CHAPTER 1
INTRODUCTION
1.1 Overview
In now day’s the wireless system has become a part of human life. Most of the electrical
and electronics equipment around are using the wireless system. An antenna is an essential
element of the wireless system. Antenna is an electrical device which transmits the
electromagnetic waves into the space by converting the electric power given at the input into
the radio waves and at the receiver side the antenna intercepts these radio waves and converts
them back into the electrical power. There are so many systems that uses antenna such as remote
controlled television, cellular phones, satellite communications, spacecraft, radars, wireless
phones and wireless computer networks. Day by day new wireless devices are introducing
which increasing demands of compact antennas. Increase in the satellite communication and
use of antennas in the aircraft and spacecraft has also increased the demands a low profile
antenna that can provide a reliable communication.
A microstrip antenna is one who offers low profile and light weight. It is a wide beam
narrowband antenna can be manufactured easily by the printed circuit technology such as a
metallic layers in a particular shape is bonded on a dielectric substrate which forms a radiating
element and another continuous metallic layer on the other side of substrate as ground plane.
Not only the basic shapes any continuous shape can be used as the radiating patch. Instead of
using dielectric substrate some of the microstrip antennas use dielectric spacers which results
in wider bandwidth but in the cost of less ruggedness. Microstrip antennas are low profile
antenna and mechanical rugged and can be easily mounted on any planar and non-planar
surfaces.
The size of microstrip antenna is related to the wavelength of operation generally λ/2. The
applications of microstrip antennas are above the microwave frequency because below these
frequency the use of microstrip antenna doesn’t make a sense because of the size of antenna.
At frequencies lower than microwave, microstrip patches don't make sense because of the sizes
required. Now a day’s microstrip antenna is used in commercial sectors due to its
inexpensiveness and easy to manufacture benefit by advanced printed circuit technology. Due
to the development and ongoing research in the area of microstrip antenna it is expected
that in future after some time most of the conventional antenna will be replaced by microstrip
antenna due to its innovative features, small size and acceptable cost too.
1.2 Ultra Wideband (UWB) Technology
Ultra-wideband (UWB) technology for communications and radar has been a topic of research
since the early 1960s. However, research and development in this area gained momentum only
in recent years for several reasons. The principal reason is the handiness of high-speed
semiconductor switching device technology. Another reason is that these systems were ratified
for the first time only in 2002 for unlicensed use under the Federal Communications
Commission Part 15 (Title 47 of the Code of Federal Regulations) [FCC. 2002]. The use of
UWB in the range of 3.1 to 10.6 GHz was unlicensed by FCC [15]. This permitted the
unlicensed use of deliberate UWB wireless emissions within restricted frequency bands at very
low power spectral density and is history from the viewpoint of frequency overlay. Finally, as
the wireless spectral bands are getting herded with the development of wireless devices, the
need for high-bandwidth wireless communications is also forcing the development of UWB
communication systems.
Fig. 1.1: UWB Frequency spectrum along with its application
UWB is any radio transmitter with a spectrum that engages more than 20% of the center frequency
or a minimum of 500 MHz and that meets the power limits allotted by the regulatory bodies to
minimize the menace to legacy systems. UWB draws gains of broad spectrum in terms of the bit
rates it can handle. By Shannon's theorem, the channel capacity C is given by,
C
Where, B is the bandwidth and
1.1
is the signal-to-noise ratio. Range of operation of such systems
is ascertained by the Frii’s formula.
d
1.2
d being the distance, Pt the transmitted power and Pr the received power, the above equations
suggests that channel capacity can be increased by increasing bandwidth instead of power.
Thus, UWB has primarily been a high bit, short range system. The advantages, disadvantages
and applications of UWB are listed in Table 1.
Table 1.1: UWB advantages, disadvantages and applications
UWB Property
Very
Applications
wide High-rate Wireless
fractional and
Personal Area
absolute
Network
Advantages
Disadvantages
High rate
Potential
Communications
interference
to/from
bandwidth
existing
Low-power,
Very short pulses
Persistence
communications,
Hardware
indoor localization
simplicity
systems
of
Small hardware
multipath
reflections
Low power
Multiple access
Large number of
multi-paths
Low power
combined
Carrier-less
Transmission
communications
and localization
Low fade margins
Scatter in angle of
Arrival
1.3 Objectives
The objectives of this thesis are
To design a high gain with better return loss multiband antenna having simple geometrical
structure.
To design and develop multiband compact hexagonal microstrip patch antenna for UWB
applications.
To investigate and analyze the electrical and geometrical properties of the compact multiband
hexagonal microstrip patch antenna.
To fabricate the compact multiband hexagonal microstrip patch antenna and test the
performance of the antenna experimentally.
1.3 Literature Review
Table 1.2: Literature Review
Sr.No.
1.
Author Name
Paper Name
Kailas Kantilal Sawant,
Dr. C.R. Suthikshn Kumar
CPW Fed Hexagonal
Micro Strip Fractal
Keywords
CPW fed ,Hexagon
Microstrip fractal
antenna, UWB
Communication
Antenna for UWB
Wireless
Communications
2.
Jacob Abraham, Thomaskutty
Mathew
David Fractal
Antenna for
Multiband Wireless
Communication
David fractal,
hexagon patch,
multiband antenna
3.
Sonu Agarwal, Ravi Dutt
A Hexagonal Shaped
Gupta, Santanu Kumar
Fractal Antenna for
Behera
UWB application
Fractal,
ultra
monopole,
wide
band
(UWB), modified
ground plane
4.
Norzaniza AT, Matin MA.
Design of microstrip
UWB antenna with
band notch
characteristics
Band-notched
antenna, microstrip
antenna, ultra wide
band (UWB)
1.4 Thesis Outline
Chapter 1: This chapter presents the introduction, objective and literature review involved in
accomplishing the project.
Chapter 2: This chapter presents the fundamentals of antenna. It contains introduction of antenna and
antenna properties.
Chapter 3: This chapter deals with the basics of microstrip antennas, the advantages and
disadvantages. The basic geometries, feeding techniques, features and applications of planar
antennas are exemplified here.
Chapter 4: This chapter presents the fractal antenna and its types.
Chapter 5: This chapter describes the design of proposed hexagonal microstrip fractal antenna.
Chapter 6: This chapter presents simulation results using the HFSS software and results of
fabricated antenna using VNA.
Chapter 7: This chapter contains the conclusion and it also describes future scope of the thesis.
CHAPTER 2
FUNDAMENTALS OF ANTENNA
2.1 Antenna
An antenna is defined by Webster’s Dictionary as “a usually metallic device (as a rod or wire)
for radiating or receiving radio waves”. The IEEE Standard Definitions of Terms for Antennas
(IEEE Std. 145–1983) defines the antenna or aerial as “a means for radiating or receiving radio
waves” [16]. Many different structures can act as antennas. Generally, antennas are constructed
out of conducting material of some nature and can be constructed in many shapes and sizes.
The size is related to the wavelength of operation of the antenna. An antenna designed for
operation at 10 kHz is almost always much larger than an antenna designed for operation at 10
GHz.
2.2 Antenna Properties
The antenna forms a critical component in a wireless communication system. A good design of
the antenna can relax system requirements and improve its overall performance .The
performance of the antenna is determined by several factors that also called antenna properties
as follows:
2.2.1 Input Impedance
Generally, input impedance is important to determine maximum power transfer between
transmission line and the antenna. This transfer only happen when input impedance of antenna
and input impedance of the transmission line are match. If not match, reflected wave will be
generated at the antenna terminal and travel back towards the energy source. This reflection of
energy results causes a reduction in the overall system efficiency. It is also important that the
input impedance of the antenna is mostly resistive, so that most of the power introduced to the
antenna is radiated. Input impedance has real and complex parts and its general form is:
Zin = Rin + jXin
2.1
Where, Rin represents the resistance or power radiating portion of the impedance.
Xin represents the reactive portion or power storage component of the impedance.
2.2.2 VSWR
Voltage Standing Wave Ratio (VSWR) is the ratio between the maximum voltage and the
minimum voltage along transmission line. The VSWR, which can derived from the level of
reflected and incident waves, is also an indication of how closely or efficiently an antenna’s
terminal input impedance is matched to the characteristic impedance of the transmission line.
Increasing in VSWR indicates an increase in the mismatch between the antenna and the
transmission line. A decrease VSWR means good matching with minimum VSWR is one. Most
wireless system operates at 50 Ohm impedance. Hence the antenna must be designed with an
impedance as close to 50 ohm as possible. A VSWR of 1 indicate an antenna impedance of
exactly 50 ohms. Mostly, the ratio of VSWR≥1.5:1 is needed for antenna functional.
2.2.3 Gain
The gain of an antenna is essentially a measure of the antenna’s overall efficiency. If an antenna
is 100% efficient, it would have a gain equal to its directivity. There are many factors that affect
and reduce at the overall efficiency of an antenna. Some of the most significant factors that
impact antenna gain include impedance, matching network losses, material losses and random
losses. By considering of all factors, it would appear that the antenna must overcome a lot of
adversity in order to achieve acceptable gain performance. Gain is a directional function; it
changes with position around the antenna and is defined as
G (θ, φ) = 4πU (θ, φ) / Pin
(2.2)
Where 4πU (θ, φ) the radiation intensity and Pin is is the input power.
2.2.4 Radiation Pattern
The radiation patterns of an antenna provide the information that describes how the antenna
directs the energy it radiates. All antennas, if 100% efficient, will radiate the same total energy
for equal input power regardless of pattern shape. Radiation patterns are generally presented on
a relative power scale. It can be show in polar plot 360 degree. Example of radiation pattern is
shown in Fig. 3.3. In many cases, the convention of an E-plane and Hplane pattern is used in
the presentation of antenna pattern data. The E-plane is the plane that contains the antenna’s
radiated electric field potential while the H-plane is the plane that contains the antenna’s
radiated magnetic field potential. These planes are always orthogonal [16].
Fig. 2.1 Example of radiation pattern
2.2.5 Directivity
Directivity, D, is important parameter that shows the ability of the antenna focusing radiated
energy. Directivity is the ratio of maximum radiated to radiate reference antenna. Reference
antenna usually is an isotropic radiator where the radiated energy are same in all direction and
have directivity of 1. Directivity can be definition as
D = Umax / Uo
(2.3)
Where Umax = Maximum radiated energy, Uo = Isotropic radiator radiated energy
2.2.6 Polarization
The polarization of an antenna describes the orientation and sense of the radiated wave’s electric field
vector. There are three types of basic polarization:
•
Linear polarization (linear)
•
Elliptical polarization
•
Circular polarization
Generally most antennas radiated with linear or circular polarization. Antennas with linear polarization
radiated at the same plane with the direction of the wave propagate.
For circular polarization, the antenna radiated in circular form.
2.2.7 Frequency Bandwidth (FBW)
The term bandwidth simply defines the frequency range over which an antenna meets a certain set
of specification performance criteria. The important issue to consider regarding bandwidth is the
performance trade-offs between all of the performance properties described above. Antennas form
three classes in terms of frequency coverage: Narrow band – These antennas cover a small range
of the order of few percent around the designed operating frequency.
FBW =
(2.4)
Where f max, f min are the maximum and minimum frequencies. Fo is center frequency.
Wideband or broadband – These antennas cover an octave or two range of frequencies.
FBW =
(2.5)
Frequency independent – these antennas cover a ten to one or greater range of frequencies.
2.2.8 Front to Back Ratio (FBR)
Front to back isolation ratio is defined as the difference in gain from the front of the antenna
and the gain from the back of the antenna. FBR is of concern to communication engineers when
the antenna is to be used in a crowded frequency band. Amateur radio operator’s frequency use
front-to-back isolation as a parameter when comparing Yagi-Uda antennas.
2.2.9 3 dB Beamwidth (half power beam width, HPBW)
Once the antenna pattern information is detailed in a polar plot, some quantitative aspects of
the antenna pattern properties can be described. These quantitative aspects include the 3 dB
beamwidth (1/2 power level), directivity, side lobe level and front to back ratio. To further
understand these concepts, first consider the fundamental reference antenna, the point source.
A point source is an imaginary antenna that radiates energy equally in all directions such that
the antenna pattern is perfect sphere. These antennas is said to be an omnidirectional isotropic
radiator and has 0 dB directivity. In practice when antenna is said to be an omnidirectional, it
is inferred that this is referenced only to the horizontal or azimuth sweep plane. For any practical
the 3 dB beamwidth of antenna is simply a measure of the angular width of the -3dB points on
the antenna pattern relative to the pattern maximum. These -3dB points on the pattern represent
the point on the pattern where the power level is down 3 dB of the value at the pattern maximum.
Generally, the 3 dB beamwidth is expressed separately for each of the individual pattern sweep
planes antenna, there will always be some specific direction of maximum radiated energy.
2.2.10 Return Loss
Return Loss is a measure of the reflected power from an impedance discontinuity, such as the input
port of an antenna, and can be defined in terms of ZA as:
Return loss (dB) = -
(2.6)
Impedance. Since the reflected power from an antenna input port reduces the radiated power,
it is a good practice to minimize return loss for maximizing antenna efficiency. Return loss can
be measured accurately by using a calibrated network analyzer. A comparison of the measured
results with the predicted data can then provide a guideline for how to proceed with the rest of
the design process.
Department of Electronics and Telecommunication Engineering
Dr. Babasaheb Ambedkar Technological University, Lonere.
10
CHAPTER 3
MICROSTRIP ANTENNA
3.1 Introduction to Microstrip patch Antenna
The thought of microstrip antenna was traced in 1953 and a patent in 1955. But it gained
significant attention in the start of 1970s. High performance application where weight, size
installation and robustness is the main requirement microstrip antenna is used such as aircraft,
space craft, automobile vehicles, and satellite and missile applications. The antennas discussed
earlier are 3-dimensional antennas which are bulky and need more space to be deployed.
Microstrip antennas are used to meet these requirements. The demand for compact and low-cost
antennas has brought the microstrip antenna to the forefront due to gaining demand for personal
and hand held mobile communications. Microstrip antennas are also called as patch antennas as
radiating patch. One of the major advantages is that we can fabricate the feeding and matching
networks with the radiating patch on the dielectric substrate. The ground plane is placed on the
back side of the substrate. The top and side views of a rectangular microstrip
antenna are shown in Figure below The radiating patch may be square, circular, elliptical,
circular ring, ring sector shapes etc. shown in Fig.. Because of ease of fabrication, analysis and
attractive radiation properties the rectangle and circular patches are frequently used. They are
having low cross-polarization radiation. The basic properties of microstrip patch antennas have
been numerously discussed in literature .The different types of radiators are broadside and endfire radiator. Broadside radiators have its maximum radiation pattern directed normal to the
patch or axis of the antenna element. The end-fire radiators have its maximum radiation pattern
directed along the axis of the antenna element. The maximum pattern of the patches is normal
to the patch i.e. n in general it acts as broadside radiator. By properly choosing the mode i.e.
field configuration of the excitation under the patch, the broadside radiator pattern can be
achieved. By judicious mode selection, the End-fire radiation can be achieved. The strip and
ground plane are separated by a dielectric sheet as shown in Fig. 3.1(a).
There are varieties of dielectric material that can be used as substrates in microstrip antenna
design with the dielectric constant in the range of 2.2 ≤ εr ≤ 12. For designing an antenna with
good efficiency, larger bandwidth the substrate height should be more and its dielectric constant
should be low, because the low dielectric constant material provides loosely bounded fields
which leads to release the of more radiation s into space, but it costs in increase of antenna size.
While substrate with higher dielectric constants and lower thickness is used in the applications
where tightly bounded fields are required such as waveguides and microwave circuitry.
(a)
(b)
(c)
Fig 3.1(a) Microstrip Patch Antenna Structure, (b) Analysis of Fringing Field, (c) Coordinal Visualization
But uses of high dielectric constant material will cost in poor efficiency and smaller
bandwidths or greater losses. Various types of shapes that used as a microstrip radiating patch
are shown in figure below. Rectangular and circular are the most widely used shapes.
(a)
Square
(f) Disk Sector
(b) Rectangle
(g) Triangle
(c) Dipole
(d) Elliptical
(h) Circular disk
(e) Circular
(i) Ring Sector Fig
3.2: Different Shapes of an Antenna Structures
3.2 Feeding Methods:
There are different feeding configurations used to feed the microstrip antennas. The four
of them which are popularly used are listed below.
1. Microstrip or CPW feed line
2. Coaxial probe feed
3. Aperture coupling
4. Proximity or EMC coupling
The microstrip feed line is also a metallic strip has smaller width as compared to that of the
patch. The main advantage of using the microstrip line feed is that it is very easy to manufacture
one can easily fabricate the fed line with the radiating patch on substrate. It is easy to achieve
the impedance matching by feeding at the inset position and also it is very simple to model.
However microstrip feed line suffers from surface waves and spurious feed radiation
especially when a substrate with high thickness is used, also has the narrow bandwidth (typically
2-5%). A microstrip patch antenna using this feed line is shown in figure with its equivalent
circuit in Figures. 3.3(b) and 3.4(b), respectively. In the coaxial-line feeding a hole is made
in the ground plane and substrate through which the core conductor cable is soldered to the
radiating element. While the outer cable of feed line is made connected to the ground plane.
The coaxial probe feed can also easy to fabricate but difficulty arises in drilling the core
conductor in ground plane and substrate and proper soldering required. Coaxial probe feed
has the advantage that a designer can easily get the impedance match by feeding at the driving
point (is a point where antenna impedance is equal to characteristic impedance of feeding cable
usually 50 ohm).
It has low spurious radiation. However, It also has narrow bandwidth and difficult to model. A
typical coax feed and its equivalent circuit is also shown in Figure 3.4(a) and 3.4(b) respectively.
(a)
(b)
Fig 3.3 (a) Microstrip Feed Line (b) Equivalent Circuit
(a)
(b)
Fig 3.4 (a) Coaxial line feed (b) Electrical Equ. Circuit for coaxial line
These contacting feeding methods microstrip line and probe feed shows asymmetry which
results in generation of the higher order modes. Therefore to avoid this problem, non- contacting
feeding are used. The aperture coupled feed is most difficult to fabricate among four feeding
techniques. But, it is easy to model and having less unwanted radiation. Aperture coupling
having a ground plane sandwiched between two substrates. The slot is made on ground plane
to couple the energy from feed line to patch. Various types of shapes are used in this feed.
Rectangular shape is mostly used of them. Lower substrate has high dielectric 12 material for
tightly bounding the fields. The selection of substrate material plays a major role on antenna
performance. The Upper substrate is responsible for releasing the electromagnetic waves into
space therefore for better radiations low dielectric material is used as upper substrate. While the
lower one supports in coupling the energy from feed line to radiating patch, therefore a thin
substrate with higher dielectric constant is used in the lower substrate.
The amount of energy coupled to radiating patch depends on the slot dimensions and position,
they can be optimize in order to get the maximum coupling optimize. This feed also has low
spurious feed radiation. An antenna using aperture coupled feed with its electrical equivalent
circuit in Fig.3.5 (a) and 3.5(b) respectively. The Proximity coupling has the advantage of
large bandwidth, easy to model and has less unwanted radiation. However it is difficult to
fabricate because of proper alignment of feed line and radiating patch is required. As in aperture
coupled, proximity coupled feed also uses two substrates which are selected in the same manner
as in aperture coupled. Feed line is placed between the substrate and the ground plane is kept
below the lower substrate. Feed line is extended a more as a stub.
There are basically two types of bandwidths, impedance bandwidth, defined as the bandwidth
over which the antenna remains matched to the feed line to some specified level such as VSWR
≤ 2 and the pattern bandwidth, defined as the bandwidth over which the pattern remains
constant. The ideal broadband element will meet both the standards.
Fig 3.5(a) aperture coupled feed (b) Electrical Equ. Circuit for aperture.
Fig 3.6(a) Proximity coupled feed
Fig 3.6(b) Electrical Equivalent Circuit of proximity coupled feed
Analysis Of Microstrip Antenna:
There are different methods to analyze the microstrip antenna. The popular models are
transmission line model, cavity model and full wave model. The simplest model of among all is
transmission line model. It is easy to analyze using this model and it is more accurate when
it employed on thin substrate. The disadvantage of this model is, as it gives less precise results
and lacks in versatility. The transmission line model constitutes the microstrip antenna by two
slots distinguished by a low-impedance transmission line of length L, width W and height
H, as shown in Figure 3.6(a).
Fig 3.7 Microstrip line
Fringing Effects Since the patch dimensions are finite along the length and width, the fields
at the patch edges undergo fringing. This is instanced in Fig.3.1 (a, b) for the two radiating
slots of the microstrip antenna. The amount of fringing fields coming from the radiating edges
mainly depends on the dielectric material used in substrate and the height of substrate.
Lower the dielectric constant material results more bowed fringing fields that leads to better
radiation. Therefore lower the dielectric material constant better the radiation. For a microstrip
line, typical electric field lines are shown in Fig.3.12. It can be observed that the fringing
field line is not only travels in substrate but also in the air. Therefore for the more accurate
prediction of the performance of antenna the air should also take into consideration. As L/h ratio
increases and εr >> 1, the fringing field will more concentrate on the substrate. Due to these
fringing fields coming out from the edges the operating length becomes more than the
physical length. Therefore to consider the fringing effect in patch an antenna, an effective
dielectric constant (EDC) is calculated. The value of εreff is nearer to the value of the actual
dielectric constant used. The effective dielectric constant is a function of frequency. For the
higher frequency the value of effective dielectric constant reaches to the actual value. Effective
dielectric constant will be in the range of 1< εreff < εr. As the frequency of operation increases
the fringing fields disappears, because the electric field lines will concentrate inside the
substrate. The graph showing in figure 3.8. The variation of εreff value with frequency for three
different types of materials.
Fig 3.8 Microstrip Electric field lines and effective dielectric constant
For low frequencies, effective dielectric constant is almost constant. At the intermediate
frequencies its values start to gain slowly and finally approach the values of the actual value of
dielectric constant. The initial values also referred to as static values (at lower frequencies) of
effective dielectric constant is given by Eq. 3.1. This value is sensible for W/h > 1.
Fig 3.9 Effective dielectric constant vs. frequency for different substrates
3.1
3.2
Fig 3.10: Physical and effective lengths of rectangular microstrip patch
The characteristics impedance Zo of the microstrip feed line depends on the width W of the
feed line and height h of the substrate and it holds different values for W/h ≤ 1 and W/H
≥ 1 as exemplified in the Eqs 3.1 and 3.2 respectively.
3.3
Eq.3.3 The width W of the line and height h of the substrate is decided by parameters A and B
which are functions of the characteristic impedance of the line and dielectric constant of the
substrate as shown in Eqs. 3.1, 3.2 and 3.3.
3.4
Generally, the characteristics impedance of the feed line is taken as 50Ω. The reason behind choosing
this are described below:
Practically, all source ports that are available having 50Ω internal impedance. Therefore
by Maximum Power Theorem, there is a need to select a feed line of 50Ω characteristic
impedance for the microstrip antenna to transfer maximum power from source to load.
Theoretically, it is determined that 76Ω is required for minimum attenuation in the line
and 37Ω is needed for maximum power transfer from the line. Therefore to compromise
between these two, we are choosing the average of the above two values i.e. 50Ω as the
characteristics impedance for the feed line.
3.3 Advantages and Disadvantages of Microstrip antenna
Microstrip patch antenna has various advantages over conventional microwave
antenna with one similarity of frequency range from 100 MHz to 100 GHz. There are
enormous numbers of advantages and having few disadvantages also, which helps
us to analyze and use it for different applications.
3.3.1 Advantages
•
Light weight, low volume, low profile.
Printed circuits are thin, therefore they require less volume than their waveguide or coaxial. Printed
circuits antennas consist primarily of nonmetallic materials like foam materials as substrates, such
antennas have an extremely low weight compared to conventional antennas.
•
Polarization
Any polarization can be obtained with the versatility of patch geometries. We can realize multi
polarization capability in antennas with single or multiple ports.
•
Dual frequency antennas
We can realize dual-frequency operation in antennas by employing either dual-stacked patches or
a loaded diode or a stub on patch.
•
Excitation technique
Patches allow a various excitation techniques to be employed, compatible with any technology of the
active circuitry and beam forming networks.
•
Suitable for integration with MICs (Microwave Integrated Circuits)
MICs are in good deal to handle and less expensive than the alternative waveguides
3.3.2 Disadvantages
a) Narrow bandwidth
b) Low efficiency
c) Low Gain
d) Extraneous radiation from feeds and junctions
e) Poor end fire radiator except tapered slot antennas
f) Polarization purity is difficult to achieve
g) Low power handling capacity.
h) Surface wave excitation
3.4 Applications
Since the microstrip antenna having advantages like light weight and ease to design and
installation, low cost etc. it is having enormous applications. Initially it brings the applications
in military and satellite. Recently, they are extended to commercial applications.
Some of the applications are listed below:
a) Mobile and satellite applications
b) Global positioning system
c) Radar application
d) Medical applications
e) Worldwide Interoperability for Microwave Access (WiMAX)
f)Radio Frequency Identification (RFID
Department of Electronics and Telecommunication Engineering
Dr. Babasaheb Ambedkar Technological University, Lonere.
20
CHAPTER 4
FRACTAL ANTENNA
4.1 Fractal’s Definition
According to Webster's Dictionary a fractal is defined as being "derived from the Latin fractus
meaning broken, uneven: any of various extremely irregular curves or shape that repeat
themselves at any scale on which they are examined." Mandelbrot offered the following
definition: “A fractal is a shape made of parts similar to the whole in some way” [29].
4.2 Background
A fractal is a rough or fragmented geometric shape that can be subdivided in parts, each of
which is (at least approximately) a reduced-size copy of the whole. Fractals are generally
selfsimilar and independent of scale. There are many mathematical structures that are fractals;
e.g.
Sierpinski gasket, Cantor‟s comb, von Koch‟s curve. Fractals also describe many real-world
objects, such as clouds, mountains, turbulence, and coastlines that do not correspond to simple
geometric shapes. As we see fractals have been studied for about a hundred years and antennas
have been in use for as long. Fractal antennas are new on the scene. The geometry of the fractal
antenna encourages its study both as a multiband solution and also as a small (physical size)
antenna. First, because one should expect a self-similar antenna (which contains many copies
of itself at several scales) to operate in a similar way at several wavelengths. That is, the antenna
should keep similar radiation parameters through several bands. Second, because the
spacefilling properties of some fractal shapes (the fractal dimension) might allow fractal shaped
small antennas to better take advantage of the small surrounding space.
4.3 Why Fractal as Antenna Elements?
Small antennas are of prime importance because of the available space limitation on devices
and the oncoming deployment of diversity and multi-input multi-output (MIMO) systems. The
basic antenna miniaturization techniques can be summarized into lumped element loading,
material loading, and use of ground planes, short circuits, the antenna environment and finally
the antenna geometry. Among these techniques the antenna environment and finally the antenna
geometry optimization and use of ground planes can achieve miniaturization or compactness of
the antenna while maintaining the good antenna performance in terms of bandwidth and
efficiency. However the classical small antennas suffer from inefficient performance. Fractal
geometry provides the solution by designing compact and multiband antennas in a most
efficient and sophisticated way. The general concepts of fractals can be applied to develop
various antenna elements [30]. The properties of these fractal designed antennas allows for
smaller, resonant antennas that are multiband and may be optimized for gain. When antenna
elements or arrays are designed with the concept of self-similarity for most fractals, they can
achieve multiple frequency bands because different parts of the antenna are similar to each other
at different scales. Application of the fractional dimension of fractal structure leads to the gain
optimization of wire antenna and the self-similarity makes it possible to design antennas with
very wideband performance.
4.3.1 Fractals as Space-filling Geometries
A fractal is mathematically defined to be infinite in intricacy, this is not desirable if antennas
are to be fabricated using these geometries. For example, the complexity and repetition of a
cloud does not extend to infinitely small or large scales, but can be approximated as doing so
for a certain band of scales. From the scale of human perception, a cloud does seem to be
infinitely complex in larger and smaller scales. The resulting geometry after truncating the
complexity is called a “prefractal”. A prefractal drop the intricacies that are not distinguishable
in the particular applications. While Euclidean geometries are limited to points, lines, sheets,
and volumes, fractals include the geometries that fall between these distinctions, a fractal can
be a line that approaches a sheet. The line can meander in such a way as to effectively almost
fill the entire sheet. These space-filling properties lead to curves that are electrically very long,
but fit into a compact physical space and can lead to the miniaturization of antenna elements.
As mentioned earlier and indicated by Gianvi (2002) that prefractals drop the complexity in the
geometry of a fractal that is not distinguishable for a particular application. For antennas, this
can mean that the intricacies that are much, much smaller than a wavelength in the band of
useable frequencies can be dropped out. This now makes this infinitely complex structure,
which could only be analyzed mathematically, manufacturable [31]. The Hilbert curve is an
example of a space-filling fractal curve that is self-voiding (i.e., has no intersection points).
The first four steps in the construction of the Hilbert curve are shown in Fig. 4.1.
Fig. 4.1 Generation of four iterations of Hilbert curves
4.3.2 Fractals as Miniaturized Antennas
A fractal can fill the space occupied by the antenna in a more effective manner than the
traditional Euclidean antenna. This leads to more effective coupling of energy from feeding
transmission lines to free space in less volume. Fractal loop and fractal dipole wire radiators
are contrasted with linear loop and dipole antennas, fractals effectively fills the space and
because of fractal dimensions allows antenna miniaturization. Fractal antennas do not need to
be limited to only wire antennas. 4.3.3 Fractals as Multiband Antennas
Fractal antennas show multiband or log periodic behavior that has been attributed to selfsimilar
scale factor of the antenna geometry. Fractal loop shows improved impedance and SWR
performance on a reduced physical area when compared to non-fractal Euclidean geometries.
In order to enable more operating bands within lower spectrum, a higher scaling factor is
required. Fractal antenna represents a class of electromagnetic radiators where the overall
structure is comprised of a series of repetition of a single geometry and where repetition is at
different scale.
4.4 Fractal Geometry
There are many fractal geometries that have been found to be useful in developing new and innovative
design for antennas. Fig. 3.2 shows some of these unique geometries [32].
Fig. 4.2 Example of other fractal antennas
4.4.1 Sierpinski Carpet
Sierpinski Carpet fractal antenna is realized by successive iterations applied on a simple square
patch as shown in Fig. 4.3(a), which can be termed as the zeroth order iteration. A square of
dimension equal to one third of the main patch is subtracted from the center of the patch to
retrieve first order iteration, as shwn in Fig. 4.3(b). The next step is to etch squares which are
nine times and twenty seven times smaller than the main patch as demonstrated in Fig. 4.3(c)
and 4.3(d) respectively. The second and third order iterations are carried out eight times and
sixty four times respectively on the main patch. This fractal can be termed as third order fractal
as it is designed by carrying out three iterations. The pattern can be defined in such a way that
each consequent etched square is one-third in dimension as compared to the previous one
sharing the same center point. This procedure of design carried out on a square shaped patch
can be implemented on any of the four geometries named above [33].
Fig. 4.3 Four stages in construction of sierpinski carpet
4.4.2 Koch Curves
The geometric construction of the standard Koch curve is fairly simple. It starts with a straight
line as an initiator. This is partitioned into three equal parts, and the segment at the middle is
replaced with two others of the same length. This is the first iterated version of the geometry
and is called the generator. The process is reused in the generation of higher iterations [34].
4.4.3 Hilbert Curves
Fig. 4.5 shows the first few iterations of Hilbert curves. It can be noticed that each successive
stage consists of four copies of the previous, connected with additional line segments. This
geometry is a space-Filling curve, since with a larger iteration, one may think of it as trying to
fill the area it occupies. Additionally the geometry also has the following properties:
selfAvoidance (as the line segments do not intersect each other), simplicity (since the curve can
be drawn with a single stroke of a pen) and self-similarity [32].
Fig. 4.4 Step of construction of Koch curves geometries
Fig. 4.5 Four stage in construction of Hilbert curves
4.4.4 Sierpinski Gasket Geometry
Sierpinski gasket geometry is the mostly widely studied fractal geometry for antenna
applications. Sierpinski gaskets have been investigated extensively for monopole and dipole
antenna configurations. The self-similar current distribution on these antennas is expected to
cause its multi-band characteristics. It has been found that by perturbing the geometry the multiband nature of these antennas can be controlled. Variations of the flare angle of these geometries
have also been explored to change the band characteristics of antenna. Antennas using this
geometry have their performance closely linked to conventional bow-tie antennas.
However some minor differences can be noticed in their performance characteristics. It has
been found that the multi-band nature of the antenna can be transformed into wideband
characteristics by using a very high dielectric constant substrate and suitable absorbing
materials [35].
4.4.5 Circular Microstrip Patch Antenna
First, a nearly circular metallic patch is designed. Then a point star shaped fractal geometry
with some sharpness factor and dimension is subtracted from solid nearly circular patch as
shown in Fig. 4.6 to expose substrate material to create first fractal iteration. Proper care has
been taken to maintain electrical connectivity throughout the circular boundary. Such four
electrically interactive iterations are included in the antenna geometry to design the final fractal
geometry for the proposed antenna as shown in Fig. 4.6 [36].
Fig. 4.6 Circular microstrip patch antenna
4.4.6 Giuseppe Piano fractal
The recursive procedure of the Giuseppe Piano fractal is shown in Fig. 4.7, which is applied to
the edges of the square patch up to the second iteration as depicted in Fig. 4.8. The iterations of
Sierpinski carpet fractal are shown in Fig. 4.3. The proposed antenna applies the above two
fractals to the square patch as shown in Fig. 4.9. The antenna feed is through a microstrip line
with a matching section over a semi-elliptical ground plane. The ground plane is selected as a
combination of the rectangular and semi-elliptical shapes in order to obtain an approximately
linear phase variation for the transmission and reception of narrow pulses in UWB systems.
The group delay should be nearly constant across the frequency band [37]. As the iteration of
fractal geometry increases, its resonance frequency decreases, this may lead to an effective
antenna miniaturization. However, for iterations higher than the second iteration, the reduction
of operating frequency is not achievable since the antenna design becomes quite complicated
and its fabrication becomes difficult [38].
Fig. 4.7 Initiator and generator of the Giuseppe piano fractal
Fig. 4.8 Giusepe peano fractal as applied to the edges of the metallic patches
Fig. 4.9 Proposed geometry
4.4.7 Pythagorean tree fractal
The construction of the Pythagoras tree begins with a square. Upon this square are constructed
two other squares, each scaled down by a linear factor, such that the corners of the squares
coincide pairwise. The same procedure is then applied recursively to the two smaller squares,
ad infinitum [39]. Fig. 4.10 shows an illustration of the first five iterations in the construction
process. Iteration in the construction adds squares of size, for a total area of 1. Thus, the area of
the tree fractal might seem to grow without boundary [40]–[42]. However, starting at the fifth
iteration, some of the squares overlap, and the tree fractal actually has a finite area because it
snuggles into a box. We design an MPTF by eliminating the first iterations large side square and
change the isosceles right-angled triangle to an isosceles triangle with steep angles to reduce the
fractal height to design compact antennas. This triangle change is our fractal freedom degree
that helps the antenna designer to make a novel fractal shape. Our purpose in designing an MPTF
is to use this fractal to control impedance bandwidth and resonances [43].
Fig. 4.10 Illustration of the first five iterations for Pythagorean tree fractal
4.4.8 Fractal Arrays
The idea of using fractal spacing for arrays has been investigated by several researches. In
references [20, 21, 22], the spacing of the array was shaped using fractal geometries, while the
elements were standard Euclidean shapes. Arrays with the distribution of a cantor set has been
the topic of these papers, [23, and 24]. A cantor set distribution is also implemented in [25,] for
the spacing of an array .Similar features of the patterns of the arrays compare to similarities in
the spacing geometry. Also, Cantor Sets of the different fractal dimension are simulated,
showing a correlation between the maximum side lobe level and the fractal dimension. A
Department of Electronics and Telecommunication Engineering
Dr. Babasaheb Ambedkar Technological University, Lonere.
30
derivation of the radiation patterns for cantor sets distributed currents is presented in [49]. In
paper [50], the authors present an analysis of array elements in a Sierpinski carpet configuration
to create sum and difference patterns. Simple procedure for evaluating the impedance matrix of
the Peano-Gosper fractal array has been presented in [51]. Phased array antennas can be focused
by deploying proper phase excitations on the array elements. Fractal geometry has been applied
to focused arrays.
CHAPTER 5
PROPOSED HEXAGONAL MICROSTRIP FRACTAL ANTENNA
5.1 Introduction
Fractals are defined by Beniot Mandelbrot as structures whose dimensions are not whole
numbers. These structures are occurring in and around us in nature. Some of the natural
fractals are flowers, branching of leaves and plants. Fractals have two common properties
irrespective of its structure they are (1) self-similarity and (2) space filling [6].By using the
fractal geometry in the design of antennas, antenna with reduced size and antenna with
multiband resonance is achieved. Fractals also have an attractive feature of infinite perimeter
[11].Ultra-wideband (UWB) wireless communication technique which is suitable for short
range wireless systems. UWB is an emerging technology which is used in academia, industry
and research [8]. UWB technology offers numerous advantages like low transmission power,
higher data rate, and ability to work in low SNR, resistance to jamming, immune to multipath
propagation [1] .Federal Council Commission (FCC) allotted the bandwidth 3.1GHz to
10.6Hz and named this spectrum as UWB frequency band. UWB technology finds its
application in Ground Penetrating Radar systems, wall-imaging systems, communication
systems measurement systems, automotive collision avoidance system and radar level
gauges [12]. Coplanar Waveguide (CPW) is used to power the antenna [3].This feeding
mechanism provides immense advantages over conventional microstrip line feed. Some of
the merits of CPW are shorter power delivery paths causing low inductance, structural
strength, thermal cooling, layout density, minimum cross talk and excellent isolation [2].
This hexagonal shaped microstrip fractal antenna which operates on UWB frequencies. The
radiating patch consists of a total of 44 hexagons within the monopole hexagon [1] [14]. To
match the input impedance and output impedance a two-step feed configuration is used. A
monopole hexagonal patch is first designed, simulated and results are analyzed then fractals
are introduced within the monopole hexagonal patch.
Fig. 5.1.Basic Hexagonal Dimensions: Horizontal Radius (h), Vertical Radius (v), Edge Radius
5.2 Antenna Design
A Hexagonal Shaped Monopole patch is designed and it is depicted in Fig.5.2. (a). A Co-Planer
Waveguide feed mechanism is used to feed the antenna. The patch consists of a hexagon and
input is fed through of its edges through CPW mechanism [5]. Polyamide substrate is used as a
substrate of thickness 1.6mm. Dielectric constant of substrate is 4.4 and the relative
permeability is 1 [1].The monopole patch has basic dimensions namely horizontal radius (h),
vertical radius (v) and edge radius (e). For the transformation of monopole patch into fractal
three different dimensions of hexagons are designed and they are subtracted from the monopole
patch. As mentioned above three important dimensions are defined namely vertical radius,
horizontal radius and edge radius. For monopole patch the dimensions are horizontal radius (h)
=10.3mm, vertical radius (v) =13.7 mm and edge radius (e) =13.7 mm. Then one edge of the
hexagonal patch is chosen and CPW feed is given and the antenna radiates. The results of the
antenna are analyzed and fractals are introduced in the patch [4]. The first level fractal of a big
hexagon is made at the center of the patch because the current distribution is very less at the
center of the radiating patch. The big fractal hexagon has the parameters of horizontal radius
(h1) =3.096mm, vertical radius (v1) =3.73 mm and edge radius (e1) =3.73 mm. Then six
hexagons of medium dimensions of horizontal radius (h2) = 1.365mm, vertical radius (v2)
=1.577 mm and edge radius (e2) = 1.577 mm are subtracted from the monopole patch and these
are placed on all the sides of the biggest hexagonal cut made earlier from the monopole patch.
In a similar fashion the next fractal structures of very smaller dimensions of horizontal radius
(h3) = 0.454mm, vertical radius (v2) =0.525 mm and edge radius (e2) = 0.525 mm are
subtracted from the monopole patch [12]. After performing all the subtractions of 44 hexagons,
a new hexagonal shaped fractal antenna is formed as shown in Fig.5.2.
(i)
(ii)
(iii)
( iv)
Fig.2. Iterations on
(a)
(b)
(c)
(d)
Fig.5.2. Iterations on monopole patch
Fig.5.2 (a) shows a monopole patch in which one of its edges is selected to give the feed for
the antenna [1].For the first iteration a big hexagon of above specified dimension is designed
and it is subtracted from the monopole patch which is depicted in Fig.5.2 (b). For the second
iteration hexagons of medium dimensions are designed and subtracted from the first iterated
fractal, this is depicted in Fig.5.2(c). For the third iteration smaller hexagons are designed
around the four sides of medium hexagon and it is subtracted from the second iterated fractal,
it is depicted in Fig.5.2 (d).
5.3 Calculations
Input Parameters-
Frequency (fo) = 3.1 GHz
Material=FR4
Dielectric Constant (Ɛr) = 4.4
Thickness of material (h) = 1.6mm
Width
W=
= 29.44mm
Effective dielectric constant
Ɛreff =
Resonant length
Effective Length = Leff =
24.12 mm
Length Extension =
∆L = 0.412 h
= 0.735 mm
Length =
L = Leff - 2∆L = 22.65 mm
CHAPTER 6
RESULTS
6.1 Simulated Results of Hexagonal Microstrip Patch
The simulation of the microstrip patch antenna designed in a HFSS simulator is shown in the
figure below: Width [W] = 29 mm, Length [L] =22 mm, Substrate thickness=1.6 mm, Relative
permittivity of substrate = 4.4
(a)
(b)
(c)
Fig. 6.1. Monopole (a) Return Loss (b) VSWR(c) Radiation Pattern 2D
Fig.6.1 (a) shows the return loss of monopole. The parameter return loss is a figure of merit that
mathematically describes the impedance matching between transmission line and antenna. This
transfers happens only when characteristic impedance is matched with input impedance of
antenna otherwise reflected waves are generated which results in the degraded performance of
an antenna. Ideally reflected waves must be zero. In this iteration gives good impedance
matching at 9.6 GHz with return loss of -29.49 dB for X band application, 5.9 GHz with return
loss of -22.95 dB for C band application, 7.8 GHz with return loss of -21.59 dB for C band
application, 6.8 GHz with return loss of -16.10 dB for C band application, 3.5 GHz with return
loss of -14.98 dB for Wi-MAX, 5.1 GHz with return loss of -11.45 dB for WLAN.
Reflected waves are responsible for VSWR. It’s worthy to note that at the particular frequencies
the VSWR plot goes below 2 which essentially mean the S11 plot goes beyond -10 dB levels
at those points. The VSWR plot is well below 2 at 3.5 GHz, 6.8 GHz, 5.1 GHz, 5.9 GHz, 7.8
GHz and 9.6 GHz frequencies. The VSWR plot for the proposed geometry is shown in fig.6.1
(b).
The term radiation pattern is otherwise called as far-field pattern (farthest region away from
antenna, irrespective of the distance). The distribution of field and power are independent of
distance. It refers to the angular (directional) dependence of the strength of radio waves from
the antenna (power radiated from the antenna). The polar plot of far-field at central operating
frequency at 6.5 GHz is shown in fig. 6.1(c).
(a)
(b)
(c)
Fig. 6.2. 1st Iteration (a) Return Loss (b) VSWR(c) Radiation Pattern 2D
In this iteration gives good impedance matching at 5.9 GHz with return loss of -20.43 dB for C
band application, 6.7 GHz with return loss of -21.89 dB for C band application, 7.01 GHz with
return loss of -13.99 dB for C band application, 7.8 GHz with return loss of -25.30 dB for C
band application, 9.8 GHz with return loss of -11.37 dB for X band application, 10.3 GHz with
return loss of -11.09 dB for X band application shown in fig.6.2 (a).
The VSWR plot is well below 2 at 5.9 GHz, 6.7 GHz, 7.01 GHz, 10.3 GHz, 7.8 GHz and 9.8 GHz
frequencies. The VSWR plot for the proposed geometry is shown in fig.6.2 (b).
The polar plot of far-field at central operating frequency at 6.5 GHz is shown in fig. 6.2 (c).
(a)
(b)
Department of Electronics and Telecommunication Engineering
Dr. Babasaheb Ambedkar Technological University, Lonere.
40
(c)
Fig. 6.3. 2nd Iteration (a) Return Loss (b) VSWR(c) Radiation Pattern 2D
In this iteration gives good impedance matching at 9.2 GHz with return loss of -42.54 dB for X
band application, 5.8 GHz with return loss of -25.59 dB for Wi-MAX, 6.6 GHz with return loss
of -23.63 dB for C band application, 7.05 GHz with return loss of -18.57 dB for C band
application, 7.7 GHz with return loss of -20.81 dB for C band application, 8.8 GHz with return
loss of -15.30 dB for X band application shown in fig.6.3 (a).
The VSWR plot is well below 2 at 5.8 GHz, 6.6 GHz, 7.05 GHz, 7.7 GHz, 8.8 GHz and 9.2 GHz
frequencies. The VSWR plot for the proposed geometry is shown in fig.6.3 (b).
The polar plot of far-field at central operating frequency at 6.5 GHz is shown in fig. 6.3 (c).
(a)
(b)
(c)
Fig. 6.4. 3rd Iteration (a) Return Loss (b) VSWR(c) Radiation Pattern 2D
In this iteration gives good impedance matching at 5.8 GHz with return loss of -22.19 dB for
Wi-MAX, 6.6 GHz with return loss of -17.18 dB for C band application, 7.01 GHz with return
loss of -12.9 dB for C band application, 7.7 GHz with return loss of -15.98 dB for C band
application, 8.8 GHz with return loss of -20.32 dB for X band application, 9.1 GHz with return
loss of -20.54 dB for X band application 10.1 GHz with return loss of -11.80 dB for X band
application shown in fig.6.4 (a).
The VSWR plot is well below 2 at 5.8 GHz, 6.6 GHz, 7.01 GHz, 10.1 GHz, 9.1 GHz, 7.7 GHz and
8.8 GHz frequencies. The VSWR plot for the proposed geometry is shown in fig.6.4 (b).
The polar plot of far-field at central operating frequency at 6.5 GHz is shown in fig. 6.4 (c).
Table 6.1: Comparisons for All Results of Antennas
Sr.No.
Iterations
Frequencies (GHz)
Return Loss (dB)
VSWR
Bandwidth (MHz)
1
Monopole
6.8
-16.1
1.37
190
8.9
-12.11
1.65
168
9.6
-29.49
1.06
232
5.9
-20.43
1.21
132
6.7
-21.89
1.17
192
7.8
-25.3
1.11
142
6.6
-23.63
1.14
183
8.8
-15.3
1.41
240
9.2
-42.54
1.01
261
5.8
-22.19
1.16
151
6.6
-17.18
1.32
201
8.8
-20.32
1.21
205
9.1
-20.54
1.2
204
2
3
4
First
Iteration
Second
Iteration
Third
Iteration
6.2 Fabricated Results of Hexagonal Microstrip Patch
Antenna design is fabricated using PCB Prototyping machine and it is tested using vector
network analyzer (VNA). VNA is one of the most essential RF and microwave measurement
approaches. A network analyzer is an instrument that measures the network parameters of
electrical networks. Today, network analyzers commonly measures S-parameters because
reflection and transmission of electrical networks are easy to measure at high frequencies, but
there are other network parameter sets as Y-parameters, Z-parameters and H-parameters.
Fig. 6.5: Report of Reflection Coefficient
Fig.6.6: Report of voltage standing waves ratio (VSWR-Fabricated)
CHAPTER 7
CONCLUSION AND FUTURE SCOPE
7.1 Conclusion
This Work demonstrates the outline of hexagonal molded monopole fractal antenna where littler
components are subtracted from the monopole fix to get the fractal geometry. It is appropriate
for UWB recurrence of around 3.1GHz to 10.6GHz with not exactly - 5dB in the whole band.
Main concern of the thesis is to study of multiband patch antenna using different techniques
and frequency ratio of the microstrip antenna.
As the very less copper portion from patch is etched out which helps to achieve the high gain
of the antenna to use it for satellite application. The simulated results and measured results are
matched well up to great extent.
7.2 Future Scope
The furthermore miniaturization is possible to compact the size of the antenna. Reconfigurable antenna
elements can be used to enhance diversity performance.
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