RADIAL LINE SLOT ARRAY (RLSA) ANTENNA DESIGN FOR POINT TO POINT
COMMUNICATION AT 5.8 GHz
MD.RAFI UL ISLAM
This project report submitted in partial fulfillment of the requirement for the award of
the
Master of Engineering (Electrical-Electronics & Telecommunication)
FACULTY OF ELECTRICAL ENGINEERING
UNIVERSITI TEKNOLOGI MALAYSIA
MAY 2007
To my beloved mother and father
iii
ACKNOWLEDGEMENT
First and foremost, the author is grateful to the Lord Almighty for showing the
lights and paths to fulfill the dream to complete this postgraduate degree. Secondly, the
author is deeply indebted to a number of individuals who helped make this thesis
possible.
Besides, I deeply appreciate the inspirations and guideline that I have received
from my supervisor Professor Dr. Tharek b. Abdul Rahman for his personal kindness,
skill, patience, valuable advice and encouragement. I would like take this opportunity to
thanks everyone who has contributed either directly or indirectly throughout this thesis
and project.
Wireless Communication Centre, UTM has provided sophisticated facilities and
constructive environment in the process of this research. Special thanks are dedicated to
the technician of Wireless Communication Centre who offered invaluable technical
assistance and supports, especially Mr. Mohammed Abu Bakar.
Finally, I would also like to thank all of my family and my friends for their moral
support on me.
iv
ABSTRACT
A new type of linearly polarized Radial Line Slot Array (RLSA) antenna
developed by FR-4 substrate is proposed for outdoor WLAN point to point application.
The WLAN is based on the IEEE 802.1 la standard and the operating frequency range is
the upper UNII band (5.725-5.875) GHz. The frequency range is use for point to point
microwave link. Point to point microwave link is a wireless connection between one
point to another point. Before this all the research has been conducted on conventional
RLSA antenna developed by polypropylene and two layers copper foil. But these
antennas are quite expensive and difficult to develop. Typically the point to point system
uses standard parabolic dish antenna. However the uses of these antennas have some
disadvantages such as aperture blockage. To overcome this drawback, a new antenna
design is proposed and investigated. Therefore, this research was conducted in order to
design and develop antenna with aesthetic, low cost, high performance, durable and flat
antenna that could be utilized in point to point microwave link. The development of
linearly polarized radial line slot antenna (RLSA) with experimental performance is
presented. The method is beam squinted design. The prototype development processes
included specification definition, selection of the cavity's dielectric material and
construction of prototype. The production of prototypes is divided into two stages. The
first stage is to simulate the radiation pattern of slot arrangement at operating frequency.
The second stage is to produce the prototype and to evaluate its performance. The
prototype of RLSA has been successfully constructed and tested for outdoor WLAN
point to point application
v
ABSTRAK
Satu rekabentuk baru linearly polarized Radial Line Slot Array (RLSA) antena
telah dihasilkan menggunakan bahan FR-4 sabagai dielektrik. Antena ini telah
dicadangkan untuk penggunaan di luar bangunan, bagi aplikasi LAN secara wayarles
(outdoor WLAN). Aplikasi WLAN adalah berdasarkan standard IEEE 802.11a dan
frekunsi operasi adalah atas UNII band (5.725-5.875) GHz. Julat frekunsi digunakan
untuk gelombang micro titik-ke-titik. Gelombang micro titik-ke-titik ialah sambungan
tanpa wayar dari satu titik ke titik yang lain. Banyak kajian telah dilakukan terhadap
antena RLSA yang dibangunkan daripada polypropylene dan kepipis dua lapisan.
Tetapi, kos pembangunanya sangat mahal dan kompleks. Kebiasaanya sistem titik ke
titik menggunakan parabola dish yang standard. Namun demikian, penggunaan antena
ini mempunyai sedikit kelemahan seperti aperture blockage. Untuk mengatasi
kelemahan ini, satu rekabentuk antena baru di cadangkan dan dikaji. Maka, kajian ini
dijalankan untuk merekabentuk dan membangunkan antena yang estetik, berkualiti,
kuat, flat dan berkos rendah yang boleh di laksanakan dalam gelombang micro point to
point. Pembangunan linearly polarized radial line slot antenna (RLSA) telah dijalankan
dengan kajian eksperimen. Kaedah yang digunakan ialah rekabentuk beam squinted.
Proses pembangunan prototaip ini termasuk spesifikasi definasi, pemilihan cavity's
dielectric material dan pembinaan prototaip. Proses pembangunan prototaip ini
dibahagikan kepada dua peringkat. Peringkat pertama ialah mensimulasi alunan radiasi
penyusunan slot di frekunsi operasi dan peringkat kedua ialah menghasilkan prototaip
dan menilai keberkesananya. Prototaip antena RLSA telah berjaya dihasilkan dan telah
diuji pada aplikasi “outdoor WLAN”.
vi
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF FIGURES
x
LIST OF TABLES
xii
LIST OF SYMBOLS ANAD ABBREVIATION
xiii
LIST OF APPENDICES
XV
1
INTRODUCTION
1.1
Objective
2
1.2
Research Background
2
1.3
Research Scope
3
1.4
Thesis Structure
4
vii
2
LITERATURE REVIEW
2.1
3
History of Research of Radial Line Slot Array Antenna
5
2.1.1
Antenna Characteristics
8
2.1.1.1 Radiation Pattern
9
2.1.1.2 VWSR and Return Loss
10
2.1.1.3 Gain
11
2.1.1.4 Efficiency
12
2.1.1.5 Polarization
13
2.1.1.6 Directivity
15
2.1.1.7 Bandwidth
15
2.1.1.8 Beamwidth
16
SLOT DESIGN AND RETURN LOSS IMPROVEMENT
3.1
Introduction
18
3.2
The RLSA Antenna Structure
18
3.2.1
21
Advantage of RLSA antenna
3.3
Operation of the Radial Cavity
22
3.4
Antenna Polarization
24
3.5
General Conditions for Linearly Polarized Wave
25
3.6
Theoretical Slot Surface Design
26
3.7
Arraying the Linearly Polarized Unit Radiator
28
3.7.1
29
Choosing the Azimuthal Element Spacing
3.8
Reflection Canceling slot Design
31
3.9
Beam Squinted Design
34
3.10
Determination of RLSA Antenna Design Parameters
36
3.11
RLSA Antenna Slot Surface Design Software
37
viii
3.12
4
5
37
SIMULATION AND ANTENNA PROTOTYPE MANUFACTURING
4.1
Introduction
40
4.2
Simulation of radiation pattern and return loss
41
4.3
Define proper material for antenna to be fabricated
42
4.4
Technological Aspects of Antenna Prototype Manufacturing
43
4.5
Etching process
44
4.6
Advantages of etching
45
4.7
Procedure Etching Process
45
MEASUREMENT, RESULT AND ANALYSIS
5.1
Introduction
47
5.2
Availability Equipments or Instruments
47
5.3
Procedure Return Loss
49
5.3.1
51
5.4
5.5
6
The Resultant Linearly Polarized RLSA Slot Surface
Return loss result and analysis
Radiation Pattern Measurement
52
5.4.1
54
Radiation pattern result and analysis
Test-Bed
56
5.5.1
57
Test-bed results and analysis
DISCUSSION AND FUTURE WORKS
6.1
Conclusion
59
6.2
Future works
60
REFERENCES
61
APENDIX
65-75
ix
LIST OF FIGURES
FIGURE
TITLE
PAGE
2.1
Structure Single-Layered RLSA Antenna
Recommended by Takahashi
6
2.2
Structure Layout Radial Line Slot Array Antenna
7
2.3
Typical Radiation Pattern of a Microwave Antenna
9
2.4
Linear Polarization
13
2.5
Circular Polarization
14
2.6
Elliptical Polarization
14
2.7
Graphical of Beamwidth
16
3.1
Single layer linearly polarized RLSA antenna structure
20
3.2
Power Flow within the Radial Cavity
22
3.3
linear polarizations
24
3.4
Slot Geometry for a Linearly Polarized Unit Radiator
26
3.5
The positioning of additional reflection canceling slots
34
3.6
The beam squint geometry
35
3.7
Linearly Polarized RLSA Slot Pattern
And Associated Parameters
38
4.1
Radiation pattern (Simulation)
41
4.2
Return loss (Simulation)
42
5.1
Return-loss and VSWR measurement experiment
49
x
5.2
Marconi instrument transmission line test head
50
5.3
Calibration kit (WILTRON MODEL 22S50)
50
5.4
Measured return loss
51
5.5
Return loss (simulation)
52
5.6
Radiation pattern measurement
53
5.7
Near field measurement system block diagram in
54
5.8
Measured radiation pattern (E-co) in the anechoic chamber 55
Anechoic chamber
5.9
Radiation pattern (simulation)
55
5.10
A partial view of the test-bed
56
5.11
Signal strength using built-in wireless router
57
5.12
Signal strength using conventional RLSA antenna
57
5.13
Signal strength using developed prototype
58
xi
LIST OF TABLES
TABLE
2.1
TITLE
Specification of RLSA Antenna recommended
PAGE
6
By Masaharu
2.2
Specification of RLSA Antenna recommended by
7
Malaysia Researchers
4.1
parameter of FR-4
43
4.2
Antenna design parameters
44
5.1
Type of measurement with respective device
48
xii
LIST OF SYMBOLS AND ABBREVIATIONS
2D
-
Two dimension
3D
-
Three dimension
εeff
-
Effective dielectric constant
εo
-
dielectric constant of free space
εr
-
dielectric constant / permittivity
λ
-
wavelength
λg
-
guided wavelength
λo
-
free space wavelength
µo
-
Permeability of free space
c
-
velocity of light
D
-
directivity
dB
-
decible
f
-
Frequency
b
-
Radial Cavity Height
IL
-
Insertion Loss
L
-
Inductance
Pi
-
Incident Power
Pmax
-
Peak handling Capacity
Pr
-
Reflected Power
Pt
-
Transmitted Power
R
-
Resistance
RL
-
Return Loss
xiii
TEM -
Transverse Electromagnetic
V
-
Voltage
ρa
-
Slot Array Radius
ρ sc
-
Short circuit distance
ρw
-
waveguide radus
Ls
-
ws
-
Slot width
Zo
-
characteristics impedance
Slot length
xiv
LIST OF APPENDICES
APPENDIX
TITLE
PAGES
A
The Design Software Description
65
B
Antenna Important Parameters
73
xv
CHAPTER 1
INTRODUCTION
A Radial Line Slot Array (RLSA) antenna is attractive for point-to-point
communications as well as for receiving Direct-to-Home television programs. Geometrical
simplicity, low profile feature and cost effectiveness make the Radial Line Slot Antenna
(RLSA) a unique choice for outdoor wireless LAN point to point application. Generally, an
RLSA antenna is an attractive candidate for high efficiency and high gain antenna for direct
broadcast from satellite (DBS) application. This type of antenna belongs to the slotted
waveguide arrays family, which is the only and the most promising candidates for high gain
planar antennas, having the smallest conductor losses among all the planar feeding structures
such as the microstrip lines [1]. It is composed of two circular parallel plate waveguide,
which can supports two types of wave traveling or standing wave. Therefore, this antenna is
more versatile in comparison of other antennas. Outstandingly, the RLSA can achieve more
than 30dBi of gain in the 5.8 GHz band [30]. The: low cost feature of RLSA has motivated
some researchers to expand its design for lower frequency applications, such as WLAN [26]
and Solar Power Satellite antenna [31]. The research is to design antenna using FR-4
substrate by chemical etching process for outdoor/indoor WLAN point to point microwave
link for Upper Unlicensed National Information Infrastructure (UNII) band at (5.725 5.875)GHz. Under the Malaysian Communications and Multimedia Commission (MCMC)
regulations, the maximum ElRP over the frequency band shall not exceed 1000mW or
30dBm [29].
1
Typically point to point communication system uses standard parabolic disk
antenna. However, the use of this type of antenna has some disadvantages. In a
primary fed design, there is a considerable aperture blockage. An offset design,
which eliminates the blockage, is on the other hand susceptible to physical damages
as its feed is significantly exposed from the body of reflector. Furthermore, in the
latter design, the alignment procedure is quite involved.
A more beneficial design is the linearly polarized Radial Line Slot Array
(RLSA) type antenna. Advantages of this antenna include its high radiation
efficiency, low profile due to it can be mounted at roof and wall, ease of installation,
feed rear-mounted, not subject to leaf and water build-up due to its flat surface.
1.1
Objective
The Project Objective was to develop a Linearly Polarized Radial Line Slot
Array (RLSA) Antenna by using photo etching process on FR-4 Substrate
Operating at 5.8 GHz.
1.2
Research Background
Point to point communication can be happened via small aperture antennas
with gain about 30-35dBi. Various types of appropriate antennas, including parabolic
reflectors, microstrip arrays and RLSA have been proposed. Parabolic reflectors are
the most widely used. However, a more compact, with low profile, robust, easy
2
installation and pleasing aesthetics antenna is desirable. Microstrip antennas possess
the above characteristics, but they are generally expensive and poor efficiency.
A good alternative solution could be a slot antenna array. Such an array is the
RLSA antenna which initially has been proposed by Ando M. [1]. Much research
into RLSA antenna has been performed in Japan, with a lot of success reported for
circularly polarized antennas. The RLSA antenna is expected to have a high
efficiency of more than 80% for the antenna gain of 30-35 dBi. However, the design
of a linearly polarized RLSA, which would be required for the reception of direct
broadcast satellite TV programs, applications for microwave point-to-point and
point- to-multipoint communication links in Malaysia is still not fully established.
Because of this, further investigation of linearly polarized RLSA antenna is required.
Based on earlier study done by Paul W. Davis [2], a procedure for the design of
linearly polarized RLSA antennas have been developed. This procedure will be
explained in the next sections.
1.3 Research Scope
This research works involve investigation and development of the linearly
polarised RLSA antenna as an extension of the research works conducted by
University of Queensland [3], [4], [5]. The design of RLSA antenna in this project is
a combination of theoretical and experimental approaches. Firstly, theoretical slot
surface design is developed by the slot surface design software. The resulting slot
surface parameters such as slot position and orientation are fed into the theoretical
radiation pattern modelling software developed using Matlab to evaluate the
expected radiation pattern. After the obtained theoretical radiation pattern result is
satisfied, the design software will output desired Computer Aided Manufacturing
(CAM) file format in order to develop the antenna prototypes.
3
The original slots surface design software of RLSA is developed using
Borland C++ 5.00 by University of Queensland that runs on Unix platform. This
RLSA slot surface software was slightly modified to enable it run successfully in
IBM compatible personal computer. In the slots surface design, some parameters in
the input data file have to be changed according to the frequency, relative
permittivity of dielectric material and dimension of RLSA antenna.
A coaxial-to-waveguide feeding structure has to be developed to convert the
travelling radial waveguide field mode present in the RLSA antenna’s radial cavity
into the coaxial field mode to travel along the coaxial feed line. The construction of
the feeding structure is required to match the impedance of the radial waveguide to
the coaxial feeding line.
In order to evaluate the performance of RLSA antenna in receive point to
point signal, two test-beds have been setup. The test-bed is utilizing the conventional
RLSA antenna provided developed by polypropylene and copper foil and another
one is setup by using the RLSA antenna prototypes. The performance of RLSA
antenna prototypes have been compared to conventional RLSA antenna in receiving
point to point signal.
1.4 Thesis Structure
This thesis is organized into six chapters to completely cover the whole research
works that have been conducted for the radial line slot array antenna project.
The second chapter discusses about the literature review.
The third chapter describes about the radiating slot design and return loss
improvement
Chapter four discusses about simulation and antenna prototype manufacturing
The chapter five describes about the experimental setup of the antenna.
The chapter six discuss about result analysis, discussion and future works
4
CHAPTER 2
LITERATURE REVIEW
2.1 History of Research of Radial Line Slot Array Antenna
This kind of antenna which is “Radial Line Slot Array” antenna is recommended
by Kelly at the end of 1950 [7]. Followed by it’s usage of this antenna is proposed at
early of 1960 [8], [9]. Further modification has been made and it becomes the reciprocal
antenna where it is able to receive and transmit the signal according to the polarization
design [9]. Concept of this RLSA antenna is consist a set of slot antennas arranged on
the upper face of a radial line and the rear conductive surface incorporates the coaxial
feeding element at the center of the antenna [8]. This concept or configuration of this
antenna has been used by Goto and Yamamoto at 1980 [10] with the new design which
is circularly polarized Radial Line Slot Array antenna in double-layered version. There
is a drawback for the double-layered version which is relatively complicated waveguide
structure and the cost fabrication is high after some researches have been done. One of
the researchers, Takahashi [11] has been recommending the single layered RLSA
antenna structure which has solved the problem at 1989 as shown in the Figure 2.1.
Single-layered structure has been found much suitable for the point-to-point
communications. Furthermore, this antenna can be mounting easily. As a result, this
structure of antenna has been spotted for the future research.
5
Figure 2.1: Structure Single-Layered RLSA Antenna recommended by Takahashi [6]
On the other hand, this antenna also has been used for Direct Broadcast Satellite (DBS)
application in Ku-Band frequency due to some researchers, Goto & Yamamoto has been
worked out in that [9]. At the 1995, Masaharu Takahashi has invented the circularly
polarized Line Slot Array Antenna which based on the structure recommended by Goto
& Yamamoto. The characteristic of this antenna has been summarized as below:-
Table 2.1: Specification of RLSA antenna recommended Masaharu [9]
Diameter
Gain
550mm
33.5 dBi
Efficiency
62%
Side-lobe
10 dB
Return-loss
20 dB
This has been encouraged another researcher; Paul Davis who has fabricated Linearly
Polarized Slot Array antenna. He has been using the idea from Takashi for fabrication
structure of antenna. Besides, reflection canceling slot technique has been applied in this
antenna.
6
Researchers Malaysia, Imran, M. I. and Tharek, A. R. has been continuing the research
for Outdoor Point to Point Application in Malaysia. They have been succeeded apply the
beam squint technique in the RLSA antenna which based on Paul Davis’s design. This
antenna characteristic is summarized as below:Table 2.2: Specification of RLSA Antenna recommended by Malaysia Researchers
Waveguide, radius (mm)
Waveguide, height (mm)
Number of slots, N
Slot length, (mm)
Slot width, (mm)
Probe length (mm)
Dielectric relative
permittivity, εr
300
8
264
17
1
7
2.33
This beam squint technique which has been applied in RLSA antenna has improved the
poor return losses performance.
Figure 2.2: Structure Layout Radial Line Slot Array Antenna [2]
7
Figure 2.2 shows the structure layout of radial line slot array antenna which
indicates the top view and cross view respectively. The antenna consists of two plates
spaced a distance d, where the upper plate has a radiating slot pattern and the rear part
has additional non-radiative slots. The radial cavity formed between these plates is filled
with a dielectric material which relative permittivity,
> 1.The purpose of dielectric is
used to create a slow wave structure with guided wavelength λg being smaller than the
free wavelength λo to avoid grating lobes in the radiation pattern. This orientation of
slots is such that to receive waves of proper polarization, linear in this case, are coupled
inside the cavity.
Generally, the operation of the antenna is called reciprocal theorem; can be
considered in either receive or transmit modes. For the transmit mode, energy fed to the
antenna via the coaxial symmetric wave inside the radial cavity. An area on the upper
guide surface of radius min ρ around the feeding mechanism is left devoid of slot allow
the radial cavity mode to stabilize and form an axially symmetric traveling wave. In
turn, this cavity mode is coupled into radiated free wave via the pattern on the upper
cavity surface. All residue waves which traveling in the outward direction will be
eliminated due to the perimeter of the antenna are left open.
2.1.1 Antenna Characteristics
Normally, antennas are reciprocal devices [16]; this is a property of
interchangeability of the same antenna for transmitting and receiving the same kind of
signal. Besides, the directional pattern also does not depend on transmit or receive mode
usage. Antenna reciprocity is possible because antenna characteristics are essentially the
same for sending and receiving electromagnetic energy. The most interest antenna
8
parameters include radiation pattern, return loss, gain, efficiency, polarization,
directivity, bandwidth, beamwidth [16].
2.1.1.1 Radiation Pattern
Radiation Pattern is a graphical representation of the radiation (far field)
properties of an antenna where the shape depends on directivity and gain of the antenna
[16]. The typical radiation pattern of antenna consists of main lobe and side lobes as
shown in Figure2.3.
Figure 2.3: Typical Radiation Pattern of a Microwave Antenna
Most of the antennas consist of a main lobe and a number of side lobes. The main lobe is
usually very narrow and directs the wanted power from the antenna, meanwhile the side
lobes are those lobes that are near to the main lobe and radiate the energy that is
9
regarded as “wasted”. The plot of the relative strength of radiated field as a function of
the angular parameters θ and φ, for a constant radius r .
It is necessary to define the antenna’s radiation characteristics by measure or calculate
the absolute amplitude, phase and polarization of the radiated field over the surface of a
sphere and to do this at every frequency of operation. In practice, relative amplitude and
phase is sampled at a number of points and the absolute level calibrated by a
measurement of power gain in the direction of the peak radiated field.
2.1.1.2 VWSR and Return Loss
VSWR in other words stands for voltage standing wave ratio [6]. In general, it is
a measure of impedance mismatch between the transmission line and its load. It is a ratio
of the maximum voltage to minimum voltage of the corresponding field components
appearing on a line that feeds an antenna, as shown in formula below:-
Where r is the reflection coefficient
The combination of the original wave traveling down towards the antenna and
the reflecting wave for antenna is called a standing wave. The ratio of the two above
described waves is known as the Standing Wave Ratio [6]. This phenomena occurred is
because of any part of chain from radio to antenna fails to show a same impedance due
to bad connections or incorrect antenna length. In order to decrease the reflected wave
from the antenna, we need to minimize VSWR as much as possible.
10
Return loss is basically the same thing as VSWR. This parameter is often
expressed as the ratio in decibels of the power incident on the antenna terminal to the
power reflected from the terminal at a particular frequency or band of frequencies.
Return loss can be expressed by formula:-
If 50% of the signal is absorbed by the antenna, and 50% is reflected back, we
say that the Return Loss is 3dB [6]. A very good antenna might have -10dB which
means that 90% of signal absorbed, and 10% is reflected.
2.1.1.3 Gain
The formal definition of gain in any direction is "Power density radiated in
direction divided by the power density which would have been radiated by a lossless
(perfect) isotropic radiator having the same total accepted input power. If the direction is
not specified, the value for gain is taken to mean the maximum value in any direction for
that particular antenna, and the direction along which the gain is maximum called the
antenna “boresight” [12]. For the power gain antenna is specified direction (φ, θ) is
defined as:-
11
Gain can be expressed in dBi or dBd, where dBi is the units’ decibels that referenced to
an isotropic antenna element; meanwhile dBd is the units’ decibels that referenced to a
dipole antenna element. The relationship between these two units is 0 dBd = 2.1 dBi and
the ideal reference point is defined to be 0 dBi. Nowadays, new antenna designs are
often refer to dBi common reference level because the isotropic antenna is theoretically
radiate equal power in all directions, resulting in a perfect spherical pattern [13].
2.1.1.4 Efficiency
During the process of transfer energy to and from space, some amount of energy
will dissipated as heat in the antenna. A 100% efficient antenna would theoretically
convert all input power into radiated power, with no loss to resistive or dielectric
elements. However, 75% of efficiency is already very good in real life, while 50%
efficiency antenna is acceptable. Efficiency (β) can be calculated by the below formula
[14],
Efficiency has no unit and the ideal figure is 1. Normally, the figure for efficiency will
dramatically decrease when the antenna is built into a device [14]. This will be a good
figure of merit especially for small antennas.
12
2.1.1.5 Polarization
Polarization is defined as the orientation of the electric field of an
electromagnetic wave. Basically there are three types which are linear polarization,
circular polarization and elliptical polarization [15]:-
Figure 2.4: Linear Polarization
Figure 2.4 illustrates the graphical which the electric vector remain the same plane
during the propagation. The electric field that is propagated parallel to the ground is said
to be horizontal polarization. Followed by electrical field that is propagated
perpendicular to the ground is said to be vertical polarization. Other specialized antennas
exist with Cross Polarization which means that it is having both vertical and horizontal
components.
Figure 2.5 illustrates the circular polarization which the plane of polarization has the
electric lines of force rotating through 360° with every cycle of radio frequency energy,
for example helix and crossed array of dipole fed in quadrate. It can either be righthanded or left-handed circular polarization depending on which way it is spinning [15].
13
Figure 2.5: Circular Polarization [15]
Followed by elliptical polarization which is a cross between linear (horizontal or
vertical) and circular polarization as shown in Figure 2.6. It can be either is right handed
or left-handed circular polarization depending on which way it is spinning. If the
rotation is clockwise direction to the direction of propagation, it categorized as righthanded, otherwise, it is referred to as left-handed [12].
Figure 2.6: Elliptical Polarization
14
2.1.1.6 Directivity
Directivity of an antenna is defined as “The ratio of the radiation intensity in
a given direction from the antenna, to the radiation intensity averaged over all
directions.” This average radiation intensity is equal to the total power of the antenna
divided by (4π). The Directivity (Dm) can be defined generally as the formula below
[15]:
Where Gm is the maximum gain
ηA is the antenna efficiency
Directivity is a measure of how focused an antenna can concentrates in a given direction.
A theoretical 100% efficient antenna referred to as an isotropic element, has 0 dBi
directive gain equally distributed in all 3 dimensions.
2.1.1.7 Bandwidth
The term bandwidth is used to describe the frequency range over which the
antenna will operate satisfactorily [16]. In order to perform well on one or more
frequencies bands over a range of different frequencies, we need to make it resonant in
the middle of each band. The percentage bandwidth can be defined as:
15
Where fU is the upper frequency
fL is the lower frequency
fC is the center frequency between fU and fL
2.1.1.8 Beamwidth
Beamwidth defined as the defectiveness of a directional antenna. It also means as
the angle where the transmitted power has dropped by 3 dB from the maximum power
on either side of the main lobe of radiation where the intensity falls off by half power as
shown in Figure 2.7.
Figure 2.7: Graphical of Beamwidth [6]
16
Beamwidth is measured in degrees, representing an angular measurement of how wide
the pattern is dispersed. Due to the RF radiation is in 3-dimension, it is usually measured
in two angular directions which is azimuth and elevation.
17
CHAPTER 3
SLOT DESIGN AND RETURN LOSS IMPROVEMENT
3.1
Introduction
This chapter begins by covering the RLSA antenna structure. This is follow by
operation principles of the radial cavity and some background theory on polarization. In
particular, the general condition to produce linear polarization was discussed. Following
that, the theoretical slot surface design was covered. The requirements for arraying the
unit radiators to produce a linearly polarized RLSA were then explained.
The most significant shortcoming of the standard linearly polarized RLSA
antenna was the poor return loss [24], [25], [26], [27] of the manufactured prototypes.
This poor return loss was due to the reflections that added in phase that come from the
slots of the linearly polarized RLSA antenna. Two methods proposed in this chapter for
overcoming this problem including the addition of reflection canceling slots and
squinting the beam away from the boresight direction. The first method is referred as the
reflection canceling slot design. The second method for improving the antenna return
18
loss is requires only modifying the distribution of primary radiating slots without
introducing extra slots. This method is referred as the beam squint design.
In addition, determination of RLSA antenna design parameters was covered. The RLSA
antenna slot surface design software was then explained. Finally, the resultant linearly
polarized RLSA slot surface was discussed.
3.2
The RLSA Antenna Structure
A radial line slot array antenna is a high gain antenna. It is a kind of slotted
waveguide array that is filled with dielectric material that suppresses the grating lobes.
The slots are arrayed so that their radiation is added in phase in the beam direction. The
structure of the investigated single-layer linearly polarised RLSA antenna is shown in
Figure 2.1. In this configuration, the antenna consists of two plates spaced a distance d
apart, the upper plate bearing a radiating slot pattern and the rear plate has additional
non radiative slots. The radial cavity formed between these plates is filled with a
dielectric material of relative permittivity εr>1. The purpose of this dielectric is to create
a slow wave structure with the guided wavelength λg being smaller than the free space
wavelength λ0 to avoid grating lobes in the radiation pattern. The orientation of slots is
such that to receive waves of proper polarisation, linear in this case, are coupled inside
the cavity.
The operation of the antenna can be considered in either receive or transmit
modes of operation. Both are equally valid due to the reciprocity theorem. In the
transmit mode of operation, energy fed to the antenna via the coaxial cable is launched
by the feeding mechanism into an outward travelling axially symmetric wave inside the
radial cavity. An area on the upper guide surface of radius ρmin around the feeding
19
mechanism is left devoid of slots to allow the radial cavity mode to stabilise and form an
axially symmetric travelling wave. In turn, this cavity mode is coupled into a radiated
free space wave via the pattern on the upper cavity surface.
Any residual wave
travelling in the outward direction is dissipated because the perimeter of the antenna is
left open.
Figure 3.1:
Single layer linearly polarised RLSA antenna structure [4]
20
3.2.1
Advantages of RLSA Antenna
Every antenna has its advanteges, so does RLSA antenna. The following are a
few advantages of the radial line slot array antenna.
a)
Robust
RLSA antenna is made of conductor and dielectric material. The
conductor material in this project is the copper shim and dielectric material is
polypropylene. Polypropylene is a hard material and this makes the body of
RLSA antenna become robust.
b)
Not subject to water and leaf build up
RLSA antenna is a flat surface antenna. As a result any water and leaf
will slide down from the antenna when they fall on the antenna. There will no
build up on antenna surface and will not affect the receiving efficiency of the
antenna.
c)
Easy to align with satellite
The antenna alignment is easier if suitable beam squint approach is
applied in antenna design. This is because the beam squint approach can squint
the antenna beam to suit the pointing angle of the antenna to the specified
satellite.
d)
Feed and downconverter rear mounted
21
Unlike the parabolic antenna, the fed and downconverter is mounted on
the rear side of the antenna. As a result, there is no blockage of the incoming
signal. This will improve the efficiency of the antenna. In addition, if feed and
downconverter are mounted on the rear side it is not easily susceptible to
damages.
3.3
Operation of the Radial Cavity
In order to analyze the operation of the radial cavity, it is necessary to understand
the nature of the electromagnetic waves propagating within it. This analysis will be
performed with reference to a single layered RLSA structure. The power flow [28], [29]
within the radial cavity for single layered RLSA antenna is shown in Figure 3.2.
Figure 3.2:
Power Flow within the Radial Cavity [5]
22
Due to the reciprocity theorem, the following analysis will be performed with reference
to the transmitting mode of operation of the antenna. In the transmit mode,
electromagnetic power is fed from a coaxial transmission line into the center of the
radial slow wave cavity by the disk ended feed probe. The feed probe is needed to
convert power from the (Transverse Electromagnetic) TEM transmission line mode into
a TEM cavity mode that traveling axially outward wave inside the radial cavity. A
region of radius is left devoid of slots, in order for the radial mode to stabilize within the
guide before encountering the discontinuities related to the presence of the slots.
The slot arrangement on the upper plate needs to be designed such that it couples
as much of the cavity energy into the radiated pencil beam as possible. Any energy not
radiated by the slot surface is lost. The design of RLSA antenna in this project, use the
open edges of the radial cavity, therefore the residual energy escapes through the open
edges of the radial cavity.
The slots must therefore be designed to intercept the currents on the upper
waveguide surface and produce radiation of the desired polarization. Thus, it is the form
of the magnetic field within the radial cavity to be investigated. For simplicity, the
height of the radial cavity, d has to be limited to be less than one half of the guide
wavelength. d <
λg
2
(2.1)
Where d is the height of the radial guide, and λg is the wavelength inside the
radial guide, here is referred to as the guide wavelength. Under these conditions, the
only possible symmetric waveguide mode that can propagate within the radial cavity is a
TEM mode.
23
3.4
Antenna Polarization
In general, the polarization of a wave can be defined [9] in terms of a wave
radiated (transmitted) or received by an antenna in a given direction. In more detail
definition, the polarization of a wave is the figure the instantaneous electric fields trace
out with time at a fixed observation point [10].
Polarization can be classified as linear, circular and elliptical. If the electric field
vector moves back and forth along a line, it is said to be linearly polarized as depicted in
Figure 3.3
linear polarizations [10]
To obtain the characteristics of a radiated wave that determine its resulting
polarization, the instantaneous field [9] of a plane wave traveling in free space in the ẑ
direction can be written as:
~
E ( z, t ) = E x (z , t ) xˆ + E y ( z, t ) yˆ
(2.2)
24
The instantaneous components can be written as:
E x = Ax cos(ωt − kz + ϕ x )
(2.3)
(
(2.4)
E y = A y cos ωt − kz + ϕ y
)
where Ax , A y are the maximum magnitudes of the x̂ and ŷ components respectively,
and ϕ x , ϕ y account for the fact that the two constituent components may be excited by
independent sources having different instantaneous phases, as is the case for the unit
radiator pair in an RLSA antenna.
3.5
General Conditions for Linearly Polarized Wave
For a wave to have linear polarisation, requires ϕ x and ϕ y of the field
components given in (2.3), (2.4) to satisfy the following condition [9]:
∆φ = ϕ y − ϕ x = nπ
n = 0,1,2,...
(2.5)
From equation (2.5), this means that two orthogonal linear components that are in time
phase or 1800 (or multiples of 1800) out of phase.
25
3.6
Theoretical Slot Surface Design
The theoretical slot design procedure for a single layered, linearly polarized Radial
Line Slot Array Antenna (LPRLSA) was developed with reference to the slot geometry
shown in Figure 2.4, which resides on the upper surface of a single layered cavity. The
desired polarization for the radiated wave is linear which is in direction of x-axis.
ρ1
θ1
#1
E1
E2
Desired
Polarization
θ2
#2
ρ2
#2m-1
#2m
Figure 3.4:
Slot Geometry for a Linearly Polarized Unit Radiator [2]
26
A unit radiator is defined as an adjacent slot pair #1, #2, as depicted in Figure 2.4
lying along the φ = constant direction. In order to achieve the requirements of utilising
this slot pair to produce a linearly polarised radiation, the following requirements [1], [5]
have to be enforced:
i. The co-polar components must combine in phase.
ii. The cross-polar components must cancel each other out. These requirements can be
expressed mathematically as follows:
Co-polarisation
Cross-polarisation
sin θ1 sin (θ1 + φ ) − sin θ 2 sin (θ 2 + φ ) = 1
(2.6)
− sin θ1 cos(θ1 + φ ) + sin θ 2 cos(θ 2 + φ ) = 0
(2.7)
The unit radiator can be placed at an arbitrary position on the radiating surface and
obtain the desired + x̂ linearly polarized radiation if the solutions for θ1 and θ 2 satisfy
equations (2.6) and (2.7) for all values of φ . This can be achieved by choosing:
θ1 =
π
2
−
θ2 =π −
φ
2
φ
2
(2.8)
(2.9)
The derivations above apply to an arbitrary slot pair in a given circular ring of
radius, ρ . Other unit radiators lying on that same ring will be located on a constant phase
front of the inner guide field, and hence will also be subject to the conditions present
throughout the entire derivation above.
27
3.7
Arraying the Linearly Polarised Unit Radiator
Using the equations (2.8) and (2.9), suitable oriented unit radiators comprising two
slots can be arrayed around a given ring of radius ρ . The resultant radiated fields will
combine to produce linearly polarized radiation in the + x̂ direction. The angular spacing
S φ of the unit radiators placed on a ring of fixed radius can be chosen arbitrarily.
Now the requirements to obtain a ring of unit radiators to achieve linear broadside
polarization have been identified. The next step is to establish the arraying of these rings to
achieve a more directive pattern. In order to obtain broadside radiation, the successively
arrayed slot rings must satisfy the 0° phase shift requirement. This is achieved by a radial
spacing [1] between successive unit radiators in the radial direction of one guide
wavelength, λ g . This requirement can be expressed as:
for slot 2m-1
ρ odd = ρ1 ± nλ g
(2.10)
for slot 2m
ρ even = ρ 2 ± nλ g
(2.11)
where n and m are integers, m being the order of the unit radiator within the nth
ring. m will assume values starting from 1, up to the number of unit cells in the current
ring, and n shall assume values starting from 0, with n=0 represents the innermost ring,
of radius ρ min . ρ1 and ρ 2 are the radial distance to slot #1 and slot #2, which are
taken here to be the first unit radiator in the first ring (m=1, n=0). As can be seen from
(2.10), (2.11), the radial spacing, S ρ , between adjacent slot pairs will therefore be equal
to λ g .
The dielectric material is needed to form the radial cavity for the RLSA antenna.
With the rings of unit radiators being arrayed with a spacing of S ρ = λ g , the guided
wavelength, if there were no dielectric material loading the radial cavity, the situation
28
will become λg = λ0, and the radiation pattern would be plagued with intolerable grating
lobes, drawing power from the main beam and hence reducing overall antenna gain.
3.7.1
Choosing the Azimuthal Element Spacing
The density of energy radiated from a given area on the radiating surface of the
RLSA antenna will be proportional to the product S ρ ⋅ S φ . The product would be kept
constant, and aperture illumination control to counter the effect of a radially tapering
cavity field strength would be handled by varying the radiating slot length on the
antenna surface. Under this condition, since S ρ is itself a constant for the LPRLSA,
the Sφ also to be a constant. There are three restrictions that must be placed on each of
i)
the number of unit radiators in the innermost ring
ii)
ρ min
iii)
Sφ
in order to obtain a physically realizable boresight LPRLSA antenna.
The difficulty comes from trying to fit an integer number of unit radiators,
evenly spaced around a ring, with the same azimuthal spacing in every ring on the
antenna radiating surface. If the inner radius is taken to be ρ min , successive radii will be
given utilizing (2.10) and (2.11) as ρ min + λ g , ρ min + 2λ g , …, ρ min + nλ g . For these radii,
the
(
corresponding
2π ρ min + nλ g
circumferences
will
be
(
)
2π ρ min + λ g ,
(
)
2π ρ min + 2λ g ,
…,
) . Hence, to ensure an integer number of unit radiators are placed along a
ring, the value of Sφ need to be chosen as:
29
(
2π ρ min + n λ g
)
Sφ
= a whole number greater than 1
(2.12)
For (2.12) to hold, the following relationship is written:
(2.13)
ρ min = m λ g
where m is an integer greater than or equal to one. This will allow (2.12) to be rewritten
as:
2π λ g (m + n )
Sφ
(2.14)
=p
where p is a whole number greater than or equal to two. Rearranging (2.14) for any
arbitrary ring on the radiating surface can be express the condition:
Sφ =
(m + n ) ⋅ 2π λ
p
g
(2.15)
For the case of the innermost ring, which is obtained by setting n = 0 in (2.15)
Sφ =
m
⋅ 2π λ g
po
(2.16)
where p o is the number of unit radiators in the innermost ring. Since the value of Sφ has
to be the same for every ring (all values of n ), the equation (2.15) is equates to (2.16) to
obtain:
(m + n ) ⋅ 2π λ
p
g
=
m
⋅ 2π λ g
p0
(2.17)
which with simple rearrangement gives:
30
n⎞
⎛
p = p o ⎜1 + ⎟
⎝ m⎠
(2.18)
Recalling that our initial stipulation was that p must be an integer to obtain a
whole number of unit radiators within a ring, equation (2.18) can only be satisfied for all
values of n by choosing p o to be divisible by m . Put into words, the preceding
derivation indicates that in order to obtain a uniform value for S φ over the entire
boresight LPRLSA antenna radiating surface, the radius of the innermost ring, ρ min
need to be chosen as an integer number of guide wavelengths, λ g . Additionally, the
number of unit radiators placed in this innermost ring must be divisible by the number of
wavelengths in ρ min .
3.8
Reflection Canceling slot Design
The first method to improve return loss of linearly polarized RLSA antenna is by
introducing additional reflection canceling slot. As the sole purpose of the additional
reflection canceling slots is to produce reflections, therefore it is necessary the
introduced reflection canceling slots produce suitable phased reflections without having
any net radiation effect.
Figure 3.5 shows the upgraded unit radiator that incorporates the reflection
canceling slots. As can be seen, the unit radiator now consists of the radiation slots, #1,
#2 which are arranged so as to satisfy the linear polarization condition. The additional
slots, #3, #4, are the reflection canceling slots, are positioned to satisfy the two
requirements of producing canceling reflections and no net radiation.
31
From Figure 3.5, in order to ensure that the reflections produced by the
additional slots are in antiphase to the reflections from the corresponding radiating slots,
consider the additional distance traveled by the wave from the reflection canceling slot
#3 compared to the radiating slot #1, the reflection canceling slot #3 will travel an
additional distance of 2 × d 2 when compared to the reflection canceling slot #1. This
additional travel equates to a phase difference, ∆β :
∆β = 2d 2 k g
(2.19)
where k g = 2π / λ g is the wave number in the radial cavity. For the reflected waves to
destructively interfere, a phase difference of 1800 or π is required, this will result in:
π = 2d 2
2π
λg
(2.20)
rearrange lead to:
d2 =
λg
4
(2.21)
As the radiation slots, #1, #2 are spaced by one half guide wavelength, we see that we
require the distance d1 to satisfy the requirement:
d1 =
λg
2
(2.22)
Due to this chosen spacing, these additional reflections will combine in antiphase with
those from the radiation slots, producing reflection cancellation at the feed point.
To satisfy the requirement that the additional slots #3, #4 have no net radiation
effect, they must be placed perpendicular to the current flow line at all points on the
antenna surface. This requirement ensures that as parallel slots fed in alternating phase,
32
their combined net radiation will cancel out each other. This requirement can be
expressed in terms of the parameters in Figure 2.5 as:
θ r3 = θ r 4 =
π
(2.23)
2
The standard linear polarized RLSA design leads to radially adjacent slots being
spaced a distance λg/2 apart. From equation (2.23), the reflection canceling slots are
placed on the front (radiating) surface of antenna and are spaced a radial distance of λg/4
from the radiating slots. In order to avoid slots overlap, they are offset [2], [4] from the
radiating cell centerline in the direction by an amount given by
r3 =
Sφ
2
r4 = −
−
Sφ
2
λg
φ⎞
⎛
⋅ tan⎜ π − ⎟
2⎠
4 2
⎝
+
λg
φ⎞
⎛
⋅ tan⎜ π − ⎟
4
2⎠
⎝
−
−
π
2
π
2
≤φ <
≤φ <
π
2
π
2
(2.24)
(2.25)
The reflection canceling slots radial distant r3 and r4 are shown in Figure 3.5. S φ
represents the azimuthal interslot spacing and φ represents the azimuthal position on the
radiating surface of the unit radiator being modified. In the new method, these additional
slots are placed on rear (non-radiating) surface of the antenna. In this case, there is no
need to offset them in the azimuthal ( φ ) direction of equations (2.26) and (2.27) as there
is no fear of overlap with primary (radiating) slots.
33
d1
d2
Figure 3.5: The positioning of additional reflection canceling slots [5]
3.9
Beam Squinted Design
Although the method of placing the reflection canceling slots on the antenna’s rear
surface avoids the problem of slot overlap, it still requires generating extra slots and this
adds to the manufacturing cost of the antenna. An alternative method to improve the return
loss in a standard LPRLSA is use a beam squinting technique. This method is explained
using the geometry shown in Figure 2.6. Assuming that the desired squint angle is θT, φT
in the respective planes shown in Figure 2.6, the analysis for co-phased superposition of
slot radiation contributions at the observation point result in the following expressions [3],
[4] for new slot inclination angles θ1, θ2:
34
θ1 =
θ2 =
π
4
+
⎛ cos θ T
1 ⎧⎪
⎨arctan⎜⎜
2 ⎪⎩
⎝ tan φT
⎫⎪
⎞
⎟⎟ − (φ − φT )⎬
⎪⎭
⎠
⎛ cos θ T
3π 1 ⎪⎧
+ ⎨arctan⎜⎜
4 2 ⎪⎩
⎝ tan φT
(2.26)
⎫⎪
⎞
⎟⎟ − (φ − φT )⎬
⎪⎭
⎠
Figure 3.6
(2.27)
The beam squint geometry [5]
35
In addition to the change in slot inclinations, one also needs to modify the radial spacing
between consecutive slot pairs, Sρ as given by following expression:
Sρ =
λg
(2.28)
1 − ξ sin θT cos(φ − φT )
where,
ξ= 1
(2.29)
εr
The return loss and VSWR measurements of all the 18 prototypes developed
were carried out by another research officer in the research team. Appendix A shows
detailed results for the return loss and VSWR of 18 prototypes measured using
microwave network analyzer. The return loss and VSWR improvement from prototype
#1 to prototype #18 could be notice as shown in Appendix A. It can be seen that the
return loss is reasonable for all prototypes and falls into the 10 dB range.
3.10
Determination of RLSA Antenna Design Parameters
Since the RLSA antenna is going to be commercialized in the Malaysia market in
future, it is necessary to determine the design parameters of the antenna according to the
available resources. A survey was done to determine the type of materials that is
available in the Malaysian market. According to the availability and cost single sided
FR-4 has been selected which has a dielectric constant 5.4
The frequency range that should be covered by the antenna should be between
5.725-5.875 GHz. As a result, the center frequency for the design antenna is:
fc =
5.725 + 5.875
= 5.8GHz
2
(2.30)
36
3.11
RLSA Antenna Slot Surface Design Software
For the purpose of producing prototypes, a slot surface design software has been
developed [30], [29] to implement the required slot location and orientation calculations
that making up the radiating surface of the RLSA antenna. The slot surface design
software is the practical implementation of the theoretical concept presented, in the form
of Computer Aided Manufacturing (CAM) format for slot surface layout development.
When provided with specific design information such as desired frequency of operation,
diameter of antenna and the dielectric permittivity of the cavity material, this software
produces an output data file containing the positional information for every slot required
to be on the radiating surface.
The original slot surface design software [30], [29] was developed in Unix
operating system. Therefore, some modifications have to be made to the original source
code to enable it to run in Windows 98 operating system. The slots surface design source
code was modified was found run successfully under Borland C++ 5.02 in Microsoft
Windows 98 platform. The RLSA source code is used to generate the slots pattern of the
radiating and canceling slots as well as beam squinted design. The design software
descriptions are shown in Appendix A.
3.12
The Resultant Linearly Polarized RLSA Slot Surface
The parameters of interest described in the preceding sections are accurately
graphically depicted in Figure 2.7. Also defined in Figure 2.7 for convenience of
reference are the parameters Sρ, which represents the ring separation in the radial
37
direction, and Sφ, which represents the unit radiator separation in the azimuthal direction
within a given ring.
Figure 3.7 Linearly Polarized RLSA Slot Pattern and Associated Parameters [5]
From equations (2.10) and (2.11), the theoretical Linearly Polarised RLSA slot pattern,
S ρ and S φ can be defined by:
S ρ = λg
(2.31)
S φ = chosen arbitrarily
(2.32)
38
These two parameters S ρ and Sφ will determine the ring separation and azimuthally
direction in the slot surface design for production of RLSA antennas.
39
CHAPTER 4
SIMULATION AND ANTENNA PROTOTYPE MANUFACTURING
4.1
Introduction
This chapter discusses about the simulation result that has been done prior to
antenna fabrication and antenna manufacturing procedure. RLSA antenna has very
attractive features due to its mass production opportunities i.e. possibility to employ the
printed-circuit technology like one for microstrip patch antennas and arrays. The real
behavior of the antenna prototype may cause differentiation in the measurement analysis
due to many favors such as the fabrication errors and neglected parameters. The
development stage of the RLSA antenna prototype and prototyping technique along with
the simulation result has been discussed.
40
4.2
Simulation of radiation pattern and return loss
A radial line slot array antenna is a high gain antenna. It is a kind of slotted
waveguide array that is filled with dielectric material that suppresses the grating lobes.
The slots are arrayed so that their radiation is added in phase in the beam direction. For
the radiation pattern simulation, a software has been used which was developed using
Borland C++ 5.00 by University of Queensland. To achieve the desired radiation pattern
in the slots surface design, some parameters in the input data file have to be changed
according to our desired frequency, relative permittivity of dielectric material and
dimension of RLSA antenna. The resulting slot surface parameters such as slot position
and orientation are fed into the theoretical radiation pattern modeling software
developed using Matlab to evaluate the expected radiation pattern. After the obtained
theoretical radiation pattern result is satisfied, the design software will output desired
Computer Aided Manufacturing (CAM) file format in order to develop the antenna
prototypes
Fig 4.1: Radiation pattern (Simulation)
41
Frequency (GHz)
-10.00
5
5.05 5.1 5.15 5.2 5.25 5.3 5.35 5.4 5.45 5.5 5.55 5.6 5.65 5.7 5.75 5.8 5.85 5.9 5.95
6
(dB)
-10.50
-11.00
-11.50
-12.00
Return Loss Graph
Fig 4.2: Return loss (Simulation)
4.3
Define proper material for antenna to be fabricated
Since the RLSA antenna is going to be commercialized in the Malaysia market in
future, it is necessary to determine the design parameters of the antenna according to the
available resources. A survey was done to determine the type of materials that is
available in the Malaysian market. According to the availability and cost single sided
FR-4 has been selected which has a dielectric constant 5.4. FR-4 is widely used for
antenna manufacturing and FR-4 can be etched very easily. Conventional RLSA antenna
use very thin copper foil on the top and another copper plate at the rear which acts as a
ground plane. A dielectric material is used which is sandwiches with upper plate and
rear plate. The major problem of this technology is when we etch the prototype we need
to immerse the prototype into the etching solution and as the copper foil is attached to
the dielectric by double sided tape the etching solution may seep beneath the copper foil
which may cause the copper spoil. But the FR-4 has one side copper so there is no
chance of spoil and we can etch that without any difficulty.
42
Table 4.1: parameters of FR-4
Dielectric constant
5.4
Thickness
1.6 mm
Size (height)
13.5 cm
Size (width)
13.5 cm
Layer used
3
Density
1.91 kg/L
Dissipation Factor (Loss tangent)
0.02 @1 MHz, 0.01 @ 1 GHz
Copper foil thickness
18-35-70 µ
Some advantages of FR-4 are given below:
Unmodified epoxy resin, flame resistant, with good thermal and mechanical
properties for rigid laminates, thin laminates and prepregs for multilayer boards, unclad
laminates for electro-mechanical applications.
4.4
Technological Aspects of Antenna Prototype Manufacturing
The upper slot-consisted plate of radial line can be fabricated by using mask
produced by the CAD software in accordance with final antenna features required. The
mask of slots’ topology has been accomplished by computer laser jet printing on
transparent film under control of special component of the CAD software. Later, the
slotted antenna aperture can be fabricated by employment of common chemical etching
technology with negative mask setting etc. The prototyping phase is divided into two
major structures, which are the RLSA slotted radial waveguide and the feed probe. The
slotted waveguide consist of three separate parts, which are then combined together.
43
Here 3 layers of single sided FR-4 have been used. Later on these three parts is attached
together with double sided tape. Note that the using of double sided tape is neglected in
simulation analysis. Each layer on FR-4 is 1.6 mm thickness.
Table 4.2: Antenna design parameters
Design Frequency
5.8 GHz
Dielectric constant (fr4)
5.4
Height of radial cavity
4.86mm
Number of slot pairs in the inner
16
ring
Radiating slot length
15mm
Radiating slot width
1mm
Radiating slot counts
94
4.5 Etching process
Etching is a process of removing the unwanted material by chemical reaction. This
technology was first commercially used during World War Two for producing gun sight
reticules.
44
4.6 Advantages of etching
ƒ
SUPERIOR FOR PROTOTYPES: Excellent precision, repeatability, and
accuracy, quick turn around. Engineering changes and modifications can be
made quickly, easily, and inexpensively
•
LOW COST: Compared to hard tooled parts, the chem-etch photo tool is more
efficient and far less expensive than typical hard tooling. Unlike hard tooling,
complexity of a part is not a cost driver.
•
NO HARD TOOLING: no major investments no die maintenance and repair
costs, no long tooling delivery delay. Photo tool lead time is normally 1-2 weeks
and can be shortened if necessary. Photo tool charges are substantially lower than
conventional die shop charges.
•
NO METAL STRESS OR PART DEFORMATION: Chem-etched parts remain
flat since the metal removal is achieved chemically, not mechanically
•
BURR FREE: Secondary debarring operations are eliminated since no metal to
metal contact stress occurs.
•
MATERIAL PROPERTIES: Remain unchanged. Temper of metal is not
changed. Magnetic property is not affected.
4.7 Procedure Etching Process
ƒ
Generate artwork on transparent sheet from CAD/CAM system.
ƒ
Clean and pre-treat the material.
45
ƒ
Apply 1.5 mil dry PHOTO RESIST (photographically resistant) Mylar layered
film to the metal sheet to be etched.
ƒ
Expose the film via UV (Ultraviolet) light to transfer the piece part
photographically to the previously treated sheet of metal.
ƒ
Develop the metal sheet thereby exposing the piece part image.
ƒ
Chemically remove the unprotected material areas by etching.
ƒ
Rinse and strip the photographically resistant film from the sheet.
ƒ
Proceed to next steps of manufacturing operations, forming, dip brazing,
hardware insertion, silk screening, etc.
46
CHAPTER 5
MEASUREMENT, RESULT AND ANALYSIS
5.1 Introduction
This chapter describes about the step-by-step procedure which will be implemented
for measuring and analyzing the characteristics of RLSA antenna. The measurement will
include the return-loss, radiation pattern and gain of antenna. In order to test the antenna
performance a standard test bed has been designed.
5.2 Availability Equipments or Instruments
Currently, there are quite numerous instruments which are use for the same purpose
but may be with different feature which is more user-friendly to the user. The devices
(available at Wireless Communications Centre) are needed to perform the measurement
is summarized in the Table 5.1 as below:-
47
Table 5.1: Type of measurement with respective device
Type of Measurement
Radiation Pattern
&
Gain
Return-loss
&
VSWR
Device or Facilities
1. Anechoic Chamber Room
Dimension: (8.04m
(L)X5.04m(W)X5.04m(H))
Dimension (Internal within Absorber):
(6.7mX3.4mX3.4m)
2. Device which is available
- “turntable” with adjustable movement
(computer controlled)
- Agilent Network Analyzer 8722ES
(50MHz~40GHz)
- position table controller (NSI Part No.
11-00025)
- transmitter (NSI-RF-WR90)
8.4GHz~12.4GHz
- computer (Antenna Measurement
Software, NSI developed)
3. Antenna under test (AUT)
- RLSA antenna
- Horn antenna (Model Dorado
1GHz~18GHz)
1. Device which is available
- Marconi 6204 Microwave test set
- Calibration kit (WILTRON MODEL
22S50)
2. Antenna under test (AUT)
- RLSA antenna
48
5.3 Procedure Return Loss
An experiment has been setup to measure the return-loss and VSWR of RLSA
antenna which is illustrated by Figure 5.1.
Figure 5.1: Return-loss and VSWR measurement experiment
As shown in the Figure 5.1, the antenna under test needed to connect to the
Marconi instrument transmission line test head which is indicates ‘Return-loss’ terminal.
In order to set the range of frequency, identify the operation of frequency for the antenna
which is going to be measured and makes sure that the operation frequency is within the
range of the frequency which has been set. To set the start frequency, push the ‘start’
button and key in the desired frequency by the keypad and followed by the ‘stop’ button
as well. Besides, this device also enable user to key in the center frequency which is
operation frequency of the antenna by push the ‘center’ button. The range of the
frequency has been set automatically if the ‘span’ button has been set. The next step is
self calibration of the device when the start and stop frequency has been set. In order to
49
calibrate the device, the calibration kit (WILTRON MODEL 22S50) which is indicates
short and open at both ends (Figure5.3) is needed
Figure 5.2: Marconi instrument transmission line test head
Figure 5.3: Calibration kit (WILTRON MODEL 22S50)
The correct setting should be done by follow the instruction on the screen and connect
the calibration kit for shot or open which is instructed by the Marconi Instrument. After
the calibration, the calibration kit can be replaced by the antenna under test for return-
50
loss measurement. The data prompted is recorded and using certain program to
transform to readable data.
5.3.1
Return loss result and analysis
The following graph shows the measured return loss
-2
-4
Return Loss(dB)
-6
-8
-10
-12
-14
-16
-18
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
Frequency (GHz)
Col 1 vs Col 2
Figure5.4: Measured return loss
From the Graph 5.1, it can be concluded that the return loss for this antenna is roughly
between -16 to -17db at the 5.8 GHz (Which means that 98% signal transmitted and 2%
signal reflected back). It can be said that the antenna posts a good return-loss in 5.8 GHz
so it is expected to be receive and transmit point to point signal.
51
Frequency (GHz)
-10.00
5
5.05 5.1 5.15 5.2 5.25 5.3 5.35 5.4 5.45 5.5 5.55 5.6 5.65 5.7 5.75 5.8 5.85 5.9 5.95
6
(dB)
-10.50
-11.00
-11.50
-12.00
Return Loss Graph
Figure 5.5: Return loss (simulation)
From the above two graphs it is clear that the measured return-loss shows a good return
loss compared to simulation result. In simulation the return loss at 5.8 GHZ was about 11 to -12 db whereas in measured return loss it is about -16 to -17 at the same frequency.
5.4
Radiation Pattern Measurement
An antenna radiation pattern is a graphical representation of the field magnitude
at the fixed distance from the antenna as a function of direction. It starts from the
antenna at the origin of a spherical coordinate system, radiation field E and H are
Perpendicular to each other and both are transverse to the direction of propagation rˆ. On
the other hand, the field intensities vary as 1 − r . Only the electric field will be
discussed but magnetic field behavior follows directly since its intensity is proportional
to the electric field and its direction is perpendicular to E and rˆ [23]. Basically, in order
to obtain the radiation pattern measurement results, the vital feature is to maintain the
constant large distance between the antennas and to vary the observation angle. This is
accomplished by rotating the test antenna (antenna under test, AUT) which is illustrated
by Figure 3.5. The fields from the motionless source antenna provide a constant
illumination of the test antenna whose output varies with its angular position. This leads
to the rule that the pattern of the rotated antenna that is being measured.
52
Figure 5.6: Radiation pattern measurement
There are many ways of displaying antenna patterns. As such, a principle plane pattern
could be plotted in polar or rectangular form. In addition, the scale could be either linear
or logarithmic (in dB). Here polar-linear type pattern has been used.
The facility used to measure antenna radiation characteristic is referred to as an antenna
range. Generally, the entire measurement facility consists of the space (indoor or
outdoor) for the source and test antennas, antenna petitioners, a transmitter a receiving
system and data display and recording equipment. The most suitable antenna ranges for
this project is depends the availability facilities in UTM which the Wireless
Communications Centre (WCC, P15) offers an Anechoic Chamber for this
measurement. The anechoic chamber is composed of an RFI Modular Shield Room with
RFI high performance foam Absorber applied to walls ceiling and floor. The chamber
dimensions are, external 8.04m long X 5.04m wide X 5.04m high, internal within the
absorber 6.7m X 3.4m X 3.4m. Followed by that, the design antenna distance is 5m and
the quiet zone is a minimum of 1m diameter. There is one ace door 1.2m wide X 2.0m
high for the assessment inside the chamber. The suitability for the antenna under test
(AUT) is from 300MHz ~ 40GHz with the extended option to 60GHz. Figure 5.6
illustrates the block diagram of near field measurement system in anechoic chamber.
This system is fully automatic during the measurement.
53
Figure 5.7: Near field measurement system block diagram in anechoic chamber
5.4.1
Radiation pattern result and analysis
The figure shows the measured radiation pattern.
From the graph of the measured E-plane radiation pattern we may say that the received
signal is -36 dBm while transmitting power was -25 dBm that means some signal has
been dropped. This is due to multipath fading and some other losses (medium loss, free
space loss). The area of the main lobe is between -30deg to +30 deg which exhibit a very
close agreement between simulated pattern and measured pattern
54
Polar Plot
0
-25
330
30
-30
-35
-40
300
60
-45
-50
-55
-60
270
90
-65
-25 -30 -35 -40 -45 -50 -55 -60 -65 -60 -55 -50 -45 -40 -35 -30 -25
-60
-55
-50
-45
240
120
-40
-35
-30
210
150
-25
180
E_Co vs degree
Col 3 vs degree
Figure 5.8: Measured radiation pattern (E-co) in the anechoic chamber
Figure 5.9: Radiation pattern (simulation)
55
5.5
Test-Bed
A test-bed has been prepared to compare the antenna performance with
commercially developed antenna prototype. Also the antenna has been compared with
conventional RLSA antenna which was developed by using polypropylene as a dielectric
sandwiched between two copper foil. Both two antennas are of the same size. For the
test-bed setup we used two wireless router (Mikrotik) connected in point to point at 5.8
GHz. The main objective of the test-bed was to measure the RSSI strength (received
signal strength) and compare among the signal strength for different types of antenna.
Figure 5.10: A partial view of the test-bed
56
5.5.1
Test-bed results and analysis
Test-bed results are shown in terms of figure
Figure 5.11: Signal strength using built-in wireless router
Figure 5.12: Signal strength using conventional RLSA antenna
57
Figure 5.13: Signal strength using developed prototype
The figure-5.11 shows the received signal level of built in wireless router
antenna which is about -76 dBm. Figure 5.12 refer the signal strength of conventional
RLSA antenna which is -56 dBm and figure- 5.13 shows the signal strength of the
developed prototype using FR-4 which is about -46 dBm. After evaluating these figures
it can be concluded that the developed prototype can receive more signal power compare
to conventional RLSA and commercial built in antenna.
58
CHAPTER 6
CONCLUSION AND FUTURE WORKS
6.1
Conclusion
This chapter draws the conclusion from the research works done. It also suggests
the future works, which can be done to further improve the project
In this project a linear-polarized RLSA antenna has been studied theoretically and
fabricated it using FR-4 by etching process for point to point application. The main
purpose of this project was to find out a suitable locally available material for RLSA
antenna which is very convenient to fabricate by etching process. To meet the purpose
the mostly available and widely used material FR-4 has been chosen and been shown that
a better result can be achieved by using this. The return loss of the developed prototype
shows a better result than simulation. The radiation pattern showed a close agreement
between the measured and simulation radiation pattern at main lobe. However there is
some signal receive power drop in the measured radiation pattern. To make it usable for
outdoor a waterproof casing need to be fabricated. The characteristics of the RLSA
antenna have been measured at WCC, Universiti Teknologi Malaysia. Now it can be
installed for outdoor WLAN point to point application.
59
Two test-beds have been setup using conventional RLSA antenna and
commercially developed wireless router antenna to evaluate the performance of
developed RLSA antenna in receiving point to pint signal. The test-bed results validate
the potential for developed RLSA antennas to be used in reception of point to point
signal.
6.2
Future works
The RLSA antenna can be designed for indoor wireless LAN application which
required wider beamwidth of antenna radiation pattern. This can be achieved by using
beam squint design approach by varying the squint angle of every ring. However, further
investigation must be done including the determination of the appropriate squint angle for
each ring using the developed radiation pattern modeling software.
For the radiation pattern modeling software, enhancement could be done with the
addition of the Graphical User Interface (GUI) to make the software easy to use.
Furthermore, the radiation pattern modeling software developed using Matlab needs to
convert to stand-alone application by using Matlab and Microsoft Visual C++ compilers.
60
REFERENCES
[1] Ando M., Numata T., Takada J., Goto N. (1988) “ A linearly polarized radial
line slot antenna.” IEEE Transactions on antennas and propagation, Vol.
36. No. 12. 1675-1680
[2] LIM TIEN SZE, “Linearly Polarised Radial Line Slot Array Antenna
Radiation Pattern Modelling & Test-Bed Development for Direct Broadcast
Satellite,” Universiti Teknologi Malaysia.
[3] Mazlan Othman, “Space Application Development in Malaysia,” National
Space Agency in Malaysia, 14 January 2004.
[4] “TV Reception Solution,” Stallions Satellite and Antenna,
http://www.tvantenna.com/support/tutorials/lnbs.html
[6] David M. Pozar, “Microwave Engineering, Second Edition,” John Wiley &
Sons, Inc. 1998.
[7] K.C. Kelly (March 1957). “Recent Annular Slot Array Experiments.” IRE
National convention Record. Vol 5, Part 1, 144-151.
[8] F.J. Goebles, Jr. K.C. (July 1961), “Arbitrary Polarization from Annular
Planar Antenna.” IRE Trans. On Antennas and Propagation, Vol. AP-9, 342349.94
61
[9] K.C. Kelly, F.J. Goebles, JR (July 1964), “Annular Slot Monopulse Antenna
Array.” IEEE Trans, Antennas and Propagation, Vol AP-12, 391-403.
[10] N. Goto, M. Yamamoto (August 1980), “Circularly Polarized Radial Line
Slot Antennas.” IEEE Technical Report, AP 89-54, 43.
[11] M. Takahashi, J. Takada, M. Ando, N. Goto (October 1989), “A SingleLayered Radial Slot Line Slot Antenna.” IEEE Technical Report, AP 89-54.
[12] D.Jefferies, “Antenna”
http://www.ee.surrey.ac.uk/Personal/D.Jeffries/antenna.html (current August
[13] SkyCross, “Antenna Terminology”
http://www.skycross.com (current August 2003)
[14] Moteco Group Website, “Antenna Basics”
http://www.moteco.com (current July 2003)
[15] Dennis Roddy, “Microwave Tehcnology,” Prentice-Hall, 1986, pp. 405-414.
[16] Allan W.Scott, “Understanding Mircowave,” John Wiley & Sons, Inc, 1993.
[17] Bruce R. Elbert, “The Satellite Communication Applications Handbook,”
ARTECH HOUSE, INC, 1997.
[18] M. RICHHARIA, “SATELLITE COMMUNICATIONS SYSTEMS DESIGN
PRINCIPLES,” THE MACMILLAN PRESS LTD, 1st published 1995.
[19] G. MARAL, M. BOUSQUET, “Satellite Communications Systems,” JOHN
WILEY & SONS, Second Edition 1993.
62
[20] James Wood, “SATELLITE COMMUNICATIONS POCKET BOOK,”
OXFORD BOSTON JOHANNESBURG, Revised edition 1996.95
[21] B. G. Evans, “SATELLITE COMMUNICATION SYSTEMS,” The Institution
of Electrical Engineers, London, United Kingdom, 3rd edition, 2000.
[22] Stephenson D. J. “Newnes Guide to Satellite TV: Installation, Reception and
Repair,” Butterworth-Heinemann LTd., Oxford, Great Britain, 1994.
[23] WARREN L. STUTZMAN, GRARY A. THIELE “ANTENNA THOERY
AND DESIGN,”JOHN WILEY & SONS, INC, Second edition, 1998.
[24] Davis, P.W. and Bialkowski, M.E. (1997). ” Improving return loss in RLSA
antennas using beam squinting.” Proceedings of the IWTS’97, Kuala Lumpur.
[25] Takada, J., Ando M. and Goto N. (1992). “A reflection cancelling slot set in a
linearly polarized radial line slot antenna.” IEEE Transactions on Antennas and
Propagation. Vol. 40. No. 4, 433-438.
[26]
Davis, P.W. and Bialkowski, M.E. (1997). “Comparing Beam Squinting and
Reflection Cancelling Slot Methods for Return Loss Improvement in RLSA
Antennas.” Proceedings of The IEEE Antennas And Propagation Symposium
(IEEE AP-S), 1938-1941, Montreal, Canada.
[27] Takada, J., Ando M. and Goto N. (1989). “A Beam-Tilted Linearly Polarized
Radial Line Slot Antenna.” Electronics and Communications in Japan, Part 1,
Vol. 72, No. 11, 27-34.
63
[28] A.I.Zagghoul, R.K. Gupta, E.C. Kohls, L.Q. Sun & R.M. Allnutt, “Low Cost
Flat Antenna for Commercial & Military SATCOM Terminals,” Lockheed
Martin Global Telecommunications (LMGT) Systems & Technology
Clarksburg, MD.
[29] Davis, P.W. (2000). “A linearly polarized radial line slot antenna for direct
broadcast satellite services.” University of Queensland, Australia, Ph.D. Thesis.
[30] Davis, P.W. and Bialkowski, M.E. (1997), ” Experimental investigations into
a linearly polarized radial slot antenna for DBS TV in Australia.” IEEE
Transactions on Antennas and Propagation, Vol. 45. No. 7. 1123-1129.
[31]
Davis, P.W. and Bialkowski, M.E. (1999).” Linearly Polarized Radial Line
Slot Array Antennas with Improved Return Loss Performance”, IEEE Antennas
and Propagation Magazine, Vol. 41. No. 1. 52-61.
64
APPENDIX A
The Design Software Description
The RLSA antenna software is divided into a number of executable programs,
each of which is for specific task in the design phase. The descriptions of the programs
are showed in table 1.1.
Table 1.1: The Design Software Descriptions
Program code name
Program description
Slot1.c
This
is
the
executable
source
code
file
responsible for the generation of slot patterns for
the linearly polarised RLSA with its main beam
in
the
boresight
direction.
The
antenna
parameters are placed in the data input file name
‘indata’. This software is capable of producing
the slot pattern in the following file formats:
DAT, DXF, HPGL, and POSTSCRIPT. These
file formats allow for the antenna to be modelled
with the Matlab software using (DAT), CAM
manufactured using (DXF) and (HPGL) as well
as etched using (DXF) file formats.
Stslot1.c
This
is
the
executable
source
code
file
responsible for the generation of slot patterns for
the linearly polarised RLSA with its main beam
squinted at an angle θt in the E-plane. The
antenna parameters are placed in the data input
file name ‘instdata’. This software is capable of
producing the slot pattern in the following file
65
formats: DXF, DAT, HPGL, and POSTSCRIPT.
These file formats allow for the antenna to be
modelled with the Matlab software using (DAT),
CAM manufactured using (DXF and (HPGL) as
well as etched using (DXF) file formats.
Matlab software (Full.m)
This is the software developed using Matlab for
modelling the theoretical antenna radiation
pattern. It takes its input data file (design.dat)
produced by using the modelling output option
of each of the slot program. After performed all
the necessary calculations, it will plot the
theoretical
radiation
pattern
for
the
corresponding design.dat file.
With respect to the three programs above, there are a few input data files have to
be used to get the patterns of the slot’s surface. The input data files descriptions could be
found in the following section.
1.1
The Input Data Files
Each of the programs listed above initially reads from a particular standard
ASCII text data input file. The corresponding names of these files are indicated in the
table above. Below is a description of the parameters currently contained in each of
these files.
a)
Indata
innerrad
innernum
epsr
f0
slotspercell
maxradius
margins
romax
66
Where the parameters have the following meanings:
Innerrad
The equivalent of Pmin, the radius of the inner area left devoid of slots,
the only difference being that innerrad must express as an integer
number of guide wavelengths. For example, if innerad = 2, the radius of
the blocking area is 2*λg.
Romax
The equivalent of Pmax, the outer radius of the slot pattern. This variable
has the units of millimetres.
Innernum
This variable specifies the number of slot pairs in the innermost array
ring. The value must be divisible by the innerrad variable for the
antenna to be realisable.
epsr
The relative permittivity of the dielectric material used for the antenna
cavity.
f0
The centre frequency for the antenna under design.
Slotspercell
Indicates whether or not reflection cancelling slot are to be included. If
the value of slotpercell=2 mean there is no reflection cancelling slot, If
the value of slotpercell=5 mean reflection cancelling slot will be
included.
Maxradius
Indicates the maximum radius of an antenna that can fit on the output
media.
Margins
When it is necessary to split the pattern into two halves, an area of
overlap is allowed for that is produced on both halves. This value of
margins indicates how far (in millimetres) in a given half will encroach
past the joining line into the other half of the pattern will also be
included on this half of the design, and vice-versa for the design of the
other half.
The input data file (indata) is manually edited by the user. As a matter of fact
any numerical figures that is entered by the user will have the effect of the patterns and
coordinates of the slots in radiating and cancelling surface. Therefore user must be very
67
careful when keying the numerical data’s in order to get an antenna which will meets the
specified requirements.
b) Instdata
The instdata file contains the following parameters in the shown order and format:
innerrad
innernum
epsr
f0
slotspercell
maxradius
margins
squint
romax
The parameters have the same meanings as in indata, with the following additions:
This value specifies the squint angle θT, in
Squint
the E-plane for the main beam direction.
c) Design.dat
The design.dat file is produced using the modelling output option of the slot
surface design software and contains the following parameters in the shown order and
format:
Slot_count
Slot_coord_pair[1]
Slot_coord_pair[2]
………………
Slot_coord_pair[slot_count]
68
Where the parameters have the following meanings:
The total number of slot (x y coord pairs)
Slot_count
in the following list
This is a list of x y coordinate pairs of
Slot_coord_pairs
length slot_count of the form x_value
y_value with each successive x y value pair
being separated by a carriage return. The x
y value pairs represent the centre of the
slot.
1.2
Designing the radiating slots surface
In order to design the coordinates, width and length of each slots, the executable
program slot1.exe and the file indata are needed as shown in Figure 1.1. Below is the
methodology of designing the radiating slots surface.
slot1.exe
indata
design.dxf
Figure 1.1
Radiating Slot Design
69
The slot1.exe will read the necessary information from indata file when it is
executed. The design.dxf file will be produced when the slot1.exe program execution is
terminated. A DXF file is an ASCII or binary file format of an AutoCAD drawing file
for exporting AutoCAD drawings to other applications or for importing drawings from
other applications. Finally the design.dxf file will be sent to CAM machines to produce
the physical layout of the slots pattern in copper shim.
1.3
Designing the cancelling slots surface
As for the cancelling slots surface design, the executable program slot1_r.exe
and indata is needed as shown in Figure 1.2. Below is the methodology:
slot1_r.exe
indata
design.dxf
Figure 1.2
Cancelling Slot Design
70
1.4
Plotting the theoretical antenna radiation pattern
In the modelling program Full.m which run under Matlab software, it will first
define input parameters including design frequency (fo), relative permitivity ( ε ), slot
r
length (l), slot width (w), free space propagation constant (k), phi (φ) and observation
distance (r). After all the input parameters has been defined, the program will read total
number of slot and every slot’s coordinate from file and store them in memory. Where
the parameters have the following meanings:
Relative permitivity ( ε )
r
This is the relative dielectric permitivity of
the radial cavity dielectric.
Slot length
The length of the surface slots
slot_width
The width of the surface slots
k
This is the free space propagation constant
for the wave frequency
φ
This is the observation plane that will
recording the field pattern. For example the
E field in the plane φ =constant = φcut
observation_distance (r)
This is the radial distance from the antenna
The theoretical antenna radiation pattern could be plotted using the Matlab
program as explained in Chapter 3. The design.dat file is needed from the slot surface
design software. The Matlab program and design.dat will generate the theoretical
antenna radiation pattern.
71
Full.m
design.dat
Radiation
Pattern Graph
Figure 1.3
Theoretical Radiation Pattern Plotting
72
APPENDIX-B
Antenna Important Parameters
1
Gain
The signal level from the satellite is very weak when it reaches the earth station,
therefore the antenna must has high gain in order to bring up the signals to be above
noise in order to produce a viewable picture.
The gain (G) of an antenna can be expressed by:
⎛ η (πd ) 2
G = 10 log⎜⎜
2
⎝ 100λ
⎞
⎟⎟dB
⎠
(1)
where d = the antenna diameter (m)
η= antenna efficiency (%)
λ = wavelength (m)
The value of η is always less than unity and is often expressed as a percentage.
2
Beamwidth
The beamwidth is a very important characteristic of an antenna. It is a measure of
the gain of the antenna versus the angle of deviation form its centerline projected out
into space. This specification is usually given as the 3 dB beamwidth of the antenna.
73
It can be somewhat likened to the selectivity specification of a receiver, as it
gives you an indication of how well the antenna can select the desired station (satellite)
while rejecting all others at which the antenna is not aimed. The satellites in the world
are angularly spaced fairly close together (2°) in geo-synchronous orbit. The narrower
the beamwidth of the antenna, the less chance you will have that unwelcome
interferences from adjacent satellites will invade your system.
3
Sidelobes Level
Ground noise enters the antenna system principally through side lobes so these
are arranged to be as low as possible in relation to the main lobe amplitude.
4
Antenna Efficiency
The antenna efficiency is the percentage of the incoming signal finally arriving
to be collected at the focal point. Efficiencies of antennas generally in the range of 6080%.
5
Antenna Noise
Antenna noise is any signal received is combined with an element of noise which
degrades the overall performance:
Signal = wanted signal + noise
74
The noise component must be kept as small as is possible, taking into account
cost and available technology. Noise can come from many sources and is produced by
the thermal agitation of atoms and molecules above absolute zero (-273°C or 0 K). This
is why noise is said to have an equivalent noise temperature. The noise temperature of
the earth is normally standardised at 290 K. There are three main sources of noise in the
environment:
1. Extraterrestrial noise sources – wide bandwidth radiation caused by the energy
conversion in stars and the residual background radiation of the ‘big bang’.
2. Man-made noise – noise come from microwave pollution due to man’s electrical
activities.
3. Ground noise – the major component of noise incident on the antenna aperture, and
depends mainly on the antenna diameter, antenna depth and elevation setting. The
smaller the diameter of the dish the wider and more spread out will be the side lobes,
so more noise will enter from the warm earth. The noise temperature also increases
as the elevation angle decreases, since lower elevation settings will pick up more
ground noise due to side lobes intercepting the ground. A deep dish picks up less
ground noise at lower elevations.
75