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RADAR ANTENNA
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Antenna Definition
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Means for radiating or receiving radio waves
*A radiatedelectromagneticwaveconsists ofelectricandmagnetic fields
which jointlysatisfyMaxwells Equations
Transitionalstructure betweenguidingdeviceand freespace
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Antenna Characteristics
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Antenna Gain
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Antenna Pattern
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Antenna Pattern Characteristics
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Effect of Aperture Size on Gain
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Reflector Comparison
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Polarization
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Circular Polarization (CP)
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Parabolic Reflector Antenna
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Cassegrain Reflector Antenna
Thecassegrain reflector system usestwo reflectors,theprimary being
aparabloidandthesecondary beingan hyperboloid
Illuminationcontrol -> Difficult,
Theaperture Blockage -> Large13
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Cassegrain Reflector Antenna
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Arrays
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Array Controls
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Radar Antenna Architecture
Comparison
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Large Phased Arrays
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PHASED ARRAY ANTENNA
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ELECTRONICALLY STEERED PHASED ARRAY
Advantage:
Agile, rapid beam steering
Potential for largepeakandlargeaveragepower.
Multipletargettracking.
A convenient meansto employsolidstatetransmitters
Control ofapertureillumination because of manyantennaelements
available
Operation with morethan one function.
Disadvantage
Complexandcan be of high cost
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two antennaelements, fed with the
samephase,
two antennaelements, fed with
differentphaseshift
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Beamsare formed byshiftingthephase ofthesignal
emitted from each radiatingelement,to provide
constructive/destructive Interference so asto steer the
beamsinthedesireddirection.
The beam sharpnessisimproved bythedestructive
interference.
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TYPES OF ARRAYS
Linear array:
Antennaelementsarrangedinastraightlinein onedimension.
Can be usedasthe feed for parabolicantenna.
Planar array:
Two dimensionalconfiguration ofantennaelementsarrangedto lie on
theplane .
Broadside array:
Thedirection of maximum radiationisperpendicular to theplane ofantenna.
Endfire array:
Hasmaximum radiationparallelto thearray.23
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Linear array ofaphased-arrayantenna
Theseantennaeconsist oflineswhose
elementsare fedaboutacommonphaseshifter.
A number ofverticallyabouteach other
mountedlinear arrays form a flatantenna
Advantage: simplearrangement
Disadvantage: Raydeflection onlyinasingle
planepossible
Examplesgiven:
PAR-80 (horizontal beam-deflection)and
RRP-117 (vertical beam-deflection)Large
Vertical Aperture (LVA) Antenna (fixed
beam form)
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PAR 80ThePAR-80 works with aphasedarrayantenna.
The beam iselectronicallysteeredinthehorizontalplaneandthe wholearrayswivels
vertically with help ofanellipticalgearbox.Most
ofthepartsareprovidedtwice for a better
redundance.
The radar must bechecked byaperiodical flight-
checkwith high requirements ontheprecision.
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RRP-117y Theair-defense radar RRP-117 (RemoteRadar
Post)isacompletelyD-Band radar system in
solidstatetechnology. Itconsists ofaprimary
radar,asecondary radar andasubsystem for
simulation. The RRP-117provides3D- target
informationsin realtime over the whole
range ofdetectioncorrelated witthein
formations ofthesecondary radar. It usesa
phasedarrayantenna turning within6 rpm
andanelectronicallysteared beam in
elevation.
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Planar array ofaphased-arrayantenna
Theseantennaarrayscompletelyconsist
ofsingles radiatingelementsandeach of
itgetsan ownphaseshifter.
Theelementsare orderedinamatrix
array.Advantage: Raydeflectionintwo planes
possible
Disadvantage: complicatedarrangement
andmoreelectronicallycontrolledphase
shifter needed
Examplesgiven:AN-FPS-85 andThomson
Master-A27
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AN/FPS-85y The world's firstlargephasedarray radar,the
AN/FPS-85 wasconstructedinthe 1960sat
Eglin Air Force Base, Florida. Other large
radars have beenintroducedsincethen, but
the Grand Old Lady ofthe South,asthe radar
installationisknownat Eglin, remainsthe
nation'sprimaryspacesurveillance radar
because ofits unsurpassedpower and
coverage.
y The beam formed bythephasedarray
antennaextends from 155 to 205 inazimuth
andisscannedat35 elevation.28
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ThomsonMaster-A
y
Thepashedarrayantenna rotateswith6
r.p.m.andscanstheairspacewith pecil-beams upto an
altitude to 100 kft. Thephasedarrayiscomposed
of35 linear arrayswith 35 phaseshifters. During
the receivingtimethe radar usesthemonopulse
procedure.
y
Two truckswith aloadcapacity of 10 tonsarenecessary for this fastlymovable radar. The radar
isair-transportable with two C 130 aircrafts or
Chinook- helicopters.
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BEAM STEERING NAD ARRAY FEED
NETWORKS
Parallel fedarraySeries fedarray
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Radar Transmitter/Receiver Timeline
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Sensitive radar receiver must beisolated from thepowerful radar transmitter
Transmittedpower typically 10 kW 1MW
Receiver signalpower in 10s W 1 mW
Isolationprovided byduplexer switching
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Duplexer Function
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DUPLEXERS
A pulse radar cantimeshareasingleantenna between
transmitter and receiver byemployinga fastactingswitching
devicecalledduplexer.
For high power applications,theduplexer isgas-discharge
devicecalleda TR (transmit-receive)switch.
The high power pulse from thetransmitter causesthegas-
dischargedeviceto breakdownandshortcircuitthe receiver to
protectit from damage.
On receive,the RF circuitry ofthecoldduplexer directsthe
echo signalto the receiver thanthetransmitter.
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Based ontheshort slot hybrid junction which consists oftwo
sections of waveguides joinedalong one oftheir narrow walls with
aslotcutinthecommon wallto providecoupling betweenthetwo.
Two TR tubesare used,oneineach section of waveguide.
Inthetransmitcondition,power isdividedequallyinto each
waveguide bythe first hybrid junction (ontheleft).
Both gas-discharge TR tubes breakdownand reflectincidentpower
out ofantennaarm.
Theshort-slot hybrid junction hasthepropertythateach timeit
passesthrough theslotineither direction,itsphaseisadvanced by
90 0
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Anypower thatleaksthrough the TR tube isdirectedtowards
thearm with matchedloadandnotto the receiver
On receiveconditionthe TR tubesdo not fireandecho signals
passthrough theduplexer andinto the receiver.
Thepower splitsequallyatthe first junctionand because ofthe
900 phaseadvance onpassingthrough theslot,thesignal
recombinesinthe receivingarm andnotinthearm with dummy
load.37
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TR TUBES AS WAVEGUIDES
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TR TUBE
keep- alive electrode
main gap
DC ground
The TR tube is a gas-discharge device designed to break down and ionize quickly
at the onset of high RF power , and to deionize quickly once the power is removed.
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Photograph of Traveling Wave Tubes
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How Big are High Power Klystron Tubes ?
MillstoneHill Radar TransmitterRoom