Rectangular Patch Antenna

MINIATURIZATION OF RECTANGULAR PATCH ANTENNAS USING DEFECTED GROUND STRUCTURES (DGSs)

This thesis is presented in partial fulfillment for the award the Bachelor of Electrical Engineering (Honors) Universiti Teknologi MARA (UiTM)

SULAIM BIN AB QAIS FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MARA (UiTM) SHAH ALAM JULY 2012

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MINIATURIZATION OF RECTANGULAR PATCH ANTENNAS USING DEFECTED GROUND STRUCTURES (DGSs)

SULAIM BIN AB QAIS

FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MARA MALAYSIA

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ABSTRACT

Microstrip patch antennas (MPA) is a very popular in mobile and radio wireless Communication. The antenna is very easy to fabricate and has good radiation characteristics. However, future technology needs smaller patch antenna to go well with future gadgets. The limitation of conventional MPA is that the size is comparable to half wavelength. In order to overcome this limitation, a new solution method of using defected ground structure (DGS) having metamaterial characteristic can reduce the size. The metamaterial response is verified in simulation based on scattering parameter using Nicolson-Ross-Weir (NRW) method. This thesis presents design, simulate and fabrication of miniaturized antenna operating at 2.4GHz and 4.75GHz.

Two DGS

structures have been designed. The first design is the circular rings and second design is the split rings. Both of the circuits are able to reduce the size from 34% and 81% respectively. Those antennas were simulated using Computer Simulation Technology Microwave Studio (CST MWS) and measured using Vector Network Analyzer (VNA). Both the simulated and measured data were compared. From the simulation, the DGS antennas design at 2.4 GHz and 4.75 GHz have return loss value of negative 24.4 dB and negative 35.6 dB respectively. The 4.75 GHz antenna also increases the bandwidth up to 80% compared to conventional one.

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TABLE OF CONTENT CONTENTS

PAGES

DECLARATION

i

ACKNOWLEDGEMENT

ii

ABSTRACT

iii

TABLE OF CONTENT

iv

LIST OF FIGURES

vii

LIST OF TABLES

x

LIST OF SYMBOLS/ABBREVIATIONS

xi

CHAPTER 1: INTRODUCTION

1

1.1 INTRODUCTION

1

1.2 PROBLEM STATEMENT

3

1.3 OBJECTIVES OF THE PROJECT

3

1.4 SCOPE OF WORK

4

1.5 ORGANIZATION OF THE THESIS

4

1.6 SUMMARY

5

CHAPTER 2: LITERATURE REVIEW

6

2.1 INTRODUCTION

6

2.2 ANTENNA PROPERTIES

7

2.2.1

Polarization

7 4

2.2.2

Radiation Pattern

8

2.2.3

Half Power Beam Width (HPBW)

10

2.2.4

Antenna Gain

11

2.2.5

Voltage Standing Wave Ratio (VSWR)

12

2.2.6

Bandwidth

12

2.3 MICROSTRIP ANTENNA

13

2.3.1

Overview of Microstrip Antenna

13

2.3.2

Surface Wave Effect

13

2.3.3

Microstrip Line

14

2.3.4

Rectangular Patch Antenna Design

15

2.3.5

Feeding Technique

17

2.4 METAMATERIAL

21

2.4.1

Metamaterial Theory

21

2.4.2

Behavior of Wave

23

2.4.3

Refraction and Snell‟s Law

24

2.4.4

Metamaterial Structures

27

2.4.5

Nicholson Ross Weir (NRW)

28

2.5 Relation Between DGS, EBG and Metamaterial

30

CHAPTER 3: METHODOLOGY

32

3.1 METHODOLOGY OVERVIEW

32

3.2 FLOW CHART OF DESIGN PROCESS

33

3.3 DGS STRUCTURES

34

3.4 INVESTIGATION OF METAMATERIAL SUBSTRATE

35

3.5 ANTENNA DESIGN

38

3.5.1

Designing Rectangular Patch Antenna

38

3.5.2

Designing Rectangular Patch Antenna with DGS

43

3.6 FABRICATION PROCESS

43

3.7 MEASUREMENTS

45

5

CHAPTER 4: RESULTS AND DISCUSSION

47

4.1 INTRODUCTION

47

4.2 4.75GHZ ANTENNAS

47

4.3 2.4 GHZ ANTENNAS

55

4.4 CONCLUSION

63

CHAPTER 5: CONCLUSION AND FUTURE IMPROVEMENTS

64

5.1 CONCLUSION

64

5.2 SUGGESTION FOR FUTURE WORKS

65

REFERENCES

66

APPENDICES

69

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LIST OF FIGURES FIGURES

PAGES

Figure 2.1

Types of antenna polarization

8

Figure 2.2

Polar radiation pattern

9

Figure 2.3

Diagram of radiation pattern

10

Figure 2.4

Rectangular plot of radiation pattern

11

Figure 2.5

Graph return loss versus frequency

12

Figure 2.6

Microstrip line

14

Figure 2.7

Microstrip diagram and equivalent circuit

15

Figure 2.8

EM fields around the microstrip line

16

Figure 2.9

Microstrip line feed

17

Figure 2.10

Top view of rectangular patch antenna structure with coaxial feed 19

Figure 2.10

Top view of rectangular patch antenna structure with coaxial feed 19

Figure 2.11

Diagram of aperture coupled feed

20

Figure 2.12

Proximity coupled feed

21

Figure 2.13

Electromagnetic wave

22

Figure 2.14

Permittivity-permeability(ε-μ) diagram which shows the material classifications 23

Figure 2.15

Wave incidents on a positive index material

23

Figure 2.16

Wave incidents on a negative index material

24

7

Figure 2.17

Incident wave travelling through positive-index and negative materials

25

Figure 2.18

Refracted rays in conventional material

26

Figure 2.19

Refracted rays in Left-handed material

26

Figure 2.20

Structure used for metamaterial synthesis (a) SRRs, (b) metal wire lines, (c) CSRRs, (d) Slot lines

27

Figure 2.21

Metamaterial Structure (a) Mushroom structure, (b) Planar CRLH structure 28

Figure 2.22

Different DGS geometries

31

Figure 3.1

Flow chart of antenna design

33

Figure 3.2

Bottom view of DGS structures

35

Figure 3.3

Transmission line method

35

Figure 3.4

Permittivity, εr and permeability, µr value versus frequencies for substrate with circular rings DGS

36

Permittivity, εr and permeability, µr value versus frequencies for substrate with slip rings DGS.

37

Figure 3.6

Designed metamaterial substrates dimension

38

Figure 3.7

Top view dimension (mm) of conventional rectangular patch antenna: (a) 4.7 GHz antenna (b) 2.4 GHz antenna

40

Figure 3.5

Figure 3.8

Result of return loss simulation on variations inset cut length for conventional rectangular patch antenna: (a) 4.7 GHz antenna (b) 2.4 GHz antenna

41

Figure 3.9

3D view structure DGS antenna

43

Figure 3.10

Anechoic chamber

46

Figure 4.1

Top view dimension of the of 4.7 GHz antennas (a) conventional (b) metamaterial antenna with circular rings DGS.

Figure 4.2

48

Comparison of return loss simulation data of conventional antenna and DGS metamaterial antenna. 48

8

Figure 4.3

4.7 GHz radiation pattern

50

Figure 4.4

Size comparison of fabricated 4.7 GHz antenna

51

Figure 4.5

Graph simulation and measurement of 4.7 GHz conventional antenna versus frequency

52

Graph simulation and measurement of 4.7 GHz DGS antenna versus frequency

52

Graph comparing between 4.7 GHz conventional and DGS antennas

53

Figure 4.8

Radiation pattern measurement

54

Figure 4.9

Top view dimension of the of 2.4 GHz antennas

56

Figure 4.10

Comparison of return loss simulation data of conventional antenna and DGS metamaterial antenna 56

Figure 4.11

2.4 GHz antenna radiation pattern

57

Figure 4.12

Size comparison of fabricated 2.4 GHz antenna

59

Figure 4.13

Graph simulation and measurement of 2.4 GHz conventional antenna versus frequency

60

Graph simulation and measurement of 2.4 GHz DGS antenna versus frequency

60

Graph comparing between 4.7 GHz conventional and DGS antenna

61

Measurement radiation pattern

62

Figure 4.6

Figure 4.7

Figure 4.14

Figure 4.15

Figure 4.16

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LIST OF TABLES FIGURES

PAGES

Table 3.1 Parameter of substrate characteristics (RO3003)

34

Table 3.2 Parameter goals of conventional rectangular patch antenna

39

Table 3.3 Settings for CST MWS

39

Table 3.4 Antenna design optimization

42

Table 4.1 Comparison between 4.7 GHz conventional and DGS antenna

50

Table 4.2 Comparison between 2.4 GHz conventional and DGS antenna

58

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LIST OF SYMBOL/ABBREVIATIONS

SYMBOL

NAME

εr

Relative permittivity

µr

Relative permeability

n

Refraction index

W

Patch width

L

Patch length

c

Speed of light

t

Copper thickness

h

Substrate thickness

l

Inset cut

Ws

Substrate width

Ls

Substrate length

Zo

Characteristic impedance

RF

Radio frequency

3D

Three-dimensional

MPA

Microstrip patch antenna

dB

Decibel

GHz

Gigahertz

VSWR

Voltage standing wave ratio

BW

Bandwidth

LH

Left-handed

LHM

Left-handed material

EM

Electromagnetic

UV

Ultravioletlm 11

SRR

Split-ring resonator

CSRR

Complementary Split-ring resonator

NRW

Nicolson-Ross-Weir

EBG

Electromagnetic band gap

DGS

Defected ground structure

SMA

Subminiature version A

VNA

Vector Network Analyzer

CST MWS

Computer Simulation Technology Microwave Studio

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CHAPTER 1

INTRODUCTION

This chapter consists of a brief introduction to my project including problem statement, objectives, scope of work and outline of my thesis. This chapter highlighted the important of the project and the arrangement of this thesis. 1.1 INTRODUCTION Antenna is one of the important elements in the RF system for receiving or transmitting signals from and into the air as medium. Without proper design of the antenna, the signal generated by the RF system will not be transmitted and no signal can be detected at the receiver. Antenna design is an active field in communication for future development. Many types of antenna have been designed to suit with most devices. One of the types of antenna is the microstrip patch antenna (MPA). The microstrip antenna has been said to be the most innovative area in the antenna engineering with its low material cost and easy to fabricate which the process can be made inside universities or research institute. The idea of microstrip antenna was first presented in year 1950‟s but it only got serious attention in the 1970‟s [1].

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Due to MPA advantages such as low profile and the capability to be fabricated using lithographic technology, antenna developers and researchers can come out with a novel design of antenna which will reduce the cost of its development. Through printed circuit technology, the antenna can be fabricated in mass volume which contributes to cost reduction. MPA are planar antennas used in wireless links and other microwave applications. It uses conductive strips and patches formed on the top surface of a thin dielectric substrate separating them from a conductive layer on the bottom surface which is the ground for the antenna. A patch is typically wider than a strip and its shape and dimension are important features of the antenna. MPA are probably the most widely used antennas today due to their advantages such as light weight, low volume, low cost, compatibility with integrated circuits and easy installation on the rigid surface. Furthermore, they can be easily designed to operate in dual-band, multi-band application, dual or circular polarization. They are important in many commercial applications [7]. They are extremely compatible for embedded antennas in handheld wireless devices such as cellular phones, pagers etc. These low profile antennas are also useful in aircraft, satellites and missile applications, where size, weight, cost, ease of installation, and aerodynamic profile are strict constraints [8]. MPA with inset feed has been studied extensively over the past two decades because of its advantages. However, length of MPA is comparable to half wavelength with single resonant frequency with bandwidth around 2%. Moreover, some applications of the MPA in communication systems required smaller antenna size in order to meet the miniaturization requirements. So, significant advances in the design of compact MPA have been presented over the last years. Many methods are used to reduce the size of MPA like using planar inverted F antenna structure (PIFA) or using substrate with high dielectric constant [9]. Defected Ground Structure (DGS) is one of the methods to reduce the antenna size.

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DGS is relatively a new area of research and application associated with printed circuit and antennas. The evolution of DGS is from the electron band gap (EBG) structure [10]. The substrate with DGS must be designed so that it becoming metamaterial. The substrate with DGS is considered as metamaterial substrate when both relative permittivity, εr and permeability, µr are negative. The invented metamaterial antenna will have comparable performance and smaller size to conventional one. The substrate that I used was RO3003. The metamaterial antenna behaves as if it were much larger than it really is. By designing the antenna with the DGS makes possible to reduce the size for a particular frequency as compared to the antenna without the DGS. MPA inherently has narrow bandwidth and bandwidth enhancement is usually demanded for practical applications, so for extending the bandwidth, DGS approaches can also be utilized. 1.2 PROBLEM STATEMENT In the microstrip antenna application, one of the problem is to reduce its size while having considerable performance. Moreover, the propagating of surface wave will reduce the efficiency of the antenna. This is due to the increment of the side and back radiation. When this happen, the front lobe or main lobe will decrease which lead to reduction in gain. 1.3 OBJECTIVES OF THE PROJECT Conventional antenna follows the right-hand rule which determine how electromagnetic wave behaves. This antenna radiates at frequency of half wavelength of the patch length while metamaterial antenna able to omit this rule. Metamaterial antenna able to radiates while having smaller size of antenna. Moreover, metamaterial offers an alternative solution to widen the antenna applications using the left-hand rule. The unique properties of metamaterial enable the enhancement of the conventional antenna, thus open more opportunities for better antenna design. This project will emphasize on obtaining the metamaterial using DGS with optimized parameters of negative index behaviour in which both permittivity and permeability co-exist simultaneously in the required frequency region. 15

The main objectives of this project are: i)

To prove the concept of metamaterial.

ii)

To reduce the size of rectangular patch antenna by implementing metamaterial as substrate.

iii)

To compare the performance of DGS and conventional antenna.

1.4 SCOPE OF WORK This project focuses on the development of two miniaturized rectangular patch antenna using DGSs that functions at 4.7 GHz and 2.4GHz. The scope of the project will includes the study of metamaterial having both negative permittivity and permeability. Then, produce the metamaterial substrate by using DGS. The work includes the designing of DGS that will change RO3003 substrate into metamaterial. These substrates are then tested through simulation using NRW method to find the metamaterial functional frequency. Then, both conventional and DGS antennas are designed to resonate at the obtained frequency. The parameter of conventional rectangular patch antenna is based on formula [11] and [12] while DGS patch antennas size are tuned to work at the desire frequency. All simulation for conventional and DGS antennas had been done in CST MWS. Thus, the size and performance of conventional and DGS antenna are compared. Fabrication was made to verify the simulation results.

1.5 ORGANIZATION OF THE THESIS This thesis consists of six chapters and the overview of all the chapters are as follows: Chapter 1: This chapter provides a brief introduction on the background, the objectives of the project and scope of work involved in accomplishing the project.

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Chapter 2: Literature review of fundamental of antenna properties, equation of conventional rectangular patch antenna, metamaterial and defected ground structure (DGS) are described in this chapter. This chapter focuses on fundamentals and theories of metamaterial and how it‟s able to reduce antenna size. Chapter 3: This chapter gives an overview of the antenna design methodology, simulation, fabrication and testing (measurement) procedures. Chapter 4: This chapter describes the simulation and measurement results obtained from this project, including description and discussion. Chapter 5: A final conclusion is made in chapter 5 based on the outcome of the project, followed by the recommendations for the future work. 1.6 SUMMARY Understanding of metamaterial is the most important topic in this project to develop miniaturized antenna. By obtaining metamaterial substrate will introduce „left handed‟ region. The size reduction is due to the antenna radiates or resonates at lower frequency. Moreover, conventional antenna radiates at frequency of half wavelength of patch length while metamaterial antenna omit this rule. The metamaterial substrates are realized using DGS. DGS is used to alter the electrical properties of RO3003 so that both permittivity and permeability becoming negative.

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CHAPTER 2

LITERATURE REVIEW

This chapter consists of explanation to antenna fundamentals, microstrip patch antenna (MPA), metamaterial, negative refractive index and defected ground structure (DGS). 2.1 INTRODUCTION Antenna is a device used for radiating and receiving an electromagnetic wave in free space [13]. The antenna works as an interface between transmission lines and free space. Antenna is designed for certain frequency band. Beyond the operating band, the antenna rejects the signal. Therefore, we might look at the antenna as a band pass filter and a transducer. There are many different types of antennas. The isotropic point source radiator is one of the basic theoretical antenna which is considered as a radiation reference for other antennas. In reality such antenna cannot exist. The isotropic point source radiator radiates equally in all direction in free space. Antennas‟ gains are measured with reference to an isotropic radiator and are rated in decibels with respect to an isotropic radiator (dBi). Antennas can be categorized into 9 types which are [7]: 18

i.

Active integrated antennas

ii.

Antenna arrays (including smart antennas)

iii.

Dielectric antennas (such as dielectric resonant antennas)

iv.

Microstrip antennas (such as patches)

v.

Lens antennas (sphere)

vi.

Wire antennas (such as dipoles and loops)

vii.

Aperture antennas (such as pyramidal horns)

viii.

Reflector antennas (such as parabolic dish antennas)

ix.

Leaky wave antennas

2.2 ANTENNA PROPERTIES This part describes the performance of antenna, definitions of its various parameters. Some of the parameters are interrelated and not at all of them need to be specified for complete description of the antenna performance. 2.2.1 Polarization Polarization is the direction of wave transmitted (radiated) by the antenna. It is a property of an electromagnetic wave describing the time varying direction and relative magnitude of the electric field vector. Polarization may be classified as linear, circular, or elliptical as shown in Figure 2.1. Polarization shows the orientation of the electric field vector component of the electromagnetic field. In line-of-sight communications it is important that transmitting and receiving antennas have the same polarization (horizontal, vertical or circular). In non-line-of-sight the received signal undergoes multiple reflections which change the wave polarization randomly.

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Elliptical

Linear

Circular

E = electrical field vector Figure 2.1: Types of antenna polarization [2]

2.2.2 Radiation pattern An antenna radiation pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates. In most cases, the radiation pattern is determined in the far-field region and is represented as a function of the directional coordinates. Radiation properties include power flux density, radiation intensity, field strength, directivity phase or polarization. Radiation pattern provides information which describes how an antenna directs the energy it radiates and it is determined in the far field region. The information can be presented in the form of a polar plot for both horizontal (azimuth) and vertical as figure 2.2.

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Figure 2.2: Polar radiation pattern [3]

Figure 2.3 shows radiation pattern in 3D. The radiation pattern could be divided into: i.

Main lobes This is the radiation lobe containing the direction of maximum radiation.

ii.

Side lobes These are the minor lobes adjacent to the main lobe and are separated by various nulls. Side lobes are generally the largest among the minor lobes.

iii.

Back Lobes This is the minor lobe diametrically opposite the main lobe.

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First null beamwidth

Major lobe

Half power beamwidth (HPBW)

Minor lobes

Side lobe

Back lobe Minor lobes

Figure: 2.3 Radiation pattern [3].

2.2.3 Half Power Beam width (HPBW) The half power beam width is defined as the angle between the two directions in which the radiation intensity is one half the maximum value of the beam. The term beam width describe the 3dB beam width as shown in Figure 2.4. The beam width of the antenna is a very important figure-of-merit, and it often used to as a trade off between it and the side lobe level. By controlling the width of the beam, the gain of antenna can be increased or decreased. By narrowing the beam width, the gain will increase and it is also creating sectors at the same time [14].

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Figure 2.4: Rectangular plot of radiation pattern [2].

2.2.4 Antenna Gain Antenna gain is a measure of directivity properties and the efficiency of the antenna. It is defined as the ratio of the radiation intensity in the peak intensity direction to the intensity that would be obtained if the power accepted by the antenna were radiated isotropically. The gain is similar to directivity except the efficiency is taken into account. Antenna gain is measured in dBi. The gain of the antenna can be described as how far the signal can travel through the distance. When the antenna has a higher gain it does not increase the power but the shape of the radiation field will lengthen the distance of the propagated wave. The higher the gain, the farther the wave will travel concentrating its output wave more tightly [15]. The gain of an antenna will equal to its directivity if the antenna is 100% efficient. Normally there are two types of reference antenna can be used to determine the antenna gain. Firstly is the isotropic antenna where the gain is given in dBi and secondly is the half wave dipole antenna given in dBd. I will be using only dBi as a reference.

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2.2.5 Voltage Standing Wave Ratio (VSWR) Voltage Standing Wave Ratio is the ratio of the maximum to minimum voltage on the antenna feeding line. Standing wave happen when the matching is not perfect which the power put into antenna is reflected back and not radiated. For perfectly impedance matched antenna the VSWR is 1:1. VWSR causes return loss or loss of forward energy through a system. 2.2.6 Bandwidth The bandwidth of an antenna means the range of frequencies that the antenna can operate. The bandwidth of an antenna is defined as the range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard [16].

In other words, there is no unique characterization of the

bandwidth and the specifications are set to meet the needs of each particular application. There are different definitions for antenna bandwidth standard. I considered the bandwidth at -10 dB at the lower and upper centre frequency from the return loss versus frequency graph as shown in figure 2.5.

Figure 2.5: Graph return loss versus frequency [4]

Return loss is a measure of reflection from an antenna. 0 dB means that all the power is reflected; hence the matching is not good. -10dB means that 10% of incident power is reflected; meaning 90% of the power is accepted by the antenna. So, having -10dB as a bandwidth reference is an assumption that 10% of the energy loss.

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Referring to figure 2.5, the value of bandwidth can be calculated in the form of percentage as formula (1) below: √

(1)

2.3 MICROSTRIP ANTENNA 2.3.1 Overview of Microstrip Antenna A microstrip antenna consists of conducting patch and a ground plane separated by dielectric substrate. This concept was undeveloped until the revolution in electronic circuit miniaturization and large-scale integration in 1970. The early work of Munson on microstrip antennas for use as a low profile flush mounted antennas on rockets and missiles showed that this was a practical concept for use in many antenna system problems. Various mathematical models were developed for this antenna and its applications were extended to many other fields. The number of papers, articles published in the journals for the last ten years. The microstrip antennas are the present day antenna designer‟s choice.

Low dielectric constant substrates are generally preferred for

maximum radiation. The conducting patch can take any shape but rectangular and circular configurations are the most commonly used configuration. A microstrip antenna is characterized by its length, width, input impedance, gain and radiation patterns. Various parameters, related calculation and feeding technique will be discussed further through this chapter. The length of the antenna is about half wavelength of its operational frequency. The length of the patch is very critical and important that result to the frequency radiated. 2.3.2 Surface wave effect The surface wave gives effect of reduction to the amplitude of the input signal before propagate through the air. Additionally, surface waves also introduce spurious coupling between different circuit or antenna elements. This effect severely degrades the performance of microstrip filters because the parasitic interaction reduces the isolation in the stop bands. Surface waves reaching the outer boundaries of an open microstrip structure are reflected and diffracted by the edges. The diffracted waves provide an 25

additional contribution to radiation which degrading the antenna pattern by raising the side lobe and the cross polarization levels. Surface wave effects are mostly negative, for circuits and for antennas, so their excitation should be suppressed if possible. 2.3.3 Microstrip line Microstrip line is a conductor of width W printed on a thin grounded dielectric substrate of thickness h and relative permittivity,

. Microstrip line is used as a feeding technique

for inset feed. Moreover, the electrical properties which are the permittivity and permeability can also be achieve using this method. The diagram of the microstrip line is shown in figure 2.6.

Figure 2.6: Microstrip line with width W and thickness h.

The effective dielectric constant of a microstrip line is given by [9] as equation (2): (2) The value of characteristic impedance used mostly 50Ω and 75 Ω. The value that I am using is 50 Ω transmission line.

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The characteristic impedance, Zo can be calculated as:

√

*

+ (3)

*

{√

+

2.3.4 Rectangular patch antenna design The figure 2.7 shows the microstrip diagram of rectangular shape of microstrip patch antenna and the equivalent circuit. The arrows show how the wave flows through the patch and the ground plane [11].

Figure 2.7: Microstrip diagram and equivalent circuit [4].

The conventional patch antenna equations were taken from [11] and [12]. The equation to realize the conventional rectangular patch antennas are shown as below:

( Where W is the patch antenna width,

)

(4)

is the operational frequency and εr is the substrate

permittivity. 27

Another point to note is that the EM fields are not contained entirely within the microstrip patch but propagate outside of the patch as well. This phenomena result to fringing effect. Two parallel plates of microstrip and ground will form a capacitor; the electric field does not end abruptly at the edge of the plates. There is some field outside that plates that curves from one to the other. This causes the real capacitance to be larger than what I calculate using the ideal formula. Thus I will have larger capacitance compare to ideal equation. While considering the fringing effect the effective length and the effective permittivity will change. Figure 2.8 shows that EM field are not contained entirely within the microstrip patch.

Figure 2.8: EM fields around the microstrip line [5].

(5)

√ε

Where L is the patch antenna length and c is speed of EM wave in vacuum which equal to 3x108m/s.

(

)(

)

The substrate length and width dimensions are calculated as equation (7) and (8):

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(6)

(7) (8) Where Ws is the substrate width and Ls is the substrate length 2.3.5 Feeding Techniques Antenna feeding technique can generally divide into two categories which are contacting and non-contacting. The four most popular feed techniques used in patch antenna are the microstrip line, coaxial probe (both contacting schemes), aperture coupling and proximity coupling (both non-contacting). 2.3.5.1 Microstrip Line Feed Microstrip line feed is a feeding method where a conducting strip is connected to the patch directly from the edge as shown in figure 2.9. The microstrip line is etched on the same substrate surface which gives advantage of having planar structure. The method is easy to fabricate because it only need a single layer substrate and no hole.

Microstrip feed

Patch

Substrate

Ground plane Figure 2.9: Microstrip line feed [5].

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Another point to note is that microstrip line feed need an inset cut in the patch. The purpose of the inset cut in the patch is to match the impedance of the feed line to the patch without the need for any additional matching element. This is achieved by properly controlling the inset position. Hence this is an easy feeding scheme, since it provides ease of fabrication and simplicity in modelling as well as impedance matching. This feeding method will be used in my antenna design. The simplified calculation for the length of the inset cut shown by equation (9):

(9) where: l = the inset cut length εr = Permittivity of the dielectric L = Length of the microstrip patch 2.3.5.2 Coaxial feed The Coaxial feed is a very common technique used for feeding Microstrip patch antennas. As seen from Figure 2.10, the inner conductor of the coaxial connector extends through the dielectric and is soldered to the radiating patch, while the outer conductor is connected to the ground plane.

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Substrate

Patch

(a) Substrate

Coaxial connector

Ground plane

(b) Figure 2.10 Rectangular patch antenna structure with coaxial feed: (a) Top view (b) side view [5]. The benefit of this feeding method is that the connector can be put at any location within the patch to match the impedance. Moreover, this feed method is easy to fabricate and has low spurious radiation. However, there is some disadvantage. When dealing with thicker substrate the inductance increase could effect to the matching problem.

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2.3.5.3 Aperture Coupled Feed

Patch

Aperture slot

Microstrip line

Substrate 1 Ground plane

Substrate 2

Figure 2.11: Diagram of aperture coupled feed [5].

The coupling aperture is usually centred under the patch, leading to lower crosspolarization

due to symmetry of the configuration. The amount of coupling from the

feed line to the patch is determined by the shape, size and location of the aperture. Since the ground plane separates the patch and the feed line, spurious radiation is minimized. Generally, a high dielectric material is used for bottom substrate and a thick, low dielectric constant material is used for the top substrate to optimize radiation from the patch [18]. The major disadvantage of this feeding technique is that it is difficult to fabricate due to multiple layers, which also increases the antenna thickness. This feeding scheme also provides narrow bandwidth. 2.3.4.3 Proximity Coupled Feed This type of feed technique is more less the same as aperture couple feed except that the microstrip line is optimized to get the best matching. The main advantage of this feed technique is that it eliminates spurious feed radiation and provides high bandwidth [18]. Matching can be achieved by controlling the length of the feed line and the width-to-line ratio of the patch. The main disadvantage of this method is using double layer substrate and needs proper alignment which is tedious in fabrication process.

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Patch Microstrip line

Substrate 1 Substrate 2

Figure 2.12: Proximity Coupled feed [5] 2.4 METAMATERIAL 2.4.1 Metamaterial theory Metamaterial define as artificial effective electromagnetic structures with unusual properties not readily found in nature. However, „unusual‟ is generally meant to imply material properties which have been hardly explored before the turn of 20th century, in particular negative refractive index [19]. Metamaterial is a material having negative relative permittivity and permeability. These two properties determine how a material will interact with electromagnetic radiation. When both permittivity and permeability are simultaneously negative, it‟s then having a negative refractive index (NRI) or left-handed material (LHM). This relationship is shown by the following Maxwell‟s equation for refractive index: √

(8)

Maxwell‟s equation tells us how the electromagnetic wave behaves which contain both electric and magnetic field. Figure 2.13 shows the electric and magnetic field which propagates in perpendicular to each other. The field directions in a plane wave also form 33

right angles with respect to their direction of travel (the propagation direction). When an electromagnetic wave enters in a material, the fields of the wave interact with the electrons and other charges of the atoms and molecules that compose the material, causing them to move about. For example, this interaction alters the motion of the wavechanging its speed or wavelength.

Electric field Electric field Propagation direction

Magnetic field

Figure 2.13: Electromagnetic wave [6]

Knowing that the permittivity and permeability are the only material properties that relevant in changing the wave behaviour, thus changing or tuning this value gives higher degree of freedom in designing an antenna. Another point to notes is that only when both permittivity and permeability are positive and negative is useful in antenna design as shown is figure 2.14. Single negative region which is region II and IV impede the signal. Region I is where the permittivity and permeability are both positive. This region is most explored. This is where most of material behaves. However, region III is less explored. This is where the matamaterial exist. Materials that exist and behave in this region are not readily available in nature. At the intersection (point A) it is the zero refractive index (n) diagrams.

34

Figure 2.14: Permittivity-permeability(ε-μ) diagram which shows the material classifications [14].

2.4.2 Behaviour of wave An electromagnetic wave can be depicted as a sinusoidal varying function that travels to the right or to the left as a function of time. Figure 2.15 shows that the wave travels into the material having positive refractive index from n1 to n2. The speed of the wave decreases as the refractive index of the mataterial increase.

S O U R C E

n1

n2>n1

Figure 2.15: Wave incidents on a positive index material [6].

35

When the refractive index is negative, the speed of the wave, given by c/n is negative and the wave travels backwards toward the source as shown in Figure 2.16. Therefore, in left-handed metamaterial, wave propagates in the opposite direction to the energy flows.

S O U R C E

n1

n2>-n1

Figure 2.16: Wave incidents on a negative index material [6].

2.4.3 Refraction and Snell's Law One of the most important fundamental theories that govern the optical effect or bending of light between two material is the refraction. Refraction is a basic theory behind lenses and other optical element that focus, changing direction and manipulates light. The principle and theory applicable for other wave as well such as RF signal. Highly sophisticated and complex optical devices are developed by carefully shaping materials so that light is refracted in desired ways. Every material, including air, has an index-of-refraction (or refractive index). When an electromagnetic wave travels from a material with refractive index n1 to another material with refractive index n2 the change in its trajectory can be determined from the ratio of refractive indices n2/n1 by the use of Snell's Law shown in equation (9). (9) The Snell‟s law is also applicable for left-handed material with the same refraction angle with respect to normal line except in different direction as shown in figure 2.17. These 36

figures show how travelling waves from free space entering right-handed (having positive-index of refraction) and left-handed (having negative-index of refraction) material.

Positive-index material

Negative-index material

Figure 2.17: Incident wave travelling through positive-index and negative-index material.

In addition, figure 2.18 and 2.19 show how the spreading patterns of the waves on entering and exiting the conventional and LHM material respectively. For conventional material, the refracted waves are spreading away after entering and exiting the medium. For LHM, the waves are refracted in such a way as to produce a focus inside the material and then another just outside. The radiation pattern is more a beamlike, which leads to the creation of highly directional antennas and also may allow more antennas to be placed in closely packed space.

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Vacuum

Conventional material

Figure 2.18: Refracted rays in conventional material [6].

Vacuum

Conventional material

Figure 2.19: Refracted rays in Left-handed material [6].

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2.4.4 Metamaterial structures There are four basic structures that had been discussed in literatures to realize metamaterial substrates as in figure 2.20. Combining the electric dipole and magnetic dipole structure can result to metamaterial substrate. The summary of the different elements used for metamaterial synthesis shows in figure 2.20. Each one of the element can be considered as either electric or magnetic dipole. Split-ring resonators (SRRs) and slot lines can be considered as magnetic dipole while metal wire lines and complementary split-ring resonator (CSRRs) are regarded as electric dipole.

(a)

(b)

(d)

(c)

Figure 2.20: Structure used for metamaterial synthesis (a) SRRs , (b) metal wire lines, (c) CSRRs, (d) slot lines

Metamaterial substrates are synthesized by combining electric and magnetic dipole elements. These structures are able to realize metamaterial if properly aligned. Specifically, there are four combination methods as listed below [19]: • SRR and Wire dipole: This is the widely used combination structure.

39

• SRR and CSRR: This structure does no gain much popularity because the arrangement is difficult. • Slot dipole and wire dipole: The mushroom as figure 2.21a structures falls in this category. • Slot dipole and CSRR: The structure as shown in figure 2.21b. This structure had been used for wideband filter applications

(a)

(b) Figure 2.21: Metamaterial structure (a) Mushroom structure, (b) Planar CRLH structure

2.4.5 Nicholson Ross Weir (NRW) NRW is a tool or method converting scattering parameter from the simulation or measurement into electrical properties which are permittivity, εr and permeability, µr. In the NRW algorithm, the reflection coefficient is

√

(10)

Where, (11) 40

As a step to acquire the correct root, X is must be in the form of S-parameter, the magnitude of the reflection coefficient, must be less than one. The following stage is to calculate the transmission coefficient of the metamaterial. (12)

( )

( )

(13)

Where (14) Where

we define: -[

(15)

Then we can solve for the permeability using (16)

√

Where

is the free space wavelength and

is the cutoff wavelength.

The permittivity is given by ( )

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(17)

2.5 RELATION BETWEEN DGS, EBG AND METAMATERIAL The concept of DGS arises from the studies of Photonic Band Gap (PBG) structure which dealing with manipulating light wave. PBG is known as Electron Band Gap (EBG) in electromagnetic application. They are actually artificial periodic structures that can give metamaterial behavior. These structures would have periodic arrangement of metallic, dielectric or metalodielectric bodies with a lattice period

λ being the guide

wavelength. In 1999, a group of researchers further simplified the geometry and discarded the periodic nature of pattern. They simply use a unit cell of dumbbell shape to the same response as EBG which known as „Defected Ground Structure‟ (DGS). Therefore, a DGS is regarded as a simplified variant of EBG on a ground plane [23]. Thus DGS can be described as a unit cell EBG or an EBG with limited number of cells. The DGS slots are resonant in nature. The presence of DGS can realize metamaterial substrate. Figure 2.23 shows different geometries that have been explored with the aim of achieving improved performance in term of stopband and passbands, compactness, and ease of design [23].

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(a)

(b)

(c)

(f)

(g)

(h)

(k)

(l)

(p)

(q)

(r)

(e)

(d)

(i)

(j)

(n)

(o)

(s)

(t)

Figure 2.23 : Different DGS geometries : (a) dumbbell-shape (b) Spiral-shaped (c) H-shaped (d) U-shaped (e) arrow head dumbbell (f) concentric ring shaped (g) split-ring resonators (h) interdigital (i) cross-shaped (j) circular head dumbbell (k) square heads connected with U slots (l) open loop dumbbell (m) fractal (n)half-circle (o) V-shaped (q) meander lines (r) U-head dumbbell (s) double equilateral U (t) square slots connected with narrow slot at edge.

DGS has been widely used in the development of miniaturized antennas. In, our design, DGS is a defect etched in the ground plane that can give metamaterial behavior in reducing the antenna size. DGS is basically used in microstrip antenna design for different applications such as antenna size reduction, cross polarization reduction, mutual coupling reduction in antenna arrays, harmonic suppression etc. DGS are widely used in microwave devices to make the system compact and effective [21].

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CHAPTER 3

METHODOLOGY

3.1 METHODOLOGY OVERVIEW This chapter consists of the design methodology used for this project which includes the description on the selected DGS structures, conventional and DGS antenna simulation and fabrication process. The methodology of this project starts by understanding of the microstrip antenna technology. This includes the properties study of the antenna such as operating frequency, radiation pattern, and antenna gain. The related literature reviews are carried out from reference books and IEEE publish paper. The simulation has been done by using Computer Simulation Technology Microwave Studio (CST MWS). To meet the theoretical expectation, the design is being optimized accordingly. The methodology described here is illustrated in Figure 3.1. The simulations performed also have limitation. The fabrication process is started after the optimization result from the simulation.

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3.2 FLOW CHART OF DESIGN METHODOLOGY The implementation of this project includes two main parts which are software design and hardware design. The approaches of the project design are represented in the flow chart in figure 1. The software simulation includes the designing of conventional antennas and DGS metamaterial antennas. CST Microwave Studio software is used for antenna simulation. The designing of DGS antennas are implemented by finding the metamaterial response of the substrate having both permittivity, εr and permeability, µr negative. The bottom copper of Rogers3003 must critically shape for these purposes. Start

Design possible metamaterial structure NO CST simulation

Metamaterial structure used

Optimize antennas design

Antenna fabricate

Measurement

Data analysis

End

Figure 3.1: Flow chart of antenna design. 45

3.3 DGS STRUCTURES Before designing the antenna with DGS substrate, two DGS structures were designed first. The target parameter is for the substrate to behave as metamaterial at 4.75 GHz and 2.45 GHz. The characteristics parameters of the substrate are described in the Table 3.1. RO3003 has good permittivity accuracy with deviations of only 0.04 mm. The substrate thickness is 0.5 mm. The loss tangent is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy. This substrate has low tangent loss which is 0.0013. Lower tangent loss means lower signal loss. Thus the substrate is suitable for antenna design. Table 3.1: Characteristic parameter of substrate (RO3003)

Characteristics

Values

Permittivity, εr

3.00

Permeability, µr

1.00

Loss tangen, tan

0.0013

Thickness, h

0.5 mm

Copper cladding, t

0.035 mm

0.04

The DGSs are designed to resonate at 2.4 GHz and 4.7 GHz. These structures are designed based on [4]. Two DGS structures have been designed. The first design (a) is the circular rings and the second design (b) is the split rings. Structure (b) is a modification made from (a) so that the substrate behaves as metamaterial at 2.45 GHz.

46

DGS Copper

(a)

(b)

Figure 3.2: Bottom view of DGS structures: (a) circular rings behave as metamaterial at 4.75 GHz, (b) split rings behave as metamaterial at 2.45 GHz.

3.4 INVESTIGATION OF METAMATERIAL SUBSTRATE The designing of DGS antennas are implemented by finding the metamaterial response of the substrate having both permittivity, εr and permeability, µr negative at desired frequency. The bottom copper of Rogers 3003 must be critically shaped for this purpose. The transmission line method is selected for analysis purposes as shown in figure 3.3. The substrate two-port S-parameter was extracted using this technique in simulation. The NRW method act as a converter to obtain the permittivity, εr and permeability,µr of the material. The transmission line technique has been introduced by Zijie lu [19].

50

line

DGS structure

Figure 3.3.Transmission line method [17].

47

As mention before the value of permittivity and permeability of the substrate can be changed, influenced by introducing DGS. The location of negative permittivity, εr and permeability, µr were observed base on NRW calculation. Microsoft Excel was used to make calculation faster. Optimization was done so that the substrate becomes metamaterial at desired frequency. In figure 3.4 the blue line represents the permittivity, εr value and the red line represents permeability, µr value. Figure shows that for circular rings structure having both permittivity and permeability becoming negative at 4.7 GHz frequency band. Therefore, we can realize that this substrate operate as metamaterial at 4.7 GHz. Moreover, this substrate also operates as metamaterial at higher frequency. As we can see at 9.6 GHz, both electrical properties are negative.

4.75 GHz εr= -154 µr= -0.000818

Figure 3.4: Relative permittivity, εr and permeability, µr value versus frequencies for substrate with circular rings DGS

48

In figure 3.5 the blue line represents the permittivity, εr value and the red line represents permeability, µr value. Figure shows that for circular rings structure having both permittivity, εr and permeability, µr becoming negative at 2.4 GHz frequency band. Therefore, we can realize that this substrate operate as metamaterial at 2.4 GHz. Moreover, this substrate also operates as metamaterial at higher frequency. As we can see at 9.6 GHz, both electrical properties are negative.

2.45 GHz εr= -132 µr= -0.000461

Figure 3.5: Permittivity, εr and permeability, µr value versus frequencies for substrate with slip rings DGS.

Final physical parameters of both substrates are shown as in figure 3.6. These substrate structures had been tuned to get the metamaterial functional at desired frequencies. Figure 3.6 shows two rings with inner radii of 5 mm and 6 mm. Both of rings have 0.5mm width operate with (a) 4.7 GHz and (b) 2.45 GHz. The rings have 1.29 mm gap. The slip rings structure is a modification made from double rings so that the substrate behaves as metamaterial at 2.45 GHz. The 1.29 mm gap increase the copper area thus increases the total capacitance value. 49

17.9

17.5

1.29

20.0

20.0

(a)

(b)

Figure 3.6: Designed metamaterial substrates dimension (mm) (a) double rings and (b) slip ring physical parameter.

3.5 ANTENNA DESIGN This part discusses the process of designing the rectangular patch and DGS antenna. Antenna design process also includes optimization, fabrication and measurement process. 3.5.1 Designing rectangular patch antenna The simulation of conventional antenna is designed for the purpose of comparison to DGS one. Two conventional MPA antennas were designed at 4.75 GHz and 2.4 GHz respectively. The conventional patch antenna equations were taken from [10] and [11] as mention previously in chapter 2. The objectives of the antenna design are described in the Table 3.2. The antenna design should have the value of return loss less than negative 10dB at the operational frequency. The antenna considered to radiate as return loss is less than negative 10dB. Specifically, the return loss should be as low as possible to reduce the matching impedance. Moreover, the design antenna should have linear polarization.

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Table 3.2: Characteristics goals of conventional rectangular patch antenna

Frequency of operation

4.7 GHz and 2.4 GHz

Return loss (dB)