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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
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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
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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 ivLIST 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
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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
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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
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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
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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 Figure 3.5 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 3.8 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. 48 Figure 4.2 Comparison of return loss simulation data of conventional antenna
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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
Figure 4.6 Graph simulation and measurement of 4.7 GHz DGS antenna
versus frequency 52
Figure 4.7 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
Figure 4.14 Graph simulation and measurement of 2.4 GHz DGS antenna
versus frequency 60
Figure 4.15 Graph comparing between 4.7 GHz conventional and DGS
antenna 61
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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 DecibelGHz Gigahertz
VSWR Voltage standing wave ratio BW Bandwidth
LH Left-handed
LHM Left-handed material EM Electromagnetic UV Ultravioletlm
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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
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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.
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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]:
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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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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.
E = electrical field vector
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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
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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].
First null beamwidth Half power beamwidth
(HPBW) Minor lobes Major lobe Side lobe Back lobe Minor lobes
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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
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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:
(
)
(4) Where W is the patch antenna width, is the operational frequency and εr is the substrate28
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].
√ε
(5)
Where L is the patch antenna length and c is speed of EM wave in vacuum which equal to 3x108m/s.
(
) (
)
(6) The substrate length and width dimensions are calculated as equation (7) and (8):29
(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.
Figure 2.9: Microstrip line feed [5]. Patch
Substrate
Ground plane Microstrip feed
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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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(a)
(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.
Substrate Patch Ground plane Coaxial connector Substrate
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2.3.5.3 Aperture Coupled Feed
Figure 2.11: Diagram of aperture coupled feed [5].
The coupling aperture is usually centred under the patch, leading to lower cross-polarization 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.
Aperture slot Patch Microstrip line Substrate 1 Substrate 2 Ground plane
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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
Patch
Substrate 1
Substrate 2 Microstrip line
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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 wave-changing its speed or wavelength.
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.
Electric field
Propagation direction
Electric field
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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.
Figure 2.15: Wave incidents on a positive index material [6].
n2>n1
n1
S O U R C E36
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.
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
S O U R C E
n2>-n1
n1
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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.
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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Figure 2.18: Refracted rays in conventional material [6].
Figure 2.19: Refracted rays in Left-handed material [6].
Vacuum
Conventional
material
Conventional
material
Vacuum
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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.
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.
(a) (b)
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• 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
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) (a) (b)
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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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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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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].
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (t) (n) (o) (p) (q) (r) (s)
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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.
Figure 3.1: Flow chart of antenna design. End
Data analysis Measurement Antenna fabricate Optimize antennas design Metamaterial structure used
CST simulation Design possible metamaterial
structure Start
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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 0.04
Permeability, µr 1.00
Loss tangen, tan 0.0013 Thickness, h 0.5 mm Copper cladding, t 0.035 mm
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.
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(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].
Figure 3.3.Transmission line method [17]. 50 line
DGS structure DGS
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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.
Figure 3.4: Relative permittivity, εr and permeability, µr value versus frequencies for substrate
with circular rings DGS 4.75 GHz
εr= -154
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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.
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.
2.45 GHz εr= -132
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(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.
17.9 20.0 17.5 20.0 1.29
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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) <-10dB Feeding method Microstrip line Polarization Linear
Base on simulation each of conventional rectangular patch antennas are able to operate at 4.75 GHz and 2.4 GHz respectively. The dimension is as shown in figure 3.7a and 3.7b. The simulation template used was „Antenna (mobile phone)‟ which have the following default settings as table 3.3. The setting units of mm and GHz in simulation are due the ease of simulation. The boundaries of simulation are set to free space for all direction. These settings are used to calculate the farfield radiation pattern so that the antenna surrounding set as open space.
Table 3.3 Settings for CST MWS
Measurement unit length mm Measurement unit frequency GHz
Boundaries Free space
(a) 3.9 21.3 25.6 21.3 6.1 17.9
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(b)
Figure 3.7 Top view dimensions (mm) of conventional rectangular patch antenna : (a) 4.7 GHz antenna (b) 2.4 GHz antenna
The simulation was done from 1GHz to 10 GHz. All the calculation of EM field was done using „transient solver‟ with default settings. The optimization was done so that the antennas having lowest reflection coefficient at the desired frequency. The length of the inset cut were tune to get the result.
Figure 3.8a shows return loss of 4.7 GHz conventional patch antenna having different inset cut length. Changing the length of inset cut will change the impedance value. Different values of inset cut length are observed for the purpose of optimization. Having short inset length such 3.0 mm the return loss is negative 5.1 dB and with deeper inset length of 6.5 mm the return loss become negative 16.1 dB. The best inset length is 6.1 mm with return loss of negative 28.0 dB.
Figure 3.8b shows return loss of 2.4 GHz conventional patch antenna having different inset cut length. Having inset length of 11.4 mm the return loss become negative 18.3 dB. The best inset length is 11 mm with return loss of negative 21.7 dB. Thus it can be understood that if the inset cut is far from the matching impedance the return loss will increase. 47.2 43.5 11.0 39.0 4.3 35.0
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(a)
(b)
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
: -28.0 : -16.1 : -5.1 : -5.5 : -0.8 : -5.5 : -17.0 : -5.5 : -23.4 : -5.5 : -19.7 : -18.3 : -21.7 : -21.0 : -5.5 : -21.6
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Table 3.4 summarized the physical parameters that change the antenna performance. By increasing or decreasing the inset cut, patch width or length the design antenna is able to achieve the desired frequency and performance.
The correct length of inset feed will have the lowest reflection coefficient. Shorter or longer inset cut will increase the reflection coefficient.
Reducing the antenna width will increase the antenna resonance frequency. Increasing the length will reduce the resonance frequency. Excessive reduction or increment will increase reflection coefficient.
Reducing the length will increase the antenna resonance frequency. Increasing the length will reduce the resonance frequency. Antenna resonates mainly due to antenna length which is half wave length of the operational frequency. Thus the length of patch length affects the resonance frequency more than the patch width.
Table 3.4 Antenna Design Optimization
Design Parameters Comment
Inset cut length Need specific length. Longer or shorter increase the reflection coefficient.
Antenna Width Reduce width = increase resonance frequency.
Increase width = reduce the resonance frequency.
Antenna Length Reduce length = increase the antenna resonance frequency. Increase length = reduce the resonance frequency.
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3.5.2 Designing rectangular patch antenna with DGS
The metamaterial antennas were designed using two DGS substrates that were discussed before. The two rings structure used to implement metamaterial antenna that operates at 4.75 GHz and the slip rings at 2.45 GHz. Figure 3.9 shows diagram of DGS antenna.
Figure 3.9: 3D view structure DGS antenna.
Then, the patch size including the length, width was tuned so that the antenna operates at the desired frequency. Moreover, the inset cut was also tuned to obtain the best matching impedance.
3.6 FABRICATION PROCESS
The fabrication process involves 5 steps which are:
Generate mask on transparency film
Photo exposure process
Etching in developer solution
Etching in Ferric Chloride
Soldering the probe.
Each of the listed process will be explained further below.
Rectangular patch
RO3003 Substrate
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3.6.1 Generate mask on transparency film
First step is to transfer the antennas top and bottom layer structure into .DXF format that is compatible with Auto-Computer Added Drawing (AutoCAD) and print it onto the transparency film.
3.6.2 Photo exposure process
Second step is the Ultraviolet exposure process. It is done to transfer the image of the circuit pattern with a film in a UV exposure machine onto to the photo resist laminated board.
3.6.3 Etching in developer solution
The third step is to ensure the pattern will be fully developed, during the developing process. The photo resist developer solution was used to wash away the exposed resist. Then the solution was removed by spray wash. In this process, water was added with Sodium Hydroxide (NaOH).
3.6.4 Etching in Ferric Chloride
Fourth step is the etching in ferric chloride. It will remove the unwanted copper area and this process was followed by the removal of the solution by water.
3.6.5 Soldering the probe
The second process in the fabrication is the soldering process. This process only can be done after the etching process has finish. In this process, a port and a solder will be used. The soldering process actually is to connect the port to the antenna. After that, the feeder will be soldered together with the antenna. 1mm SMA connector was soldered to the microstrip antenna.
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3.7 MEASUREMENTS
The measurement is done to investigate the performance of the fabricated antenna. The return loss and the radiation pattern are analyzed and investigated. From the return loss, we also can observe the transmission loss and bandwidth. Then, the measurement will be compared to the simulation. The return loss was measured using Vector Network Analyzer (VNA). The equipment was calibrated before taking any measurements. One-port scattering parameter for both conventional and DGS antenna were measured. Then, all the measurement data was stored.
The radiation patterns were measured in anechoic chamber room. Anechoic chamber are guarded with absorbers located around the walls. The absorbers are used to absorb the unwanted signal to reduce the reflected signal. It can increase the efficiency of the measurement process. The equipments were divided into two sections. The first section consist of the horn antenna and VNA, the second section consist of a spectrum analyzer, a positioner and a rotator.
The first section is the transmitter part and the second section is the receiving part. The antenna under test (AUT) will be placed at the rotator. The positioner will control the rotator to rotate the tested antenna from 0 degrees to 360 degrees. The received signal of the antenna will be measured by the spectrum analyzer at different angle in the term of signal to noise ratio. This measurement is done to see the radiation pattern of the antenna. To use these device, the experience user is needed to guide inexperience user to use these device because it can make hazardous effect by higher frequency and quite expensive.
Figure 3.10 shows all the connection of the devices for anechoic chamber. The VNA is connected to reference antenna (horn antenna). The rotator are connected to the antenna under test (AUT). From that the measurement value decision can be made whether the value is same from the expected requirement.
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Figure 3.10: Anechoic chamber VNA Spectrum analyzer Horn antenna Antenna under test (AUT) Rotator
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CHAPTER 4
RESULT AND DISCUSSION
4.1 INTRODUCTION
In Chapter 3, metamaterial substrate and microstrip patch antenna had been designed. The method and procedures in designing metamaterial structure and microstrip patch antenna had been elaborated extensively in chapter 3. This chapter presents the results and findings of both 4.75 GHz and 2.45 GHz antennas.
4.2 4.75 GHZ ANTENNAS
This part presents the simulation and measurement of 4.75 GHz antenna. Both conventional and DGS antenna had been build. This part also consists of comparison between conventional and DGS antenna performance in term of return loss, bandwidth and radiation pattern.
4.2.1 Antenna parameters
The simulation result shows that metamaterial antenna can reduce the of rectangular patch antenna. The antenna with circular rings DGS can reduce the size up to 34.3% as shown in figure 4.1. This antenna operates at 4.75 GHz.
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(a) (b)
Figure 4.1: Top view dimension of the of 4.7 GHz antennas (a) conventional (b) metamaterial antenna with circular rings DGS.
4.2.2 Simulation: Return loss comparison between conventional and DGS
Figure 4.2: Comparison of return loss simulation data of conventional antenna and DGS metamaterial antenna. 3.9 21.3 21.3 6.1 17.9 25.6 20.0 2.9 19.0 17.5 13.6 7.1
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Figure 4.2 describes the return loss response of metamaterial microstrip antenna compared to return loss response of conventional antenna. Noticed that the rectangular patch antenna with DGS gives better return loss which is -35.6dB compared to the simulated result of patch antenna without DGS which is just -25.7dB. The operating frequency of the DGS antenna is between 4.65 GHz to 4.74 GHz. The bandwidth of DGS antenna is 96 MHz which is greater than conventional antenna which is just 50.8 MHz. Notice that DGS antenna increase the bandwidth by 89%.
4.2.3 Simulation: Radiation pattern of 4.75 GHz antennas
Figure 4.3 (a) and 4.3 (b) shows the 3D radiation pattern for the conventional antenna and DGS metamaterial antenna at 4.7 GHz. The simulated gain for the DGS metamaterial antenna is 3.5 dBi as for the conventional antenna the simulated gain is 4.41 dBi. This shows that DGS antenna have considerable gain as compared to conventional one.
Figure 4.3 (c) and (d) show the polar plot for the conventional antenna and DGS metamaterial antenna. The beam main lobe direction for conventional antenna is 4 degree with beam width 102.5 degree. The beam main lobe direction for DGS antenna is 5 degree with beam width 95.4 degree.
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Figure 4.3 : 4.7 GHz radiation pattern (a) 3D conventional antenna (b) 3D Antenna with DGS (c) Polar plot Conventional Antenna (d) Polar plot of DGS Antenna.
Table 4.1 describes all data obtained from simulation in CST Microwave Studio. The directivity of DGS metamaterial antenna is 4.28 dBi slightly less than directivity of conventional antenna that has 6.3 dBi of directivity change is due to the introduction of DGS. Based on the data analyzed, it can be concluded that the performance of both the antenna through simulation more less the same.
Table 4.1: Comparison Between 4.7GHz Conventional and DGS Antenna Parameter 4.7GHz conventional antenna 4.7GHz DGS antenna Return loss,dB -25.7 -35.6 Bandwidth, MHz 50.7 97.2 Gain, dBi 4.41 3.5 Directivity, dBi 6.3 4.28 Size, mm2 545.28 358 Total Efficiency, % 64.4 83.6
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4.2.4 Comparison between simulation and measurement
Figure 4.4 shows the size comparison between conventional, DGS antenna and 50 cent Malaysian coin. These antennas operate at 4.7 GHz. These antennas are connected with SMA connector. The smaller antenna is the 4.7 GHz metamaterial antenna. The metamaterial antenna has a DGS structure at the ground.
(a) (b)
Figure 4.4:Size comparison of fabricated 4.7 GHz antenna(a) front view (b) bottom view
The simulation results indicate that the metamaterial structure has an effect to the
conventional patch antenna by shifting the frequency regions to different value as well as affecting the S11 parameters. As far as current simulations performed, it is shown that the metamaterial structure will be able to reduce the size of the patch antenna.
Figure 4.5 shows comparison between measurement and simulation result of 4.7 GHz conventional antenna. The measurement shows negative 15.5 dB return loss which is higher compared to the simulation with negative 25.7 dB. The fabricated antenna operates at 4.7 GHz which is in good agreement with simulation.
Figure 4.6 shows comparison between measurement and simulation result of 4.7 GHz DGS antenna. The measurement result shows negative 15.6 dB return loss which is higher compared to the simulation with negative 25.7 dB. The fabricated antenna
operates at 4.7 GHz which is in good agreement with simulation. It can be concluded that the build antenna has a lower performance compared to simulation.
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Figure 4.5: Graph simulation and measurement of 4.7 GHz conventional antenna versus frequency.
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4.2.5 Comparison of conventional and DGS antenna
Figure 4.7 shows the comparison of 4.7 GHz DGS and conventional antenna that I have fabricated and measured. Figure 4.2 describes the return loss response of metamaterial microstrip antenna compared to return loss response of conventional antenna. Notice that the rectangular patch antenna with DGS having comparable return loss which is -15.6dB.
Figure 4.7: Graph comparing between 4.7 GHz conventional and DGS antennas.
It can be concluded from the S11 comparison graphs that the resonant frequency has shifted in the magnitude from the designed frequency for most of the designs. The root cause of the shift is could be due to the patch and substrate dimension that varies in fabrication process.