Design and Implementation of a Dual Excited Planar Circular
Array Antenna for Base Stations
Except where reference is made to the work of others, the work described in this
thesis is my own or was done in collaboration with my advisory committee. This
thesis does not include proprietary or classified information.
Veneela Ammula
Certificate of Approval:
Prathima Agrawal
Professor
Electrical and Computer Engineering
Sadasiva M. Rao, Chair
Professor
Electrical and Computer Engineering
Stuart M. Wentworth
Associate Professor
Electrical and Computer Engineering
George T. Flowers
Dean
Graduate School
Design and Implementation of a Dual Excited Planar Circular
Array Antenna for Base Stations
Veneela Ammula
A Thesis
Submitted to
the Graduate Faculty of
Auburn University
in Partial Fulfillment of the
Requirements for the
Degree of
Master of Science
Auburn, Alabama
Dec 18, 2009
Design and Implementation of a Dual Excited Planar Circular
Array Antenna for Base Stations
Veneela Ammula
Permission is granted to Auburn University to make copies of this thesis at its
discretion, upon the request of individuals or institutions and at
their expense. The author reserves all publication rights.
Signature of Author
Date of Graduation
iii
Vita
Veneela Ammula, daughter of Raghu Ammula and Padmavathy Ammula was
born on September 7th, 1985, in Guntur District, India. She received the Bachelor
of Engineering degree in Electrical Engineering from Vasavi college of Engineering in
2007. She joined the Masters program in the Department of Electrical and Computer
Engineering at Auburn University in August 2007. Her current area of research is
focused on Design of Base Station Array Antennas.
iv
Thesis Abstract
Design and Implementation of a Dual Excited Planar Circular
Array Antenna for Base Stations
Veneela Ammula
Master of Science, Dec 18, 2009
(B.E., Osmania University, 2007)
63 Typed Pages
Directed by Sadasiva M. Rao
Smart antennas present a promising solution to the present day capacity and
coverage shortage in mobile wireless communications. These intelligent antennas
when used at the base station can avoid a lot of interference by transmitting and
receiving signals only in desired directions. Circular array antennas have gained
popularity among various antenna configurations used for direction agile applications.
The purpose of this thesis is to design a planar circular array antenna for base
stations in mobile wireless communication systems. The antenna could be electroni-
cally steered to give a complete 360 coverage around the base station. The model was
created using FEKO Suite 5.4, a Method of Moments (MoM) based electromagnetic
simulation software. Particle Swarm Optimization algorithm was applied to maxi-
mize the gain of the antenna in a single azimuth direction. Ideal dimensions for the
antenna structure were obtained from the optimization process.
v
The designed antenna was fabricated and tested in an anechoic chamber to verify
its radiation characteristics. The experimental results were found to be in good agree-
ment with the simulation results. An improved design configuration was simulated
for future development of this array antenna.
vi
Acknowledgments
First and foremost, I would express my sincere gratitude to my advisor Professor
Sadasiva M Rao for his guidance and help during my study and research at Auburn.
Thanks for leading me into the research area of wireless communications and antenna
design. Without his patience and support, this thesis would not be possible.
I thank Professor Prathima Agrawal for supporting me through a graduate re-
search fellowship which made this work possible.
My thanks also go out to Professor Stuart M Wentworth and Calvin Cutshaw for
rendering their professional skills to the completion of the prototype and Professor
Lloyd Stephen Riggs for his patient assistance in using the facilities in the Antenna
Laboratory.
I would like to again specially thank all my advisory committee members Dr.
Sadasiva M. Rao, Dr. Prathima Agarwal and Dr. Stuart M. Wentworth for all the
constant support and guidance throughout my studies at Auburn.
I am grateful to my parents for their consistent support and encouragement
during my study and thanks to my friends for their kind presence whenever needed.
vii
Style manual or journal used Journal ofApproximation Theory (together with the
style known as ?aums?). Bibliograpy follows van Leunen?s A Handbook for Scholars.
Computer software used The document preparation package TEX (specifically
LATEX) together with the departmental style-file aums.sty.
viii
Table of Contents
List of Figures xi
1 Introduction 1
1.1 Evolution of Cellular Network . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Cell Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Cell Sectoring . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Smart Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Organization of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Design of the Dual Excited Planar Circular Array Antenna 8
2.1 Circular Array Antenna Theory . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 Radiation pattern of a general array . . . . . . . . . . . . . . 11
2.1.2 Array factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.3 Array pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Array Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 SIMULATION SOFTWARE: FEKO 15
3.1 Method of Moments . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 FEKO Suite components . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3 Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3.1 OPTFEKO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3.2 Particle Swarm Optimization . . . . . . . . . . . . . . . . . . 19
3.4 Simulation of the antenna model in FEKO . . . . . . . . . . . . . . . 20
3.5 Optimal Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.6 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.6.1 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.7 Antenna design with improved gain . . . . . . . . . . . . . . . . . . . 28
3.7.1 Radiation Patterns . . . . . . . . . . . . . . . . . . . . . . . . 30
3.7.2 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4 Fabrication of the Antenna 33
4.1 Feed Network for Antenna Array . . . . . . . . . . . . . . . . . . . . 35
ix
5 Experimental Verification 40
5.1 Measuring the Resonant Frequency and input impedance . . . . . . . 40
5.2 Measuring the gain of the antenna . . . . . . . . . . . . . . . . . . . . 42
5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6 Conclusion 47
Bibliography 49
x
List of Figures
1.1 Cellular system geometry.(Figure obtained from [1]) . . . . . . . . . . 2
1.2 Omnidirectional Antenna and Coverage Patterns. . . . . . . . . . . . 2
1.3 Cell Splitting.(Figure obtained from [3]) . . . . . . . . . . . . . . . . 3
1.4 Cell Sectoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.5 Sectorised antenna for Base stations. . . . . . . . . . . . . . . . . . . 4
1.6 Comparison of Coverage patterns of different antenna configurations.
(Figure from [6]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 An arbitrary array antenna . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Geometry of the array antenna. . . . . . . . . . . . . . . . . . . . . . 13
2.3 Geometry of the antenna with only essential elements. . . . . . . . . 14
3.1 Diverse applications of FEKO software(Figure obtained [11]) . . . . . 16
3.2 Particle Swarm Optimization as modeled by a swarm of bees searching
for flowers (Figure obtained from [13]) . . . . . . . . . . . . . . . . . 20
3.3 CADFEKO view showing the meshing and far field parameters of the
antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.4 Parameters for Particle swarm optimization . . . . . . . . . . . . . . 23
3.5 Geometry of the complete antenna array . . . . . . . . . . . . . . . . 24
3.6 Radiation patterns for the antenna . . . . . . . . . . . . . . . . . . . 26
3.7 Radiation pattern of antenna . . . . . . . . . . . . . . . . . . . . . . 27
3.8 Comparison of radiation patterns for different antenna configurations 28
xi
3.9 Geometry of the new antenna design . . . . . . . . . . . . . . . . . . 29
3.10 Radiation patterns for the new antenna . . . . . . . . . . . . . . . . . 31
4.1 Top view of the fabricated antenna . . . . . . . . . . . . . . . . . . . 33
4.2 Side view of fabricated antenna . . . . . . . . . . . . . . . . . . . . . 34
4.3 Block Diagram of power divider . . . . . . . . . . . . . . . . . . . . . 35
4.4 Wilkinson power divider and it equivalent microstrip line implementation 36
4.5 The transmission parameters of Wilkinson power splitter . . . . . . . 37
4.6 Reflection parameters of Wilkinson power splitter . . . . . . . . . . . 37
4.7 Isolation parameters of Wilkinson power splitter . . . . . . . . . . . . 38
4.8 Fabricated Wilkinson Power Divider . . . . . . . . . . . . . . . . . . . 38
4.9 Power Splitter connected to the ports of the antenna . . . . . . . . . 39
5.1 An RF anechoic chamber . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.2 SWR measurement of the antenna . . . . . . . . . . . . . . . . . . . . 42
5.3 DAMS tripod stand (Figure obtained from [12]) . . . . . . . . . . . . 43
5.4 Antenna placed on wooden block for support . . . . . . . . . . . . . . 44
5.5 Initializing a measurement in DAMS (Figure obtained from [12]) . . . 44
5.6 Radiation pattern in Horizontal plane . . . . . . . . . . . . . . . . . . 45
5.7 The simulated and measured radiation patterns of antenna . . . . . . 45
xii
Chapter 1
Introduction
In recent years there has been an explosive growth in the number of wireless users,
particularly in the area of mobile communication. This growth has triggered need
for improving the capacity, coverage and quality of service. Researchers are exploring
several new technologies to make effective use of the limited resources. Smart antenna
technology provides a key to these problems.
1.1 Evolution of Cellular Network
To accommodate all the users in limited available frequencies or the channels a
cellular structure has been designed to systematically provide service to the mobile
users. In this design, a coverage zone is divided into small hexagonal segments called
cells. Fig. 1.1 shows the Cellular system geometry.
Each cell has an omnidirectional antenna located at the center of the cell. Fig.
1.2 shows the radiation pattern of an omnidirectional antenna. Each cell is allotted a
set a frequencies which are different from the neighboring cells. The size of the cell is
defined by various factors such as the local terrain, the frequency band in which the
network operates and capacity requirement in that region.
The same frequencies could be used in different cells provided they are situated
at a distance large enough to avoid interference. In Fig. 1.1, the cells with the same
alphabetical notation use the same set of frequencies. This is known as frequency
reuse. The cells are grouped into clusters such that the frequencies allocated to a
1
Figure 1.1: Cellular system geometry.(Figure obtained from [1])
Figure 1.2: Omnidirectional Antenna and Coverage Patterns.
particular cell within a cluster can only be re-used by a corresponding cell in adjacent
2
clusters. Since each of these cells reuses the frequency spectrum, a significant increase
in capacity can be achieved.
1.1.1 Cell Splitting
The cells can be further divided into micro cells to aid the congestion problem
as shown in Fig. 1.3. Each of these micro cells is provided with an individual base
station with smaller range and less transmission power. This helps in improving the
capacity but at the same time it increases the number of hand offs and processing
required per user. Hand off is a phenomenon of transferring the communication from
one base station to another when the mobile user is moving from one cell to another.
In cell splitting the radius of the cell R is decreased while maintaining the D/R ratio
value to be constant, where D is the distance between the centers of clusters [6].
Figure 1.3: Cell Splitting.(Figure obtained from [3])
1.1.2 Cell Sectoring
With further requirement of increased number of channels, cells are divided into
regions called sectors. Each cell is divided into 3 sectors of 120 degrees each. Fig. 1.4
shows the cellular structure with cells divided into sectors.
In cell sectoring the radius of the cell remains constant decreasing the D/R ratio.
Sector antennas limit the direction of radiation by resolving directivity of antenna
3
Figure 1.4: Cell Sectoring.
into sectors. Omni directional antennas were replaced by directional antennas such
as Monopole Yagi-Uda arrays antennas. Fig. 1.5 shows a sectorised antenna for Base
station.
Figure 1.5: Sectorised antenna for Base stations.
Co-channel interference, which is a phenomenon of disturbance or cross talk that
occurs when two users in different cells use the same frequency, is greatly reduced
in cells which use sector antennas compared to the ones which use omnidirectional
antennas. The signal to interference ratio (S/I) increases greatly.
4
1.2 Smart Antennas
Unlike wireless systems in the past, which used fixed antenna systems, smart
antennas or adaptive arrays are dynamically able to adapt to the changing traffic
requirements. Smart antennas, usually employed at the base station, radiate narrow
beams to serve different users. When the users are well separated spatially the same
frequency can be reused, even if the users are in the same cell. This additional intra-
cell channel reuse based on spatial separation is the key in achieving an increase in
the capacity of the system. So the coverage of smart antennas is greater than the
conventional base transceiver station (BTS) antennas.
When smart antenna technology was first introduced in the 1960?s it was used
in military communication systems, were narrow beams where used in order to avoid
interference arising from noise and other jamming signals. Extending the smart an-
tenna concept further, researchers worked on the technology to apply it to the per-
sonal communication industry to accommodate more users in the wireless network by
suppressing interference.
Smart antenna systems extend cell sectoring by providing coverage to the sector
through multiple beams. These multiple beams are obtained by using antenna arrays,
where the number of beams covering a sector is determined as a function of array
geometry. As they can reject interference in unwanted directions and direct radiation
towards the desired user they provide greater coverage for each base station. The
capacity of the system is also improved as the bit error rate (BER) is lower. There
are two types of smart antennas. They are Switched beam antennas and Adaptive
array antennas.
Switched beam forming is a smart antenna approach in its simplest form. Here,
multiple fixed beams in predetermined directions are used to serve the users. In
5
this approach the base station switches between several beams that give the best
performance as the mobile user moves through the cell. But the drawback is that if
an interferer is present at the center of the directive beam then the undesired user
gets enhanced.
Adaptive beam forming is the most advanced approach based on smart antenna
techniques. It uses an antenna array backed by strong signal processing capability to
automatically change the beam pattern in accordance with the changing signal en-
vironment. It directs maximum radiation in the direction of the desired mobile user
and also introduces nulls in the interfering directions while tracking the desired mo-
bile user at the same time. The adaptation is achieved by multiplying the incoming
signal with complex weights and then summing them together to obtain the desired
radiation pattern. These weights are computed to adapt to the changes in the signal
environment. The complex weight computation based on different criteria is incorpo-
rated in the signal processor in the form of software algorithms. Fig. 1.6 shows the
comparison among switched beam systems, adaptive arrays and sector antennas in
terms of their coverage in both low interference and high interference environments.
Although better performance is expected in the adaptive array, the switched
beam system holds the advantage of being easier to integrate with the current system.
1.3 Organization of Thesis
Chapter 2 starts with the literature review of various antennas proposed for
producing directive beams with beam steering properties. The radiation pattern
of an arbitrary antenna array is explained. This is followed by an overview of the
configuration of the proposed Dual Excited Planar Circular Array Antenna
6
Figure 1.6: Comparison of Coverage patterns of different antenna configurations.
(Figure from [6])
Chapter 3 gives a brief introduction to the Electromagnetic simulation software,
FEKO. It explains the particle swarm optimization method used in optimizing the
dimensions of the model and the methodology followed for simulation of antenna
design. The simulation results are also presented along with an improved design for
future study.
Chapter 4 explains the fabrication of the antenna structure. It is followed by
design and fabrication of a feed network for the antenna which is a Wilkinson power
splitter. Performance of the splitter is validated by experimental measurement of its
electromagnetic properties.
Chapter 5 deals with the measurements of the radiation pattern characteristics
of the fabricated antenna in the anechoic chamber using the help of Desktop Antenna
Measurement System.
The thesis is then concluded in Chapter 6 by summarizing the design, fabrica-
tion and experimental characterization of the Dual Excited Planar Circular Array
Antenna. Ideas for improving the results obtained in this work are also discussed.
7
Chapter 2
Design of the Dual Excited Planar Circular Array Antenna
The Yagi-Uda antenna array is used for many applications where a narrow
beamwidth and high gain is required. Basically, it comprises a driven element, a
reflector element placed behind the driven element and at least one director element
placed in front. The reflector and director elements are parasitic elements, which
when suitably designed,[2] , provide a directive radiation pattern.
Circular arrays have become very popular in mobile and wireless communications
because of their advantages over other configurations. They provide symmetry. As
they do not have edge elements, directional patterns synthesized with a uniform cir-
cular array (UCA) can be electronically steered in 360 degrees without any significant
change in the beam shape [16]. Reference [20] compares UCA with uniform rectan-
gular array (URA) and planar uniform circular array (PUCA) antennas to find the
best suitable configuration for smart antenna applications. Simulation results showed
very good beamformed patterns with PUCAs with deep nulls towards the angles of
interference. Of all three geometries, the UCA was shown to have the best patterns
with narrowest main beam pointing towards the direction of the signal of interest
(SOI). This might be due to the large separation between nearly diametrically oppo-
site elements. When URA and the PUCA antennas with similar areas were compared
it was found that slightly greater directivity was obtained when PUCAs were used.
The beam forming properties of circular array antennas have been studied in
reference [8]. An array of monopoles arranged along the circular ring on a ground
plate was considered. In the first case simulations were done to find the optimum
8
number of elements that could be accommodated in a circular ring and it was found
that a six element array was enough to produce a directive beam in the desired
direction. In the second case simulations were carried on to find the effect of different
excitation schemes. In this, experiments were conducted by changing the number of
excited elements in the array. It was observed that when two element were excited
best beam focusing properties were observed in both azimuth and vertical planes. In
the third case the radius of the ground plane was varied and the radiation pattern
for each configuration has been studied. It was observed that as the radius of the
ground plate, Rg, decreased, the angle of elevation of the main beam increased till
Rg reached one wavelength and further increase in Rg lowered the elevation of the
beam. The antenna was fabricated to compare its results with the simulations and a
close match between them was found. The directivity produced in the azimuth plane
remained to be 6 dB which requires an improvement.
Various configurations of switched parasitic arrays which could provide angular
diversity were proposed in reference [19]. In these designs the signal source remained
constant and other surrounding parasitic elements were either short-circuited or open-
circuited to provide directive radiation pattern.
Reference [9] presents design of a circular array composed of 25 wire monopole
elements. The configuration comprises an element fed with supply positioned at
the center of the ground plane. The remaining 24 elements are uniformly dispersed
along two concentric rings around the center element. These elements are reactively
loaded to change the electrical length of the elements to produce directive beams. By
changing the loads a 360 degree beam scan is produced with 30 degree increments.
The antenna array was designed to operate at 1 GHz frequency. Manual optimization
was performed to find the values for 50 ? impedance matching and required beam
width such that the elements behind the driven element act as reflectors and the ones
9
in front of it act as directors. To extend operation to other frequencies, optimization
should be carried out at each desired frequency and parameters that direct the pattern
need to be recalculated. And the manual optimization method followed to find the
best values for the parameters is often tedious and may not provide the best optimized
values for the considered configuration.
A configuration similar to the above design has been proposed in reference [5]
to operate at 2.4 GHz frequency. The configuration consists of an excited element at
the center of a hollow cylindrical ground structure and six elements arranged around
it in a circle. The elements were loaded at the bottom using varactors to control their
reactances and help produce beam patterns in different directions. The ground is
chosen to be a hollow cylinder to accommodate the driver circuit and also to reduce
the angle of elevation of the radiation pattern. A genetic algorithm has been applied
to the antenna configuration using HFSS software to optimize the dimensions of the
structure so as to obtain the best beam pattern with highest gain possible. The
optimized antenna produced a gain of 8.08dBi in azimuth plane at an elevation of
4 degrees from the ground plane. The antenna was fabricated and tested to verify
the radiation pattern. It was found that there was a very close match between the
designed and fabricated antenna.
Particle swarm optimization (PSO) is another optimization technique which re-
ceived recent popularity for applications related to the design of antennas and mi-
crowave components. Reference [15] proposed a design of a non uniform circular
antenna array using PSO. The PSO method was used to optimize the positions of
elements arranged in a circular array. It was observed that PSO gave better results
than GA optimization.
10
2.1 Circular Array Antenna Theory
Single antennas usually provide low directivity with a wide radiation pattern.
Several antennas can be arranged in space and can be interconnected to produce a
directional radiation pattern. Such configuration is referred to as an array antenna.
Array antennas offer an efficient means for providing azimuth beam scan with limited
elevation coverage. When the elements of the array are placed along a circular ring
it is known as a circular array. Its applications are found in radio direction finding,
air and space navigation, underground propagation, radar, sonar, and other systems
including smart antennas [6].
Figure 2.1: An arbitrary array antenna
2.1.1 Radiation pattern of a general array
An arbitrary array antenna in three dimensional space as shown in Fig. 2.1 is
considered. It is assumed that the source of incident wave is a plane wave and is
present in the far field of the array. The spherical coordinates of a vector from origin
to the mth element of the array is given by vectorrm = (?m,?m,?m) and a unit vector in the
11
direction of the incident wave is given by ??k = (1,?,?). The phase of the received
plane wave at the mth element is the phase constant ? = 2?/? multiplied by the
projection of the element position vectorrm onto the plane wave arrival vector ??k. This is
given by vectork? vectorrm with the dot product taken in rectangular coordinates. In rectangular
coordinates, ??k = sin?cos??x + sin?sin??y + cos??z = ?r and vectorrm = ?msin?mcos?m?x +
?msin?msin?m?y + ?mcos?m?z and the relative phase of the incident wave at the mth
element is ?m = ?vectork ? vectorrm = ??m(sin?cos?sin?m +sin?sin?sin?msin?m +cos?cos?m)
= ?(xmsin?cos? + ymsin?sin? + zmcos?)
2.1.2 Array factor
For an array of M elements, the array factor is given by
AF(?,?) =
Msummationdisplay
m=1
Imej(?m+?m)
where Im is the magnitude and ?m is the phase of the weighting of the mth element.
The normalized array factor is given by
f(?,?) = AF(?,?)max|AF(?,?)|
2.1.3 Array pattern
If each element has a pattern gm(?,?), which may be different from other ele-
ments, the normalized array pattern is given by
F(?,?) =
summationtextM
m=1 Imgm(?,?)e
j(?m+?m)
max(|summationtextMm=1 Imgm(?,?)e(j(?m+?m)|)
In the above equation, the element patterns must be represented such that the pattern
maxima are equal to the element gains relative to a common reference.
12
2.2 Array Design
This work describes a planar circular array antenna. It consists of an array of
17 elements placed on a hollow cylindrical ground structure. Among the 17 elements
16 are distributed uniformly on the circumference of the 2 concentric rings and the
other element is placed on the center of the ground structure.
Figure 2.2: Geometry of the array antenna.
Fig. 2.2 shows the geometry of the antenna. Two driven elements shown with
red ports in Fig. 2.2 are chosen such that they are placed 135 degrees apart on
the inner ring. The elements placed diagonally opposite to the excited elements
are grounded to act as reflectors. The elements behind the excited elements in the
outer ring, the element at the center are also grounded and the remaining parasitic
elements are removed from the system. So there are only 9 active elements which
provide the desired directional beam in every case. The main beam of the radiation
pattern is observed along the line bisecting the angle of the driven elements. The
13
driven elements are fed equally through a Wilkinson power splitter located beneath
the cylindrical ground.
Figure 2.3: Geometry of the antenna with only essential elements.
Fig. 2.3 shows the geometry of the design with only the essential elements
required in producing the desired beam pattern. The dimensions of the antenna
structure are obtained by applying a particle swarm optimization technique with the
help of FEKO simulation software. The dimensions and parameters for optimization
are presented in detail in the next chapter.
14
Chapter 3
SIMULATION SOFTWARE: FEKO
FEKO is simulation software used for analyzing a wide range of electromagnetic
problems involving objects of arbitrary shapes. FEKO is an abbreviation derived
from the German phrase FEldberechnung bei Korpern mit beliebiger Oberflache (Field
computations involving bodies of arbitrary shape). Some of its applications include
EMC analysis, antenna design, microstrip antennas and circuits, dielectric media,
scattering analysis [11]. It could be installed on MS Windows or Linux based PC?s or
work stations. The time required for FEKO to simulate a design is very low compared
to other electromagnetic softwares like HFSS which takes several hours for a single
basic simulation.
FEKO is based on the method of moments (MoM) technique. In this method
the electric surface currents on the conducting structures are calculated to ascertain
the electromagnetic fields. Multilevel Fast Multipole Method (MLFMM), the Finite
Element Method (FEM), Uniform Theory of Defraction (UTD), Geometrical Optics
(GO) and Physical Optics (PO) are also used when solving electrically large problems
and inhomogeneous dielectric bodies of arbitrary shape. The PO approximation or the
UTD are hybridized with the MoM in order to be utilized by FEKO. Because of this
hybridization technique, any critical region of a structure can be evaluated using the
MoM while the remaining regions, that may be larger, flat or curved, can be analyzed
using the PO approximation or UTD. Fig. 3.1, shows the diverse applications of
FEKO software and various techniques used depending on the electrical size of the
structure.
15
Figure 3.1: Diverse applications of FEKO software(Figure obtained [11])
3.1 Method of Moments
The MoM is a full wave solution of Maxwell?s integral equations in the frequency
domain. MoM is a ?source method? which means that only the geometric structure
of the antenna is discretized, not free space. It is used for modeling open boundary
radiating structures where no 3D space discretization is required.
The conductors in the antenna structure can be broken into ?segments?, and
the current flowing through each of these segments can be determined. The value
obtained by multiplying the current with the vector describing the little segment
(length and orientation) is termed as ?Moment? [18].
A set of ?basis functions? may be assumed into which the current distributions
are decomposed. These basis functions could be a rectangular approximation to the
Dirac delta function. Because the widths of the rectangular sections are non-zero,
only a finite number of them are enough to cover the antenna wire structure. Another
16
type of basis function is triangular in shape. They give a smoother approximation
to the current distribution. The current distribution is piecewise-linear between the
matching points. In general, the ?nth moment? is obtained by integrating the product
of the Green?s function with the nth basis function [18].
The ?method of moments? starts from deriving the currents on each segment,
or the strength of each moment, by using a coupling Green?s function. The Green?s
function includes electrostatic coupling between the moments which helps in calculat-
ing the buildup charge at all the points if the spatial change of the currents is known
accurately [18].
A Green?s function is a kind of 3-dimensional version of the impulse response
function familiar in linear electronic circuit analysis. One sets up a structure for
the space under consideration, specifying where the excitations can be and what the
boundary conditions are. One then excites the structure with a single little region of
excitation, with all the other possible excitations set to zero. The Green?s function
is the response of all the other regions in the problem to this excitation. Since the
system is assumed linear, the principle of superposition applies and the total response
to an arbitrary set of excitations can be obtained for the problem by direct summation
or integration over all the excitations. The far field can be computed from moments
defined at each segment of the antenna [18].
3.2 FEKO Suite components
FEKO consists of the following major user interfaced units which aid from de-
signing the antenna to obtaining the radiation patterns: CADFEKO, EDITFEKO,
POSTFEKO [11].
17
CADFEKO is a graphical environment in which the geometry and mesh can
be created. We can specify the solution settings and calculation requirements here.
From CADFEKO other modules of FEKO can also be accessed. It displays three
dimensional antenna geometry, which allows the user to view the antenna structure
from all angles and verify the geometry. It also displays other information, such as
meshing and three-dimensional field patterns.
EDITFEKO is used to construct advanced models both in terms of the geometry
and solution requirements with the help of a high level scripting language.
POSTFEKO displays polar and linear plots of the S-parameters, 3D far fields,
2D and 3D near fields.
3.3 Optimization
Optimization is a procedure for choosing the best possible solution for a given
problem among the available alternatives. This could be achieved by changing the
parameters which could affect the desired result or response. In a geometric model
the parameters are mostly dependent on the dimensions or material properties or the
excitation of the structure. Optimization is based on various aspects like
? Method to be used, which defines the optimization technique that should be
used for each particular type of problem.
? Parameters, which define the range in which the search is conducted.
? Goals, which define the final desired output of the search [11]
3.3.1 OPTFEKO
The optimization process in FEKO is carried out by the OPTFEKO unit. Various
optimization methods like Genetic Algorithm (GA), Particle Swarm Optimization,
18
Simplex Nelder- Mead algorithm and Grid search could be performed in FEKO.
After running various simulations to identify the optimization method suitable for
the antenna design in this work, it was observed that the particle swarm optimization
method offered the best configuration for the antenna.
3.3.2 Particle Swarm Optimization
The particle swarm optimization (PSO) algorithm was first introduced by James
Kennedy and Russell Eberhart in 1995. It is less cumbersome compared to algorithms
like genetic algorithm (GA) as it could be implemented with minimal mathematical
processing. It has evolved as a powerful optimization tool in solving mathematical
functions as well as electromagnetic problems. Therefore in this work PSO algorithm
is used to optimize the dimensions of the design so as to maximize gain of the antenna
in desired direction.
The algorithm could be easily explained by considering an analogy to a swarm
of bees in search of flowers in a garden. The objective of bees is to find a location
in the garden where maximum number of flowers are concentrated. An assumption
is made that the bees do not have prior knowledge of the garden. The bees fly
randomly in different directions with different velocities. When a bee finds a location
with maximum flowers, i.e. its personal best location, it remembers the location and
informs the whole swarm about the location. In this way, each bee has the knowledge
about the best location, i.e. global best location found so far. Based on nostalgia
or social influence the bee continues its navigation through locations between the
personal best location and global best location. This is a continuous process and a
bee can be in any location, even the exact location where the other bees have been.
Along the way, a bee might find a new location with higher concentration than
before. It would then be attracted to this new location as well as the location of
19
Figure 3.2: Particle Swarm Optimization as modeled by a swarm of bees searching
for flowers (Figure obtained from [13])
the most flowers found by any bee in the whole swarm. In this way the bees explore
the whole garden and eventually find the location with the highest concentration of
flowers.
For carrying out a particle swarm optimization in FEKO, the default swarm
size is set to 20 and the number of iterations to 50, resulting in 1000 solver runs.
The maximum number of solver runs, (C), could also be specified by the user by
maintaining a condition that when the number of solver runs is converted into a
population size (A) and number of iterations (B), A ? B ? C. A is selected as a
function of the number of parameters (Np), and B ? 5 must always be satisfied.
3.4 Simulation of the antenna model in FEKO
The antenna design is created in the CADFEKO component of FEKO which
supports the creation and setup of FEKO models. Fig. 3.3 shows the CADFEKO
display containing the antenna design and various tool bars. The tool bar at the top
is used to make modifications in the display of the model. The tool bar at the left is
20
used for geometry creation, modification, transform, solutions, network creation and
optimization.
First of all the unit for the models dimensions is selected(meters) and then the
variables are assigned values in terms of wavelength. These variables define the various
dimensions of the design model. Following is the list of variables used:
? hc= 0.2500? (Height of the cylindrical ground plane)
? rc= 0.6000? (Radius of the cylindrical ground plane)
? hi= 0.2500? (Height of elements on inner circle)
? ho= 0.2750? (Height of elements on outer circle)
? horg= 0.2667? (Height of element at origin)
? ri= 0.2667? (Radius of inner circle)
? ro= 0.5334? (Radius of outer circle)
Next the antenna design is created by choosing the required shapes from the
geometry tool bar. A hollow cylinder with all the elements mounted on it is created.
The elements I, II with ports marked at the bottom as shown in Fig. 3.3 are chosen
for excitation and 1V is applied to them. The frequency of operation of the antenna
is designated as 2GHZ.
The far field request is created named ?H-Plane? with parameters set at
? theta = 85? to 90?
? phi = 0? to 360?
21
Figure 3.3: CADFEKO view showing the meshing and far field parameters of the
antenna
The model is then meshed choosing edge length = 0.2?, Segment length = 0.2?,
Wire radius = 0.01?.
A Particle Swarm Method (PSO) optimization search is included. The parame-
ters shown in Fig.3.4 have been defined for optimization.
The optimization goal is selected so as to maximize the gain of the antenna
design. The simulation is run and 1000 optimization solvers are completed before the
final optimized dimensions are obtained. The optimization was carried out in several
steps. In the first step the dimensions of the cylinder (i.e. the height and the radius
of the cylinder) was optimized. In the second step the lengths of the wire antenna
elements on the ground plane were optimized.
22
Figure 3.4: Parameters for Particle swarm optimization
3.5 Optimal Geometry
After optimization was conducted a new set of optimized values for the dimen-
sions were obtained which could maximize the gain of the antenna. Following is the
list of optimized dimensions of the design
? hc= 0.2000 ? (Height of the cylindrical ground plane)
? rc= 0.5600 ? (Radius of the cylindrical ground plane)
? hi= 0.2545 ? (Height of elements on inner circle)
? ho= 0.7990 ? (Height of elements on outer circle)
? horg= 0.7970 ? (Height of element at origin)
? ri= 0.2667 ? (Radius of inner circle)
? ro= 0.5334 ? (Radius of outer circle)
23
3.6 Results
Fig. 3.5 presents the antenna configuration with all the elements shown i.e. the
configuration that is practically used.
Figure 3.5: Geometry of the complete antenna array
Fig. 3.6[a] shows the radiation pattern of the antenna when elements (shown in
Fig. 3.5) 1 and 4 are excited and the elements 5, 8, 9, 12, 13, 16, 17 are grounded
and other elements are removed from the system (i.e, they are open circuited).Fig
3.6[b] is obtained when the 2 and 5 elements are excited and the elements 1, 9, 6, 10,
13, 14, 17 are grounded and other elements are removed from the system. Thus the
pattern rotates in the azimuth plane with change in the antenna coniguration.
A switching circuit is placed underneath the antenna to select and excite the
required elements in order to steer the beam in the desired direction.
24
The following figures show the simulated radiation pattern of the antenna. The
gain of the pattern is 9.8dB and the beamwidth is 55 degrees. The angle of elevation
of the beam was measured as 5 degrees from the horizontal plane.
[a]
Far Field Gain in Horizontal Plane
Angle (Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
[b]
Far Field Gain in Horizontal Plane
Angle (Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
[c]
Far Field Gain in Horizontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
[d]
Far Field Gain in Horizontal Plane
Angle (Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
25
[e]
Far Field Gain in Horizontal Plane
Angle (Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
[f]
Far Field Gain in Horizontal Plane
Angle (Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
[g]
Far Field Gain in Horizontal Plane
Angle (Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
[h]
Far Field Gain in Horizontal Plane
Angle (Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   10
    5
    0
   -5
  -10
  -15
  -20
Figure 3.6: Radiation patterns for the antenna
3.6.1 Observations
It is observed that the elements in the outer circular ring acted as reflectors
and the elements in the inner circle directed the beam such that the main beam is
concentrated in the direction of diagonal between the excited elements. The element
present at the center of ground plate helped in reducing the side lobe value of the
26
radiation pattern. Simulations should be carried out by changing the positions of
the elements and the number of elements being used in the radiation mechanism to
obtain better radiation pattern with a higher gain.
Fig. 3.7 shows the radiation pattern, when the 17 element antenna configuration
shown in Fig. 3.5 system is considered.
Figure 3.7: Radiation pattern of antenna
The gain of the pattern is 8dB and the beamwidth is 58 degrees at 5 degrees
elevation from the horizontal plane. Fig. 3.8 compares the radiation pattern of the 9
element system with the 17 element system. It is observed that the unused elements
contributed to the interaction and coupling which reduced the gain of the system.
It is also observed that the amount of side lobe for the 17 element system is less
compared to the 9 element system.
27
Figure 3.8: Comparison of radiation patterns for different antenna configurations
3.7 Antenna design with improved gain
Though the previous design has been sent for fabrication further simulations
have been carried out to design an antenna with better gain characteristics. In these
optimizations the limits of the design parameters were further extended and the
radius of the circular rings on which the elements lie has also been optimized. It
was observed that by increasing the diameter of the cylindrical ground plane the gain
of the structure improved notably. In the new configuration elements were added in
both the inner and outer circles in the positions behind the excited elements such that
there are 4 elements 45 degrees apart from each other behind the excited elements
in both the concentric circular rings. Fig. 3.9 shows the configuration of the new
antenna design. The elements displayed with red and yellow ports are the excited
elements.
Following is the list of optimized dimensions of the design
28
Figure 3.9: Geometry of the new antenna design
? hc = 0.1775? (Height of the cylindrical ground plane)
? rc = 3.5000? (Radius of the cylindrical ground plane)
? hi = 0.5974? (Height of elements on inner circle)
? ho = 0.8933? (Height of elements on outer circle)
? horg = 0.7844? (Height of element at origin)
? ri = 0.2667? (Radius of inner circle)
? ro = 0.5334? (Radius of outer circle)
29
3.7.1 Radiation Patterns
A gain of 12dB was observed with this configuration at 10 degrees elevation from
the horizontal plane. The following figures show the radiation patterns of the design.
[a]
Far Field Gain in HoriZontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
[b]
Far Field Gain in Horizontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
[c]
Far Field Gain in Horizontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
[d]
Far Field Gain in Hoizontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
30
[e]
Far Field Gain in Horizontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
[f]
Far Field Gain in Horizontal Plane
Angle(Phi, Angle)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
[g]
Far Field Gain in Horizontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
[h]
Far Field Gain in Horizontal Plane
Angle(Phi, Degrees)
  330
  300
  270
  240
  210
  180
  150
  120
   90
   60
   30
    0
   15
   10
    5
    0
   -5
  -10
  -15
Figure 3.10: Radiation patterns for the new antenna
3.7.2 Observations
Various simulations have been performed to examine the effect of every element
in the array on the radiation pattern. It was found that the presence of an element
at the center of the ground plate reduced the amount of side lobe in the radiation
31
pattern. The elements in the outer circular ring acted as reflectors and the elements
in the inner circle directed the beam such that the main beam is concentrated in the
direction of diagonal between the excited elements. By adding the other elements in
the circle behind the excited elements the pattern became stronger and achieved a
higher gain. Hence these elements increased the directivity of the pattern. It was also
observed that by increasing the radius of ground plate from 0.6? to 2.5? the gain of
the pattern improved drastically. But when the radius of ground plane was further
increased to 3.5? the improvement in the gain of the pattern was very small only
about 0.5dB. Therefore these changes were optimized following the particle swarm
optimization to obtain the best dimensions for the antenna structure which could
increase the gain. Though the new antenna dimensions made it bulkier it has greatly
improved the gain of the system and decreased the angle of elevation of the pattern.
But with the increase in the radius of the ground plane, the angle of elevation
of the pattern at which high gains were obtained reached about 10 degrees above the
horizontal plane.
32
Chapter 4
Fabrication of the Antenna
For experimental validation, the antenna design acquired from the first opti-
mization is fabricated. The antenna was fabricated to operate at 2 GHz frequency.
The antenna was fabricated in Auburn University?s Electrical Engineering work shop.
Copper sheets were used to fabricate the cylindrical ground and copper rods of stan-
dard diameter very close to that of the optimized value were used for the elements.
All the elements excluding the ones which are excited are screwed to the ground plate
so that they could be removed if required. The two excited elements are connected to
the power supply through a power splitter designed to distribute the power equally
between the two excited elements. The complete version of the antenna is shown in
Figures 4.1, 4.2
Figure 4.1: Top view of the fabricated antenna
Dimensions of the fabricated antenna are as follows:
33
Figure 4.2: Side view of fabricated antenna
? hc= 1.1811 inches (Height of the cylindrical ground plane)
? rc =3.3070 inches (Radius of the cylindrical ground plane)
? hi =1.5029 inches (Height of elements on inner circle)
? ho = 4.7238 inches (Height of elements on outer circle)
? horg =4.7096 inches (Height of element at origin)
? ri = 1.5750 inches (Radius of inner circle)
? ro = 3.1500 inches (Radius of outer circle)
? rr = 0.0625 inches (Radius of antenna elements)
34
4.1 Feed Network for Antenna Array
For an array antenna, the feed network is used to regulate the amplitude and
phase requirements of the radiating elements. Thus designing and implementing
the feed network is an important part of antenna design. For the present design
two elements are given voltage excitation which requires a power splitter which can
distribute the power equally and in-phase between the two ports. Among the various
power splitter circuits, a Wilkinson power divider is selected as it is lossless when
the output ports are matched. By definition, it is a -3dB three port power divider
device that divides power equally in magnitude and phase. Fig. 4.3 shows the block
diagram of a power divider.
Figure 4.3: Block Diagram of power divider
Quarter wave transformers are used to match the split ports to the common port.
Since a lossless reciprocal three port network cannot have all ports simultaneously
matched, Wilkinson introduced a resistor for the purpose of matching. Apart from
matching, the resistor also improves the isolation between the output ports. Fig.
4.4 shows the equivalent circuit and the microstrip implementation of a Wilkinson
two-way power divider. The impedances required for the quarter wave transformer
and the isolating resistor are also shown in this figure.
The dimensions of the design are as follows: Z1 = 50?, Z2 = 70.7? and R =
100?. The length of Z2 elements for copper substrate is quarter of a guide wavelength
35
Figure 4.4: Wilkinson power divider and it equivalent microstrip line implementation
which equals to 3.75cms for a frequency of 2 GHz. Here the input port, port1, of the
wilkinson circuit is terminated in 50?, and the output ports 2 and 3 are measured
by ports 1 and 2 of the VNA. S22 is the reflection looking into port 2 and S32 is
the signal coupled from port2 to port3. Hence S23 is a measure of isolation. The
individual response of the power dividers are presented below. The transmission
parameters (S21 and S31) observed from Fig.4.5 shows they are very close to -3dB. It
is of note that the performance of the divider is more acceptable at 1.8 GHz rather
than 2 GHz.
It is evident from the Fig. 4.6 that the reflection parameters (S22 and S33) are
below - 20dB at 2GHz. The isolation parameters (S32 and S23) shown in Fig.4.7 are
less than -20dB.
Following figures show the fabricated Wilkinson power splitter being connected
to the ports of the antenna.
36
Figure 4.5: The transmission parameters of Wilkinson power splitter
Figure 4.6: Reflection parameters of Wilkinson power splitter
37
Figure 4.7: Isolation parameters of Wilkinson power splitter
Figure 4.8: Fabricated Wilkinson Power Divider
38
Figure 4.9: Power Splitter connected to the ports of the antenna
39
Chapter 5
Experimental Verification
The radiation pattern measurements have been carried out in the anechoic cham-
ber of Antenna Laboratory at Auburn University. Indoor anechoic chambers have
been developed to provide a controlled environment for testing the performance of
the antennas. These can overcome problems related to outdoor testing. An ane-
choic chamber is an enclosure lined completely with radiation absorbent material
(RAM) to simulate a quiet, free-space like environment for testing the radiation pat-
terns of antennas. Only the antenna to be tested and equipment required to perform
the measurements are allowed inside the anechoic chamber. Far field measurements
can be carried out in this chamber to measure the antenna characteristics like gain,
beamwidth, and side lobe level. The RAM is responsible for absorbing the incident
RF radiation from all possible directions. They generally comprise an array of pyra-
mid shaped pieces made up of rubberized foam material impregnated with controlled
mixtures of carbon and iron. Fig. 5.1 shows an RF anechoic chamber.
5.1 Measuring the Resonant Frequency and input impedance
The network analyzer was first calibrated before proceeding with the measure-
ments. The resonance frequency and the input impedance measurement values of the
fabricated Planar Circular array antenna were obtained as follows.
? Resonant Frequency= 2GHz
? Input impedance = 25+j12 ?
40
Figure 5.1: An RF anechoic chamber
The impedance of an antenna is the impedance at the antenna terminals with no
load attached, impedance may be defined as the ratio of the voltage to current at the
antenna terminals. Maximum power transfer can be achieved when the impedance of
the antenna is matched to that of the load. If there is an impedance mismatch then
part (or all) of the power is reflected rather than radiated. The amount of the wave
reflected depends on how bad the mismatch is.
Voltage Standing Wave Ratio (VSWR) is a measure of impedance mismatch between
the transmission line and its load. The higher the VSWR, the greater is the mismatch.
? The VSWR ratio was measured as 1.8?0.6 at 2GHz frequency as shown in the
Fig. 5.2
41
Figure 5.2: SWR measurement of the antenna
5.2 Measuring the gain of the antenna
In the anechoic chamber there are two platforms for placing the source antenna
and test antenna. The platforms were separated with a distance of about 136 inches.
A Scientific Atlanta?s standard gain horn antenna SGH 1.7 was used as source an-
tenna. The antenna under test (the fabricated Planar Circular Array antenna) was
placed at the receiving end on the tripod stand of the Desktop Antenna Measurement
System (DAMS). Fig. 5.3 shows the (DAMS) tripod stand.
The DAMS is designed to aid the testing of small antennas. It can calculate the
path loss and gain of the antenna. The setup comprises of a tripod stand to place the
test antenna, platform controller which connects the computer to the tripod stand
and controls the movement of the platform. The platform on the tripod stand can
rotate 360 degrees in the horizontal direction and from +45 to -45 degrees in the
vertical direction.
42
Figure 5.3: DAMS tripod stand (Figure obtained from [12])
As the antenna had an SMA connector at the bottom it could not stay flat when
placed on the tripod. To fix this problem a small hollow rectangular wooden block
was fixed on the stand with the help of foam tapes and the antenna was fixed on
the block as shown in Fig. 5.4. Before the measurements were made the source
and test antenna were properly aligned with the help of a laser aligner. A supply
power of 25dBm was provided to the horn antenna through the analyzer. Then the
DAMS software was activated so that the platform of the tripod stand rotates 360
degrees horizontally. The far field radiation pattern measurements at each position
were recorded. The measurements were taken with an interval of 10 degrees.
Fig. 5.5 shows the step wise procedure followed in performing the radiation
pattern measurements. The measurement setup was calibrated by selecting the range
about which the platform moves in horizontal and vertical direction. In this case the
platform is subjected to rotate 360 degrees in the horizontal direction. A horizontal
scan was then performed and the data is saved to plot the radiation pattern using
DAMS software.
43
Figure 5.4: Antenna placed on wooden block for support
Figure 5.5: Initializing a measurement in DAMS (Figure obtained from [12])
5.3 Results
Fig. 5.6 shows the radiation pattern of the antenna in the horizontal plane.
The gain of the antenna was measured as 9.3dB. Beamwidth is the directiveness of
44
a directional antenna, defined as the angle between two half-power or -3 dB points
on either side of the main lobe of radiation. Our antenna had a measured BW of 60
degrees.
Figure 5.6: Radiation pattern in Horizontal plane
Figure 5.7: The simulated and measured radiation patterns of antenna
45
Fig. 5.7 compares the radiation pattern of both simulated and experimental
radiation pattern. It is observed that the side lobe value of fabricated antenna is more
than that of the designed antenna. The difference in the gain of the fabricated antenna
when compared to the simulation result may be due to the impedance mismatch at
the inport port of the antenna. An impedance matching circuit can be designed to
overcome this problem.
46
Chapter 6
Conclusion
In this work, a dual excited planar circular array antenna implementing elec-
tronic beam steering over 360 degrees azimuth useful for direction agile applications
is presented. A PSO algorithm was applied to optimize the antenna?s structure so
as to provide maximum gain in the horizontal plane in the direction of a diagonal
between the two excited elements. This antenna has been successfully developed after
running numerous simulations using FEKO software package to get the best combi-
nation of parameters for overall performance. This was followed by the fabrication
and testing phases. From the FEKO simulations, a gain of 9.8dB was obtained at 5
degrees elevation from the azimuth plane with a beam width of 55 degrees.
The experimental validation presented in this work is restricted to the electro-
magnetic behavior of the antenna, and ignores the driving circuit. The experimental
verification was carried out using a scale model at 2 GHz. The impedance mea-
surements were carried out using the network analyzer and the radiation pattern
measurements were conducted in the anechoic chamber of the Antenna Laboratory
at Auburn University. The results were in good agreement with the simulated re-
sults, even though there were minor differences. The gain of the fabricated antenna
was 9.3dB, the input impedance was measured to be 20+j12 ? and the beam width
measured as 60 degrees.
Another improved design was simulated and its results were presented. A gain of
12 dB was achieved at an elevation of 10 degrees from the horizontal.The elements that
were theoretically considered to be open were removed from the system during the
47
experiment. Although, theoretically it is possible to open theseelementsand eliminate
them from radiation mechanism, practically the unused elements do contribute for
the interaction and coupling which may deteriorate or alter the pattern.
Hence the future work in developing the array antenna would constitute a design
in which all the elements of the system are used and to optimize the antenna structure
to reduce the angle of elevation of the main beam and to improve the radiation pattern
such that the value of side lobes is reduced much further.
48
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