Appendix REPORT

8/6/2019 Appendix REPORT

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Design and development of S- band Circularly Polarized microstrip patch antenna

Dept. of E.C.E., SVCE Page 3

Example: 2.4GHz signal completes a cycle as it travels through the air every 12.5 cm.

In vacuum and air, c is equal to the speed of light (299 793 077 m/s), but radio waves

are slower when passing through other materials and hence the wavelength will be shorter.

This is of great importance when designing antennas.

Antennas demonstrate a property known as reciprocity , which means that an antenna will

maintain the same characteristics regardless if it is transmitting or receiving

In this chapter we briefly describe the microstrip antenna and its parameters while the design

of the project are described in the next chapters.

M I CROSTR I P PATCH ANTENNAS - A microstrip patch antenna is a narrowband,

wide-beam antenna fabricated by etching the antenna element pattern in metal trace bonded

to an insulating dielectric substrate with a continuous metal layer bonded to the opposite side

of the substrate which forms a ground plane.

Main Properties of Patch Antenna

Patch Antennas offer effective low-profile designs for a wide range of wireless applications.

They are inexpensive to fabricate, light in weight, and can be made conformable with planar

and non-planar surfaces. The patch antennas are compact and compatible with microwave

integrated circuits (MICs) for high-frequency applications.

Unfortunately, they have some shortcomings, including relatively low gain, narrow

bandwidth, and sensitivity to fabrication errors. Despite that; and because of rising

demands for multiple frequencies in wireless designs, patch antennas support multiple

function circuits that will force us to use it as it until we overcome on its disadvantages.

Common microstrip antenna radiator shapes are square, rectangular, circular and elliptical,

but any continuous shape is possible. More detailed description on the working and general

designs of microstrip antennas are given in APPENDIX I.

Here we will just discuss the common parameters and characteristics we need to assess while

designing the antenna.

.


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Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter

to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz)

and 300 GHz. This broad definition includes both UHF and EHF (millimeter waves), and

various sources use different boundaries. In all cases, microwave includes the entire SHF

band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower

boundary at 1 GHz (30 cm), and the upper around 100 GHz (3mm). The different frequency

bands and their ranges are given below.

Microwave frequency bands Letter Designation Frequency range

L band 1 to 2 GHz S band 2 to 4 GHz

C band 4 to 8 GHz X band 8 to 12 GHz

K u band 12 to 18 GHz K band 18 to 26.5 GHz

K a band 26.5 to 40 GHz Q band 33 to 50 GHz

U band 40 to 60 GHz V band 50 to 75 GHz

E band 60 to 90 GHz W band 75 to 110 GHz

F band 90 to 140 GHz D band 110 to 170 GHz

Here we have designed the antenna to operate in the S Band specifically reserved for

military communications.

ANTENNA PARAMETERS

RAD I AT ION PATTERN : The radiation pattern of an antenna is a plot of the far-field

radiation properties of an antenna as a function of the spatial co-ordinates, which are

specified by the elevation angle and the azimuth angle . More specifically it is a plot of the power radiated from an antenna per unit solid angle, which is nothing but the radiation

intensity.


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Fig 2: Radiation pattern of a generic directional antenna

HALF POWER BEAM W I DTH : The half power beam width (HPBW) can be

defined as the angle subtended by the half power points of the main lobe.

MA I N LOBE: This is the radiation lobe containing the direction of maximum radiation.

M I NOR LOBE: All the lobes other than the main lobe are called the minor lobes.

These lobes represent the radiation in undesired directions. The level of minor lobes is

HPBW usually expressed as a ratio of the power density in the lobe in question to that of the

major lobe. This ratio is called as the side lobe level (expressed in decibels).

BACK LOBE: This is the minor lobe diametrically opposite the main lobe.

SI DE LOBE : These are the minor lobes adjacent to the main lobe and are separated by

various nulls. Side lobes are generally the largest among the minor lobes.


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fig 3: half power beam width pattern

AVERAGE S I DE LOBE LEVEL: The average value of the relative power pattern of

an antenna taken over a specified angular region, which excludes the main beam, the power

pattern being relative to the peak of the main beam.

DI RECT I VI TY : The directivity of an antenna has been defined by as the ratio of the

radiation intensity in a given direction from the antenna to the radiation intensity averaged

over all directions. In other words, the directivity of a nonisotropic source is equal to the

ratio of its radiation intensity in a given direction, over that of an isotropic source. The

directivity of a nonisotropic source is equal to the ratio of its radiation intensity in a given

direction, over that of an isotropic source.

D= =

Where D is the directivity of the antenna

U is the radiation intensity of the antenna.

Ui is the radiation intensity of an isotropic source.

Dmax is the maximum directivity.

P is the total power radiated.


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POWER GA I N OR S I MPLY GA I N: The power gain or simply gain Gp , of an

antenna referred to an isotropic source is the ratio of its maximum radiation intensity to the

radiation intensity of a loss less isotropic source with the same power input.

G p = (4Pi U max ) / (P input )

RAD I AT ION EFF I C I ENCY: The ratio of the gain to the directivity of an antenna is

called the radiation efficiency

L = Gp / D

NULLS: In an antenna radiation pattern, a null is a zone in which the effective radiated

power is at a minimum. A null often has a narrow directivity angle compared to that of the

main beam. Thus, the null is useful for several purposes, such as suppression of interfering

signals in a given direction.

RAD I AT ION RES ISTANCE: The resistance that, if inserted in place of an antenna,

would consume the same amount of power that is radiated by the antenna.

I NPUT I MPEDANCE: The input impedance of an antenna is defined by as the

impedance presented by an antenna at its terminals or the ratio of the voltage to the current at

the pair of terminals or the ratio of the appropriate components of the electric to magnetic

fields at a point.

RETURN LOSS (RL): The Return Loss (RL) is a parameter, which indicates the

amount of power that is lost to the load and does not return as a reflection. The RL is given

by,

(dB)

ANTENNA EFF I C I ENCY : The antenna efficiency is a parameter, which takes into

account the amount of losses at the terminals of the antenna and within the structure of the

antenna. These losses are given by

Reflections because of mismatch between the transmitter and the antenna R I 2 losses (conduction and dielectric).


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BANDW I DTH : The bandwidth of an antenna is defined by as the range of usable

frequencies within which the performance of the antenna, with respect to some characteristic,

conforms to a specified standard. The bandwidth can be the range of frequencies on either

side of the center frequency where the antenna characteristics like input impedance, radiation

pattern, beam width, polarization, side lobe level or gain, are close to those values whichhave been obtained at the center frequency. The bandwidth of a broadband antenna can be

defined as the ratio of the upper to lower frequencies of acceptable operation. The bandwidth

of a narrowband antenna can be defined as the percentage of the frequency difference over

the center frequency.

Where f H= Upper frequency .

f L=Lower frequency.

f C=Center frequency.

Fig 4: Measuring bandwidth from the plot of the reflection coefficient


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BEAM OF AN ANTENNA: The major lobe of the radiation pattern of an antenna.

BEAMW I DTH: In a radiation pattern containing the direction of the maximum of a lobe,

the solid angle subtended between the half-power power points of the main lobe.

POLAR ISAT ION: Polarization of a radiated wave is defined by as that property of anelectromagnetic wave describing the time varying direction and relative magnitude of the

electric field vector. The polarization of an antenna refers to the polarization of the electric

field vector of the radiated wave. In other words, the position and direction of the electric

field with reference to the earths surface or ground determines the wave polarization. The

most common types of polarization include the linear (horizontal or vertical) and circular

(right hand polarization or the left hand polarization).

Fig 5: A linearly (vertically) polarized wave

C I RCULAR POLAR ISAT ION : In a circularly polarized wave, the electric field

vector remains constant in length but rotates around in a circular path. A left hand circular

polarized wave is one in which the wave rotates counterclockwise whereas right hand circular

polarized wave exhibits clockwise motion as shown in Figure.

Fig 6: circularly polarization


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L I NEAR POLAR I SAT I ON : If the path of the electric field vector is back and forth

along a line, it is said to be linearly polarized. Figure shows a linearly polarized wave.

fig 7: vertical and horizontal linear polarization

More details on the types and contrast between the polarizations are provided in

APPENDIX - II

VOLTAGE STAND I NG WAVE RAT I O (VSWR) : The VSWR is basically a

measure of the impedance mismatch between the transmitter and the antenna. The higher the

VSWR, the greater is the mismatch. The minimum VSWR which corresponds to a perfect

match is unity. The VSWR is given by Makarov as,

Where is the reflection coefficient?

Vs is the amplitude of the reflected wave.

Vi is the amplitude of the incident wave.


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REACT I VE NEAR-F IELD REG I ON: In this region, the reactive field dominates.

The reactive energy oscillates towards and away from the antenna, thus appearing as

reactance. In this region, energy is only stored and no energy is dissipated. The outermost

boundary for this region is at a distance,

Where R 1 is the distance from the antenna surface.

D is the largest dimension of the antenna and is the wavelength.

RAD I AT I NG NEAR-F I ELD REG I ON (FRESNEL REG I ON): Radiating

near-field region (also called Fresnel region) is the region, which lies between the reactive

near-field region and the far field region. Reactive fields are smaller in this field as compared

to the reactive near-field region and the radiation fields dominate. In this region, the angular

field distribution is a function of the distance from the antenna. The outermost boundary for

this region is at a distance,

Where R 2 is the distance from the antenna surface.

FAR-F I ELD REG ION (FRAUNHOFER REG ION):

Far-field region (also called Fraunhoffer region): The region beyond is the

far field region. In this region, the reactive fields are absent and only the radiation fields exist.

The angular field distribution is not dependent on the distance from the antenna in this region

and the power density varies as the inverse square of the radial distance in this region.


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Fig 8 : Field regions of an antenna

DECIBELS: - Decibels (dB) is commonly used to describe gain or loss in circuits. The

number of decibels is found from:

Gain in dB = 10 log(gain factor)

Q FACTOR: The Q-factor of an antenna is proportional to the ratio of energy stored to

the energy lost per cycle.


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CHAPTER TWO

L ITERATURE SURVEY

M ICROSTR IP PATCH

ANTENNA


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dimensions length=14mm, width=20mm, height=4.16mm with embedding 3 layer of

substrates in stack manner including air as dielectric material.

In our work, we have obtained a smaller size antenna by incorporating parallel slots on a

rectangular patch. By this realized Circular shape patch gives 30 to 40% reduction in size.

patch dimensions length=8.7mm, width=11mm, height=3.16mm selecting RT duriod as

dielectric material.

The other parameter where we have improved on is the bandwidth which is 26% (2.5GHz),

return loss of 40dB at center frequency, with relative directive gain of 6.2dB and half power

beam width of 110 0.


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CHAPTER THREE

ANTENNA DES IGN

CONS IDERAT IONS


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calculations involved and gives out prominent field radiations. The procedure assumes that

the specified information including the dielectric constant of substrate ( r ) the resonant

frequency (f 0) and height of substrate (h).

r = 2.2

f 0 = 3.25 GHz

h = 0.079 mm

0 = c/ f 0, where c is light wave velocity =300M mt/sec.

0 = 92.30 mm

Radius of Circular patch : In 3dB Power divider the distance between two ports Power

divider is hypotenuse of circular patch, where the height & base is equal to the radius of

circular patch. Hence from the above we can calculate radius of the required circular patch.

Hy 2 =B2 + H 2

Where Hy = hypotenuse, B = base, H = height.

Note that for the circular patch to operate in the desired frequency, the required conditions

were satisfied in the power divider itself where the distance between the two ports were

calculated using the centre

Length & Width of Substrate : It should be greater than the length & width in which the

circular patch is enclosed.


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POLAR IZ AT I ON

An antenna is a transducer that converts radio frequency (RF) electric current to

electromagnetic waves that are then radiated into space. Antenna polarization is an important

consideration when selecting and installing antennas. Most wireless communication systems

use either linear (vertical, horizontal) or circular polarization. Knowing the difference between polarizations can help maximize system performance for the user.

Circularly Polarized Patch

A microstrip patch is one of the most widely used radiators for circular polarization. Below

shows some patches, including square, circular, pentagonal, equilateral triangular, ring, and

elliptical shapes which are capable of circular polarization operation. However square and

circular patches are widely utilized in practice.

A single patch antenna can be made to radiate circular polarization if two orthogonal patch

modes are simultaneously excited with equal amplitude and out of phase with sign

determining the sense of rotation. Two types of feeding schemes can accomplish the task as

given in figure below. The first type is a dual-orthogonal feed, which employs an external

power divider network. The other is a single point for which an external power divider is not

required. We will restrict our self only to first method i.e. Dual-Orthogonal Fed circularly

Polarized Patch

Dual-Orthogonal Fed circularly Polarized Patch

The fundamental configurations of a dual-orthogonal fed circularly polarized patch using an

external power divider. The patch is usually square or circular. The dual-orthogonal feeds

excite two orthogonal modes with equal amplitude but in phase quadrature. Several power

divider circuits that have been successfully employed for CP generation such as the

Wilkinson power divider, and the 3DB power divider. In this we have used 3db power

divider, it divide the amplitude equally with 90 degree phase shift, then its fed to the patch.


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Fig 9: Typical configurations of dual-fed circularly polarized micro-strip antennas:

(a)circular patch and (b) square patch

Wave port design : Another important component is the wave port through which the

excitation to the patch antenna is supplied. It has been found that for effective design

y Width of wave port = 5 (width of feed line)

y Height of wave port = (width of feed line) + 6 (height of substrate)

Where width of feed line is calculated using Line Calculator in ADS, width of feed

line = 2.408mm corresponding to 50 ohms transmission line


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SECTION -2

SIMULATION SOFTWARES

CHAPTER ONE

HFSS


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Two simulation softwares are used to implement our design.

While the main patch antenna element, waveport, boundary conditions and excitations were

designed and provided in HFSS, the power divider for the dual feed was designed in ADS for

simplicity and convenience and was then exported to the main design in HFSS after

necessary formatting. Their implementations in our design are described in next sections.

Below we briefly describe the principles and the tools used in HFSS, while ADS is discussed

in the next chapter.

HFSS:

HFSS stands for High Frequency Structure Simulator. It is a full wave EM field

simulator for arbitrary 3-D volumetric passive device modelling. It integrates simulation,

visualization, solid modelling and automation. It employs Finite Element Method and

adaptive meshing and can be used to calculate S-parameters, resonant frequency and fields.

In general, the finite element method divides the full problem space into thousands of smaller

regions and represents the field in each sub-region (element) with a local function. In HFSS,

the geometric model is automatically divided into a large number of tetrahedral, where a

single tetrahedron is a four-sided pyramid. This collection of tetrahedral is referred to as the

finite element mesh.

The value of a vector field quantity (such as the H-field or E-field) at points inside each

tetrahedron is interpolated from the vertices of the tetrahedron. At each vertex, HFSS stores


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the components of the field that are tangential to the three edges of the tetrahedron. In

addition, HFSS can store the component of the vector field at the midpoint of selected edges

that is tangential to a face and normal to the edge (as shown below). The field inside each

tetrahedron is interpolated from these nodal values.

By representing field quantities in this way, the system can transform Maxwells

equations into matrix equations that are solved using traditional numerical methods.

The wave equation that is solved by HFSS is derived from the differential form of

Maxwells Equations. For these expressions to be valid, it is assumed that the field vectors

are single-valued, bounded and have continuous distribution along with their derivatives.

Along boundaries or sources the fields are discontinuous and the derivatives have no

meaning. Therefore boundary conditions define the field behavior across discontinuous

boundaries .

The adaptive meshing constructs a mesh that conforms to the electrical performance

of the device. By employing adaptive meshing, the mesh is automatically tuned to give the

most accurate and efficient mesh possible. The adaptive meshing algorithm searches for the

largest gradient in the E-field or error and sub-divides the mesh in those regions. It also

targets singularities such as the edge of a conductor, as locations to add extra elements. After

the mesh is refined a full solution is performed and the process is repeated until convergence.


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Fig 10: A flowchart for solution of models in HFSS.

After each adaptive pass, HFSS compares the S-parameters from the current mesh to the

results of the previous mesh. If the answers have not changed by the user defined value or Delta-S, then the solution has converged and the current or previous mesh can be used to

perform a frequency sweep. HFSS uses the previous mesh to perform the frequency sweep if

they have been requested.

The Delta-S is the default criteria use to determine the mesh/solution convergence.

Delta-S is defined as the maximum change in the magnitude of the S-parameters between two

consecutive passes.

Max ij[mag(S Nij-S N-1 ij)] where i and j cover all matrix entries and N is the pass

number.

The adaptive frequency for which the solution is to be found should be the end

frequency since the structure being simulated is a broadband structure, so that all the lower

frequencies are considered.


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CHAPTER TWO

ADS


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Advance Design System

ADS is a sophisticated circuit simulator and can take a significant amount of time to learn all

the complex features. ADS is a sophisticated circuit simulator and can take a significant

amount of time to learn all the complex features. There is a graphical user interface to draw

the circuit diagram (Schematic entry). The software comes with significant number of

predefined libraries. Since the focus of ADS is RF and microwave design, the majority of the

devices in the library are rf and microwave devices. There are, however, a few low frequency

FETs and BJTs. If you want to simulate power electronic circuits you should use a more

appropriate package.

The are several different simulations that ADS can perform. Some of these can be found in

traditional SPICE simulators. The more complex simulation modes are also available in other

design software like Microwave Office. The simulation mode which we are using is S-

Parameter Analysis, which is microwave equivalent of AC analysis.

Advantage of advance design system

y Efficient operation of the ADS user-interface

y Use of ADS built-in design examples and Design Guides

y Fast and efficient schematic capture

y Simulation using: DC, AC, S -parameter, Transient, Harmonic Balance, Envelope, and Data Flow

y Optimization and Tuning

y Control and display of simulation data and measurement equations

y Use of components such as SDDs, noise controllers, and more

y Use of various sources, including modulated sources such as CDMA and GSM

y Brief use of ADS Momentum from Layout


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y Use of behavioral system models

y Use of data access components DAC

y Simulation of Network Analyzer data

y Sub-circuit and hierarchy creation

y Plus a wide variety of tips and techniques that apply to all RF, Microwave, and RFIC

designs, including amplifiers, filters, mixers, and oscillators

To get a clear picture of ADS we are taking an example of Wilkinson Power divider

Schematic diagram

Fig 11: Schematic view Wilkinson power divider in ADS


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Layout

Fig 12 : Layout view of Wilkinson power divider in ADS


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SECT I ON 3

D ESIGN and RESULTS

CHAPTER ONE

DES IGN


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Fig 13: Design Flow


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Design Specifications: -

y Frequency of operation ( ): The resonant frequency of the antenna must be

selected appropriately. The microstrip antenna is designed in S Band. The S band is

part of the microwave region of the electromagnetic spectrum. Its frequency range is

from 2 to 4 GHz. Hence the antenna designed must be able to operate in this

frequency range. The resonant frequency selected for my design is 3.25 GHz.

y Return Loss : It should be less than -10 dB.

y Gain: It should be greater than 5 dB.

y Dielectric constant of the substrate (r): The dielectric material selected for my

design is RT Duroid which has a dielectric constant of 2.2.for designing of antennas

dielectric constant should be in the range of A substrate in the lower

range of dielectric constant has been selected since it provides better efficiency,

larger bandwidth.

y Height of dielectric substrate ( h ): In many applications it is essential that the

antenna is not bulky.The height of the substrate should lie in the

range .Hence, the height of the dielectric substrate is

selected as 0.079 mm.


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DES I GN

The procedure assumes that the specified information including the dielectric constant of

substrate ( r ) the resonant frequency (f 0) and height of substrate (h).

r = 2.2

f 0 = 3.25 GHz

h = 0.079 mm

0 = c/ f 0, where c is light wave velocity =300M mt/sec.

0 = 92.30 mm

Radius of Circular patch : In 3dB Power divider the distance between two ports Power

divider is hypotenuse of circular patch, where the height & base is equal to the radius of

circular patch. Hence from the above we can calculate radius of the required circular patch.

Hy 2 =B2 + H 2

Where Hy = hypotenuse, B = base, H = height.

Fig 14: Radius of circle


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Length & Width of Substrate : It should be greater than the length & width in which the

circular patch is enclosed.

Wave port design :

y Width of wave port = 5 (width of feed line)

y Height of wave port = (width of feed line) + 6 (height of substrate)

Where width of feed line is calculated using Line Calculator in ADS, width of feed

line = 2.408mm corresponding to 50 ohms transmission line

Far field Setup: It should be 0/4 from all the side of the Substrate.

ELEMENT DES IGN

ADS:

Schematic view of 3dB Power divider in ADS.The difference between the length of two port

should be equal to 0/4 in order to have 90 degree phase shift to get circular polarized.

Note that the width at the input port corresponds 50 ohm transmission line, at the two divided

arms corresponds to 70 ohms transmission line and at the port terminals it changes back to

that of 50 ohms line as shown below:

Fig 15: Concept of power divider


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Fig 16: Schematic view

After designing 3dB Power divider we will go for Layout view for further optimization


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Fig 17 : Layout view

Once the proper Layout is obtained, we will convert the power divider designed in ADS into

HFSS format using CST Software, for further design of Micro strip Patch Antenna.


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HFSS

Importing 3dB Power divider in HFSS

Fig 18: 3dB power divider


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Fig 19:Substrate view

Fig 20: Ground

Then after this design we will create circular Patch of radius = 23.077 mm


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Fig 21: circular patch

After creating circular Patch, we will import & unite Power divider in Circular Patch.

Fig 22: view after uniting power divider & circular patch

Assigning wave port to antenna


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Fig 23: Wave port

Then after this we will assign Boundary condition to get the final Antenna

Fig 24: Circularly Polarized Microstrip Patch Antenna


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Fig 25: Excitation state of circular patch

Fig 26: Excitation state of boundary of microstrip patch antenna


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CHAPTER TWO

SIMULAT ION

RESULTS


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Simulated Results: Software simulation and experimental tests were used in order to

evaluate the performance of the antenna design. Experimental results are compared with

simulation performance estimates in order to verify that the designs perform as intended.

y Return Loss:

Fig 27: Return Loss

Conclusion : As per the requirement the the return loss is less than -10dB.


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y Gain:

Fig 28: Gain

Conclusion :Graph shows Gain is greater than 5dB


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y Polar Plot

Fig 30(a):Simulated polar plot


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View at phi=0 deg

View at phi=90 deg

Fig 30(b): 3D Radiation Pattern of Single element


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y Axial Ratio

Fig 31: Simulated axial ratio

Conclusion: Axial ratio is defined as Ey/Ex = 1,in dB it should be 0. For Circular ratio the

axial ratio should be in range of 0 to 3 dB.

The obtained graph shows the axial ratio is less than 3dB. Hence proves that radiation

obtained from the designed Microstrip Patch Antenna is Circularly Polarized.


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Summary of simulated results

Performance parameters Value

R esonant Frequency 3. 25 GHz

Single element R eturn Loss Better than -10 dB over the band of 0. 5 GHz

Single element Beam width (HPBW) 85 0 in Azimuth plane and 60 0 in Elevation plane

Single element directive gain 5 .9dB

Table 1: simulated results


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Advantages and Disadvantages

Some of their principal advantages of designed antenna are given below:

Light weight and low volume.

Low profile planar configuration which can be easily made conformal to host surface. Low fabrication cost, hence can be manufactured in large quantities. Can be easily integrated with microwave integrated circuits (MICs). Mechanically robust when mounted on rigid surfaces. Light weight, low volume and low profile planar configurations that can be made

conformal.

Some of their major disadvantages are given below:

Narrow bandwidth Low efficiency Extraneous radiation from feeds and junctions Low power handling capacity.


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Applications

This designed patch is been utilized for the following purpose at LRDE Bangalore.

Costal surveillance antenna

Satellite communications Missile telemetry Satellite navigation receiver Biomedical radiator


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CONCLUS ION


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CONCLUS ION

A novel (Circular-shape) technique for enhancing bandwidth of microstrip patch

antenna is successfully designed in this project. Simulation results of a wideband microstrip

patch antenna covering 3.0GHz to 3.5GHz frequency have been presented. Techniques for

microstrip broad banding, size reduction, and side lobe reduction are applied with significant

improvement in the design by employing proposed circular patch shaped design.The

proposed microstrip patch antenna achieves a fractional bandwidth of 0.5GHz (3.0 to

3.5GHz) at -10 dB return loss, with -25dB return loss at centre frequency is been achieved.

The achievable gain of the antenna is 5.9dB (greater than 5db).The proposed patch has a

compact dimension (radius) of 2.3718cm.

The wideband characteristic of the antenna is achieved by using the orthogonal

feeding techniques. Better radiation performance is achieved by making circular patch and by

suitably selecting the microstrip 3db power divider, the antenna is improved. The composite

effect of integrating these techniques offers a low profile, broadband, high gain, and compact

antenna element suitable for array applications.

This wideband compact single element antenna is used to design a circular patch

microstrip antenna. Simulation results of this circular patch microstrip antenna covering

3.0GHz to 3.5GHz frequency at -10dB return loss have been presented.The axial ratio for

circular polarization should be 0 ideally. Practically it is expected between 0 to 3. Simulatedresult shows that the axial ratio is between 1 and 2.

This designed patch is been utilized for costal surveillance antenna for purpose of

DEFENCE at LRDE Bangalore.


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REFERENCES

[1] Ramesh Garg, Prakash Bartia, Inder Bhal and Apsiak Ittipiboon, M icrostrip Antenna

D esign Hand Book , Artech House, Norwood, MA, 2001.

[2] Constanantine A Balanis, Antenna Theory Analysis and D esign , John Weily & Sons,

New York, 1997

[3] Fan Yang, Xui-Xia Zhang, Xioning Ye, and Yahya Rahmat-Sami, Wideband E-shaped

Patch Antenna for Wireless Communications , IEEE Transactions on Antennas and

Propagation, vol. 49, no.7, July 2001.

[4] D.M.Pozzar M icrostrip Antenna Coupled to M icrostripline , Electron Lett., vol. 21,

no.2, pp. 49- 50, January 1995.

[5] Yunbo Pang, Baoxin Gao, Novel Compact Multi-frequency Microstrip Patch Antenna

*State Key Laboratory on Microwave and Digital Communications, Department of

Electronic Engineering, Tsinghua University,Beijing 100084, P. R. China

[6] A. Danideh and R. Sadeghi-Fakhr, Wideband Co-Planar Microstrip Patch Antenna

Progress In Electromagnetic R esearch Letters, Vol. 4, 8189, 2 008

[7] ADS manual

[8] James, J. R., P. S. Hall, and C. Wood. Theory and Design of Micro strip antenna, IEEE

Transaction.

[9] Kin-Lu-Wong, Compact and Broad band micro strip antenna

[10] HFSS ELECTROMAGNETIC SIMULATOR SOFTWARE by ANSOFT.COM


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APPEND IX - I

M ICRO-STR IP PATCHES

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 as shown in Figure

2.1.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.

Fig A1(a): Structure of a Microstrip Patch Antenna

In order to simplify analysis and performance prediction, the patch is generally square,

rectangular, circular, triangular, elliptical or some other common shape as shown in Figure

2.2. For a rectangular patch, the length L of the patch is usually ,

where is the free-space wavelength. The patch is selected to be very thin such that

(where t is the patch thickness). The height h of the dielectric substrate is

usually . The dielectric constant of the substrate ( ) is typically in

the range


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Figure A1(b): Common shapes of microstrip patch elements

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 [5]. However, such a configuration leads to a larger antenna

size.

In order to design a compact Microstrip patch antenna, higher dielectric constants must be

used which are less efficient and result in narrower bandwidth. Hence a compromise must be

reached between antenna dimensions and antenna performance.

Microstrip patch antennas have a very high antenna quality factor (Q). Q represents the losses

associated with the antenna and a large Q leads to narrow bandwidth and low efficiency. Q

can be reduced by increasing the thickness of the dielectric substrate. But as the thickness

increases, an increasing fraction of the total power delivered by the source goes into a surface

wave. This surface wave contribution can be counted as an unwanted power loss since it is

ultimately scattered at the dielectric bends and causes degradation of the antenna

characteristics.

However, surface waves can be minimized by use of photonic band gap structures. Other problems such as lower gain and lower power handling capacity can be overcome by using an

array configuration for the elements.


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RAD IAT ION MECHAN ISM

The most popular models for the analysis of Microstrip patch antennas are the transmission

line model, cavity model, and full wave model (which include primarily integral

equations/Moment Method). The transmission line model is the simplest of all and it gives

good physical insight but it is less accurate. The cavity model is more accurate and gives

good physical insight but is complex in nature. The full wave models are extremely accurate,

versatile and can treat single elements, finite and infinite arrays, stacked elements, arbitrary

shape elements and coupling. These give less insight as compared to the two models

mentioned above and are far more complex in nature.

Transmission Line Model:

This 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.

Fig A1(c): Microstrip line Fig A1(d):Electric field lines

Hence, as seen from Figure 2.4, 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 must be obtained in order to

account for the fringing and the wave propagation in the line. The value of is slightly

less then because the fringing fields around the periphery of the patch are not confined in

the dielectric substrate but are also spread in the air as shown in Figure 2.4 above. The

expression for is given by:


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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.

Fig A1(f): Top View of Antenna and, Side View of Antenna

It is seen from Figure 2.6 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 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 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 L., Which is given empirically by Hammerstad as:

The effective length of the patch now becomes:


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For a given resonance frequency , the effective length is given by as:

For a rectangular Microstrip patch antenna, the resonance frequency for any mode is

given by James and Hall as:

Where m and n are modes along L and W respectively.

For efficient radiation, Bahl and Bhartia as give the width W:

CAV I TY MODEL:

Although the transmission line model discussed in the previous section is easy to use, it has

some inherent disadvantages. Specifically, it is useful for patches of rectangular design and it

ignores field variations along the radiating edges. These disadvantages can be overcome by

using the cavity model. A brief overview of this model is given below. In this model, the

interior region of the dielectric substrate is modeled as a cavity bounded by electric walls on

the top and bottom. The basis for this assumption is the following observations for thinsubstrates ( )


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The electric field is z directed only, and the magnetic field has only the transversecomponents and in the region bounded by the patch metallization and the ground

plane. This observation provides for the electric walls at the top and the bottom.

Fig A1(g): Charge distribution and current density creation on the microstrip patch

Consider Figure as shown above. When the microstrip patch is provided power, a charge

distribution is seen on the upper and lower surfaces of the patch and at the bottom of the

ground plane. This charge distribution is controlled by two mechanisms-an attractive

mechanism and a repulsive mechanism as discussed by Richards. The attractive mechanism

is between the opposite charges on the bottom side of the patch and the ground plane, which

helps in keeping the charge concentration intact at the bottom of the patch. The repulsivemechanism is between the like charges on the bottom surface of the patch, which causes

pushing of some charges from the bottom, to the top of the patch. As a result of this charge

movement, currents flow at the top and bottom surface of the patch. The cavity model

assumes that the height to width ratio (i.e. height of substrate and width of the patch) is very

small and as a result of this the attractive mechanism dominates and causes most of the

charge concentration and the current to be below the patch surface. Much less current would

flow on the top surface of the patch and as the height to width ratio further decreases, the

current on the top surface of the patch would be almost equal to zero, which would not allow

the creation of any tangential magnetic field components to the patch edges. Hence, the four

sidewalls could be modeled as perfectly magnetic conducting surfaces. This implies that the

magnetic fields and the electric field distribution beneath the patch would not be disturbed.

However, in practice, a finite width to height ratio would be there and this would not make


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the tangential magnetic fields to be completely zero, but they being very small, the side walls

could be approximated to be perfectly magnetic conducting.

Since the walls of the cavity, as well as the material within it are lossless, the cavity would

not radiate and its input impedance would be purely reactive. Hence, in order to account for

radiation and a loss mechanism, one must introduce a radiation resistance Rr and a loss

resistance . A lossy cavity would now represent an antenna and the loss is taken into

account by the effective loss tangent which is given as:

is the total antenna quality factor and has been expressed by in the form:

represents the quality factor of the dielectric and is given as :

where is the angular resonant frequency.

is the total energy stored in the patch at resonance.

is the dielectric loss.

tan is the loss tangent of the dielectric.

represents the quality factor of the conductor and is given as :


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where is the conductor loss.

. is the skin depth of the conductor.

h is the height of the substrate.

represents the quality factor for radiation and is given as:

where is the power radiated from the patch.

Substituting equations (3.8), (3.9), (3.10) and (3.11) in equation (3.7), we get

Thus, above equation describes the total effective loss tangent for the microstrip patch

antenna.


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FEED TECHNIQUES

Microstrip patch antennas can be fed by a variety of methods. These methods can be

classified into two categories- contacting and non-contacting

In the contacting method , the RF power is fed directly to the radiating patch using aconnecting element such as a microstrip line.

The two popular feed methods of this type are :

a. Microstrip line feed

b. Coaxial probe feed

In the non-contacting scheme , electromagnetic field coupling is done to transfer power

between the microstrip line and the radiating patch.

The two popular feed methods of this type are:

a. Aperture coupled feed

b. Proximity coupled feed

Microstrip Line Feed:

In this type of feed technique, a conducting strip is connected directly to the edge of themicrostrip patch as shown in the figure below. The conducting strip is smaller in width as

compared to the patch and this kind of feed arrangement has the advantage that the feed can

be etched on the same substrate to provide a planar structure.

Fig A1(h):Microstrip line feed


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The purpose of the inset cut in the patch is to match the impedance of the feed line to the

patch without the need for any additional matching element. This is achieved by properly

controlling the inset position. Hence this is an easy feeding scheme, since it provides ease of

fabrication and simplicity in modelling as well as impedance matching. However as the

thickness of the dielectric substrate being used, increases, surface waves and spurious feed

radiation also increases, which hampers the bandwidth of the antenna. The feed radiation also

leads to undesired cross polarized radiation.

Coaxial Feed:

The Coaxial feed or probe feed is a very common technique used for feeding Microstrip

patch antennas. As seen from the figure below, the inner conductor of the coaxial connector

extends through the dielectric and is soldered to the radiating patch, while the outer conductor is connected to the ground plane.

Fig A1(i): Probe fed Rectangular Microstrip Patch Antenna


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The main advantage of this type of feeding scheme is that the feed can be placed at any

desired location inside the patch in order to match with its input impedance. This feed method

is easy to fabricate and has low spurious radiation. However, its major disadvantage is that it

provides narrow bandwidth and is difficult to model since a hole has to be drilled in the

substrate and the connector protrudes outside the ground plane, thus not making it completely

planar for thick substrates ( h > 0.02 o ). Also, for thicker substrates, the increased probe

length makes the input impedance more inductive, leading to matching problems. It is seen

above that for a thick dielectric substrate, which provides broad bandwidth, the microstrip

line feed and the coaxial feed suffer from numerous disadvantages. The non-contacting feed

techniques which have been discussed below, solve these problems.

Aperture Coupled Feed:

In this type of feed technique, the ground plane as shown in Figure below separates the

radiating patch and the microstrip feed line. Coupling between the patch and the feed line is

made through a slot or an aperture in the ground plane.

Fig A1(j): Aperture-coupled feed

The coupling aperture is usually cantered under the patch, leading to lower cross

polarization due to symmetry of the configuration. The amount of coupling from the feed line

to the patch is determined by the shape, size and location of the aperture. Since the ground

plane separates the patch and the feed line, spurious radiation is minimized. Generally, a high

dielectric material is used for the bottom substrate and a thick, low dielectric constant

material is used for the top substrate to optimize radiation from the patch. The major

disadvantage of this feed technique is that it is difficult to fabricate due to multiple layers,


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which also increases the antenna thickness. This feeding scheme also provides narrow

bandwidth.

Proximity Coupled Feed:

This type of feed technique is also called as the electromagnetic coupling scheme. As shown

in the figure below, two dielectric substrates are used such that the feed line is between the

two substrates and the radiating patch is on top of the upper substrate. The main advantage of

this feed technique is that it eliminates spurious feed radiation and provides very high

bandwidth (as high as 13%), due to overall increase in the thickness of the microstrip patch

antenna. This scheme also provides choices between two different dielectric media, one for

the patch and one for the feed line to optimize the individual performances.

Fig A1(k): Proximity Coupled Feed

Matching can be achieved by controlling the length of the feed line and the width-to-line ratio

of the patch. The major disadvantage of this feed scheme is that it is difficult to fabricate

because of the two dielectric layers, which need proper alignment. Also, there is an increase

in the overall thickness of the antenna.


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Variations of Microstrip Antenna Configurations:

Various shapes of the patch

Square Rectangle Circular Ring Triangular

Types of feed arrangements Microstrip line feed Coaxial probe feed Slot or aperture feed

Based of other microstrip-like transmission line structures

Slot-line antennas Stripline slot antennas Co-planar waveguide antennas

Arrays of Microstrip antenna elements Linear arrays Two-dimensional or planar array


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Table A1(a): summarizes the characteristics of the different feed techniques


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Table A1(b): Comparison Of Feed Techniques

Technique Advantages Disadvantages

M icrostripline

Radiating Edge

Non radiating Edge

Monolithic. Good Polarization.

Impedance matching is

easier.

Spurious radiation. Must be inset or use

transformer to match

impedance.

Excites cross-polarization.

Coaxial Probe Impedance matching by

probe location.

Can be used with plated bias

for multilayer circuits.

Impedance is highlyinductive when thick

substrates are used.

Proximity Coupling

Monolithic

Multilayer

No DC contact between feed

and radiating element.

Can have large effective

thickness for patch substrate

and much thinner feed

substrate.

Several degrees of freedom

available for

matching/tuning.

Direct radiation from

coupling region.

Dimensional tolerance. Multilayer fabrication is

required.

Difficult to optimize.


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Advantages and Disadvantages

Microstrip patch antennas are increasing in popularity for use in wireless applications due

to their low-profile structure. Therefore they are extremely compatible for embedded

antennas in handheld wireless devices such as cellular phones, pagers etc... The telemetry and

communication antennas on missiles need to be thin and conformal and are often Microstrip

patch antennas. Another area where they have been used successfully is in Satellite

communication. Some of their principal advantages are given below:

Light weight and low volume. Low profile planar configuration which can be easily made conformal to host surface. Low fabrication cost, hence can be manufactured in large quantities. Supports both, linear as well as circular polarization.

Can be easily integrated with microwave integrated circuits (MICs). Capable of dual and triple frequency operations. Mechanically robust when mounted on rigid surfaces. Light weight, low volume and low profile planar configurations that can be made

conformal.

Some of their major disadvantages are given below:

Narrow bandwidth Low efficiency Low Gain Extraneous radiation from feeds and junctions Poor end fire radiator except tapered slot antennas Low power handling capacity.

However, there are ways of substantially diminishing the effect of some of thesedisadvantages. For example, surface wave excitation may be suppressed or eliminated by

exercising care during design and fabrication. Increasing or decreasing the thickness of

substrate can also control higher order excitation.


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BANDW I DTH ENHANCEMENT

I ntroduction:

Microstrip antennas have a number of useful properties, but one of the serious limitations of these antennas has been their narrow bandwidth characteristic.

The impedance bandwidth of a typical microstrip patch antenna is less than 1% to several percent for thin substrates satisfying the criteria h\ 0< 0.023 for r=10 to h/lambda0< 0.07

for r=2.3. This is in contrast to 15% to 50% bandwidth of commonly used antennaelements such as dipoles, slots and waveguide horns. Researchers have engaged inremoving this limitation for the past 20 years, and have been successful in achieving animpedance bandwidth of up to 90% and gain bandwidth up to 70% in separate antennas.

Most of these innovations utilize more than one mode, give rise to increase in size, height, or volume, and are accompanied by degradation of the other characteristic of the antenna.Increase in bandwidth can also be achieved by suitable choice of feeding technique andimpedance matching network.

Effects of Substrate Parameters on Bandwidth:

Impedance bandwidth of a patch antenna varies inversely as Q of the patch antenna.Therefore, substrate parameters such as dielectric constant r and thickness h can be variedto obtain different Q, and ultimately the increase in impedance bandwidth. Q of a resonator is defined as

Q= Energy stored/Power lost (1.1)

Figure below shows the effect of substrate thickness on impedance bandwidth and efficiencyfor two values of dielectric constants. Note that bandwidth increases monotonically withthickness. Also, a decrease in the r value increases the bandwidth. This behaviour can beexplained from the change in Q value.

Fig A1(l):Effect of substrate thickness on bandwidth,efficiency.


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Q almost linearly increase with r. Modelling of the rectangular patch as a lossy capacitor,the increase in Q is explained by the fact that the energy stored increases and power radiated decreases with increase in r . Similarly, when the substrate thickness is increased,the decrease in stored energy decreases the Q , this behaviour occurs because the fringingfield increases h and decrease in r.

In conclusion, we can say that the increase in h and decrease in r can be used to increasethe impedance bandwidth of the antenna. However, this approach is useful upto h< 0.02

0 only. The disadvantages of using thick and high dielectric constant substrates aremany, these:

Surface wave power increases, resulting in poor radiation efficiency (see Fig above). Theradiation from surface waves may lead to pattern degradation near end-fire.

Thick substrates with microstrip edge feed will give rise to increased spurious radiationfrom the microstrip step in-width and other discontinuities. Radiation from the feed linewill also increase.

Substrates thicker than 0.11 0 for r = 2.2 make the impedance locus of the probe-fed patch antenna increasingly inductive in nature, resulting in impedance matching problems.

Higher order modes along the thickness may develop, giving rise to distortions in theradiation patterns and impedance characteristics. This is a limiting factor in achieving anoctave bandwidth.

Most of the problems just listed are not experienced if thick air dielectric and aperturecoupling of the antenna to the feed are used. Surface wave effects can be controlled by theuse of photonic band gap structures.

Selection of suitable Patch Shape:

It has been found that some of the patch shapes have inherently lower Q compared to others.Correspondingly, their bandwidth is higher. These patch shapes include annular ring,rectangular/square ring, quarter wave (shorted) patch, and other geometrics. A circular ringantenna with b= 2 a when operated in the TM12 mode is found to have more than five times

the bandwidth of a rectangular patch antenna with L=1.5

W . Similarly, a rectangular/squarering antenna with an average circumference of one g can be used.

Bandwidth of annular ring and shorted quarter-wave antenna patch are comparedwith rectangular and circular patch geometries in Table 3. We can see from this table that the

bandwidth of a rectangular patch increases with an increase in the patch width.


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r = 2.32, h = 1. 5 9mm, f = 2 GHz

Table A1(c) :Comparison between Element shape, size, bandwidth.

Element Shape Element Size Bandwidth (%)

Narrow rectangular patch L = 4.92 4 cm ,W = 2 .0 cm0.7

Wide rectangular patch L = 4.79 cm, W = 7. 2 cm1.6

Square patch L = W = 4.8 2 cm1.3

Circular disk a = 2 .78 cm3.8

Annular ring b = 8.9 cm, a = 4.45 cm3.8

Quarter-wave patch L = 2 .46 2 cm, w = 2 .0 cm1.05


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APPEND IX II

POLAR IZ AT ION

The polarization of an antenna is the orientation of the electric field (E-plane) of the radio

wave with respect to the Earth's surface and is determined by the physical structure of the

antenna and by its orientation. It has nothing in common with antenna directionality terms:

"horizontal", "vertical" and "circular". Thus, a simple straight wire antenna will have one

polarization when mounted vertically, and a different polarization when mounted

horizontally. "Electromagnetic wave polarization filters are structures which can be employed

to act directly on the electromagnetic wave to filter out wave energy of an undesired

polarization and to pass wave energy of a desired polarization. Reflections generally affect

polarization

Polarization is the sum of the E-plane orientations over time projected onto an imaginary

plane perpendicular to the direction of motion of the radio wave. In the most general case,

polarization is elliptical, meaning that the polarization of the radio waves varies over time.

Two special cases are linear polarization (the ellipse collapses into a line) and circular

polarization (in which the two axes of the ellipse are equal). In linear polarization the antenna

compels the electric field of the emitted radio wave to a particular orientation.

Linear Polarization: An antenna is vertically linear polarized when its electric field is

perpendicular to the Earths surface. An example of a vertical antenna is a broadcast tower

for AM radio or the whip antenna on an automobile. Horizontally linear polarized antennas

have their electric field parallel to the Earth's surface. For example, television transmissions

in the USA use horizontal polarization. Thus, TV antennas are horizontally-oriented.

Circular Polarization: In a circularly-polarized antenna, the plane of polarization rotatesin a corkscrew pattern making one complete revolution during each wavelength. A circularly

polarized wave radiates energy in the horizontal, vertical planes as well as every plane in

between. If the rotation is clockwise looking in the direction of propagation, the sense is

called right-hand-circular (RHC). If the rotation is counterclockwise, the sense is called left-

hand circular (LHC).


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Fig A2(a): Linear & Circular Polarization

Advantages of Circular Polarization

y Reflectivity: Radio signals are reflected or absorbed depending on the material they

come in contact with. Because linear polarized antennas are able to attack" the

problem in only one plane, if the reflecting surface does not reflect the signal

precisely in the same plane, that signal strength will be lost. Since circular polarized

antennas send and receive in all planes, the signal strength is not lost, but is

transferred to a different plane and are still utilized.

y Absorption: As stated above, radio signal can be absorbed depending on the

material they come in contact with. Different materials absorb the signal from

different planes. As a result, circular polarized antennas give you a higher probability

of a successful link because it is transmitting on all planes.

y Phasing I ssues: High-frequency systems (i.e. 2.4 GHz and higher) that use linear

polarization typically require a clear line-ofsight path between the two points in order

to operate effectively. Such systems have difficulty penetrating obstructions due toreflected signals, which weaken the propagating signal. Reflected linear signals return

to the propagating antenna in the opposite phase, thereby weakening the propagating

signal. Conversely, circularly-polarized systems also incur reflected signals, but the

reflected signal is returned in the opposite orientation, largely avoiding conflict with


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the propagating signal. The result is that circularly-polarized signals are much better

at penetrating and bending around obstructions.

y Multi-path: Multi-path is caused when the primary signal and the reflected signal

reach a receiver at nearly the same time. This creates an "out of phase" problem. Thereceiving radio must spend its resources to distinguish, sort out, and process the

proper signal, thus degrading performance and speed. Linear Polarized antennas are

more susceptible to multi-path due to increased possibility of reflection. Out of phase

radios can cause dead-spots, decreased throughput, distance issues and reduce overall

performance in a 2.4 GHz system.

y I nclement Weather: Rain and snow cause a microcosm of conditions explained

above (i.e. reflectivity, absorption, phasing, multi-path and line of sight) Circular

polarization is more resistant to signal degradation due to inclement weather

conditions for all the reason stated above.

y Line-of-Sight: When a line-of-sight path is impaired by light obstructions (i.e.

foliage or small buildings), circular polarization is much more effective than linear

polarization for establishing and maintaining communication links.

Documents

Appendix REPORT