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STUDY ON THE CHARACTERISTICS OF
ANTIPODAL VIVALDI ANTENNA
HALIMAH BT ROBERT
This project is submitted in partial fulfillment of the requirements for the degree of Bachelor of Engineering with E-Ionours
(Electronics & Telecommunication Engineering)
Faculty of Engineering UNIVERSITI MALAYSIA SARAWAK
2004
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To my beloved family.
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ACKNOWLEDGEMENT
The author would like to express my sincere thanks and greatest gratitude to my
supervisor, Mr. Kismet Anak Hong Ping, for his guidance, suggestion and support given
throughout the completion of the Final Year Project. This report would not been completed
without his valuable time and advice given during the period of this project.
Special thanks to all friends, lab assistant for their co-operations and supports given
directly or indirectly.
Last but not least, the author would like to thank my parents, who gave me unlimited
love and support.
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ABSTRAK
Antipodal Vivaldi Antenna diklasifikasikan sebagai Antenna Celah Menirus (TSA),
yang merupakan "enditre traveling wave" antenna. Antenna Celah Menirus adalah sebuah Antenna Integrasi Litar yang baru, yang mudah diintegrasikan dengan komponen Integrasi Litar Gelombang Mikro (MIC) yang lain. Antenna ini adalah ringan, padat, mudah dihasilkan,
nipis dan mendatar serta meluas digunakan. Secara teorinya, Vivaldi Antenna mempunyai potensi yang baik iaitu mempunyai lingkungan frekuensi yang lebar tetapi dibatasi oleh peralihan cara tradisional iaitu daripada garisan celah ke jalur mikro. Oleh itu, laporan ini
mengkaji teknik penyaluran menggunakan jalur mikro ke garisan jalur, dan dimensi serta bentuk Antenna termasuk profil menirus yang mempengaruhi sifat antenna. Selanjutnya,
analisis ini lebih ditumpukan kepada lingkungan frekuensi dan penolakan litar elektrik khas. Akhir analisis, bentuk terbaik Antipodal Vivaldi Antenna akan dibentangkan. Analisis akan dijalankan menggunakan program komputer iaitu Microwave Office 2002 (EM Structures).
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ABSTRACT
The Antipodal Vivaldi is classified as Tapered Slot Antenna (TSA), which is an endfire traveling wave antenna. The tapered slot line antenna is a new integrated circuit antenna, which can be easily, integrated with other Microwave Integrated Circuit (MIC)
components. These antennas are light, compact, easily reproducible, thin and flat in profile and widely used. Theoretically, the Vivaldi Antenna has a good performance over a wide bandwidth but limited by the traditionally used slotline to microstrip feed transition. Therefore, this report investigated a microstrip line to parallel stripline feeding technique also the dimension and shape of antenna included the tapered profile that influences the characteristic of the antenna. Furthermore, this analysis is more focused on bandwidth and impedance characteristic. The optimum shape of Antipodal Vivaldi antenna will be presented. The analysis were carried out using Microwave Office 2002 (EM Structures).
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TABLE OF CONTENTS
PROJECT TITLE
DEDICATION
ACKNOWLEDGMENT
ABSTRAK
ABSTRACT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
CHAPTER I INTRODUCTION TO ANTIPODAL VIVALDI ANTENNA
1.1 Introduction
1.2 Project Overview
1.2.1 Objectives
1.2.2 Thesis Outlines
CHAPTER 2 LITERATURE REVIEW
2.1 Electromagnetic Wave
2.2 Transmission Line
2.2.1 Microstrip Line
2.2.2 Parallel Stripline
2.3 Antenna
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2.3.1 Types of Antenna
2.3.2 Characteristics of Antenna
CHAPTER 3 RESEARCH METHODOLOGY
3.1 Introduction
3.2 Microstrip Line Design
3.2.1 MicroWave Office 2002 Structure
3.3 Parallel Stripline Design
3.4 Tapered Profile Design (conventional Vivaldi Antenna)
3.5 Antipodal Vivaldi Antenna Design
CHAPTER 4 RESULTS AND DISCUSSIONS
4.1 Microstrip Line
4.2 Parallel Stripline Design
4.3 Tapered Profile (conventional Vivaldi antenna)
4.4 Antipodal Vivaldi antenna Design
4.4.1 Simulation 1- Top Modification
4.4.2 Simulation 2- Smoothing the Top modification
4.4.3 Simulation 3- Bottom to Top Modification
4.4.4 Simulation 4- Bottom to bottom modification
4.4.5 Simulation 5- Top Curve Modification
4.4.6 Simulation 6- Bottom Curve Modification
4.4.7 Simulation 7- Microstrip width Modification
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4.4.8 Simulation 8- Increase Length of Parallel
Stripline Modification 54
4.4.9 Simulation 9- Decrease Length of Parallel Stripline 55
4.4.10 Simulation 10 - Thickness Modification 56
4.5 Optimum Shape of the Antipodal Vivaldi antenna 56
4.6 Discussion 59
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS
REFERENCES 64
APPENDICES 67
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LISTS OF TABLES
Table Page
4.1 Antipodal Vivaldi antenna
4.2 Result for Top Modification (-5dB)
4.3 Result for Top Modification (-10dB)
4.4 Smoothing the Top modification
4.5 Bottom to Top Modification
4.6 Bottom to bottom Modification
4.7 Top Curve Modification
4.8 Bottom Curve Modification
4.9 Microstrip Width Modification
4.10 Increase Length of Paralle Stripline
4.11 Decrease Length of Parallel Stripline
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LISTS OF FIGURES
Figure Page
1.1 Flared Notch Antenna 1
1.2 Conventional Vivaldi Antenna 2
1.3 Antipodal Vivaldi Antenna and direction of dominant E Field 3
cross radiating slot
2.1 Electromagnetic Wave 7
2.2 Microstrip Line 11
2.3 Electric and magnetic field lines around the Microstrip Line 12
2.4 Parallel Stripline 14
2.5 Electric and magnetic field lines around the Parallel Stripline 14
2.6 Different type of Wire Antenna 16
2.7 Loop Antenna 17
2.8 Horn Antenna 17
2.9 Parabolic Antenna 18
2.10 Microstrip Antenna 18
2.11 Antenna radiation patterns and beamwidth measurement 21
2.12 Polarization (a) linear polarization (b) circular polarization (c) 23
Elliptical polarization
3.1 Flow Chart 27
3.2 Transmission Line Calculator 28
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3.3 Enclosure
3.4 Dielectric Layers
3.5 Boundaries
3.6 Dielectric Layers
3.7 Boundaries
3.8 Enclosure
3.9 Dielectric Layers
3.10 Boundaries
3.11 Exponentional Taper Function
3.12 Second Tapered profile modification
3.13 Dielectric Layers
3.14 Boundaries
3.15 Dimension and shape of antipodal Vivaldi antenna
4.1 3-D view for Microstrip Line
4.2 Impedance Graph
4.3 Mismatch graph
4.4 Return loss
4.5 Insertion Loss
4.6 3-D View of Parallel Stripline
4.7 Top Modification
4.8 Smoothing the Top modification
4.9 Bottom to Top Modification
4.10 Bottom to bottom Modification
4.11 Top Curve Modification
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4.12 Bottom Curve Modification
4.13 Microstrip Width Modification
4.14 Increase Length of Parallel Stripline
4.15 Decrease Length of Parallel Stripline
4.16 Optimum shape of Antipodal Vivaldi antenna
4.17 Return loss for the Optimum Shape
4.18 Impedance for the Optimum Shape
4.19 Smith Chart for the Optimum Shape
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CHAPTER I
INTRODUCTION TO ANTIPODAL
VIVALDI ANTENNA
1.1 Introduction
The Antipodal Vivaldi is classified as Tapered Slot Antenna (TSA), which is an
endfire traveling wave antenna. This antenna [1] exhibits a wide beamwidth and moderately
high directivity. The first TSA presented was the stripline tapered notch or flared notch
antenna.
A sketch of the flared notch antenna is shown in Figure I. I. A notch antenna [2,3]
formed by symmetric etching of the outer ground plane. It smoothly flaring from a narrow
slot over the center conductor transition region to a wide aperture at the board edge. The
center conductor is open circuit terminated at some distance beyond the slot.
Figure 1.1 Flared Notch Antenna
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The Transverse Electromagnetic (TEM) stripline mode excites a voltage across the
gap of the notch. Thus, the electric field lies in the plane of the boards and couples to a leaky
wave in the notch. The traveling wave is launched off the board edge and radiates [3].
A Conventional Vivaldi Antenna [2] is illustrated in Figure 1.2. The antenna is excites
by a slot line. The taper shape is designed according to the equation [4,15].
x= (az + b) x exp (mz) (1.1)
where a, b and m are constant.
This antenna is frequency independent, since at a given frequency only a section that is
exponentially is radiates. As the frequency varies, different parts of the antenna radiate, while
the size of the radiating part is constant in wavelength [3].
f1 14-1)
WI
WA
A F] EoEr Z
in
yý wýý
v
Figure 1.2 Conventional Vivaldi Antenna
This antenna is divided into two areas [1]:
(i) A propagating area defined by w, <w< wA
(ii) A radiating area defined by w, 1 <w< w0
where,
w- slot width
w, - input width
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WA - slot width at radiating area
Wo - output width
The conventional Vivaldi antenna is a member of the class of aperiodic, continuously
scaled, gradually curved, slow leaky endfire traveling wave antenna [6].
In theory, the bandwidth of a Conventional Vivaldi antenna [6] is infinite, the
limitation on bandwidth is cause by physical size of the antenna and fabrication capabilities.
But the main bandwidth limitation of a Vivaldi antenna is the microstrip to slotline transition.
To solve this problem, a microstrip to printed twinline or two sided slotline transition (parallel
stripline) used. This is known as Antipodal Vivaldi antenna [7] as shown in Figure 1.3.
ground plane
microstrip -0
twin line
r-t-_
r flared
slot
metalisation -0
dielectric
1 I
E field
Figure 1.3 Antipodal Vivaldi antenna and direction of dominant E field cross
radiating slot.
One side of board is etched to give a 50 0 microstrip line, which is then flared to
produce a half of conventional Vivaldi. On the other side the ground plane is reduced to give
50 Q twin lines. This is then flared in the opposite direction to the top, thus generating a
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Vivaldi antenna but with wings on opposite sides of the substrate. The smooth microstrip to
parallel stripline to Vivaldi transition has a very wide bandwidth capability.
This antenna exhibits similar beamwidth and gain to the Vivaldi antenna, except a
poor crosspolarization characteristic. The E field components within the antenna are skewed
with respect to the physical axis of the antenna. At the low frequency end of the band this
skew is small because the ratio of slot width to dielectric thickness is large. However as the
frequency increase, so the angle of skew increases and ultimately tends to 900 [7].
1.2 Project Overview
The purposes of this thesis are to design the optimum shape of Antipodal Vivaldi
Antenna by simulation using MicroWave Office 2002. This thesis will concentrates on the
dimension and shape of the antenna that influences the design of the Antipodal Vivaldi
Antenna.
The antenna for this thesis was based on the following requirements:
" Using RT Duroid 5880 (low dielectric constant, sr = 2.2)
" Cut off frequency = 2.45 GHz
" Substrate thickness, h=0.787 mm
" X-dimension = 16 mm
" Y-dimension = 32 mm
The optimum shape of Antipodal Vivaldi antenna will be used to form the impedance, return
loss and bandwidth.
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1.2.1 Objectives
The objectives of this project are as follows:
1) To analyze the basic concept of antenna.
2) To identify several type of antenna such as wire, aperture, reflector and
broadband antenna.
3) To distinguish the evolution of broadband antenna from Notch antenna until
Antipodal Vivaldi antenna.
4) To compute the transmission line of the Antipodal Vivaldi antenna.
5) To analyze the characteristic of the Antipodal Vivaldi antenna.
6) To design the Antipodal Vivaldi antenna.
7) To simulate the Antipodal Vivaldi antenna using MicroWave Office 2002.
8) To get the optimum shape of antipodal Vivaldi antenna and compare the results
with other researcher's results.
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1.2.2 Outline
This report divided into five chapters as follows:
Chapter 1 mainly focuses on the evolution of the TSA antenna such as the Notch antenna,
Conventional Vivaldi antenna and Antipodal Vivaldi antenna.
Chapter 2 provides an in depth look into the literature review about the antenna theory and
transmission line generally. A detailed look into the transmission line that is used in feeding
the Antipodal Vivaldi Antenna that are microstrip line and parallel stripline is available in this
chapter. Useful formula related to width w, and characteristic impedance Z0 of the
transmission lines also provided.
Chapter 3 covers the simulation set up for the transmission line such as microstrip line and
parallel stripline. The investigation on the optimum width w, conductor thickness 1 and
substrate thickness h, for microstrip line to parallel stripline transition are presented.
Chapter 4 covers the simulation on the dimensions and shape of Antipodal Vivaldi antenna
that influenced the performance parameters of the antenna and designed the optimum shape of
Antipodal Vivaldi antenna. Then the result are discusses.
Chapter 5 describes the conclusions and recommendations for the future researches and
prospects in the wide band tapered slot antenna especially for Antipodal Vivaldi Antenna.
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CHAPTER 2
LITERATURE REVIEW
2.1 Electromagnetic Waves
According to Maxwell's laws [8], electromagnetic (EM) waves are electric and
magnetic fields that travel together through space and perpendicular to each other at the speed
of light (3 x l08 ms-1). The electric field (E) was produced by stationary electric charges
while, magnetic field (H) was produced by moving electric charges. Without these charges
particles, there will be no electric force fields and thus no electromagnetic waves.
Figure 2.1 Electromagnetic wave
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The configuration of electromagnetic wave in Figure 2.1 shows that electric field
points upward and the magnetic field points out of the plane. This wave is identified as a
Transverse Electromagnetic (TEM) wave because the electric and magnetic fields are at right
angle (transverse) to each other and to the direction of propagation.
There are five characteristics of the electromagnetic waves such as wavelength,
frequency, impedance, power density and phase. The distance in which the fields of an
electromagnetic complete one cycle is known as wavelength ? ', of the signal. The wavelength
can be calculated as [9]:
x _C f
where,
? is the wavelength and expressed in units of meter (m)
c is the speed of light 3x 108 ms-1
f is the frequency (cycles per second) and expressed in units of hertz (Hz)
(2.1)
The frequency is a number of oscillations that an electrical signal complete in one
second and measured in Hertz (Hz). The period of the signal (wave) varies inversely with its
frequency and is the time of one complete cycle. [9]
fT
where,
J,
T is the period of one cycle
is the frequency and expressed in Hertz (Hz)
Impedance is defined as the ratio of the electric field to the magnetic field. The unit of
their ratio is ohm (52). In free space, the electromagnetic wave impedance is 3770.
1 (2.2)
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2.2 Transmission Line
A transmission line is a device designed to guide electrical energy from one point to
another [10]. The purpose of the transmission line is to transfer the energy output of the
transmitter to the antenna with the least possible power loss. Although the antenna can be
connected directly to the transmitter a transmission line is used to connect the transmitter and
the antenna due to the antenna usually located a far away from the transmitter.
One factor in achieving a good transmission is to ensure that the transmission lines
used are perfectly matched since a mismatch condition of the lines caused the maximum
transfer power will not be met.
Mismatch occurs when a transmission line is not properly terminated at the receiver
end. This caused some of the energy reflected back into the transmission line from the load.
However, if the load has the same impedance as the transmission line, the load will absorb the
energy delivered and no reflections will occur.
The amount of incident energy that is reflected back from a load is represent by the
reflection coefficient. The magnitude of reflection coefficient can vary from 0 to I and can be
calculated by the equation [8]:
Reflection Coefficient =F /I"'``ý = Z' Z°
vf,,,, 4, �r, i Z, + Zo
where,
Zj, is the impedance of the load.
Z() is the intrinsic impedance of the transmission line.
(2.3)
A transmission line can be defined in term of impedance. The ratio of voltage to
current at the input end is known as the input impedance, Z,,,. While the ratio of voltage to
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current at the output end is known as the output impedance, Z0,,,. Characteristic impedance,
which is labeled as Z0, is the ratio of voltage to current at any point on that transmission line
[8].
y%arward Z�=
11 1
If the line has low resistive losses the formula for characteristic impedance is [8]:
_ JL
where
L is the inductance per meter.
C' is the capacitance per meter.
If a line has significant losses due to series resistance and parallel resistive losses, the
forward (2.4)
(2.5)
equation for characteristic impedance is [8]:
Z� = R+jco L G- jwC
(2.6)
where,
R is the resistance per unit length in ohms.
G is the conductance per unit length in ohms.
L is the inductance per unit length in Henrys.
C is the capacitance per unit length in farads.
w is the angular frequency in radius per second (270.
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For free space the characteristic impedance is equal to 377 0 or 12071. The
characteristic impedance of any transmission line is a function of the conductor size, spacing
of the conductors, the conductor geometry and the dielectric constant of the insulating
material used between the conductors.
Characteristic impedance (Zo) of 500 is the most common for radio communication.
The reason for this is the best compromise between power handling ability and low loss
operation at the same characteristic impedance. The antenna also radiates only at their
resonant frequency (the frequency the antenna designed to transmit on), and this condition
only achieved for impedance about 500. Most of the cables and connectors also designed for
500 impedance. [8]
2.2.1 Microstrip Line
Figure 2.2 shows the schematic diagram of the microstrip and Figure 2.3 shows the
microwave fields in the cross section of microstrip line.
Strip
conductor
I
Dielectric
subtrate c,
Ground Plane
Figure 2.2 Microstrip line
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.ýý, ý, ̀ ., `, . ý{ ý;
rr tttr 1ý. E- Electric field
----- H- Magnetic field
Figure 2.3 Electric and magnetic field lines around the microstrip line
It consists of a conductor of width w printed on grounded dielectric substrate of
thickness h and relative permittivity e,. In stripline all the fields are contained within dielectric
region. However in microstrip some of its field's lines in the dielectric region, concentrated
between the strip conductor and the ground plane. While the others is in the air region above
the substrate. For this reason the microstrip line cannot support a pure TEM wave, since the C
phase velocity of TEM fields in dielectric region would be [13]. While, in the air f'
r
region it velocity would be c (velocity of light). Thus, because of different velocities was exist
it is not possible to form a TEM modes. Therefore, small amount of transverse fields must
exist to equalize the propagation velocities in different dielectrics. This hybrid of TE and TM
modes form the principle mode of propagation in a microstrip line, which is named quasi-
transverse electromagnetic (TEM) mode.
In microstrip line, the effective relative permittivity t is a very important parameter
and dependent on the substrate thickness h and conductor width w.
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