radar antennas

8/7/2019 radar antennas

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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

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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.

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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

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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

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