t prabhakar

8/14/2019 t Prabhakar

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INTRODUCTION

CHAPTER 1

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INTRODUCTION

Reflector antennas, in one form or another, have been in use

since the discovery of electromagnetic wave propagation in 1888 by

Hertz. Although reflector antennas take many geometrical

configurations, some of the most popular shapes are the plane,

corner, and curved reflectors. t has been shown by geometrical

optics that if a beam of parallel rays is incident upon a reflector

whose geometrical shape is a parabola, the radiation will converge

at a spot which is known as the focal point. n the same manner, if a

point source is placed at the focal point, the rays reflected by aparabolic reflector will emerge as a parallel beam. !ince the

transmitter "receiver# is placed at the focal point of the parabola,

the configuration is usually known as front fed. $he illumination of a

parabolic reflector antenna depends on the properties of the feed

used. $he widespread use of reflectors has simulated interest in the

development of feeds to improve the aperture efficiency and to

provide greater discrimination against noise radiation from ground.n order to obtain a high efficiency it is necessary that the radiation

pattern as uniform as possible and produces little spillover energy.

%esides it is desirable that the radiation pattern of the feed is

symmetrical and the feed should possess a well defined phase

center. &hen fed effectively from the focus paraboloid reflectors

produce high gain pencil beam with low side lobes and good cross

polarization discrimination characteristics. $he symmetrical focusfed paraboloid is the most widely used reflector for medium and

high gain pencil beam applications such as in Radio Astronomy and

it is considered to be a good compromise between performance and

cost. $his pro'ect describes the analysis of the focus fed parabolic

reflector and its Radiation properities in ()*)RA+ )$H- A*

A/)R$0R) A//R-A$-* )$H-.

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1. DIPOLEFEED

1. DIPOLERADIATIONPATTERN

2. DIPOLEMULTIICATI0NPATTERN

3. DIPOLEBEAMEFFICIENCY

2. SQUARECORNER FEED

3. HORNFEED

1. RADIATIONPATTERN BYVARYAINGSEVERALLENGTHS

2. SQUARECORNER BEAMEFFICIENCY

1. RADIATIONPATTERNS BYVARAYINGHORNDIMENSIONS

2. HORNPHASEVARIATION

FED TO A PARABOLIC REFLECTOR 1. f/D RATIO 2.APERTURE EFFICIENCY 3. GAIN

SELECTION OF FEED

3

1.1 PROCESSING STEPS


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1. DIPOLEFEED

1. DIPOLERADIATIONPATTERN

2. DIPOLEMULTIICATI0NPATTERN

3. DIPOLEBEAMEFFICIENCY

2. SQUARECORNER FEED

3. HORNFEED

1. RADIATIONPATTERN BYVARYAINGSEVERAL

LENGTHS

2. SQUARECORNER BEAMEFFICIENCY

1. RADIATIONPATTERNS BYVARAYINGHORN

DIMENSIONS

2. HORNPHASEVARIATION

FED TO A PARABOLIC REFLECTOR 1. f/D RATIO 2.APERTURE EFFICIENCY 3. GAIN 4. GENERAL CHARACTERISTICS

AFTER REFLECTION THE RADIATION PATTERNS OFEACH FEED ARE CALCULATED USING

1.GENERAL METHOD2.APERTURE APPROXIMATION METHOD

SELECTION OF FEED

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BASIC ANTENNA

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CHAPTER 2

BASIC ANTENNA

An antenna is a transducer designed to transmit or receive

electro magnetic waves. n other words, antennas convert

electromagnetic waves into electrical currents and vice versa.

Antennas are used in systems such as radio and television

broadcasting, point6to6point radio communication, wireless +A*,

radar, and space e7ploration. Antennas usually work in air or outer

space, but can also be operated under water or even through soil

and rock at certain freuencies for short distances.

/hysically, an antenna is an arrangement of conductorsthat

generate a radiating electromagnetic fieldin response to an applied

alternating voltage and the associated alternating electric current,

or can be placed in an electromagnetic field so that the field will

inducean alternating current in the antenna and a voltage between

its terminals. !ome antenna devices "parabolic antenna, HornAntenna# 'ust adapt the free space to another type of antenna

F! 2.1 "#$% #&'(&

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http://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Receivehttp://en.wikipedia.org/wiki/Electromagnetic_wavehttp://en.wikipedia.org/wiki/Electromagnetic_wavehttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Televisionhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Space_explorationhttp://en.wikipedia.org/wiki/Outer_spacehttp://en.wikipedia.org/wiki/Outer_spacehttp://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Electromagnetic_fieldhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Radio-frequency_inductionhttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Horn_Antennahttp://en.wikipedia.org/wiki/Horn_Antennahttp://en.wikipedia.org/wiki/Image:Moosbrunn_SW_Antenna.jpghttp://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Receivehttp://en.wikipedia.org/wiki/Electromagnetic_wavehttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Televisionhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Space_explorationhttp://en.wikipedia.org/wiki/Outer_spacehttp://en.wikipedia.org/wiki/Outer_spacehttp://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Electromagnetic_fieldhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Radio-frequency_inductionhttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Horn_Antennahttp://en.wikipedia.org/wiki/Horn_Antenna


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2.1 ANTENNA PARAMETERS

1. )ffective length

2. Resonant freuency

3. (ain

4. irectivity

5. Radiation pattern

9. +obe levels

:. mpedance

8. )fficiency

;. %and width

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R($+&' f,(-(&%

$he >resonant freuency> and >electrical resonance> is related

to the electrical length of the antenna. $he electrical length is

usually the physical length of the wire divided by its velocity factor

"the ratio of the speed of wave propagation in the wire to c


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radiated at the same distance by an hypothetical isotropic antenna.

&e write >hypothetical> because a perfect isotropic antenna cannot

e7ist in reality.!ometimes, the half6wave dipole is taken as a

reference instead of the isotropic radiator. $he gain is then given ind%d "decibels over dipole#=

(ain @"ma7imum radiation intensity in a given direction #

"ma7imum radiation intensity from isotropic antenna direction #

#B,"

#B,"ma7

E

EG

isotriopic

=

D,(%')'

Antenna directivity is usually measured in d%i, or decibels

above isotropic. $his number is obtained by measuring the gain in

the strongest lobe, and comparing it to the total gain "as if all power

was radiated uniformly in all directions#=

#"log1< 1polarization> of an antenna is the orientation of the

electric field ")6plane# of the radio wave with respect to the )arth?s

surface and is determined by the physical structure of the antenna

and by its orientation. t has nothing in common with antenna

directionality terms= >horizontal>, >vertical> and >circular>. $hus, a

simple straight wire antenna will have one polarization when

mounted vertically, and a different polarization when mounted

horizontally. >)lectromagnetic 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.

A&'(& #(,',(

As a receiver, antenna aperture can be visualised as the area

of a circle constructed broadside to incoming radiation where all

radiation passing within the circle is delivered by the antennato a

matched load. "*ote that transmittingand receiving are reciprocal,

so the aperture is the same for both.# $hus incoming power density

"watts per suare metre# 7 aperture "suare metres#@ available

powerfrom antenna "watts#. Antenna gainis directly proportional to

aperture. An isotropicantenna has an aperture of

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http://en.wikipedia.org/wiki/Bandwidthhttp://en.wikipedia.org/wiki/Feed_hornhttp://en.wikipedia.org/wiki/Polarizationhttp://en.wikipedia.org/wiki/E-planehttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Areahttp://en.wikipedia.org/wiki/Circlehttp://en.wikipedia.org/wiki/Radiationhttp://en.wikipedia.org/wiki/Antenna_(radio)http://en.wikipedia.org/w/index.php?title=Matched_load&action=edithttp://en.wikipedia.org/wiki/Transmittinghttp://en.wikipedia.org/wiki/Reciprocal_(grammar)http://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Isotropichttp://en.wikipedia.org/wiki/Bandwidthhttp://en.wikipedia.org/wiki/Feed_hornhttp://en.wikipedia.org/wiki/Polarizationhttp://en.wikipedia.org/wiki/E-planehttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Areahttp://en.wikipedia.org/wiki/Circlehttp://en.wikipedia.org/wiki/Radiationhttp://en.wikipedia.org/wiki/Antenna_(radio)http://en.wikipedia.org/w/index.php?title=Matched_load&action=edithttp://en.wikipedia.org/wiki/Transmittinghttp://en.wikipedia.org/wiki/Reciprocal_(grammar)http://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Isotropic


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M4

2

where N is the wavelength. An antenna with a gain of ( has an

aperture Ae@M4

2

G

(enerally, antenna gain is increased by directing radiation in a

single direction, while necessarily reducing it in all other directions

since power cannot be created by the antenna. $hus a larger

aperture produces a higher gain and narrower beamwidth.+arge

dish antennas, many wavelengths across, have an aperture nearlyeual to their physical area.

A&'(& (ff(%')( #,(#

n telecommunication, antenna effective area or effective

aperture is the functionally euivalent area from which an antenna

directed toward the source of the received signalgathers or absorbs

the energy of an incident electromagnetic wave.*ote 1= Antenna

effective area is usually e7pressed in suare meters.*ote 2= n the

case of parabolic and horn6parabolic antennas, the antenna

effective area is about


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M4

2GAeff =

where ( is the antenna gain "not in decibels# and N is the

wavelength. $his formula can be derived as a conseuence of

electromagnetic reciprocitywhich relates the transmit properties of

an antenna to the receiving properties. t may not hold if the

antenna is made with certain non6reciprocal materials. +ike the

antenna gain, the effective area varies with direction. f no direction

is specified, the ma7imum value is assumed

R(#'+&$* '+ *$%# #,(#

!imply increasing the size of antenna does not guarantee an

increase in effective areaO however, other factors being eual,

antennas with higher ma7imum effective area are generally

larger.n the case of wire antennas, there is no simple relationship

between physical area and effective area. n the case of aperture

antennas "for e7ample, horns and parabolic reflectors# considered in

their direction of ma7imum radiation, the aperture efficiencyis the

ratio of effective area to physical area=

physapeff AeA =

where eapis the aperture efficiency, Aphysis the physical size of

the aperture, and Aeffis the effective aperture. the definition section

above, derived from the Cederal !tandard, implies that the aperture

efficiency is


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uniform illumination of the aperture, phase variations of the

aperture field "typically due to surface errors in a reflector and high

flare angle in horns#, and scattering from obstructions. $he incident

wavefront may also not be completely phase coherent due tovariations in the propagating mediumO this results in an increase in

the effective area of an antenna not resulting in a commensurate

increase in signal, an effect known as ?aperture loss?.

R##'+& ,($$'#&%(

t is defined as that system resistance, when substituted in

series with an antenna, will consume the same power as actually

radiated.

A&'(& "(# 5'*

t is a measure of the directivity of an antenna,which

represents an angular width measured on the radiation pattern

between two points.

1:


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REFLECTORS

CHAPTER 3

REFLECTORS

A spherical wave front "one in which the energy spreads out in

all directions# spreads out as it travels away from the antenna and

produces a pattern that is not very directional. A wave front that

e7ists in only one plane does not spread because all of the wave

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front moves forward in the same direction. Cor an antenna to be

highly directive, it must change the normally spherical wave front

into a plane wave front. any highly directive microwave antennas

produce a plane wave front by using a reflector to focus theradiated energy.

Reflectors antennas in one form or other have been in use

since the discovery of electromagnetic wave propagation in 1888 by

H)R$P. Although reflector antennas take many geometrical

configurations, some of the most important shapes are planar,

corner and curved. t has been shown by geometrical optics that if a

beam of parallel rays are incident upon a reflector whose

geometrical shape is a parabola, the incident will converge at a spot

which is known as the focal point. n the same way if appoint source

is placed at the focal /ont the rays will emerge as a parallel beam.

!ince the transmitter is placed at the focal point of parabola the

configuration is known as front feed. Another arrangement that

avoids placing the feed at the focal point is known as a cassegrain

feed.cassegrian showed that incident parallel rays can be focused to

a point by utilizing two reflectors. $o accomplish this main reflector

must be a parabola, the secondary reflector must be a hyperbola

and the feed placed along the a7is of the parabola usually at or near

verte7.

$he day in, day out need of reflectors for use in radio

astronomy, micro wave communication and satellite tracking

resulted in spectacular progress in the development of sophisticated

design, analytical, fabrication techniues. n case of a parabolic

reflector the illumination over the aperture is entirely dependent on

the feed radiation characteristics. !ince the aperture efficiency,

gain, side lobe levels and beamwidth are the most important

parameters and are entirely dependent on the aperture illumination

characteristics, the feed pattern plays an important role in the

design and analysis of parabolic reflector.

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3.1 S',%',(

$he reflector dish can be solid, mesh or wire in construction

and it can be either fully circular or somewhat rectangular

depending on the radiation pattern of the feeding element. !olid

antennas have more ideal characteristics but are troublesome

because of weight and high wind load. esh and wire types weigh

less, are easier to construct and have nearly ideal characteristics if

the holes or gaps are kept under 11< of the wavelength.&ire6type

parabolic antenna "&i6Ci &+A* antenna at 2,4(hz#. -riented to

provide horizontal polarization= the reflector wires and the feed

element are both horizontal. $his antenna has a greater e7tent in

the vertical plane and hence, a narrower beamwidth in that plane.

$he feed element has a wider beam in the vertical direction than the

horizontal and hence matches the reflector by illuminating it

fully.ore e7otic types include the off6set parabolic antenna,

(regorian and Fassegrain types. n the off6set, the feed element is

still located at the focal point, which because of the angles utilized,

is usually located below the reflector so that the feed element and

support do not interfere with the the main beam. $his also allows for

easier maintenance of the feed, but is usually only found in smaller

antennas.$he /ARA%-+F R)C+)F$-R is most often used for high

directivity.

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Cig 3.1 simple parabolic antennas

Cig 3.2 Fylindrical paraboloid Cig 3.3 Forner reflctor

3.2 R(f(%'+, ,##'+&$

icrowaves travel in straight lines as do light rays. $hey can

also be focused and reflected 'ust as light rays can, as illustrated by

the antenna shown in figure. A microwave source is placed at focal

point C. $he field leaves this antenna as a spherical wave front. As

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each part of the wave front reaches the reflecting surface, it is

phase6shifted 18< degrees. )ach part is then sent outward at an

angle that results in all parts of the field traveling in parallel paths.

%ecause of the special shape of a parabolic surface, all paths from Cto the reflector and back to line Q are the same length. $herefore,

when the parts of the field are reflected from the parabolic surface,

they travel to line Q in the same amount of time.

.

Cig 3.4= /arabolic reflector radiations C6focus of paraboloid.

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PARABOLIC REFLECTOR

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CHAPTER 4

PARABOLIC REFLECTOR

A parabolic reflector, known as a parabolic dish or a parabolic

mirror, is a reflectivedevice, commonly formed in the shape of a

paraboloid of revolution. /arabolic reflectors can either collect or

distribute energysuch as light, sound, or radio waves.

$he parabolic reflector functions due to the geometricproperties of the paraboloid shape= if the angle of incidence to the

inner surface of the collector euals the angle of reflection, then any

incoming ray that is parallel to the a7is of the dish will be reflected

to a central point, or >focus>. %ecause many types of energy can be

reflected in this way, parabolic reflectors can be used to collect and

concentrate energy entering the reflector at a particular angle.

!imilarly, energy radiating from the >focus> to the dish can betransmitted outward in a beam that is parallel to the a7is of the

dish.

4.1 APPLICATIONS

Gohn Hadleyintroduced parabolic mirrors into practical

astronomyin 1:21 when he used one to build a reflecting telescope

with very little spherical aberration. %efore that, telescopes used

spherical mirrors. +ighthouses also commonly used parabolic

mirrors to collimate a point of light from a lantern into a beam,

before being replaced by more efficient fresnel lenses in the 1;th

century. $he most common modern applications of the parabolic

reflector are in satellite dishes, telescopes "including radio

telescopes#, parabolic microphones, and many lightingdevices such

as spotlights, car headlights, /AR Fansand +) housings.

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/arabolic reflectors suffer from an aberrationcalled coma.

$his is primarily of interest in telescopes because most other

applications do not reuire sharp resolution off the a7is of the

parabola.$he -lympic Clamehas been lit using a parabolic reflectorconcentrating sunlight. A parabolic reflector pointing upward can be

formed by rotating a reflective liuid, like mercury, around a vertical

a7is. $his makes the liuid mirror telescopepossible.

P#,#"+% #&'(&$

$he parabolic antenna is a high6gain reflector antenna used

for radio, television and data communications, and also for

radiolocation "RAAR#, on the 0HC and !HC parts of the

electromagnetic spectrum. $he relatively short wavelength of

electromagnetic "radio# energy at these freuencies allows

reasonably sized reflectors to e7hibit the very desirable highly

directional response for both receiving and transmitting.

/arabolic antennas at the Lery +arge Array Radio $elescope in*ew e7ico, 0!A.A typical parabolic antenna consists of a parabolic

reflector illuminated by a small feed antenna.$he reflector is a

metallic surface formed into a paraboloidof revolution and "usually#

truncated in a circular rim that forms the diameter of the antenna.

$his paraboloid possesses a distinct focal pointby virtue of having

the reflective property of parabolasin that a point light source at

this focus produces a parallel light beam aligned with the a7is of

revolution.

$he feed antenna is placed at the reflector focus. $his antenna is

typically a low6gain type such as a half6wave dipole or a small

waveguidehorn. n more comple7 designs, such as the Fassegrain

antenna, a sub6reflector is used to direct the energy into the

parabolic reflector from a feed antenna located away from the

primary focal point. $he feed antenna is connected to the

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associated radio6freuency "RC# transmittingor receiving euipment

by means of a coa7ial cabletransmission lineor hollow waveguide.

4.2 IDEAL CONDITIONS OF PARABOLIC REFLECTOR

1.&hen a bunch of parallel beams are reflected towards parabolic

reflector, then after reflecting these beams are colliminated at a

single point called C-F0!.

2.Any beam from the focus is reflected towards reflector, after

reflecting the beams travels parallel to the a7is of reflector.

3.parabolic reflector converts the spherical wave in to plane wave.

4.$he distance travelled by a any ray from focus to the parabola and

by reflection to the plane perpendicular to the parabola a7is is the

same for all rays no matter what angle they eminate from the

focus.

%ut in practical these are not possible.%ecause of losses occuringdue to spill over,tappering,illumination losses.ue to these losses

the apperture efficiency decreases and illuminated energy wasted.

4.3 CHARACTERISTICS OF PARABOLIC REFLECTOR

1. fd ratio "focal length to diameter ratio#

2. (ain

3. Radiation pattern

4. $otal Aperture efficiency

5. llumination and its losses

9. ($ ratio

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4.3.1 f/D ,#'+ 6f+%# (&!'* '+ #('(, ,#'+7

-ne of the important parameter to measure the performance

of the parabolic reflector. t determines how much power illuminated

towards it. All parabolic dishes have the same parabolic curvature, but

some are shallow dishes, while others are much deeper and more like

a bowl. $hey are 'ust different parts of a parabola which e7tends to

infinity. A convenient way to describe how much of the parabola is

used is the S ratio, the ratio of the focal length S to the diameter of

the dish. All dishes with the same f ratio reuire the same feed

geometry, in proportion to the diameter of the dish. $he values of S

ratios, typically from


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F! 4.1 V#,#'+&$ +f '*( '*('# )#( 5'* '*( f+%# (&!'* f+,

# f9( #('(,$ +f 0.2: 0.4: 1.;: #& 3

0 20 40 600

2

4

6

--->f(meters)

d=0.2 meters

0 20 40 600

5

10

15

--->f(meters)

d=0.4 meters

0 20 40 600

20

40

60

angle theta(in degrees)

-->f(meters)

d=1.5 meters

0 20 40 600

50

100

angle theta(in degrees)

--->f(meters)

d=3 meters

F! 4.2 V#,#'+&$ +f '*( '*('# )#( 5'* '*( #('(,$ f+, #

f9( f+%# (&!'*$ +f 0.1: 0.2;: 1.;: #& 3

0 20 40 600

0.05

0.1

0.15

0.2

--->d(meters)

f=0.1 meter

0 20 40 600

0.2

0.4

0.6

0.8

--->d(meters)

f=0.25 meters

0 20 40 600

10

20

30

--->angle theta(in degrees)

-->d(meters)

f=1.5 meters

0 20 40 600

50

100

150

--->angle theta(in degrees)

--->d(meters)

f=3 meters

$he general formula for finding f to ratio is given by an

antenna placed at the focal point of a parabolic reflector is said toilluminate the parabolic reflector. $he antenna has a beamwidth

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which is the how wide an angle the antenna would make if it were

radiating a beam of radio waves. $he beamwidth is a property of the

antenna itself and is the same regardless if the antenna is used for

receiving or transmitting. n designing a parabolic antenna, theantenna needs to properly illuminate its parabolic reflectorO that is,

the beamwidth of the antenna needs to match the f ratio of the

parabolic reflector. -therwise, the antenna of an over illuminated

parabolic reflector would receive a noise from behind the parabolic

reflector. +ikewise, an under illuminated parabolic reflector does not

use its total surface area to focus a signal on its antenna.

#2

cot"25.<

D

f=

&here f6focal length

6diameter

T 6 subtend angle

As U varies f become varies.

$he different relations between f, , V as shown in above plots

4.3.2 G#& +f # #,#"+% ,(f(%'+,

(ain @"ma7imum radiation intensity in a given direction #

"ma7imum radiation intensity from isotropic antenna direction #.

0sing the formula for the area of a circle, the area of the aperture of

a parabolic reflector is

#4

M"

2DA =

$his area is used in calculating the gain of a parabolic

reflector. $he gain ( of a parabolic reflector is proportional to the

2;


30/92

ratio of the area of the aperture to the suare of the wavelength l of

the incoming radio waves. W is the efficiency of the parabolic

reflector and has a practical value of 5


31/92

4.3.4 T+'# (ff%(&%

$he percentage of signal power transmitted or received

compared to the theoritical power from the proportion of a sphere

covered by the antennas beam.

Cor an antenna with a circular aperture or reflector of a

diameter"# and geometric surface" #4

M"

2DA = ,then aperture

efficiency is where is the efficiency of the antenna.

$he efficiency of the antenna is the product of several factors

which take account of the illumination law,spillover loss,surface

impairments,resistive and mismatch losses etc.

............YYY fZsi nnnnn =

$he '+& (ff%(&%6&7 specifices the illumination

law of the reflector with respect to uniform illumination. 0niform

illumination leads to high level of secondary lobes.A compromise is

achieved by attenuating the illumination at the reflector

boundaries"aperture at the taper#.n the case of cassegrain antenna

the best compromise is obtained for an illumination attenuation at

the boundaries of 1< to 12 db which leads to an illumination

efficiency& of the order of ;1X.

$he $+)(, (ff%(&% &$ is defined as the ratio of the

energy radiated by the primary source which is intercepted by the

reflector to the total energy radiated by the primary source.$he

difference constitutes the spillover energy.$he larger the angle

under which the reflector is viewed from the source,the greater the

31


32/92

spillover efficiency.However,for a given source radiation pattern,the

illumination level at the boundaries becomes less with large values

of view angle and the illumination efficiency collapses.A

compromise leads to spillover efficiency of the order of 8


33/92

3. Cocal point error

4. echanical support

$he above parameters decrease efficiency.Hence we must decrease

this loss.n the definition section above, derived from the Cederal

!tandard, and implies that the aperture efficiency is


34/92

F! 4.4 T*( )#,#'+& +f '*( #(,',( (ff%(&% 5'* '*(

'*('# #$ # f&%'+& +f '*( f(( #''(,& !)(& " 26&


35/92

0.2 0.4 0.6 0.8 1 1.2 1.4 1.60.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

---->f(meters)

---->aperture

efficiency

F! 4.> V#,#'+& & '*( #(,',( (ff%(&% 5'* '*( %*#&!(

& '*( f/ ,#'+ f+, '*( f(( #''(,& !)(& " 26&


36/92

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

--->diameter(meters)

---->aperture

efficiency

#7 I'+& #& '$ +$$($

!ome of the difficulties found in real antennas are easier to

understand when considering a transmitting antenna, but are also

present in receiving antennas, since antennas are reciprocal. -ne

difficulty is finding a point source, since any antenna, even a half6wave

dipole at 1< (Hz, is much bigger than a point. )ven if we were able to

find a point source, it would radiate eually in all directions, so the

energy that was not radiated toward the reflector would be wasted.

$he energy radiated from the focus toward the reflector illuminates

the reflector, 'ust as a light bulb would. !o we are looking for a point

source that illuminates only the reflector.

llumination

!pillover loss

39


37/92


38/92

F! 4. &f+, f(( '+&

F! 4.10 D$* '+& 5'* )#,+$ '+& '#(,$

63": >": 10": 20"7

$he above figures represent the illumination and its spillover

losses. ifferent edge tapers produce different amounts ofillumination loss and spillover loss. A small edge taper result in

38


39/92

larger spillover loss, while a large edge taper reduces the spillover

loss at e7pense of the increased illumination loss.

F! 4.11 $* 6PARABOLIC REFLECTOR7 '+& 5'*

)#,+$ f/D ,#'+$ 6f/D0.@;: 0.>;: 0.;: 0.4;7

"7S

(',

+f E

#&(

#&

H

P#&(

-n

paper,we can

only

depict

radiation in one plane. Cor simple antenna with linear polarization,

like a dipole, this is all we really care about. A dish however, is threedimensional, so we must feed it uniformly in all planes. $he usual

plane for linear polarization is the )6plane, while the plane

perpendicular to it called H6plane. 0nfortunately, most antennas not

only have different radiation patterns in the )6plane and H6planes, but

also have different phase centers in each plane, so both phase centers

cannot be at the focus.

3;


40/92

%7 F+%# L(&!'* E,,+,

$he critical focal length suggests that it is crucial to have the phase

center of the feed e7actly at the focus of the reflector. !ince the

phase center is rarely specified for a feed horn, we must determine it

empirically, by finding the ma7imum gain on a reflector with known

focal length. f we are using a feed horn with different phase centers

in the )6 and H6planes, we can also estimate the loss suffered in each

plane by referring to Cigure. +ateral errors in feed horn position are far

less seriousO small errors have little effect on gain, but do result in

shifting the beam slightly off bore sight.

7 M(%*#&%# $+,'

$here are two critical mechanical problems= mounting the feed horn

to the dish, and mounting the dish to the tripod. ost small dishes

have no backing structure, so the thin aluminum surface is easily

deformed. $he mounting structure for the feed horn is in the RC field,

so we must minimize the blockage it causes. &e do this by keepingthe support strut diameter small, by using insulating materials, and by

mounting the struts diagonally, so they aren?t in the plane of the

polarization. Ciberglass is a good materialO plant stakes or bicycle

flags are good sources.

4.3.; G/T 6GAIN TO TEMPERATURE RATIO7

&hen an antenna is receiving a signal from space, like a satellite or

)) signal there is a little background noise emanating from the sky

compared to the noise generated by the warm 3


41/92


42/92

FEEDS FOR THE

PARABOLIC REFLECTOR

42


43/92

CHAPTER ;

FEEDS FOR THE PARABOLIC REFLECTOR

;.1 FEED

Ceed is a source element in which all the energy is situated

at one point called feed element. (enerally the feed is placed at the

focus. ue to this we achieved high gain and sharp pencil beam

pattern. $he actual ?antenna? in a parabolic antenna, that is, the

device that interfaces the transmission line or waveguide containing

the radio6freuency energy to free space, is the feed element. $he

reflector surface is entirely passive. $his feed element should

usually be at the center of the reflector at the focal point of that

dish. $he focal point is the point where all reflected waves will be

concentrated. $he feed line connects the antenna to the receiver,

transmitter, or transceiver. $he line transfers radio6freuency "RC#

energy from a transmitter to an antenna, andor from an antenna to

a receiver, but, if operating properly, does not radiate or intercept

energy itself.

$he radiation from the feed element induces a current flow in

the conductive reflector surface which, in turn, re6radiates in the

desired direction, perpendicular to the directri7 plane of the

paraboloid. $he feed element can be any one of a multitude of

antenna types. &hichever type is used, it must e7hibit a directivity

that efficiently illuminates the reflector and must have the correct

polarization for the application 66 the polarization of the feed

determining the polarization of the entire antenna system. $he

simplest feed is a half6wave dipole which is commonly used at lower

freuencies, sometimes in con'unction with a closely coupled

parasitic reflector or >splash plate>. At higher freuencies a horn6

typebecomes more feasible and efficient. $o adapt the horn to a

43
http://en.wikipedia.org/wiki/Horn_antennahttp://en.wikipedia.org/wiki/Horn_antennahttp://en.wikipedia.org/wiki/Horn_antennahttp://en.wikipedia.org/wiki/Horn_antenna


44/92

coa7ial antenna cable, a length of waveguide is used to effect the

transition.

F! ;.1 D#!,#$ +f # f+%# f(( #,#"+% ,(f(%'+, #&'(&

44


45/92

D#!,# +f # f+%# f(( #,#"+% ,(f(%'+, +, $*

#&'(&with case f a feed is used as the source of transmission,

energy will be radiated from the antenna into space as well as

toward the reflector. )nergy which is not directed toward theparaboloid has a wide6beam characteristic which will destroy the

narrow pattern of the parabolic reflector. However, a

H)!/H)RFA+ !H)+ "not shown# may be used to direct most of

the radiation toward the parabolic surface and thus prevent the

destruction of the narrow pattern. irect radiation into space is

eliminated, the beam is made sharper, and more power is

concentrated in the beam. &ithout the shield, some of the radiated

field would leave the radiator directly. !ince this part of the field

that would leave the radiator would not be reflected, it would not

become a part of the main beam and could serve no useful purpose.

;.2 DIPOLE FEED

ipole antenna, developed by Heinrich Rudolph Hertz around

1889, is an antenna with a center6fed driven element for

transmitting or receiving radio freuency energy. A dipole antenna

is a straight electrical conductor measuring 12 wavelength from

end to end and connected at the center to a radio6freuency "RC#

feed line. $his antenna, also called a doublet, is one of the simplest

types of antenna, and constitutes the main RC radiating and

receiving element in various sophisticated types of antennas. $he

dipole is inherently a balanced antenna, because it is bilaterallysymmetrical.

deally, a dipole antenna is fed with a balanced, parallel6wire

RC transmission line. However, this type of line is not common. An

unbalanced feed line, such as coa7ial cable, can be used, but to

ensure optimum RC current distribution on the antenna element and

in the feed line, an RC transformer called a balun"contraction of the

words >balanced> and >unbalanced># should be inserted in the

45


46/92

system at the point where the feed line 'oins the antenna. Cor best

performance, a dipole antenna should be more than 12 wavelength

above the ground, the surface of a body of water, or other

horizontal, conducting medium such as sheet metal roofing. $heelement should also be at least several wavelengths away from

electrically conducting obstructions such as supporting towers,

utility wires, guy wires, and other antennas.

ipole antennas can be oriented horizontally, vertically, or at

a slant. $he polarization of the electromagnetic field ")# radiated

by a dipole transmitting antenna corresponds to the orientation of

the element. &hen the antenna is used to receive RC signals, it is

most sensitive to ) fields whose polarization is parallel to the

orientation of the element. $he RC current in a dipole is ma7imum at

the center "the point where the feed line 'oins the element#, and is

minimum at the ends of the element. $he RC voltage is ma7imum at

the ends and is minimum at the center.$hese antennas are the

simplest practical antennas from a theoretical /oint of view.

F! ;.2 $( *#f 5#)( +( #&'(&

A short dipole is a physically feasible dipole formed by two

conductors with a total length very small compared with the

wavelength . $he two conducting wires are fed at the centre of the

dipole. &e assume the hypothesis that the current is ma7imal at the

centre "where the dipole is fed# and that it decreases linearly to be

zero at the ends of the wires. *ote that the direction of the current

49


47/92

is the same in both the dipole branches 6 to the right in both or to

the left in both.

F! ;.3 R##'+& #''(,&$ +f $( +(

)mission is ma7imal in the plane perpendicular to the dipole and

zero in the direction of wires, that is, the current direction. $he

emission diagram is circular section torus shaped "left image# with

zero inner diameter. n the right image doublet is vertical in the

torus centre.

F! ;.4 UHF H#f 5#)( +(

;.3 DIPOLE CHARACTERISTICS

;.3.1 F,(-(&% )(,$$ (&!'*

ipoles that are much smaller than the wavelength of the

signal are called Hertzian, short, or infinitesimal dipoles. $hese have

a very low radiation resistance and a high reactance, making them

4:
http://en.wikipedia.org/wiki/Image:Elem-doub-rad-pat-pers.jpghttp://en.wikipedia.org/wiki/Image:Elem-doubl-rad-pat.jpg


48/92

inefficient, but they are often the only available antennas at very

long wavelengths. ipoles whose length is half the wavelength of

the signal are called half6wave dipoles, and are more efficient. n

general radio engineering, the term dipole usually means a half6wave dipole "center6fed#.A half6wave dipole is cut to length

according to the formula DftE, where l is the length in feet and f is

the center freuency in Hz . $his is because the impedance of the

dipole is resistive pure at about this length. $he metric formula is

DmE, where l is the length in meters. $he length of the dipole

antenna is about ;5X of half a wavelength at the speed of light in

free space.

;.3.2 R##'+& #''(,& #& !#&

ipoles have a toroidal "doughnut6shaped# reception and

radiation pattern where the a7is of the toroid centers about the

dipole. $he theoretical ma7imum gain of a Hertzian dipole is 1< log

1.5 or 1.:9 d%i. $he ma7imum theoretical gain of a N26dipole is 1l/2

-> l /4

-->l

DIPOLE BEAM EFFICIENCY

:4


75/92

0.2

0.4

0.6

0.8

1

3

1

240

90

270

1 0

300

150

3

180

tend angle(radians

intensity

0.2

0.4

0.6

0.8

10

40

9

27030

0

33

180

tend angle(radians

ntens

0 0.5 1 1.5 2 2.5 3 3.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

theta

beame

fficiency

theta (vs)dipole beam efficiency

DIPOLE MULTIPLICATION PATTERNS

*@1 *@3"number of array

elements#

:5


76/92

0.2

0.4

0.6

0.8

3

10

9

27

0

33

180

i l i l i li i

subtend angle(ra ians

i

i

0 1 2 3 4 5 6 7-

- .

.6

.4

-0.2

0

.2

.

.

.

subtend angl

--*>l/4

->l/2

-->l

0.2

0.4

0.6

0.8

0

240

27

300

3

*@5 *@;"array

elements#

INTENSITY VARIATIONS BY CHANGING SPACING BETEEN

DIPOLES

:9


77/92


78/92

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

.5

1

1. 5

2

2. 5

3

.

horn width(cm)

niu

e

o

plane

0 0 .5 1 1 .5 2 2.5 3 3.50

0. 1

0. 2

0. 3

0. 4

0. 5

0. 6

0. 7

0. 8

0. 9

1

theta

beam

efficiency

theta (vs )squrecorner beam efficiency

HORN FEED RADIATION PATTERN f/D 6VS7 APERTURE

6IDTH ISE 7 EFFICIENCY

:8


79/92

2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

.5

1

1. 5

2

2. 5

.

orn length(cm

iu

plne

f/D RATIO

0 0.5 10

2

4

6

angle theta

-->

f1

f vs theta if d=0.2

0 0.5 10

50

100

150

angle theta

-->

f2

f vs theta if d=4

0 0.5 10

500

1000

1500

angle theta

-->

f3

f vs theta if d=50

0 0.5 10

1000

2000

3000

angle theta

-->

f4

f vs theta if d=100

:;

0 0.5 1 1.5 2 2.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

f/d

apertureefficiency

f/d vs eap


80/92

0 2 4 6 8 10 12 142

1

0

1

2

ngl

ieli

ne

i

i l li l

-20 -15 -10 -5 0 5 10 15 200

2

4

6

8

10

12

14

16

i i

coor

inate

l

0 2 4 6 8 10 12 14 16

0. 2

.4

.6

0. 8

1

1. 2

.4

.6

1. 8

point on the x-axis

ii

0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20.5

1

.

2

.5

310

-

pertur

in

aper ure vs gain

PARABOLIC REFLECTOR SIMPLE

PARABOLA RADIATION PATTERN

PARABOLA CHARACTERISTICS APERTURE VS GAIN

6ESSENTRICITY7

FINAL

RESULTS6AFTER REFLECTING ON THE

PARABOLIC SURFACE7

DIPOLE RADIATION PATTERN ON THE DIRECTION OF

PARABOLA AXIS

8


81/92

0 0.5 1 1.5 2 2.5 3 3.5

.

.02

0.03

0.04

0.05

0.06

0.07

. 8

.

.

theta1(subtend angle)

n

a

e

p

0 0.5 1 1.5 2 2.5 3 3.5

0

00

15 0

20 0

00

0

theta1(radians)

iel

in

ensiy

i l l i i

0 0.5 1 1 .5 2 2.5 3 3.5

.0 1

.0 2

0.03

0.04

0.05

0.06

0.07

.0 8

.0 9

.

theta1(subtend angle)

i

l

i

l

SQUARE CORNER

RADIATION PATTERN ON SQUARE CORNER

PATTERN ALONG THETA

THE DIRECTION OF PARABOLA AXIS

81


82/92

2 3 4 5 6 7 8 9 10

00

20 0

40 0

50 0

00

00

00

theta1

il

i

i

i l l i i

1 2 3 4 5 6 7 8 9

00

300

400

500

0

0

0

phy1

ieli

nensi

i l l i i

0 0.5 1 1.5 2 2.5 3 3.5

00 0

2000

3000

00 0

00 0

phy1

il

i

i

i l l i i

0 0.5 1 1.5 2 2. .

00 0

00 0

3000

00 0

00 0

0

btend angl

ili

i

i l l i i

HORN PATTERN ALONG HORN PATTERN ALONG

THETA DIRECTION PHY DIRECTION

APERTUREAPPROXIMATION

METHOD DIPOLE FEED

ALONG THETA DIRECTION ALONG PHY

DIRECTION

SQUARE CORNER FEED

82


83/92

2 3 4 5 6 7 8 9 1 00

0 0 0

0 0 0

0 0

8 0 0 0

1 0 0 0 0

2 0 0 0

0 0 0

0 0 0

0 0

s u b t e n d a n g l e

0 0.5 1 1.5 2 2.5 3 3.5

00 0

2000

3000

00 0

00 0

phy1

il

i

i

i l l i i

0 0.5 1 1.5 2 2.5 .

00 0

00 0

3000

00 0

00 0

00

btend angl

il

i

i

i l l i i

1 2 3 4 5 6 7 8 9

0 0 0

0 0 0

0 0

8 0 0 0

1 0 0 0 0

4 0 0 0

0 0 0

0 0 0

p h y 1

i

l

i

i

i e l a l o n g p y i re c i

ALONG THETA DIRECTION ALONG PHY DIR

HORN FEED

ALONG THETA DIRECTION ALONG PHY

DIRECTION

83


84/92

0 0.5 1 1.5 2 2.5 3 3.5

10

15

subtend angle(radians)

ie

i

i

i i i

0 0 .5 1 1.5 2 2 .5 3 3.5- 0

5

-

-5

subtend ang l

ir

ii

ir i i l ir i

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

10

15

20


e

n

ens

y

0.8 0 .9 1 1 .1 1 .2 1 .3 1 .4 1 .5 1 .610

0

su b te n d a n g le

r l r

DIPOLE FEED INTENSITY6"7

DIRECTIVITY6DIPOLE7

HORN FEED INTENSITY 6B7

HORN DIRECTIVITY

APERTURE APPROXIMATION METHOD

DIPOLE FEED INTENSITY 6B7DIRECTIVITY

84


85/92

0 0.5 1 1.5 2 2.5 3 3.5-

0

5

0

0

subtend ang l

i

ii

i i i l i i

0 0.5 1 1.5 2 2.5 3 3.55

25

30

5


i

ensiy

b

i i l i i

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

10

5

subtend angl

i

ii

i i i l i i

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

5

30

5

subtend angl

i

i

i i l i i

HORN FEED INTENSITY6B7 DIRECTIVITY

85


86/92

ANALYSIS

Crom dipole feed radiation patterns , as length increases from

N4 to N the radiation intensity increases and the beam width

decreases. n the dipole multiplication patterns as the length increases

the radiation intensity increases the beam width become

decreases and the side lobes increases.Cor *@1,3,5,; cases

observed.

$he beam efficiency of dipole increases from < to 1 as

increases subtend angle for a given half wave dipole as

isotropic source.

Crom suarecorner feed radiation patterns , as length

increases from N2 to 2N the radiation intensity increases and

the beam width decreases.%ut the ma7imum intensity

decreases.

$he beam efficiency of suare corner increases from < to 1 as

increases subtend angle for a given half wave dipole as

isotropic source.

89


87/92

Crom horn feed radiation patterns , as length increases from

1 to :.5 and width varies from


88/92

CONCLUSIONS

88


89/92

CONCLUSION

$he fundamental antenna concepts and a brief introduction

to the types of feeds have been discussed. Analysis of the parabolic

reflector characteristics like f, gain, radiation patterns has been

done and the corresponding results were plotted. $he primary

radiation patterns of each feed like dipole, suarecorner and hornwere calculated and then the far field pattern of each feed was

calculated by using general and aperture appro7imation methods.

ntensity and directivity of feeds were compared.

Crom results it can be concluded Horn feed has more intensity

and more directivity among three feeds. 0sing Aperture

appro7imation method we achieved more intensity and more

directivity than general method.

8;


90/92


91/92

BIBLIOGRAPHY

REFERENCES

1. . $urrin, &20, >/arabolic Reflector Antennas and Ceeds,> $heARR+0HCicrowave )7perimenter?s anual,. ARR+,1;;Reflector Antennas,> in Antenna handbook=theory, applications, and design, Q.$. +o and !.&. +ee, editors, Lan

*ostrand Reinhold, 1;88

3. Antennas for all applications by Gohn .raus and Ronald G.arhefka,$ata c (raw Hill )dition 2


92/92

/ublications

:.%.!.(reywal,J%asic engineering athematicsJ, $ata c (raw Hill 15th

edition, revised 1;;8

t prabhakar

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