for practical designs limit the bandwidth.

Figure 2.7 Microstrip Feed for Patch Antenna. [2].

2.5.2 Coaxial Feed

In this type of feed, a coaxial probe is used to connect to the patch antenna by

drilling a hole through the dielectric substrate at a location which provides the best

impedance match for the coaxial line. Here, the coaxial line is attached to the bottom of the

ground plane and the center conductor is brought through the substrate and soldered to the

patch. The variation of the coupling of power from the coaxial line to the patch was found to

vary as the cosine of the ratio πXo /L, where L is the resonant length of the patch and X 0 being

the offset from the edge of the patch. The coaxial probe feed is also easy to fabricate and

match, and it has low spurious radiation. However it also has narrow bandwidth and it is more

difficult to model, especially for thick substrates.

15

Figure 2.8 Coaxial feed. [2].

2.5.3 Aperture-Coupled Feed

In this type, there is an additional substrate placed next to the ground plane. The order of

placement is feed line, second substrate, ground plane where the slot is present, antenna

substrate and the radiating patch. The position of the slot and the feed line with respect to

the patch determines the amount of coupling, but the size and dimensions of the slot also

vary the coupling and operating frequency to a certain degree as has been shown in the

simulations done in this work. For maximum coupling, the slot is centered over the patch

where the magnetic field of the patch is at a maximum and the feed line is placed such that

the slot is at about a quarter from the edge of the feed line. The quarter

wavelength distance enables the slot to be positioned at the point on the feed line where the

signal reaches its peak strength. The lower substrate usually has a higher dielectric

constant and the substrate immediately below the patch has a lower dielectric constant to

optimize the radiation from the patch. This feed technique is difficult to implement because

of the presence of two dielectric substrates but has the advantages that the spurious

radiation produced by the feed line is avoided as the feed line is shielded by the ground 16

plane which provides polarization purity. An additional advantage is that wider bandwidth is possible.

Figure 2.9 Aperture-Couple Feed. [2].

2.5.4 Proximity-Coupled Feed

Another form of non-contact feed mechanism which overcomes the disadvantages of the direct contact fed patches is the proximity coupled patch. This patch antenna consists of two substrates, one grounded and housing the microstrip feed line and another above the grounded substrate which has the microstrip patch etched on its surface. The power from the feed is coupled to the patch electromagnetically as opposed to a direct contact. This is the reason this form is also referred to as the electromagnetically coupled patch antenna.

An important attribute of this feed is that its coupling mechanism is inherently capacitive in nature. This is in contrast to the direct contact feeds which are predominantly inductive.

This difference in coupling significantly affects the obtainable impedance bandwidth because the inductive coupling of the edge and probe-fed patches limits the thickness of the substrate that can be used. Thus, the bandwidth in this case is inherently greater than the direct contact feed patches. The proximity coupled feed mechanism has some

17

disadvantages, the feed and antenna layers are not fully independent because power has

to be coupled between them, this results in high levels of spurious radiation from the feed

although not to the levels obtained in the edge-fed case.

Figure 2.10 Proximity-Coupled Feed. [2].

2.6 Rectangular Patch

The most commonly employed microstrip antenna is a rectangular patch. The rectangular patch antenna is approximately a one-half wavelength long section of rectangular microstrip transmission line. When air is the antenna substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-space wavelength. As the antenna is loaded with a dielectric as its substrate, the length of the antenna decreases as the relative dielectric constant of the substrate increases. The resonant length of the antenna is slightly shorter because of the extended electric "fringing fields" which increase the of the antenna slightly. An early model of the microstrip antenna is a section of microstrip transmission line with equivalent loads on either end to represent the radiation loss.

The two methods that are used to analyze are explained in the below sections.

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2.6.1 Transmission Line Model

Microstrip antennas primarily radiate because of the fringing fields created

between the patch and the ground plane on either side of its length. Basically, the

distance between the two sides considered to radiate constitutes the length and this length

determines the resonant frequency of the antenna. As can be seen from the figure 2.12 on

one side of the patch the electric field lines are directed from the ground plane to the patch

and on the other side the field lines are directed from the patch to the ground. On both

sides, there is a narrow region on top of the substrates where the field lines flow

horizontally into the patch. It is this area which produces broadside radiation from the

patch in the form of two narrow slots of depth L. When both the slots radiate, the fields along

the vertical directions from the respective patches overlap and strengthen each other,

whereas the horizontal fields directed into the patch cancels each other. Eventually, this

combined with the horizontal fields flowing out of the slots end up giving the patch a

broadside dome shaped . The length of the patch L is approximately ½ a

wavelength as determined by the dielectric material.[2].

Figure 2.11 Top View of Patch Antenna.

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Figure 2.12 Side/Horizontal view of Patch antenna.

2.6.2 Cavity Model

Another well known model for the analysis of patch antennas is the cavity model

which provides more accurate results. In this case, the following assumptions are made:

• The electric field is Z directed and the fields do not vary appreciably in that direction as

the substrate is very thin compared to the wavelength of propagation.

• The magnetic fields have only transverse components in the x and y direction in the

region bounded by the patch and the ground plane assuming electric field walls in the

+Z and –Z directions at the dielectric boundary.

When the patch antenna is fed using an appropriate feeding technique, a charge

distribution can be witnessed on the patch itself which is mirrored in a reciprocal manner on

the surface of the ground plane. The charges that is present on the lower surface of the patch

experience two types of forces acting on them – an attractive force between the charges

on the patch and the charges on the ground plane as they are opposing charges and the

like charges on the bottom of the patch. The opposing mechanism forces the charges

around the edges of the patch onto the top part of the patch metallization. This produces a

current density Jt (at the top) and Jb (on the bottom) on the patch. Most of the current

density is assumed to be on the bottom of the patch. Assuming no flow of charges to the top

of the metallization due to very low substrate thickness which would make the attractive

force strong and result in the charges remaining only at the bottom of the patch, the four

sides of the patch can be modeled as magnetic walls, wherein, there are no tangential

magnetic components to the side walls. Although this is not the case in reality since the 20

height to width ratio is finite when a design is implemented, it can be approximated

to be the case because the tangential magnetic fields produced due to the very small

surface current densities are negligible. Since the walls of the cavity and the material within it

are assumed to be lossless (the magnetic and electric walls would ensure that the magnetic

and electric fields within the cavity are not disturbed), the cavity would not radiate and its

input impedance would be purely reactive. In the absence of a loss mechanism, radiation

cannot be explained. To do so, a lossy cavity is assumed with an effective loss tangent δeff .

The total effective loss tangent for a microstrip antenna in terms of the quality factor of its

dielectric, conductor and radiation has been defined as δeff = 1/Q where Q is he quality factor.

Figure 2.13 Charge Distribution and Current Density creation on a Microstrip Antenna.

2.7 Design Procedure

2.7.1 Specification of the Design

Firstly, we should make some design specifications such as the desired operating frequency, directivity, gain, mechanical requirements etc. The environment should also be considered. In this thesis, the patch antenna will work at 2.4 GHZ and it is located inside the anechoic chamber.

Secondly, the substrate material needs be chosen prior calculation. Usually after the material is decided, the thickness of the substrate can be decided. If the substrate is thick, we get wider bandwidth and increasing driven point impedance while the dimensions increase as well. The 21

substrate used in the thesis is been manufactured by Rogers corporation and the thickness of the board is 1.575 mm.

2.7.2 Parameters of the Patch Antenna

Given the operating frequency and the physical parameters of the substrate, we would do well to use some simple equations to design the microstrip patch. The design formulas given in text

Antenna theory by Constantine Balanis can be used. It is better to begin the calculations by knowing the frequency (f r), height of the substrate (h) and the dielectric constant ( εr ). For low frequencies the effective dielectric constant is essentially constant. At intermediate frequencies its value begins to monotonically increase and eventually approach the values of the dielectric constant of the substrate. The effective dielectric constant is given by

−0.5 ε+r1 ε− r 1 h  εreff = +1 + 12 * W/ h > 1 2 2 W 

When εr and h are confirmed, εr is decided by W, while L is decided by εr. W has influence both on directivity and input impedance. The relative permittivity in the thesis is 2.33 as calculated.

Smaller values of W result in lower antenna efficiency while larger values of W leads to higher order modes. It is beneficial to have a relatively wider W, but usually lesser than half the wavelength. The practical width that leads to good radiation efficiencies is

1 2 W = 2*fr *ε0 * 0 ε r +1

The length of the antenna is calculated by the equation mentioned below. [2].

W (ε reff + 3)( + 0.264) L = 0.412* h h W (ε reff − 0.258)( + 0.8) h

Since the length of the patch is extended by L on each side, the effective length of the patch 22

now is L eff = L + 2 L where L is half wavelength. So, the actual length of the patch can now be determined by the equation written below.[2].

1 L= −2 L 2**frε reff ε0 * 0

After calculation we arrived at the length of our patch antenna to be 40.5mm and the width to be 49.5 mm.

There is no specific equation to calculate the ground dimensions, while considering the weight and dimension of the antenna, the substrate and ground metal should be as small as possible.

Most of the field is confined within a small area around the patch; consequently it is not much sense to extend the ground plane and substrate too greatly. In this project we choose a ground metal with the dimensions of 100*100 mm². The patch antenna is simulated to the conjugate input impedance of the rectifier.

2.8 Fabrication procedure for the Antenna

• Clean the board (copper) using alcohol.

• Warm up the laminator, set the temperature to 110 degree, and speed to 2. Just press

the CARRI button. When the Ready light turns green, it means temperature is ok. If it

flashes, it means the temperature is hotter.

• Turn off the light. All procedures involving photo resist must be done in a dark

environment (This means there should be no ultraviolet light).

• Cut a piece of photo resist that is a bit larger than the board.

• There are 2 layers of plastic sheets, one among which is the photo resist. Peel off one

layer a bit so that the other layer can cover back of the board.

• Wet the copper with water, but the board should not dribble drops. This is to make

sure the photo resist is bonding to board steadily.

• Cover the board with photo resist from one end, and put the board into the laminator. 23

• Take off the peeled sheet while the board is rolled into.

• When the board comes out from the laminator, put it in the drawer (Dark place) 30

minutes to cool down.

• Press the COLD button on the laminator to cool down it. When the current

temperature is below 60 cent degree (the roller stops), switch off

the power.

• Place the mask on the laminated board and then place them under the UV lamp for

exposure.

• Expose the laminated board under the UV lamp for 15 minutes.

• Keep the board in dark place for 15 minutes after exposure is complete.

• Combine developer fluid: one part pure developer and 10 parts water.

• Peel off another layer of a sheet from the board.

• Put the board in the developer for 40 to 50 seconds. Any longer than that, the exposed

resist will be dissolve.

• Put the board in clean water. Scrape the photo resist with a scoop. The exposed resist

should not be touched.

• Clean the board and dry it with paper. Do not touch the exposed resist. Put it in the

drawer 15 minutes.

• Turn on the light. And warm up the iron. Set temperature gear to five.

• Put the board on the iron for 5 minutes to make the resist adhere to the board firmly.

The color of resist will turn darker, maybe purple.

• Clean the board.

• Put the board in the etching chemical. The chemical is ready to use, needing no water.

When doing this procedure, you should wear rubber gloves and respirator and

blinders.

24

• Put the board in the chemical 40-50 minutes, and keep stirring the fluid. Time will be

much shorter if the chemical is new. Usually the chemical should be replaced after 5

boards.

• Take the board out and put into clean water. After cleaning it with alcohol and water,

put it in the developer again. Time taken to dissolve the resist is much longer.

• Clean it and the fabricated board is ready.

The picture of the fabricated board is shown in the figure below.

Figure 2.14 Fabricated Antenna.

2.9 Results

Once the design parameters of the patch antenna are calculated, it is then simulated with the help of Ansoft HFSS software. The picture below shows us the patch antenna design in HFSS.

The feeding point is found out depending up on the load which will be integrated with the patch antenna. It is been kept checking until we find a good S11 value indicating that the antenna is

25

well match with the load.

Figure 2.15 HFSS simulation picture of Patch Antenna.

The design been ready, the results of the antenna are plotted. The results like S11 (dB) and gain are been simulated using HFSS. Figure 2.16 shows the snaps of the S11 and gain of the patch antenna in db. The S11 of the antenna when measured using a network analyzer is shown in the Figure 2.17. It has a value around -17 dB which is little close when compared to the -22 dB obtained in the simulations. This small amount of error may be due to the fabrication errors which are involved during the fabrication procedure like, exposing the board for a little longer/shorter time under the UV lamp, time the board is in the developer etc.

26

Figure 2.16 Simulated S11 of the patch antenna.

Figure 2.17 Measured S11 of the Patch Antenna.

The measured and simulated S11 of the patch antenna at a frequency of 2.4 GHz. The S11 values on both do not match exactly as there might be some error involved in the fabrication of

27

the board. The input impedance is not 50 ohms because; the antenna is simulated with the complex conjugate of the rectifiers input impedance giving us the freedom to play with the impedance.

Figure 2.18 shows the 3-D graph of the gain of the patch antenna with a table of the distribution of the gain. The current distribution can also be seen in the Figure 2.19 where in the table indicates the value of the current at different points on the patch antenna which can be recognized by the variation of the color on the patch antenna.

Figure 2.18 Gain of the patch antenna (Simulated).

Figure 2.19 Current Distributions on the Patch Antenna. 28

2.9.1 Verification of the Frii’s Transmission Equation.

2D2 The condition for verifying whether the readings are taken in the far field region is . λ

The Frii’s transmission formula is given by

Pr λ 2 = ( )**Gt G r Pt 4π * r

Where

c λ = wavelength of free space which is 125 mm ( λ = ). fr

Gt = Gain of the transmitting antenna. The antenna used here is a horn antenna and the gain of this antenna is 7 dB at 2.4 GHz as given by the manufacturer.

Gr = Gain of the receiving antenna which is 6 dB from the HFSS simulation.

Below is the table which gives the verification that the antenna is working in the far field region as the LHS and RHS of the equation are both satisfied.

Table 2.1 Verification of Frii’s Transmission Equation.

Distance R Pt in dBm Pr in dBm Pr /P t Formula on (cm) RHS of the equation 41 23 2.15 8.5 * 10 -3 8.33 * 10 -3 45 23 0.83 6 * 10 -3 6.92 * 10 -3 49 23 0.41 5 * 10 -3 5.83 * 10 -3 53 23 0.66 5 * 10 -3 5.0 * 10 -3

The Experimental setup consists of a transmitting antenna connected to a microwave source and a receiving antenna connected to the power meter. The mismatching between the power meter and the antenna is been taken into to account.

29

CHAPTER 3

INTEGRATED RECTENNA

3.1 Rectifier

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. A basic half wave rectifier is considered in this thesis.

It basically consists of a capacitor to store the energy and a resistor as the load. A schottky diode is used. The input power has a strong effect on RF to DC conversion efficiency which drops when the RF voltage is below the turn on voltage of the diode. The maximum frequency of operation of the diode depends on the transit time of the charge carriers and the charge carrier mobility. The diode which is been used in this thesis is HSMS-8202. These are low cost microwave schottky diodes and are specifically designed for use at X/Ku- band. It is optimized in the GHz range, low capacitance, and low conversion loss. It is a surface mount diode and there by occupies very little space. [1].

3.1.1 Simulation

The circuit is designed using Agilent ADS (Advanced Design System). Below is the schematic diagram of the circuit along with the results from which the value of the capacitance is selected.

As the capacitance is varied from 10 pF to 30 pF, the value of the time taken to reach the peak value increases and the ripples at the rectifier output is reduced increase in discharge time. So, the value of 10 pf is selected. The resistor value is 1K ohms. The schematic diagram is shown

30

in the figure 3.1 . The circuit diagram of the rect ifier is shown below in Figure 3 .2. It is a half wave rectifier in which the diode conducts only in the positive half cycle.

Figur e 3 .1 Schematic Diagram of the Rectifier.

Figure 3 .2 Circuit Diagram of the Rectifier.

The circuit diagram consists of an AC source, an inductor which is used as an RF choke which passes the lower frequencies/DC and blocks the higher frequency signals. 31

Rectified Output 0.8

0.7

0.6

0.5

0.4 Output Voltage in V in Voltage Output 0.3

0.2

0.1

0 0 0.5 1 1.5 2 2.5 3 3.5 time in s -8 x 10

Figure 3.3 Simulated Waveform for Capacitance Value of C=10 pF.

Rectified Output 0.8

0.7

0.6

0.5

0.4 Output Voltage in V in Voltage Output 0.3

0.2

0.1

0 0 0.5 1 1.5 2 2.5 3 3.5 time in s -8 x 10

Figure 3.4 Simulated Waveform for Capacitance Value of C=15 pF.

32

Rectified Output 0.8

0.7

0.6

0.5

0.4 Output Voltage in V in Voltage Output 0.3

0.2

0.1

0 0 0.5 1 1.5 2 2.5 3 3.5 time in s -8 x 10

Figure 3.5 Simulated Waveform for Capacitance Value of C= 30 pF.

3.1.2 Input impedance of the rectifier

The input impedance of the rectifier is calculated using ADS. The circuit diagram (Figure 3.6) gives us a clear picture of finding the input impedance. ADS have a block, Zin which when placed in place of the source, gives us the input impedance of the rectifier. The rectifier should be terminated by a 1K ohm resistor. The impedance consists of real and the imaginary part and is been shown in the figure 3.7 and 3.8 respectively.

Figure 3.6 Schematic Diagram for finding Zin using ADS.

33

0

-100 m1 -200

-300

imag(Zin1) -400 m1 freq= 2.400GHz -500 imag(Zin1)=-203.675

-600 1.0 1.5 2.0 2.5 3.0 3.5 4.0 freq, GHz

Figure 3.7 Simulated Imaginary Part Input Impedance.

3.4

3.2

3.0 m2 freq= 2.400GHz 2.8 real(Zin1)=2.365 real(Zin1) 2.6

2.4 m2

2.2 1.0 1.5 2.0 2.5 3.0 3.5 4.0 freq, GHz

Figure 3.8 Simulated Real part of the Input Impedance.

So, the value of input impedance is Zin = 2.365 -j203.675 ohms. The value will not be the same when fabricated on a board as we will be having a SMA connector attached to the transmission line. The value of the impedance measured using the network analyzer is around 225 + j100 34

ohms with a phase of 10 degrees. This is shown in the smith chart shown in figure 3.10. The fabricated board of the rectifier is also shown in the figure 3.9.

Figu re 3.9 Fabricated Board of the Rectifier.

The quarter wavelength is basically used as a RF choke or DC path. It is basically used to remove the higher order frequencies and pass on the lower frequencies or DC.

Below are the waveforms of the rectifier when connected to the microwave source as an input.

The waveforms show th e half wave rectification and as the power level increases, the diode moves away from the linear region. The time period of the wave is 3.1 and when calculated, the frequency of operation is 2.4 GHz. Figure 3.11 and Figure 3.12 shows the waveforms for an input of 10 dBm and 20 dBm respectively. The type of modulation that has been used is pulse modulation. The signal which is the input to the rectifier is shown below.

Figure 3.10 Input Signal to the rectifier.

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Figure 3.11 Output of the Rectifier with an Input of 10 dBm.

Figure 3.12 Output of the Rectifier with an Input of 20 dBm.

We can observe a small change during the positive half cycle when compared with the waveform in figure 3.9 when the power is being increased from 10 dBm to 20 dBm. The Y-axis is the voltage and the X axis is the time/div.

36

Figure 3.13 Smith Chart representing the Impedance.

The figure 3.13 shows the impedance of the rectifier and the antenna. The measured and the desired impedance of the antenna are very close to each other. The desired impedance is the conjugate of the measured input impedance of the rectifier.

3.2 Integrated Rectenna

The rectifier and the antenna have been designed and fabricated. Next is the process of integrating them on a single board. Before we integrate them on a single board, we shall test the antenna and the rectifier by connecting them by a male-male adaptor.

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3.2.1 Impedance Matching

Impedance matching is an engineering procedure employed in circuit design for matching unequal source and load impedances. Impedance matching is required in order to optimize the matching networks into a circuit between the source and the load. In this model, Smith Charts are used to visualize the interactive process of impedance matching. This can be visualized in the block diagram as shown in the figure 3.14.

Microstrip Patch Matching Circuit Rectifier circuit Antenna

Figure 3.14 Simple Block Diagram of the Rectenna.

The impedance of the rectifier is known. So, the simplest way of matching both the circuits is by having the conjugate of the impedance to the antenna. The antenna is designed and simulated for the conjugate value of the input impedance of the rectifier. As, we are building the blocks separately, we should also consider the phase shift caused by the SMA connector and the male- male adaptor. This clearly explained in the smith chart shown below. It clearly gives a clear idea of understanding.

The process shall be explained in 4 steps.

Step 1: It is indicated by the number 1 in the figure 3.15. It indicates the input impedance of the rectifier. The impedance is plotted along with the phase being 225 + j100 ohms and 10 degress respectively. This value is normalized by 50 ohms and then plotted in the smith chart as the smith chart is normalized for 50 ohms.

38

Figure 3.15 Smith Chart representing the Whole Rectenna.

Step 2: This is indicates by the number 2 in the figure 3.15. It gives the value of the antenna which is simulated to the complex conjugate of the input impedance of the rectifier. The impedance and phase of the antenna being 12+j30 ohms and 150 degrees respectively. This is the impedance along with the SMA connector.

Step 3: This step is indicated by number 3 in the figure 3.15. This is the impedance of the antenna along with the SMA connector and male to male adaptor. The impedance and the phase being 30 – j 50 ohms and -70 degrees respectively.

Step 4: When the equivalent phase of the SMA connector is subtracted from step 3, we get the value of 180 – j100 ohms as indicated by number 4 in the figure 3.15. When the phase of the 39

male to male adaptor is added to number 2, we get number 3. Thus, the matching between the rectifier and the antenna is pretty close. The integrated antenna along with the effects of the male to male adaptor and SMA connector is shown in the figure 3.18. The matching of the antenna and the rectifier is shown by the measurements from the network analyzer in the smith chart pictures shown below.

Figure 3.16 Impedance of the Antenna using the Network analyzer.

40

Figure 3.17 Impedance of the Rectifier using the Network Analyzer.

Figure 3.18 Rectenna with the Connector and Adaptor.

When the rectifier was first designed, the length of the transmission line was very close to λeff .

To move the impedance to the required impedance i.e., the conjugate of the rectifier input

41

impedance, its equivalent length is calculated by moving towards the load and it is cut off from the rectifier board.

Figure 3.19 Rectenna with an LED as load.

Figure 3.19 shows the working of the rectenna successfully. Instead of a resistor at the load, an

LED (Light Emitting Diode) is connected and the glowing of the LED indicated the working of the integrated rectenna. The rectenna is placed in front of the transmitting antenna which is been connected to a microwave source.

Now, an integrated antenna is designed and fabricated without considering the connectors and the adaptors on a single board.

3.2.2 Calculations for the integrated antenna

1) Length Of the transmission line in the patch antenna = 43.3 mm

2) Length of the transmission line on the rectifier = 44 mm

3) Phase of the male to male adaptor = 140 degrees 42

We know that θ= β L

2π λ Where β = and λ eff = . λ eff ε eff

4) Phase of the SMA connector = 50 degrees.

So, the total phase from 3 and 4 is 236 degrees. Substituting these values in the equation of

θ= β L , we get L = 56.11 mm.

The total length of the whole setup is = 43.3 + 44 + 56.11 = 143.11 mm.

This when seen with respect to the wavelength ( λ eff ) is 14.5 mm more than 1.5 λ eff . This will be the value of the transmission line on the patch antenna without harming the impedance matching and the results. This can be done as one round of the smith chart is half the wavelength. The figure below shows the integrated antenna.

Figure 3.20 Integrated Rectenna on a Single Board.

The quarter wavelength is been replaced by making a hole in the center of the antenna and connecting it to the ground. This is been done to make the board as compact as possible.

43

The output of the rectenna when observed in the oscilloscope is shown in the figure 3.21.

Figure 3.21 Output of the Rectenna.

Using the method that is used for the building the rectenna shown in Figure 3.20, another rectenna is built with the frequency of operation of 2.1GHz. This rectenna is even more compact as there is an inset instead of protrusion. The antenna is successfully matched to the rectifier by having an inset of 2.5mm.Figure 3.22 shows the picture of the compact rectenna.

Figure 3.22 Integrated Rectenna with Inset Feed.

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3.2.3 Efficiency of the rectenna

The efficiency of the rectenna before integration and after integration is shown below. The results are close and there is some error related to fabrication. The table shown below clearly gives the picture of the efficiency of the rectenna.

Table 3.1 Before Integration of Patch and Rectifier.

Distance in cm I/P power to the O/P Voltage of / Efficiency in Pr = Rectifier (dBm) the rectifier % =P /P (mW) r i Pi (Vo/p) 10 10.01 ( 10mW) 2.5 6.25 62.5 15 8 (6.3mW) 2.2 4.84 76

Table 3.2 After Integration of Patch and Rectifier.

Distance in cm I/P power to the O/P Voltage of / Efficiency in Pr = Rectifier (dBm) the rectifier % =P /P (mW) r i Pi (Vo/p) 10 10.01 (10mW) 2.4 5.76 57.6 15 8 (6.3mW) 2 4 64

45

CHAPTER 4

CONCLUSION AND FUTURE WORK

In this thesis work, a compact, planar rectenna is designed for wireless power reception in

ISM(Industrial, Scientific, Medical) frequency band of operations. The system consists of 3 major blocks. (i) Antenna which is used for receiving the signal is operated in the far field region.

(ii) Impedance matching circuit which is used for matching the antenna with the rectifier. (iii) A half wave rectifier is used to convert the AC to the DC which has an efficiency of 65%. These blocks are individually and integrally verified using simulations and measurements successfully.

There are some improvements which can be done in the future work. (i) The rectifier can be replaced by the full wave rectifier for higher efficiency. (ii) The impedance matching here is generally done for the first harmonics and the other harmonics are filtered out. Instead of filtering them, they can also be matched for good impedance matching. (iii) A rectenna array can be used to increase the DC received at the output.

46

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BIOGRAPHICAL INFORMATION

Vinay Ramachandra Gowda was born in Bangalore, Karnataka, India. He obtained his

Bachelor degree in Electronics and Communication Engineering in 2009, from Visvesvaraya

Technological University, Belgaum, Karnataka, India. He has worked towards his Master of

Science in Electrical Engineering in The University of Texas at Arlington since August 2009. His research areas include Electromagnetics, Antenna Design, RF and Wireless Communications.

51