CHAPTER 1 INTRODUCTION 1.1 MICROSTRIP PATCH ......Common Microstrip antenna shapes are square,...
Transcript of CHAPTER 1 INTRODUCTION 1.1 MICROSTRIP PATCH ......Common Microstrip antenna shapes are square,...
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CHAPTER 1
INTRODUCTION
1.1 MICROSTRIP PATCH ANTENNA:
Microstrip patch antenna is a type of radio antenna with low profile, which
can be mounted on a flat surface. It consists of a flat rectangular sheet or patch of
metal, mounted over a larger sheet of metal called ground plain.
A patch antenna is a narrowband, wide-beam antenna fabricated by etching
the antenna element pattern in metal trace bonded to an
insulating dielectric substrate, such as a printed circuit board, with a continuous
metal layer bonded to the opposite side of the substrate which forms a ground
plane. Common Microstrip antenna shapes are square, rectangular, circular and
elliptical, but any continuous shape is possible. Some patch antennas do not use a
dielectric substrate and instead are made of a metal patch mounted above a ground
plane using dielectric spacers; the resulting structure is less rugged but has a
wider bandwidth. Because such antennas have a very low profile, are mechanically
rugged and can be shaped to conform to the curving skin of a vehicle, they are
often mounted on the exterior of aircraft and spacecraft, or are incorporated
into mobile radio communications devices.
They are usually employed at UHF and higher frequencies because
the size of the antenna is directly tied to the wavelength at the resonant frequency.
An advantage inherent to patch antennas is the ability to
have polarization diversity. Patch antennas can easily be designed to have vertical,
https://en.wikipedia.org/wiki/Light_beamhttps://en.wikipedia.org/wiki/Dielectrichttps://en.wikipedia.org/wiki/Printed_circuit_boardhttps://en.wikipedia.org/wiki/Ground_planehttps://en.wikipedia.org/wiki/Ground_planehttps://en.wikipedia.org/wiki/Bandwidth_(signal_processing)https://en.wikipedia.org/wiki/Mobile_radiohttps://en.wikipedia.org/wiki/UHFhttps://en.wikipedia.org/wiki/Wavelengthhttps://en.wikipedia.org/wiki/Resonancehttps://en.wikipedia.org/wiki/Polarization_(waves)
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horizontal, right hand circular (RHCP) or left hand circular (LHCP) polarizations,
using multiple feed points, or a single feed point with asymmetric patch structures.
This unique property allows patch antennas to be used in many types of
communications links that may have varied requirements.
1.1.1 RECTANGULAR PATCH:
The most commonly employed Microstrip antenna is a rectangular patch
which looks like a truncated Microstrip transmission line. It is approximately of
one-half wavelength long. When air is used as the dielectric substrate, the length of
the rectangular Microstrip antenna is approximately one-half of a free-
space wavelength. As the antenna is loaded with a dielectric as its substrate, the
length of the antenna decreases as the relative dielectric constant of the substrate
increases. The resonant length of the antenna is slightly shorter because of the
extended electric "fringing fields" which increase the electrical length of the
antenna slightly. An early model of the Microstrip antenna is a section of
Microstrip transmission line with equivalent loads on either end to represent the
radiation loss.
With the development of MIC and high frequency semiconductor devices,
Microstrip has drawn the maximum attention of the antenna community in recent
years. In spite of its various attractive features like, light weight, low cost, easy
fabrication, conformability on curved surface and so on, the Microstrip element
suffers from an inherent limitation of narrow impedance bandwidth
A WLAN is a flexible data communication network used as an extension to,
or an alternative for, a wired LAN in a building. Primarily they are used in
industrial sectors where employees are on the move, in temporary locations or
https://en.wikipedia.org/wiki/Microstriphttps://en.wikipedia.org/wiki/Wavelengthhttps://en.wikipedia.org/wiki/Dielectric_constant
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where cabling may hinder the installation of wired LAN. Increasingly more and
more wireless LANs are being setup in home and or home office situations as the
technology is becoming more affordable. Industry giants are already predicting that
90% of all notebooks will contain integrated WLAN by 2008. With progress and
expansion comes the need for faster technology and higher transfer rates. The
ongoing wireless LAN standardization and Research & Development activities
worldwide, which target transfer rates higher than 100 Mbps, justify the fact that
WLAN technology will play a key role in wireless data transmission. Cellular
network operators have recognized this fact, and strive to exploit WLAN
technology and integrate this technology into their cellular data networks.
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CHAPTER 2
FEEDING TECHNIQUES
Feeding technique influences the input impedance and polarization
characteristics of the antenna. There are three most common structures that are
used to feed planar printed antennas. These are coaxial probe feeds, Microstrip line
feeds, and aperture coupled feeds. With coaxial probe feed the centre conductor of
the coaxial connector is soldered to the patch. The coaxial fed structure is often
used because of ease of matching the characteristic impedance to that of the
antenna. Along with this, the parasitic radiation from the feed network tends to be
insignificant. Microstrip line-fed structures are more suitable compared to probe
feeds, due to ease of fabrication and lower costs. Serious drawbacks of this feed
structure are the strong parasitic radiation and it requires a transformer, which
restricts the broadband capability of the antenna. The aperture-coupled structure
has all of the advantages of the former two structures and isolates the radiation
from the feed network thereby leaving the main antenna radiation uncontaminated.
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2.1 TYPES OF FEEDING TECHNIQUES:
A feed line is used to excite to radiate by direct or indirect contact. There are
many different methods of feeding and 4 most popular methods are:
Microstrip line feed
Co-axial probe
Aperture coupling
Proximity coupling
2.1.1 MICROSTRIP LINE FEEDING:
Microstrip line feed is one of the easier methods to fabricate as it is a just
conducting strip connecting to the patch and therefore be considered as extinction
of patch. It is simple to model and easy to match by controlling the insert position..
Microstrip line feed is a conducting strip that is connected directly to the
edge of the Microstrip patch. It has an advantage that the feed can be etched on the
same substrate to provide a planar structure. However the disadvantage of this
method is that as substrate thickness increases, surface wave and spurious feed
radiation increases which limit the bandwidth
2.1.2 CO-AXIAL FEEDING:
Microstrip antennas can also be fed from underneath via a probe. Co-axial
feeding is feeding method in which the inner conductor of the co-axial is attached
to the radiation patch of the antenna while the outer conductor is connected to the
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ground plane (ie) the outer conductor of the coaxial cable is connected to the
ground plane, and the center conductor is extended up to the patch antenna.
FIG 2.1: Coaxial cable feed of patch antenna.
The position of the feed can be altered as before (in the same way as the
inset feed, above) to control the input impedance.
The coaxial feed introduces an inductance into the feed that may need to be
taken into account if the height h gets large (an appreciable fraction of a
wavelength). In addition, the probe will also radiate, which can lead to radiation in
undesirable directions.
The Advantages are
Easy to fabricate
Easy to match
Low spurious radiation
The Disadvantages are
Narrow bandwidth
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Difficult to model specially for thick substrate
Possess inherent asymmetries which generate higher order
modes which produce cross polarization radiation.
2.1.3 APERTURE COUPLING:
Another method of feeding Microstrip antennas is the aperture feed. In this
technique, the feed circuitry (transmission line) is shielded from the antenna by a
conducting plane with a hole (aperture) to transmit energy to the antenna. Aperture
coupling consists of two different substrate separated by a ground plane. On the
bottom side of the lower substrate there is a Microstrip feed line whose energy is
coupled to the patch through a slot on the ground plane separating two substrates.
This arrangement allows independent optimization of the feed mechanism and the
radiating element. Normally top substrate uses a thick low dielectric constant
substrate while for the bottom substrate; it is the high dielectric substrate. The
ground plane, which is in the middle, isolates the feed from radiation element and
minimizes interferences of spurious radiation for pattern formation and
polarization purity.
FIG 2.2: Aperture coupled feed.
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The upper substrate can be made with a lower permittivity to produce
loosely bound fringing fields, yielding better radiation. The lower substrate can be
independently made with a high value of permittivity for tightly coupled fields that
don't produce spurious radiation. The disadvantage of this method is increased
difficulty in fabrication.
The Advantages are
Allows independent optimization of fed mechanism element.
2.1.4 PROXIMITY COUPLING:
Proximity coupling has the largest bandwidth, has low spurious radiation.
However fabrication is difficult. Length of the feeding stub and width - to - length
ratio of patch is used to control the match.
2.1.5 INSET FEED:
Input impedance could be reduced by inserting an inset feed closer to the
centre of the patch. So this is used to tune the input impedance to the desired value.
Typically patch antenna yields a high input impedance, the current is low at the
ends of a half-wave patch and increases in magnitude towards the centre, the input
impedance (Z=V/I) could be reduced if the patch was fed closer to the centre. One
method of doing this is by using an inset feed (a distance R from the end). Since
the current has a sinusoidal distribution, moving in a distance R from the end will
increase the current by cos (pi*R/L) – this is just nothing that the wavelength is
2*L, and so the phase difference is 2*pi*R/(2*L)=pi*R/L.
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The voltage also decreases in magnitude by the same amount that the current
increases. Hence, using Z=V/I, the input impedance scales as:
Zin (R) = cos2(
𝜋𝑅
𝐿) Zin(0)
In the above equation, Zin(0) is the input impedance if the patch was feed at
the end. Hence, by feeding the patch antenna as shown, the input impedance can be
decreased. As an example, if R=L/4, then cos (pi*R/L)= cos (pi/4), so that
[cos(pi/4)]^2=1/2. Hence, a (1/8) – wavelength inset would decrease the input
impedance by 50%. This method can be used to tune the input impedance to the
desired value.
2.1.5.1 FED WITH A QUARTER-WAVELENGTH TRANSMISSION LINE
The Microstrip antenna can also be matched to a transmission line of
characteristic impedance Z0 by using a quarter-wavelength transmission line of
characteristic impedance Z1.
FIG 2.3:Patch antenna with a quarter-wavelength matching section
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The goal is to match the input impedance (Zin) to the transmission line (Z0).
If the impedance of the antenna is ZA, then the input impedance viewed from the
beginning of the quarter-wavelength line becomes
This input impedance Zin can be altered by selection of the Z1, so that
Zin=Z0 and the antenna is impedance matched. The parameter Z1 can be altered
by changing the width of the quarter-wavelength strip. The wider the strip is, the
lower the characteristic impedance (Z0) is for that section of line.
2.1.6 COUPLED (INDIRECT) FEEDS:
The feeds above can be altered such that they do not directly touch the
antenna. For instance, the probe feed can be trimmed such that it does not extend
all the way up to the antenna. The inset feed can also be stopped just before the
patch antenna.
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FIG 2.4: Coupled (indirect) inset feed.
The advantage of the coupled feed is that it adds an extra degree of freedom
to the design. The gap introduces a capacitance into the feed that can cancel out the
inductance added by the probe feed.
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CHAPTER 3
FOUNDATIONS FOR MICROSTRIP DESIGN
A Microstrip patch antenna is a radiating patch on one side of a dielectric
substrate, which has a ground plane on the underside. The EM waves fringe off the
top patch into the substrate, reflecting off the ground plane and radiates out into the
air. Radiation occurs mostly due to the fringing field between the patch and
ground.
FIG 3.1: Operations of a Microstrip Patch
The radiation efficiency of the patch antenna depends largely on the
permittivity (εr) of the dielectric. Ideally, a thick dielectric, low εr and low insertion
loss is preferred for broadband purposes and increased efficiency. The advantages
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of Microstrip antennas are that they are low-cost, conformable, light weight and
low profile, while both linear and circular polarization is easily achieved. These
attributes are desirable when considering antennas for WLAN systems. Some
disadvantages include such as a narrow bandwidth as well as a low gain (~6 dB)
and polarization purity is hard to achieve.
3.1 POLARIZATION TYPES:
This is the polarization of the wave radiated by the antenna in that particular
direction. This is usually dependant on the feeding technique. When the direction
is not specified, it is in the direction of maximum radiation. Shown below are two
most widely used polarization types.
3.1.1 LINEAR POLARIZATION:
A slot antenna is the counterpart and the simplest form of a linearly
polarized antenna. On a slot antenna the E field is orientated perpendicular to its
length dimension. The usual Microstrip patches are just different variations of the
slot antenna and all radiate due to linear polarization. The operations of a linearly
polarized wave radiating perpendicular to the patch plane.
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FIG 3.2: Linear Polarization
3.1.2 CIRCULAR POLARIZATION:
Circular polarization (CP) is usually a result of orthogonally fed signal input.
When two signals of equal amplitude but 90 degree phase shifted the resulting
wave is circularly polarized. Circular polarization can result in Left hand circularly
polarized (LHCP) where the wave is rotating anticlockwise, or Right hand
circularly polarized (RHCP) which denotes a clockwise rotation. The main
advantage of using CP is that regardless of receiver orientation, it will always
receive a component of the signal. This is due to the resulting wave having an
angular variation.
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FIG 3.3: Circular Polarization
3.2 BANDWIDTH:
The bandwidth of the patch is defined as the frequency range over which it
is matched with that of the feed line within specified limits. In other words, the
frequency range over which the antenna will perform satisfactorily. This means the
channels have larger usable frequency range and thus results in increased
transmission. The bandwidth of an antenna is usually defined by the acceptable
standing wave ratio (SWR) value over the concerned frequency range.
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FIG 3.4: Narrowband vs. Broadband
Most commercial antennas use a 1.5:1 ratio, suggesting that the range that is
covered between the SWR of 1 up to 1.5 is the bandwidth. To ensure comparability
with the commercial products, a decision was made to use a 1.5:1 ratio to calculate
the bandwidth of antennas.
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FIG 3.5: Bandwidth Measurement
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CHAPTER 4
DESIGN METHODOLOGY FOR RECTANGULAR PATCH
4.1 ANTENNA SHAPE:
In its most basic form, a Microstrip patch antenna consists of a radiating
patch on one side of a dielectric substrate which has a ground plane on the other
side. The patch is generally made of conducting material such as copper or gold
and can take any possible shape. The radiating patch and the feed lines are usually
photo etched on the dielectric substrate.
Microstrip patch antennas radiate primarily because of the fringing fields
between the patch edge and the ground plane. For good antenna performance, a
thick dielectric substrate having a low dielectric constant is desirable since this
provides better efficiency, larger bandwidth and better radiation.
FIG 4.1: Structure of Microstrip Patch Antenna
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4.2 METHOD OF ANALYSIS:
Transmission line model represents the Microstrip antenna by two slots of
width W and height h, separated by a transmission line of length L. The Microstrip
is essentially a non-homogeneous line of two dielectrics, typically the substrate and
air.
Most of the electric field lines reside in the substrate and parts of some lines
in air. As a result, this transmission line cannot support pure transverse electric-
magnetic (TEM) mode of transmission, since the phase velocities would be
different in the air and the substrate. Instead, the dominant mode of propagation
would be the quasi-TEM mode. Hence, an effective dielectric constant (εreff) must
be obtained in order to account for the fringing and the wave propagation in the
line.
FIG 4.2: Electric Field Lines
The value of (εreff) is slightly less then εr because the fringing
fields around the periphery of the patch are not confined in the dielectric substrate
but are also spread in the air.
εreff = 𝜀𝑟+1
2 +
𝜀𝑟−1
2 [1+12
ℎ
𝑊 ]1/2
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Where,
εreff= Effective dielectric constant
εr = Dielectric constant of substrate
h= Height of dielectric substrate
W= Width of the patch
Consider, a rectangular Microstrip patch antenna of length L, width W
resting on a substrate of height h. The co-ordinate axis is selected such that the
length is along the x direction, width is along the y direction and the height is along
the z direction.
In order to operate in the fundamental TM10 mode, the length of the patch
must be slightly less than λ/2 where λ is the wavelength in the dielectric medium
and is equal to λ0/√εreff.
where,
λ0 is the free space wavelength.
The TM10 mode implies that the field varies one λ/2 cycle along the length,
and there is no variation along the width of the patch. In the Figure 4 shown below,
the Microstrip patch antenna is represented by two slots, separated by a
transmission line of length L and open circuited at both the ends. Along the width
of the patch, the voltage is maximum and current is minimum due to the open ends.
The fields at the edges can be resolved into normal and tangential components with
respect to the ground plane.
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FIG 4.3: Microstrip Patch Antenna
It is seen that the normal components of the electric field at the two edges
along the width are in opposite directions and thus out of phase since the patch is
λ/2 long and hence they cancel each other in the broadside direction.
The tangential components which are in phase, means that the resulting
fields combine to give maximum radiated field normal to the surface of the
structure. Hence the edges along the width can be represented as two radiating
slots, which are λ/2 apart and excited in phase and radiating in the half space above
the ground plane. The fringing fields along the width can be modeled as radiating
slots and electrically the patch of the Microstrip antenna looks greater than its
physical dimensions. The dimensions of the patch along its length have now been
extended on each end by a distance ΔL.
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FIG 4.4: Top View of Antenna
∆L = 0.412 (εreff±0.3)(
W
h+0.264)
(εreff−0.258)(W
h+0.8)
The effective length of the patch Leff now becomes:
Leff = L+2∆L
For a given resonance frequency f0 , the effective length is
Leff = 𝑐
2𝑓0 √εreff
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For a rectangular Microstrip patch antenna, the resonance frequency for any
TMmn mode is
f0 = 𝑐
2√εreff [(
𝑚
𝐿)2 + (
𝑛
𝑊)2]
1
2
where m and n are modes along L and W respectively.
For efficient radiation, the width W is
W = 𝑐
2𝑓0 √(εr+1)
2
4.3 FEED POINT:
4.3.1 MICROSTRIP INSET FEED:
Previously, the patch antenna was fed at the end as shown here. Since this
typically yields a high input impedance, we would like to modify the feed. Since
the current is low at the ends of a half-wave patch and increases in magnitude
toward the center, the input impedance (Z=V/I) could be reduced if the patch was
fed closer to the center. One method of doing this is by using an inset feed (a
distance R from the end).
http://www.antenna-theory.com/antennas/patches/patch.php
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FIG 4.5: Patch Antenna with an Inset Feed.
Since the current has a sinusoidal distribution, moving in a distance R from
the end will increase the current by cos(pi*R/L) - this is just noting that the
wavelength is 2*L, and so the phase difference is 2*pi*R/(2*L) = pi*R/L.
The voltage also decreases in magnitude by the same amount that the current
increases. Hence, using Z=V/I, the input impedance scales as:
In the above equation, Zin(0) is the input impedance if the patch was fed at
the end. Hence, by feeding the patch antenna as shown, the input impedance can be
decreased. As an example, if R=L/4, then cos(pi*R/L) = cos(pi/4), so that
[cos(pi/4)]^2 = 1/2. Hence, a (1/8)-wavelength inset would decrease the input
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impedance by 50%. This method can be used to tune the input impedance to the
desired value.
The feed mechanism plays an important role in the design of Microstrip
patch antennas. A Microstrip patch antenna can be fed either by coaxial probe or
by an inset Microstrip line. Coaxial probe feeding is sometimes advantageous for
applications like active antennas, while Microstrip line feeding is suitable for
developing high-gain Microstrip array antennas. In both cases, the probe position
or the inset length determines the input impedance.
The input impedance behavior for a coaxial probe-fed patch antenna has
been studied analytically by means of various models, including the transmission-
line model and the cavity model, and by means of full-wave
analysis. Experimentally and theoretically, it has been found that a coaxial-probe
fed-patch antenna's input impedance exhibits behavior that follows the
trigonometric function:
cos2(π𝑦0
𝐿)
Where;
L = the length of the patch and
y0 = the position of the feed from the edge along the direction of the patch length
L.
On the other hand, it has been found experimentally4 that on low-dielectric-
constant materials, the input impedance of an inset-fed probe antenna exhibits
fourth-order behavior following the function:
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cos4(π𝑦0
𝐿)
Fortunately, a simple analytical approach has been developed using the
transmission-line model to find the input impedance of an inset-fed Microstrip
patch antenna. Using this approach, a curve-fit formula can be derived to find the
inset length to achieve a 50-Ω input impedance when using modern thin dielectric
circuit-board materials.
FIG 4.6
This is a graphical depiction of an inset-fed Microstrip patch antenna. The
parameters εr, h, L, W, wf, and y0, respectively, are used to denote substrate
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dielectric constant, thickness, patch length, patch width, feed-line width, and feed-
line inset distance. The input impedance of an inset-fed Microstrip patch antenna
depends mainly on the inset distance, y0, and to some extent on the inset width (the
spacing between the feed line and the patch conductor). Variations in the inset
length do not produce any change in resonant frequency, but a variation in the inset
width will result in a change in resonant frequency. Hence, in the following
discussion, the spacing between the patch conductor and feed line is kept constant,
equal to the feed line's width; variations in the input impedance at resonant
frequency with respect to inset length will studied as a function of various
parameters.
4.4 DIELECTRIC SUBSTRATE:
Considering the trade-off between the antenna dimensions and its
performance, it was found suitable to select a thin dielectric substrate with low
dielectric constant. Thin substrate permits to reduce the size and also spurious
radiation as surface wave, and low dielectric constant − for higher bandwidth,
better efficiency and low power loss. The simulated results were found
satisfactory.
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CHAPTER 5
DESIGN METHODOLOGY FOR MINKOWSKI FRACTAL STRUCTURE
Fractal antenna is an antenna that uses a fractal , self-similar design to
maximize the length or increase the perimeter of material that can transmit or
receive electromagnetic radiation within a given total surface area or volume.
An important property of any fractal geometry is the possibility of obtaining
an arbitrarily long curve confined in a given space. This property can be exploited
effectively in the process of antenna miniaturization. Another interesting property
of fractal geometry is the self-similarity property. Self similarity can be described
as the replication of the geometry of the structure at a different scale within the
same structure. Self similarity property of fractals may result in multiband
behavior of the fractal shaped antennas. The iterative-generation procedure for a
Minkowski island fractal is depicted.
FIG 5.1: Iterative generation Procedure for a Minkowski Fractal
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The fractal is formed by displacing the middle one-third of each straight
segment (indentation length) by some fraction called the indentation width.
Indentation factor (i) is defined here as the ratio of indentation width to the
indentation length. The resulting structure has five segments for every one of the
previous iteration, but not all of the same scale. Changing the indentation factor
causes a shift in the resonant frequencies, so proper tuning of the indentation factor
is necessary to obtain the frequencies required for WiMAX and Bluetooth
applications.
5.1 ANTENNA DESIGN PROCEDURE AND MODELLING:
To start with, rectangular shaped wearable antenna is designed with the
specifications using CST Microwave Studio 2010. The antenna is fed through an
aperture using Microstrip line. Width of the Microstrip line is calculated for 50
ohm characteristic impedance and its length beyond the aperture is optimized to
achieve best impedance matching. Size of the ground plane and the substrate is
taken as 76.8mm X 57.8mm. The patch layer and the ground plane of the antenna
are made up of conductive fabrics having thicknesses of 1.8mm (Flame Retardant,
FR4). The patch dimensions for each of the antenna are optimized to get a
resonance frequency of 2.4 GHz. This constitutes the basic design for zeroth
iteration of the fractal geometry for the antenna under consideration and to be used
for WiMAX and Bluetooth applications.
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5.2 DESIGN SPECIFICATIONS OF ANTENNA:
DESIGN PARAMETER VALUE
Resonant frequency (GHz) 2.4
Substrate dielectric constant 1.44
Substrate thickness(mm) 1.8
Substrate material FR4
TABLE 5.1: DESIGN SPECIFICATIONS OF ANTENNA
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CHAPTER 6
DESIGN METHODOLOGY FOR DMS STRUCTURE
A Defected Microstrip Structure (DMS) is proposed to reduce the size of a
rectangular patch antenna by increasing its electric length, without degrading its
performance.
Currently, patch antennas have become one of the principal goals in radio-
frequency and microwave system design because of their inherent characteristics.
Circuit-size reduction is another goal in order to provide a higher scale of
integration and portability, and patch antennas are a great option for this purpose.
In the present work, a defected Microstrip structure is proposed, which
behaves in a very similar way to that obtained when a one-cell Defected Ground
Structure (DGS) is used, but without any leakage through the ground plane.
Lately, the DGS has been widely employed to reduce the filter size, as well
as in amplifier implementations.
Moreover, the associated inductance of a conventional DGS is, at times, not
high enough to provide the required electric length by increasing the current path
in a certain application. Therefore, several works have been presented in which
the conventional defected ground structure is modified to obtain an increment in
the associated inductance, in such a way that the resulting electric length is bigger.
This proposed defected Microstrip line is an alternative for increasing the electric
length of certain circuits but without any manipulation of the ground plane.
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With the inductance increment, a reduction in circuit dimensions can be
achieved, firstly applied in filter design. However, the DGS behavior has not been
used in patch-antenna size reduction, since most reported applications of DGS in
patch antennas are related to harmonic suppression and bandwidth improvement.
To reduce the antenna size, a shape modification short-circuited λ /4 antenna has
been reported, which is inherently of smaller size than the conventional one, but
radiation efficiency, gain, and radiation pattern are modified.
In this work, the Defected Microstrip Structure (DMS) behavior is proposed
to reduce the antenna size by introducing a defect in a conventional patch.
Moreover, this method can also be used in other rectangular patch antennas like
conventional λ /4 patch antennas, adding an extra size reduction.
6.1 DEFECTED RECTANGULAR PATCH ANTENNA :
The proposed defected antenna is designed to perform at 2.4 GHz. The slots
can be observed along the antenna length L, that is, along the non-radiating edges.
In this position, the slots can avoid any interference or modification in the
radiation pattern.
In this case, the dimension W was obtained after an optimization procedure
to avoid high cross-polarization levels, thus we obtain a relation W/L. The antenna
dimensions are W =39.4 mm, L =28.9 mm, Ls = 10 mm, and Ws =1.0 mm. Le is
set to 9.25 mm. We is set to 2 mm. The substrate used is FR4, with dielectric
constant of 1.44 and thickness of 1.8 mm. The simulation and optimization
process were done using HFSS by Ansoft.
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Meanwhile, the simulated far-field gain pattern of both antennas is depicted.
In this figure, a comparison can be made when the elevation angle ф is zero. It is
observed that the radiation pattern in the E-plane is almost the same in both
antennas.
FIG 6.1: T-SLOT DMS STRUCTURE
6.2 SOFTWARE FOR SIMULATION:
The software used to model and simulate the Microstrip patch antenna was
HFSS (High Frequency Structural Stimulator). HFSS is a commercial finite
element method solver for electro-magnetic structures from Ansys. It is one of
several commercial tools used for antenna design, and design of RF electronic
circuit elements including filters, transmission lines and packaging. HFSS is a
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high-performance full-wave electromagnetic (EM) field simulator for arbitrary 3D
volumetric passive device modeling that takes advantage of the familiar Microsoft
Windows graphical user interface. Ansoft HFSS employs the Finite Element
Method (FEM), adaptive meshing, and brilliant graphics to give you unparalleled
performance and insight to all of your 3D EM problems. Ansoft HFSS can be used
to calculate parameters such as S-Parameters, Resonant Frequency, and Fields.
HFSS is an industry – standard stimulation tool for 3D- full wave electromagnetic
field stimulation. It is essential for the design of high frequency and high speed
component.
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CHAPTER 7
DESIGN PARAMETERS FOR PATCH ANTENNA
7.1 DESIGN FLOW:
FIG 7.1: DESIGN FLOW
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7.2 DESIGN SPECIFICATION:
The three essential parameters for the design of a rectangular Microstrip
Patch Antenna are:
Frequency of Operation
The resonant frequency of the antenna must be selected appropriately. The
resonant frequency selected for my design is 2.4 GHz.
Dielectric constant of the substrate (εr)
The dielectric material selected for my design is FR4which has a dielectric
constant of 1.44.
Height of dielectric substrate (h)
Because of using FR4, so height of dielectric substrate is 1.8 mm.
So, the essential parameters for the design are :
fO : 2.4 GHz
εr: 1.44
h : 1.8 mm
7.3 DESIGN PROCEDURE:
The transmission line model will be used to design the antenna.
Calculation the wavelength (λ) :
Because c = 3x108 and fo = 2.4 GHz,
So, λ = c/fo
By substituting c = 3x108 and fo = 2.4 GHz, we get;
λ=0.125m=125mm
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Calculation of the Width (W) :
The width of the Microstrip patch antenna with substituting εr = 1.03, we get;
W=39.4mm
Calculation of Effective dielectric constant (εreff) :
The effective dielectric constant, with substituting h =1.8mm and W=39.4mm, we
get;
εreff =1.023
Calculation of the Effective length (Leff):
The Effective length with substituting εreff =1.023, we get;
Leff =25.9mm
Calculation of the length extension (ΔL):
The length extension with substituting Leff =25.9mm, we get;
ΔL=7.78mm
Calculation of actual length of patch (L):
The actual length of patch with substituting Leff =25.9mm and ΔL=7.78mm, we
get;
L=28.9mm
Calculation of the ground plane dimensions (L(g) and W(g)) The
transmission line model is applicable to infinite ground planes only. However, for
practical considerations, it is essential to have a finite ground plane. Finite and
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infinite ground plane can be obtained if the size of the ground plane is greater than
the patch dimensions by approximately six times the substrate thickness all around
the periphery. Hence, for this design, the ground plane dimensions would be given
as:
L(g)=6h+L=6(1.8)+28.9mm=39.7mm
W(g)=6h+W=6(1.8)+39.4mm=50.2mm
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CHAPTER 8
DESIGN ANALYSIS
8.1 SIMULATION DESIGNS:
FIG 8.1: STRUCTURE OF PATCH ANTENNA
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FIG 8.2: PATCH ANTENNA WITH FRACTAL STRUCTURE
FIG 8.3: FINAL DESIGN
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8.2 SIMULATION RESULTS:
8.2.1 S-PARAMETERS:
S-parameters describe the input-output relationship between ports (or
terminals) in an electrical system. For instance, if we have 2 ports (intelligently
called Port 1 and Port 2), then S12 represents the power transferred from Port 2 to
Port 1. S21 represents the power transferred from Port 1 to Port 2. In general, SNM
represents the power transferred from Port M to Port N in a multi-port network.
A port can be loosely defined as any place where we can deliver voltage and
current. So, if we have a communication system with two radios (radio 1 and radio
2), then the radio terminals (which deliver power to the two antennas) would be the
two ports. S11 then would be the reflected power radio 1 is trying to deliver to
antenna 1. S22 would be the reflected power radio 2 is attempting to deliver to
antenna 2. And S12 is the power from radio 2 that is delivered through antenna 1 to
radio 1. Note that in general S-parameters are a function of frequency (i.e. vary
with frequency).
As an example, consider the following two-port network:
FIG 8.4: TWO-PORTNETWORK
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In the above Figure, S21 represents the power received at antenna 2 relative
to the power input to antenna 1. For instance, S21=0 dB implies that all the power
delivered to antenna 1 ends up at the terminals of antenna 2. If S21=-10 dB, then if
1 Watt (or 0 dB) is delivered to antenna 1, then -10 dB (0.1 Watts) of power is
received at antenna 2.
If an amplifier exists in the circuitry, then S21 can show gain (i.e. S21 > 0
dB). This means that for 1 W of power delivered to Port 1, more than 1 W of
power is received at Port 2.
In practice, the most commonly quoted parameter in regards to antennas is S11.
S11 represents how much power is reflected from the antenna, and hence is known
as the reflection coefficient (sometimes written as gamma: or return loss. If
S11=0 dB, then all the power is reflected from the antenna and nothing is radiated.
If S11=-10 dB, this implies that if 3 dB of power is delivered to the antenna, -7 dB
is the reflected power. The remainder of the power was "accepted by" or delivered
to the antenna. This accepted power is either radiated or absorbed as losses within
the antenna. Since antennas are typically designed to be low loss, ideally the
majority of the power delivered to the antenna is radiated.
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FIG 8.5: S11 PLOT FOR RECTANGULAR PATCH ANTENNA
FIG 8.6: S11 PLOT FOR FRACTAL DESIGN
0.00 1.00 2.00 3.00 4.00 5.00 6.00Freq [GHz]
-20.00
-17.50
-15.00
-12.50
-10.00
-7.50
-5.00
-2.50
0.00
Y1
HFSSDesign1XY Plot 1 ANSOFT
m1
m2
Curve Info
dB(St(feed_T1,feed_T1))Setup1 : Sw eep
dB(St(feed_T1,feed_T1))_1Imported
Name X Y
m1 2.4010 -17.0012
m2 3.6010 -12.2478
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FIG 8.7: S11 PLOT FOR FINAL DESIGN
8.2.2 VSWR:
For a radio (transmitter or receiver) to deliver power to an antenna, the
impedance of the radio and transmission line must be well matched to the antenna's
impedance. The parameter VSWR is a measure that numerically describes how
well the antenna is impedance matched to the radio or transmission line it is
connected to.
VSWR stands for Voltage Standing Wave Ratio, and is also referred to as
Standing Wave Ratio (SWR). VSWR is a function of the reflection coefficient,
which describes the power reflected from the antenna. If the reflection coefficient
is given by , then the VSWR is defined by the following formula:
http://www.antenna-theory.com/http://www.antenna-theory.com/basics/impedance.phphttp://www.antenna-theory.com/basics/impedance.php
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The VSWR is always a real and positive number for antennas. The smaller
the VSWR is, the better the antenna is matched to the transmission line and the
more power is delivered to the antenna. The minimum VSWR is 1.0. In this case,
no power is reflected from the antenna, which is ideal.
Often antennas must satisfy a bandwidth requirement that is given in terms
of VSWR. For instance, an antenna might claim to operate from 100-200 MHz
with VSWR
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8.2.3 GAIN:
The term Antenna Gain describes how much power is transmitted in the
direction of peak radiation to that of an isotropic source. Antenna gain is more
commonly quoted than directivity in an antenna's specification sheet because it
takes into account the actual losses that occur.
A transmitting antenna with a gain of 3 dB means that the power received
far from the antenna will be 3 dB higher (twice as much) than what would be
received from a lossless isotropic antenna with the same input power. Note that a
lossless antenna would be an antenna with an antenna efficiency of 0 dB (or
100%). Similarly, a receive antenna with a gain of 3 dB in a particular direction
would receive 3 dB more power than a lossless isotropic antenna.
Antenna Gain is sometimes discussed as a function of angle. In this case, we
are essentially plotting the radiation pattern, where the units (or magnitude of the
pattern) are measured in antenna gain.
Antenna Gain (G) can be related to directivity (D) and antenna efficiency
by:
The gain of a real antenna can be as high as 40-50 dB for very large dish
antennas (although this is rare). Directivity can be as low as 1.76 dB for a real
antenna (example: short dipole antenna), but can never theoretically be less than 0
dB. However, the peak gain of an antenna can be arbitrarily low because of losses
or low efficiency. Electrically small antennas (small relative to the wavelength of
http://www.antenna-theory.com/basics/directivity.phphttp://www.antenna-theory.com/basics/efficiency.phphttp://www.antenna-theory.com/basics/radpattern.phphttp://www.antenna-theory.com/basics/directivity.phphttp://www.antenna-theory.com/antennas/shortdipole.php
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the frequency that the antenna operates at) can be very inefficient, with antenna
gains lower than -10 dB (even without accounting for impedance mismatch loss).
FIG 8.9: GAIN PLOT
8.2.4 RADIATION PATTERN:
In the field of antenna design the term radiation pattern (or antenna
pattern or far-field pattern) refers to the directional (angular) dependence of the
strength of the radio waves from the antenna or other source.
Particularly in the fields of fiber optics, lasers, and integrated optics, the
term radiation pattern may also be used as a synonym for the near-
https://en.wikipedia.org/wiki/Antenna_(radio)https://en.wikipedia.org/wiki/Radio_waveshttps://en.wikipedia.org/wiki/Fiber_opticshttps://en.wikipedia.org/wiki/Laserhttps://en.wikipedia.org/wiki/Integrated_opticshttps://en.wikipedia.org/wiki/Near_and_far_field
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field pattern or Fresnel pattern. This refers to the positional dependence of
the electromagnetic field in the near-field, or Fresnel region of the source. The
near-field pattern is most commonly defined over a plane placed in front of the
source, or over a cylindrical or spherical surface enclosing it.
The far-field pattern of an antenna may be determined experimentally at
an antenna range, or alternatively, the near-field pattern may be found using a near-
field scanner, and the radiation pattern deduced from it by computation. The far-
field radiation pattern can also be calculated from the antenna shape by computer
programs such as NEC. Other software, like HFSS can also compute the near field.
The far field radiation pattern may be represented graphically as a plot of
one of a number of related variables, including; the field strength at a constant
(large) radius (an amplitude pattern or field pattern), the power per unit solid angle
(power pattern) and the directive gain. Very often, only the relative amplitude is
plotted, normalized either to the amplitude on the antenna bore sight, or to the total
radiated power. The plotted quantity may be shown on a linear scale, or in dB. The
plot is typically represented as a three-dimensional graph (as at right), or as
separate graphs in the vertical plane and horizontal plane. This is often known as
a polar diagram.
https://en.wikipedia.org/wiki/Near_and_far_fieldhttps://en.wikipedia.org/wiki/Electromagnetic_fieldhttps://en.wikipedia.org/wiki/Near_and_far_fieldhttps://en.wikipedia.org/wiki/Antenna_measurementhttps://en.wikipedia.org/wiki/Electromagnetic_near-field_scannerhttps://en.wikipedia.org/wiki/Electromagnetic_near-field_scannerhttps://en.wikipedia.org/wiki/Numerical_Electromagnetics_Codehttps://en.wikipedia.org/wiki/HFSShttps://en.wikipedia.org/wiki/Field_strengthhttps://en.wikipedia.org/wiki/Antenna_gainhttps://en.wikipedia.org/wiki/Antenna_boresighthttps://en.wikipedia.org/wiki/Decibelhttps://en.wikipedia.org/wiki/Vertical_planehttps://en.wikipedia.org/wiki/Horizontal_plane
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FIG 8.10: RADIATION PATTERN
8.3 COMPARISON OF SIMULATION RESULTS:
PARAMETERS
PATCH
FRACTAL
DMS
S11 PLOT
-17.0012
-31.5508
-34.7580
VSWR PLOT
1.1048
1.0543
1.0373
GAIN PLOT
3.8543
4.2504
4.4051
TABLE 8.1: COMPARISON OF SIMULATION RESULTS
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CHAPTER 9
FABRICATION & TESTING
9.1 FABRICATED ANTENNA:
FIG 9.1: FRONT VIEW
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FIG 9.2: BACK VIEW
9.2 TESTING TOOL:
9.2.1 N9926A FieldFox Handheld Microwave Vector Network Analyzer
KEY FEATURES AND FUNCTIONS:
14 GHz max frequency
Carry the world’s most integrated handheld T/R VNA analyzer
Expand your measurement flexibility with optional 2-port VNA, time-
domain, vector voltmeter, cable and antenna analyzer and more
Save time by simultaneously measuring all four S-parameters with a single
connection
Perform accurate testing with QuickCal, full 2-port unknown thru Cal, TRL
Easily measure average and pulse power with a USB power sensor
Lightest handheld VNA at only 6.6 lb. (3.0 kg)
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52
FIG 9.3: NETWORK ANALYZER
The Applications are
S-Parameters
Distance-To-Fault
Cable Trimming
Return Loss
Insertion Loss/Gain
Power Measurements
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9.3 TESTING RESULTS :
FIG 9.4: S11 PLOT
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FIG 9.5: VSWR PLOT
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9.4 CERTIFICATION FOR TESTING:
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CHAPTER 10
CONCLUSION
The designing and fabrication of Microstrip patch antenna has been
accomplished. A number of findings have been identified during the designing and
fabricating phases. The signal strength (Gain) of Microstrip patch antenna for an
unidirectional pattern is better. The area of Microstrip patch antenna is about
76.8mm * 57.8mm. There is still back lobe in radiation pattern of Microstrip
antenna. In this design, miniaturization of antennas is achieved by the use of
Minkowski fractal structure of the 1st and 2nd iterations. The material used for the
design of Minkowski fractal shaped antenna is Flame Retardant (FR4) material. In
the zeroth iteration, the antenna dimensions are chosen to suit for Bluetooth and
WiMAx applications. In the 1st and 2nd iterations the fractal geometry parameters
are tuned for optimal performance in the WiMAX and Bluetooth bands
respectively with achievement of compactness as an additional feature. In the two
frequency bands, the designed antenna gives good performance characteristics. The
DMS phenomenon is employed to reduce the physical dimensions of a rectangular
patch antenna, without degrading its radiation pattern, as well as its VSWR at the
resonant frequency (2.4 GHz). Moreover, by avoiding any etching in the ground
plane, any increasing leakage through the plane which could interfere with another
circuit of the system is not allowed. An area reduction factor of 22% was obtained.
Good agreement between the measured and simulated results was obtained for the
Microstrip patch antenna with Minkowski fractal and DMS structures.
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10.1 COMPARISON OF RESULTS:
PARAMETER
SIMULATED VALUE
MEASURED VALUE
S11 PLOT
-34.7580 dB
-26.55 dB
VSWR PLOT
1.0373
1.369
TABLE 10.1: COMPARISON OF RESULTS
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CHAPTER 11
REFERENCES
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3. A proposed defected Microstrip structure (DMS) behavior for reducing
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7. M.A. Matin, B.S. Sharif and C. C. Tsimenidis, ”Microstrip Patch Antenna
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16. Wireless. http://en.wikipedia.org/wiki/Wireless
17. www.antenna-theory.com/antennas/patches
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