DISSERTATION ON DESIGN OF FRACTAL MICROSTRIP PATCH ANTENNA FOR AEROSPACE NAVIGATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF THE DEGREE OF MASTER OF TECHNOLOGY ELECTRONICS AND COMMUNICATION ENGINEERING (SPECIALIZATION IN COMMUNICATION SYSTEMS) SUBMITTED BY Rupleen Kaur (2013ECB1235) (Reg. No. 2013 RG/A-277) UNDER THE SUPERVISION OF Er. Satbir Singh (Assistant Professor) DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING GURU NANAK DEV UNIVERSITY, RC GURDASPUR JULY, 2015
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DISSERTATION
ON
DESIGN OF FRACTAL MICROSTRIP PATCH
ANTENNA FOR AEROSPACE NAVIGATION
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE
AWARD OF THE DEGREE OF
MASTER OF TECHNOLOGY
ELECTRONICS AND COMMUNICATION ENGINEERING
(SPECIALIZATION IN COMMUNICATION SYSTEMS)
SUBMITTED BY
Rupleen Kaur (2013ECB1235)
(Reg. No. 2013 RG/A-277)
UNDER THE SUPERVISION OF
Er. Satbir Singh (Assistant Professor)
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
ENGINEERING
GURU NANAK DEV UNIVERSITY, RC GURDASPUR
JULY, 2015
Dept.of ECE, GNDU, Regional Campus Gurdaspur i
Guru Nanak Dev University, Regional Campus, Gurdaspur Department of Electronics & Communication Engineering
(Established by State Legislature Act No 21 of 1969) Accredited at “A” grade level by NAAC and awarded
“University with Potential for Excellence” status by UGC Phone: 01874-240517, Fax: 01874-242678
CERTIFICATE
Certified that the thesis entitled “Design of Fractal Microstrip Patch Antenna for
Aerospace Navigation” submitted by Rupleen Kaur (Regd. No. 2013 RG/A-277) in the
partial fulfillment of the requirements for the award of the degree of Master of
Technology (Electronics and Communication Engineering) of Guru Nanak Dev
University, is a record of student’s own work carried under my supervision and guidance.
To the best of our knowledge, this thesis has not been submitted to any University or
institute for award of any degree. It is further understood that by this certificate the
undersigned do not endorse or approve any statement made, opinion expressed or
conclusion drawn herein, but approve the thesis only for the purpose for which it is
submitted.
Supervisor
Er. Satbir Singh
Assistant Professor,
Department of Electronics and Communication
Engineering,
Guru Nanak Dev University
Regional Campus,
Gurdaspur-143521,
Punjab, India
Dated:
Co-Supervisor
Er. Naveen Kumar
(IEEE Member)
Executive Director
Elixir Publication
Chandigarh, India
Dept.of ECE, GNDU, Regional Campus Gurdaspur ii
DECLARATION
I, Rupleen Kaur, bearing University Registration Number 2013 RG/A-277, a student of
M.Tech (Regular) of Electronics & Communication Engineering Department; hereby
declare that I own the full responsibility for the information, results etc. provided in this
thesis titled “Design of Fractal Microstrip Patch Antenna for Aerospace Navigation”
submitted to Guru Nanak Dev University for the award of M.Tech (ECE) degree. I
hereby declare that this thesis is my own work and effort and that it has not been
submitted anywhere for any award. Where other sources of information have been used,
they have been acknowledged. I have taken care in all respect to honor the intellectual
property right and have acknowledged the contribution of others for using them in
academic purpose. I further declare that in case of violation of intellectual property right
or copyright, I as the candidate will be fully responsible for the same, my honorable
supervisors and Institute will not be responsible for the violation of any intellectual
property right.
Rupleen Kaur
Roll no. 2013 ECB 1235
Date:
Place: Guru Nanak Dev University, Regional Campus, Gurdaspur
Dept.of ECE, GNDU, Regional Campus Gurdaspur iii
ACKNOWLEDGEMENT
The completion of any project brings with it a sense of satisfaction, but it won’t be
complete without thanking the people who made it possible and whose constant support
crowned my efforts with success. I wish to express my deepest gratitude to Er. Satbir
Singh, Assistant Professor, Dept. of ECE, GNDU, Regional Campus Gurdaspur for his
sincere and invaluable guidance, suggestions and constant encouragement, and belief in
me, which inspired me to submit my thesis.
I am thankful to Dr. Anu Sheetal, Professor and Incharge of Dept. of ECE, GNDU,
Regional Campus Gurdaspur for providing full facilities for the execution of this thesis
work.
My sincere thanks to Er. Naveen Kumar, Director, Elixir Publications, Chandigarh for his
consistent guidance, encouragement and help in learning HFSS software. I also attribute
my sincere gratitude to NITTTR, Chandigarh for providing necessary lab facilities and
equipments.
I am grateful to all of my friends for helping me and would also like to thank all those
who have directly or indirectly contributed to the success of this work. Their intelligence
and innovation have helped me go through my every query and ended up in a huge
success.
Big thanks to my Institution and all of my faculty members for helping me in completing
my thesis and providing me with immense knowledge related to the subject.
I am extremely happy to acknowledge and express my sincere gratitude to my parents for
their constant support and encouragement.
Rupleen Kaur
Roll no. 2013 ECB 1235
Dept.of ECE, GNDU, Regional Campus Gurdaspur iv
ABSTRACT
In navigational applications antenna plays an important role in determining the location,
tracking and mapping of vehicles. In the recent years navigational antennas have
progressed rapidly and are required to perform various services like surveying, mapping
and providing geographical information without compromising size, weight and
performance. There has been a great demand for antenna designs that have multiband and
wideband properties. Hence the antenna required should be small in size, light in weight,
operates at multiband frequencies, consumes low power and provides high reliability.
Earlier Navigation was based on observations and not on scientific methods but modern
navigation determines the position by collecting the information from satellites through
the receivers. Navigational tools were initially developed for military users but with the
advent of wireless communication systems it has been adopted in civil as well. In military
applications navigational antennas are required for many applications such as
surveillance, beam steering, beam forming, tracking etc. These antennas are positioned
on aircrafts, ships or other vehicles. In aerospace navigation, antennas use radio
frequencies to communicate with air traffic control and find its destination. Therefore
different antennas are required for different purposes and hence they need a large space.
In order to overcome this problem i.e., instead of using number of antennas, a multiband
antenna that can be operated at many frequencies is the need of today.
During this thesis work a compact, mechanically robust antenna that has the capability of
operating at multiple frequencies has been designed. The antenna is a single feed fractal
microstrip patch antenna. It has two circular and two rectangular slots on the ground in
order to improve the resonance of the antenna. Moreover the antenna has conventional
Koch fractal design on the top of the patch to get multiple frequency bands. High
Frequency Structure Simulator (HFSS) software will be used for designing and obtaining
the results for the antenna. High Frequency Structure Simulator (HFSS) is an industry
standard simulation tool. It has powerful drawing capabilities to simply the antenna
design. It is seen that after simulation the antenna provides desired resonant frequencies
that have good operating bandwidth.
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TABLE OF CONTENTS
Page no.
Certificate i
Declaration ii
Acknowledgement iii
Abstract iv
Table of Contents v-ix
List of Abbreviations x
List of Figures xi-xiii
List of Tables xiv
CHAPTER 1
INTRODUCTION 1-20
1.1 Overview 1
1.2 Antenna design issues in aerospace navigation 3
1.2.1 Coverage 3
1.2.2 Space Available 3
1.3 Antenna Fundamentals 3
1.4 How an Antenna Radiates 4
1.5 Near and Far Field Regions 5
1.6 Antenna Performance Parameters 6
1.6.1 Radiation Pattern 6
1.6.2 Directivity 7
1.6.3 Input Impedance 8
1.6.4 VSWR 9
1.6.5 Return Loss 10
1.6.6 Antenna Efficiency 10
1.6.7 Antenna Gain 11
1.6.8 Polarization 11
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1.6.9 Bandwidth 12
1.7 Types of Antennas 13
1.7.1 Half Wave Dipole 13
1.7.2 Monopole Antenna 14
1.7.3 Loop Antenna 15
1.7.4 Helical Antenna 17
1.7.5 Horn Antenna 18
1.8 Organization of Thesis 19
CHAPTER 2
FRACTAL MICROSTRIP PATCH ANTENNA 21-33
2.1 Introduction 21
2.2 Advantages and Disadvantages of Microstrip Patch
Antenna
21
2.3 Basic Principle of Operation 22
2.4 Feeding Techniques 23
2.4.1 Coaxial Probe Feed 23
2.4.2 Microstrip Line Feed 24
2.5 Parameters Determining the Performance of Microstrip
Patch Antenna
25
2.5.1 Effect of Substrate 25
2.5.2 Effect of Parasitic Patches 25
2.5.3 Effect of Multilayer Configuration 25
2.6. Fractals 26
2.6.1 Introduction 26
2.7 Dimensions of Fractal Geometry 27
2.8 Fractal Geometries 28
2.8.1 Sierpinski Gasket 28
2.8.2 Sierpinski Carpet 29
2.8.3 Koch Curve 29
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2.8.4 Hilbert Curve 30
2.8.5 Minkowski Curve 30
2.8.6 Pythagorean Tree Fractal 31
2.9 Advantages and Disadvantages of Fractal Geometries 31
2.9.1 Advantages of Fractal Geometries 31
2.9.2 Disadvantages of Fractal Geometries 31
2.10 Applications of Fractal Geometries 32
2.10.1 Astronomy 32
2.10.2 Nature 32
2.10.3 Computer Science 32
2.10.4 Telecommunication and Medicine 33
CHAPTER 3
LITERATURE SURVEY 34-42
3.1 Literature Review 34
3.2 Inferences Drawn 41
CHAPTER 4
PROPOSED RESEARCH WORK 43-50
4.1 Problem Definition 43
4.2 Objective 43
4.3 Scope of Work 44
4.4 Methodology of Proposed Research work 44
4.4.1 Design Methodology 44
4.4.2 Selection of Design Parameters 45
4.4.3 Selected Geometry 45
4.5 Design of Fractal Microstrip Patch Antennas 45
4.5.1 A Simple Microstrip Patch Antenna 45
4.5.2 A Multiband Microstrip Patch Antenna with Koch
Fractal Geometry
47
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4.5.3 A Multiband Fractal Antenna with Four Slots on the
Ground Plane
49
CHAPTER 5
SIMULATED AND MEASURED RESULTS
VALIDATION
51-67
5.1 Introduction 51
5.2 Simulated Results of Microstrip Patch Antenna 51-53
5.2.1 Return Loss Characteristics 51
5.2.2 Radiation Pattern 52
5.2.3 Gain 52
5.2.4 Voltage Standing Wave Ratio 53
5.3 Simulated Results of Fractal Patch Antenna 54-56
5.3.1 Return Loss Characteristics 54
5.3.2 Radiation Pattern 55
5.3.3 Gain 55
5.3.4 Voltage Standing Wave Ratio 56
5.4 Simulated Results of Multiband Fractal Patch Antenna
with Slots on the Ground Plane
57-60
5.4.1 Return Loss Characteristics 57
5.4.2 Radiation Pattern 58
5.4.3 Gain 59
5.4.4 Voltage Standing Wave Ratio 59
5.5 Validation of Simulated Results 60
5.6 Hardware Implementation 62
5.6.1 Introduction 62
5.7 Fabrication Techniques 62
5.8 Hardware Testing 64
5.9 Performance Assessment of Fabricated Antenna 65
5.9.1 Return Loss Characteristics 65
Dept.of ECE, GNDU, Regional Campus Gurdaspur ix
5.9.2 Comparison between Simulated and Measured
Results
66
CHAPTER 6
CONCLUSION AND FUTURE WORK
68-69
6.1 Conclusion 68
6.2 Future Work 69
LIST OF PUBLICATIONS xv
REFERENCES xiv-xviii
Dept.of ECE, GNDU, Regional Campus Gurdaspur x
LIST OF ABBREVIATIONS
HF High Frequency
NAVAID Navigational Aid
RMM Remote Monitoring and Management
LLWAS Low Level Wind shear Alert System
TACAN Tactical Air Navigation
DME Distance Measuring Equipment
GPS Global Positioning System
GLONASS Global Orbiting Navigational Satellite
System
RADAR Radio Detection and Ranging
ASDE Airport Service Detection Equipment
HFSS High Frequency Structure Simulator
Dept.of ECE, GNDU, Regional Campus Gurdaspur xi
LIST OF FIGURES
Figure No. Description Page No.
Figure 1.1 Radiation Pattern from an antenna 4
Figure 1.2 Field regions nearby an antenna 6
Figure 1.3 Radiation Pattern of Directional Antenna 7
Figure 1.4 Equivalent circuit of transmitting antenna 9
Figure 1.5 Various polarization schemes 11
Figure 1.6 Bandwidth from the plot of reflection coefficient 12
Figure 1.7 Half wave dipole 13
Figure 1.8 Radiation Pattern of Half wave dipole 14
Figure 1.9 Monopole Antenna 14
Figure 1.10 Radiation Pattern of Monopole Antenna 15
Figure 1.11 Loop Antennas 16
Figure 1.12 Radiation Pattern of Loop Antenna 16
Figure 1.13 Helix Antenna 17
Figure 1.14 Radiation Pattern of Helical Antenna 18
Figure 1.15 Types of Horn Antennas 18
Figure 2.1 Microstrip Antenna Structure 21
Figure 2.2 Side view of Microstrip Patch Antenna 23
Figure 2.3 Coaxial Probe Feed for Microstrip Patch Antenna 24
Figure 2.4 Microstrip Line Feed for Microstrip Patch Antenna 24
Figure 2.5 Multilayer Configuration 26
Figure 2.6 Sierpinski Gasket 29
Figure 2.7 Sierpinski Carpet 29
Figure 2.8 Koch Curve 30
Figure 2.9 Hilbert Curve 30
Figure 2.10 Minkowski Curve 31
Figure 2.11 Pythagorean Tree Fractal 31
Dept.of ECE, GNDU, Regional Campus Gurdaspur xii
Figure 4.1 Flow graph of Design Methodology of Proposed
Research Work
44
Figure 4.2 Microstrip Patch Antenna (a) Top View (b) 3D View in
HFSS
46
Figure 4.3 3D View in HFSS (a) First iteration (b) Second iteration
(c) Third iteration
48
Figure 4.4 3D View of fractal microstrip patch antenna in HFSS 48
Figure 4.5 Multiband Fractal Patch Antenna with Slotted Ground
Plane (a) Bottom View (b) 3D view
49
Figure 4.6 Detailed Dimensions of Proposed Antenna in HFSS 50
Figure 5.1 Simulated Return Loss of basic Microstrip Patch
Antenna
51
Figure 5.2 3D Radiation Pattern of Microstrip Patch Antenna 52
Figure 5.3 Simulated 3-D Gain Plot of Microstrip Patch Antenna 53
Figure 5.4 Simulated VSWR plot of microstrip patch antenna 53
Figure 5.5 Simulated Return Loss of Fractal Patch Antenna 54
Figure 5.6 Simulated 3D radiation pattern of Fractal Patch Antenna 55
Figure 5.7 Simulated 3D Gain plot of Fractal Patch Antenna 56
Figure 5.8 Simulated VSWR plot of fractal patch antenna 56
Figure 5.9 Simulated Return Loss of Multiband Fractal Antenna 57
Figure 5.10 Simulated 3D radiation pattern of Multiband Fractal
Antenna
58
Figure 5.11 Simulated 3-D Gain Plot of Multiband Fractal Antenna 59
Figure 5.12 Simulated VSWR plot of Multiband Fractal Antenna 60
Figure 5.13 Mask generated using Coral Draw (a) Fractal geometry
(b) Ground slots
63
Figure 5.14 Screens Generated in Screen Printing technique (a)
Antenna Ground Plane (b) T-shaped Top Patch
63
Figure 5.15 Final Fabricated antenna layout (a) Top View (b)
Bottom View (c) Side view
64
Dept.of ECE, GNDU, Regional Campus Gurdaspur xiii
Figure 5.16 Fractal Patch Antenna mounted on Network Analyzer 65
Figure 5.17 Measured Return Loss of Multiband Fractal Antenna 66
Figure 5.18 Measured v/s Simulated Return Loss of Multiband
Fractal Antenna
67
Dept.of ECE, GNDU, Regional Campus Gurdaspur xiv
LIST OF TABLES
Table No. Description Page No.
Table 1.1 Various Frequency Bands for Aerospace Navigation 2
Table 4.1 Detailed Dimensions of Proposed Microstrip Patch
Antenna
46
Table 4.2 Detailed Dimensions of Proposed Multiband Fractal
Antenna
50
Table 5.1 Comparison between Proposed Multiband Fractal
Antenna with the Design Proposed in [30]
60
Dept. of ECE, GNDU, Regional Campus Gurdaspur 1
CHAPTER 1
INTRODUCTION
1.1 Overview
In 1886, Henry Hertz developed a wireless communication system in which an electric
spark occurred in dipole and loop antenna. Since then antennas are being used for
television, mobile and satellite communication. In the year 1880, Nicola Tesla suggested
a radio to transmit information and described first radio communication systems in his
papers in the year 1891. At the same time, Guglielmo Marconi was the first to patent the
telegraph and signified the importance of wireless communication. In 1940, the first
standard for communication technology was introduced [1]. A large growth in wireless
technologies was seen during 1980’s and 1990’s, due to which cheap wireless services
were introduced all over the world. In 21st century a great progress can be seen in
wireless technologies in which the devices are becoming smaller to integrate various
services.
In wireless applications antenna plays an important role as it converts electrical power
into radio waves. An antenna acts as an interface between space and transmission line.
Earlier each antenna operates at a single frequency therefore different antennas were
required for different purposes. Today’s communication requires an antenna that provides
high gain, wide bandwidth, supports multiple frequencies, is compact in size and satisfies
various requirements of the system.
Earlier Navigation was based on observations and not on scientific methods but modern
navigation determines the position by collecting the information from satellites through
the receivers. Navigational tools were initially developed for military users but with the
advent of wireless communication systems it has been adopted in civil as well. Mobile
phones, computers, laptops have GPS functionality. The GPS is a space based satellite
system that provides location information anytime and anywhere. In military applications
navigational antennas are required for many applications such as surveillance, beam
steering, beam forming, radar, tracking etc. These antennas are positioned on aircrafts,
ships or other vehicles. In aerospace navigation, antennas use radio frequencies to
Dept. of ECE, GNDU, Regional Campus Gurdaspur 2
communicate with air traffic control and find its destination. Various frequency bands
allocated for aerospace navigational applications are listed below in Table 1.1.
Table 1.1 Various Frequency Bands for Aerospace Navigation
Frequency
Band Name
2100 - 28,000 kHz HF Communications
750 MHz NAVAID (Marker Beacons)
8-12 GHz X-band
932 – 935 & 941 – 944
MHz
RMM, LLWAS, etc.
960 – 1215 MHz NAVAID (TACAN / DME, etc.)
1215 – 1390 MHz Air Route Surveillance Radar; GPS and GLONASS L1
1545 – 1559 MHz Satellite-Based Comm (To Aircraft)
1559 – 1610 MHz Satellite Navigation; GPS and GLONASS L1
1646.5 – 1660.5 MHz Satellite-Based Comm (From Aircraft)
9000 – 9200 MHz Military Precision Approach Radar
14.4 – 15.35 GHz Microwave Link
15.7 – 16.2 GHz Radar (ASDE-3)
18-19 GHz Point to Point Radio Communication
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A great improvement can be seen in aerospace navigation and various new standards are
being used throughout the world. In order to implement the new standards it is essential
to have an antenna that is low profile, supports various frequencies and provides good
transmission and reception signals. However designing such an antenna is a difficult task
because various parameters such as space, volume and presence of other objects play an
important role. Various design issues are discussed below.
1.2 Antenna Design Issues in Aerospace Navigation
Aerospace navigation requires an antenna that supports wideband/multiband frequencies
and is small in size. The major challenge in designing an antenna for aerospace
navigation is that it covers maximum frequencies while consuming less volume.
Therefore an antenna that has light weight, low cost, robust, flexible and can support
multiple frequencies is the need of today [2].
Various design issues are briefly explained below:
1.2.1 Coverage
Wireless devices should be designed in such a way that it covers maximum frequencies
that are allocated to a particular application. Earlier different antennas were used for
different purposes. The latest trend in designing an antenna for navigation purpose is that
it covers maximum frequencies. This affects in designing a complex patch antenna with
fractal shapes on its patch.
1.2.2 Space Available
Now-a-days the inclination in wireless communication is to design a compact antenna so
that it can be easily positioned even in places where volume is a major issue. While
designing a low profile antenna, complexity increases as patch is designed using fractal
geometries. Moreover bandwidth and radiation efficiency are directly proportional to the
size of the antenna. Therefore as size increases performance also increases and hence
more volume is required.
Dept. of ECE, GNDU, Regional Campus Gurdaspur 4
1.3 Antenna Fundamentals
Antennas are very essential part of communication systems. An antenna converts RF
signal that is travelling on a conductor into an electromagnetic wave. Antennas exhibits
reciprocity property i.e., antenna has same characteristics while transmitting or receiving.
In order to pair transmission and reception, antenna must be tuned to similar frequency
band of the radio to which it is linked.
1.4 How an Antenna Radiates
Firstly, let us consider how radiation takes place. Radiation in conducting wire takes
place mainly due to deceleration or acceleration of charge. No current flows, if there is no
motion of charge and hence no radiation occurs. However, when a charge moves along
bent or curved wire with uniform velocity, radiation is produced [3].
Figure 1.1 Radiation pattern from an antenna
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To understand the concept of radiation from an antenna, let us consider a voltage source
connected to a transmission line. When a voltage is applied across the conductor, a
sinusoidal electric field is produced and thus electric lines of force are also formed. The
free electrons in the conductor are dislocated by electric lines of force which in turn
produce charge by the movement of these charge carriers and hence magnetic field is
created.
Electromagnetic waves that travel between the conductors are created due to time varying
magnetic and electric fields. When these waves come close to the open space, they form
free space waves by simply joining the open ends of electric field lines. Since electrical
disturbances are created continuously by the sinusoidal source, therefore electromagnetic
waves are also radiated continuously into the free space. The electromagnetic waves are
preserved inside the antenna and transmission lines due to the presence of charged
particles, but the moment they penetrate into the free space closed loops are formed and
are radiated.
1.5 Near and Far Field Regions
The antenna field patterns are associated with two types of energy: reactive energy and
radiating energy. Therefore space around the antenna is divided into three regions as
shown in figure 1.3.
Reactive near field region: This region is influenced by reactive field. The energy in
reactive field appears as reactance by oscillating towards and away from the antenna.
Therefore the energy appears as reactance. In this no energy is dissipated, in fact it is
stored in the given region. The exterior most boundary is at a distance of R1=
0.62√D3/λ where λ is the wavelength, D is the highest dimension of antenna and R1 is
the distance from the antenna.
Radiating near field region (Fresnel region): This region resides between the far field
region and the near field region. In this field radiation fields dominate while reactive
fields are smaller. The exterior most boundary is at a distance of R2= 2D2/λ where λ is
the wavelength and R2 is the distance from the antenna [3].
Far-field region (Fraunhofer region): The region beyond radiating near field region is
the far field region. Here only the radiation fields are present and the reactive field
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does not exist. In this region, field distribution is independent from the distance of the
antenna.
Figure 1.2 Field regions nearby an antenna
1.6 Antenna Performance Parameters
The performance of the antenna can be measured from various parameters. Some
important parameters are discussed below.
1.6.1 Radiation Pattern
Radiation Pattern or Antenna Pattern is the total strength of the radiated field in different
directions from the antenna, at a given distance. The radiation pattern also describes
receiving properties of an antenna. The radiation pattern is measured in two dimensions
i.e., the vertical or horizontal planes, although it is a three dimensional pattern [3]. The
measurements of the pattern are either presented in a polar or rectangular format. The
points in the polar coordinate graph are positioned alongside a rotating radius and they
intersect with several concentric circles. In rectangular plot it is difficult to conceptualize
the behavior of antenna at different directions.
To understand the concept if radiation pattern, let us consider a directional antenna. A
directional antenna radiates more in a particular direction while less in other directions.
An omnidirectional antenna is an exceptional case of directional antenna having constant
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radiation pattern in E-plane and varies in orthogonal or H-plane. Figure 1.3 shows the
radiation pattern of a directional antenna.
Figure 1.3 Radiation Pattern of Directional Antenna
HPBW: The angle linked by half power points of the main lobe is known as Half
Power Beam width (HPBW).
Major Lobe: This lobe contains the maximum intensity of radiation in a particular
direction.
Minor Lobes: These lobes contain the radiation in undesired directions. Therefore all
the lobes other than major lobe are the minor lobes.
Back Lobe: The lobe exactly in the opposite direction of the main lobe is called back
lobe.
Side Lobe: The lobes adjacent to main lobe are called side lobes. These lobes are
unrelated by various nulls.
1.6.2 Directivity
Directivity can be defined as an ability of an antenna to transmit more power in a specific
direction while transmitting or receive more power from a specific direction while
receiving [3]. Or we can say, the directivity of an object whose physical properties vary
in different directions, is defined as the ratio of its radiation intensity in a specific
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direction, over that of an isotropic source, whose physical properties remain same
throughout all the directions. Therefore
D= U/Ui = 4πU/P (1.1)
where D = directivity of the antenna
U = the radiation strength of the antenna
Ui = the radiation strength of an isotropic source
P = the total power emitted
Directivity is the ratio of two radiation strengths; therefore it is a dimensionless quantity.
It is expressed in dBi. For the antenna to be more directive, it will have a narrow main
lobe rather than a broad one.
1.6.3 Input Impedance
The input impedance of an antenna can be defined as the ratio of the voltage to the
current at the antenna and the transmission cable connecting them [3]. The impedance of
the pair should be same for an efficient transfer of energy. The impedance of an antenna
should not be different from 50Ω. Mathematically input impedance can be represented as:
Zin = Rin + jXin (1.2)
where Zin = antenna impedance at the terminals
Rin = antenna resistance at the terminals
Xin = antenna reactance at the terminals
The imaginary part of the input impedance, Xin shows the radiation strength stored in the
near field of the antenna. Rin called the resistive part of the input impedance is composed
of two parts, loss resistance RL and radiation resistance Rr. Radiation reactance represents
the actual power transmitted by an antenna while loss resistance is the heat dissipated
from the antenna.
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1.6.4 Voltage Standing Wave Ratio
For an antenna to function effectively, transfer of maximum power between the
transmitter and the antenna should take place. This can happen only when the impedance
of the transmitter (Zs) is matched to the impedance of the antenna (Zin). Maximum Power
Transfer theorem says that maximum power can be transmitted only if the impedance of
the antenna is a complex conjugate of the impedance of the transmitter and vice-versa.
Therefore, condition for impedance matching is:
Zin = Zs* (1.3)
If the above condition is not fulfilled, then some of the radiations are reflected back,
leading to formation of standing waves, which can be distinguished by a parameter
known as Voltage Standing Wave Ratio (VSWR). Mathematically:
VSWR =1 + |Γ| /1 − |Γ| (1.4)
ᴦ = Vr / Vi = Zin – Zs/ Zin+ Zs (1.5)
ᴦ = reflection coefficient
Vi = amplitude of incident wave
Vr = amplitude of reflected wave
Figure 1.4 Equivalent circuit of transmitting antenna
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The value of VSWR should be less because higher the value, the greater is the mismatch
between the antenna and the transmitter. Ideally the value of VSWR should be unity.
1.6.5 Return Loss (RL)
Return loss can be defined as the power that is wasted to the load and is not returning as a
reflection. Return loss also indicates the mismatching between the transmitter and the
antenna. Mathematically it is represented as:
RL = -20 log |Γ| (1.6)
If impedance matching is perfect then Γ= 0 and RL = infinity, i.e., no reflected power.
Similarly when Γ= 1 and RL = 0dB, this means that all the incident power is reflected
back. The value of VSWR should not exceed 3, since this value gives RL of -10 dB.
1.6.6 Antenna Efficiency
Antenna efficiency can be defined as the amount of losses occurring within the antenna
and at various terminals of the antenna. Various losses can be defined as:
Reflection Losses: These losses occur mainly due to impedance mismatching between
the terminals and the antenna.
I2R losses: These are the conduction and dielectric losses.
Hence mathematically efficiency of the antenna can be defined as:
et= er ec ed (1.7)
where et = total antenna efficiency
er = mismatch efficiency
ec = conduction efficiency
ed = dielectric efficiency
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1.6.7 Antenna Gain
Antenna gain is closely associated with the directivity of the antenna. Directivity can be
defined as an ability of an antenna to transmit more power in a specific direction while
transmitting or receive more power from a specific direction while receiving. Therefore,
if the efficiency of the antenna is 100% then the antenna can act as an isotropic radiator
whose directivity is equal to the gain of the antenna. More precisely, we can define
antenna gain as an ability of an antenna to achieve more power in one direction at the
expense of lost power in other directions.
1.6.8 Polarization
Polarization is defined as the direction of electric field of an electromagnetic wave. It
describes the direction and position of electric field with respect to ground [3].
Polarization is of two types i.e., linear and circular polarization.
Linear polarization: In linear polarization electric field path is back and forth along a
line. Figure 1.5 represents linear polarization.
Figure 1.5 Various polarization schemes
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Circular polarization: In circular polarization, electric field vector rotates in circular
path while remaining constant in height. Circular polarization is further of two types:
Right hand circular polarized wave and Left hand circular polarized wave. In Right
hand circular polarized wave, the electric field vector rotates in clockwise motion. In
Left hand circular polarized wave; the electric field vector rotates in anticlockwise