Introduction To Radar Systems 2002 Radar Antennas

Introduction to Radar SystemsRadar AntennasMIT Lincoln LaboratoryRadar Antennas - 1PRH 6/18/02

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Signal ProcessorTargetCrossSectionReceiverAntennaMain tionRadar Antennas - 3PRH 6/18/02S / N S / N Pt G2 λ2 σ(4 π )3 R4 k Ts Bn LPav Ae ts σ4 π Ω R4 k Ts LPulseCompressionA/DTracking &ParameterEstimationConsole /DisplayRecordingG GainAe Effective AreaTs System NoiseTemperatureL tureMIT Lincoln Laboratory

Antenna Definition “Means for radiating or receiving radio waves”*– A radiated electromagnetic wave consists of electric andmagnetic fields which jointly satisfy Maxwell’s Equations Transitional structure between guiding device and free spaceFigure by MIT OCW.Radar Antennas - 4PRH 6/18/02MIT Lincoln Laboratory* IEEE Standard Definitions of Terms for Antennas (IEEE STD 145-1983)

Antenna Characteristics Accentuates radiation in some directions, suppresses in othersDesigned for both directionality and maximum energy transferCourtesy of Raytheon.Used with permission.Courtesy of Raytheon.Used with permission.Courtesy of U. S. Navy.Radar Antennas - 5PRH 6/18/02MIT Lincoln Laboratory

OutlineRadar Antennas - 6PRH 6/18/02 Introduction Fundamental antenna concepts Reflector antennas Phased array antennas SummaryMIT Lincoln Laboratory

RadiationDipole**driven byoscillatingsourceRadar Antennas - 7PRH 6/18/02Vertical Distance (m)10.50-0.5-100.511.52Horizontal Distance (m)MIT Lincoln Laboratory

Antenna GainIsotropic AntennaDirectional AntennaG GainRadiationIntensityRadiationIntensity. Same power is radiated Radiation intensity is power density over sphere (watt/steradian) Gain is radiation intensity over that of an isotropic sourceRadar Antennas - 8PRH 6/18/02MIT Lincoln Laboratory

Antenna Pattern Pattern is a plot of gainversus angleDipole example 2 π cos 2 cosθ G (θ ) 1.643 2sin θ 0230Linear Plot5301.50160-50.59090120120Gain (dBi)60Figure by MIT OCW.Gmax 1.64 2.15 dBiPolar Plot-10-15-20-25-30150180Theta θ (deg)Radar Antennas - 9PRH 6/18/02150-3503060 90 120 150 180Theta θ (deg)MIT Lincoln Laboratory

Antenna Pattern CharacteristicsFigure by MIT OCW.Aperture diameter D: 5 mFrequency: 300 MHzWavelength: 1 mRadar Antennas - 10PRH 6/18/02Gain: 24 dBiIsotropic Sidelobe Level: 6 dBiSidelobe Level: 18 dBHalf-Power Beamwidth: 12 degMIT Lincoln Laboratory

Effect of Aperture Size on GainParabolic ReflectorAntennaGain vs Diameter50FrequencyIncreases45DishGain 4πAeλ24πAλ2 πD λ 2EffectiveAreaRule of Thumb(Best Case)Maximum Gain (dBi)FeedD403530252015λ 100 cm (300 MHz)λ 30 cm (1 GHz)λ 10 cm (3 GHz)10512345678910Aperture Diameter D (m)GainGain increasesincreases asas apertureaperture becomesbecomes electricallyelectrically largerlarger(diameter(diameter isis aa largerlarger numbernumber ofof wavelengths)wavelengths)Radar Antennas - 11PRH 6/18/02MIT Lincoln Laboratory

Reflector ComparisonKwajalein Missile Range ExampleALTAIR45.7 m diameterMMW13.7 m diameterscale by1/3Operating frequency: 162 MHz (VHF)Wavelength λ: 1.85 mOperating frequency: 35 GHz (Ka)Wavelength λ: 0.0086 mDiameter electrical size: 25 λDiameter electrical size: 1598 λGain: 34 dBBeamwidth: 2.8 degGain: 70 dBBeamwidth: 0.00076 degRadar Antennas - 12PRH 6/18/02MIT Lincoln Laboratory

Polarization Defined by behavior of the electric field vector as it propagatesin timeElectromagneticWaveElectric FieldMagnetic FieldVerticalLinear(with respectto Earth)EHorizontalLinear(with respectto Earth)E(For over-water surveillance)Radar Antennas - 13PRH 6/18/02(For air surveillance looking upward)MIT Lincoln Laboratory

Circular Polarization (CP) “Handed-ness” is defined by observation of electric field alongpropagation direction Used for discrimination, polarization diversity, rain mitigationPropagation DirectionInto igure by MIT OCW.Radar Antennas - 14PRH 6/18/02MIT Lincoln Laboratory

Circular Polarization (CP) “Handed-ness” is defined by observation of electric field alongpropagation direction Used for discrimination, polarization diversity, rain mitigationPropagation DirectionInto PaperRight-Hand(RHCP)Left-Hand(LHCP)Figure by MIT OCW.Electric FieldRadar Antennas - 15PRH 6/18/02MIT Lincoln Laboratory

Field Regions Reactive Near-Field RegionFar-field (Fraunhofer) RegionR 0.62 D 3 λR 2D2 λEnergy is stored in vicinity of antennaNear-field antenna quantities–Input impedance–Mutual coupling All power is radiated outRadiated wave is a plane waveFar-field antenna quantities–Pattern–Gain and directivity–Polarization–Radar cross section (RCS)RWave PropagationDirectionDReactive Near-FieldRegionRadiating Near-Field(Fresnel) RegionRadar Antennas - 16PRH 6/18/02Wave FrontsFar-Field (Fraunhofer)RegionMIT Lincoln Laboratory

Antenna Input Impedance Antenna can be modeled as an impedance– Design antenna to maximize power transfer from transmission line– Ratio of voltage to current at feed portReflection of incident power sets up standing waveInput impedance usually defines antenna bandwidthfeedΓTransmissionLineΓ 0Incident Poweris Deliveredto AntennaΓ 1All IncidentPower isReflectedAntennaStanding WaveRadar Antennas - 17PRH 6/18/02MIT Lincoln Laboratory

OutlineRadar Antennas - 18PRH 6/18/02 Introduction Fundamental antenna concepts Reflector antennas Phased array antennas SummaryMIT Lincoln Laboratory

Parabolic Reflector AntennaParabolic SurfaceWavefrontFeed Antennaat FocusDFBeam AxisFigure by MIT OCW. Design is a tradeoff between maximizing dish illuminationand limiting spillover Feed antenna choice is criticalRadar Antennas - 19PRH 6/18/02MIT Lincoln Laboratory

Cassegrain Reflector AntennaFigure by MIT OCW.Geometry ofCassegrain AntennaRadar Antennas - 20PRH 6/18/02Ray Trace ofCassegrain AntennaMIT Lincoln Laboratory

ALTAIRDual frequencyVHF ParabolicUHF CassegrainFSS (Frequency SelectiveSurface) used for reflectorRadar Antennas - 21PRH 6/18/02MIT Lincoln Laboratory

OutlineRadar Antennas - 22PRH 6/18/02 Introduction Fundamental antenna concepts Reflector antennas Phased array antennas SummaryMIT Lincoln Laboratory

Arrays Multiple antennas combined to enhance radiation and shape r Antennas - 23PRH onseResponseCombinerPhased ArrayDirectionDirectionMIT Lincoln Laboratory

Two Antennas RadiatingDipole1*Dipole2*Vertical Distance (m)10.50-0.5*driven byoscillatingsources(in phase)Radar Antennas - 24PRH 6/18/02-100.511.52Horizontal Distance (m)MIT Lincoln Laboratory

Array Controls Geometrical configuration– Element separationPhase shiftsExcitation amplitudes– For sidelobe controlPattern of individualelements–Radar Antennas - 25PRH 6/18/02Linear, rectangular,triangular, circular gridsIsotropic, dipoles, etc.MIT Lincoln Laboratory

Increasing Array Size byAdding ElementsLinear Broadside ArrayIsotropic Elementsλ/2 SeparationNo Phase ShiftingFigure by MIT OCW.N 10 ElementsN 20 Elements10 dBi13 dBiN 40 Elements2016 dBiGain (dBi)100-10-20-300306090120150Angle off Array (deg) Radar Antennas - 26PRH 6/18/02180 0306090120150180 0Angle off Array (deg)306090120150Angle off Array (deg)Gain 2N(d / λ) for long broadside arrayMIT Lincoln Laboratory180

Increasing Array Size bySeparating Elements Linear Broadside Array N 10 Isotropic ElementsL (N-1) d No Phase ShiftingFigure by MIT OCW.d λ/4 separationd λ separationd λ/2 separation207 dBiGratingLobes10 dBi10Gain (dBi)10 dBi0-10-20-300306090120150Angle off Array (deg)180 0306090120150Angle off Array (deg)180 0306090120150Angle off Array (deg)LimitLimit elementelement separationseparation toto dd λλ toto preventpreventgratinggrating lobeslobes forfor broadsidebroadside arrayarrayRadar Antennas - 27PRH 6/18/02MIT Lincoln Laboratory180

Increasing Array Size of Scanned Arrayby Separating Elements Linear Endfire Array N 10 Isotropic ElementsL (N-1) d Phase Shifted to Point UpFigure by MIT OCW.d λ/4 separationd λ separationd λ/2 separation2010 dBiGratingLobe10 dBiGain (dBi)10GratingLobes10 dBi0-10-20-300306090120Angle off Array (deg) Radar Antennas - 28PRH 6/18/02150180 0306090120Angle off Array (deg)150180 0306090120150Angle off Array (deg)No grating lobes for element separation d λ / 2Gain 4N(d / λ) 4L / λ for long endfire array without grating lobesMIT Lincoln Laboratory180

Linear Phased ArrayScanned every 30 deg, N 15, d λ/4Figure by MIT OCW.To scan over all space without grating lobes,keep element separation d λ / 2Radar Antennas - 29PRH 6/18/02MIT Lincoln Laboratory

Planar ArraysPatternNo ScanningFigure by MIT OCW. Figure by MIT OCW.As scan to θo off broadside:– Beamwidth broadens by 1/cosθo– Directivity decreases by cosθoTo scan over all space without grating lobes,keep element separation in both directions λ / 2Radar Antennas - 30PRH 6/18/02MIT Lincoln Laboratory

Mutual CouplingDrive Both Antennas Effect of one element on another–– Can greatly complicate array design––ZZ AntennamAntennan Near-field quantityMakes input impedance dependent onscan angleHard to deliver power to antennas for allscan anglesCan cause scan blindness where nopower is radiatedCan limit scan volume and arraybandwidthBut. mutual coupling can sometimes be exploitedto achieve certain performance requirementsRadar Antennas - 31PRH 6/18/02MIT Lincoln Laboratory

Phased Arrays vs Reflectors Phased arrays provide beam agility and flexibility–– Effective radar resource management (multi-function capability)Near simultaneous tracks over wide field of viewPhased arrays are significantly more expensive thanreflectors for same power-aperture–––Radar Antennas - 32PRH 6/18/02Need for 360 deg coverage may require 3 or 4 filled array facesLarger component costsLonger design timeMIT Lincoln Laboratory

OutlineRadar Antennas - 33PRH 6/18/02 Introduction Fundamental antenna parameters Reflectors Phased arrays SummaryMIT Lincoln Laboratory

Summary Fundamental antenna parameters and array topics have beendiscussed––––––– RadiationGain, pattern, sidelobes, beamwidthPolarizationFar fieldInput impedanceArray beamformingArray mutual couplingReflector antennas offer a relatively inexpensive method ofachieving high gain for a radar– Parabolic reflectors– Cassegrain feeds Phased array antennas offer beam agility and flexibility in use– But much more expensive than reflector antennasRadar Antennas - 34PRH 6/18/02MIT Lincoln Laboratory

References Balanis, C. A., Antenna Theory: Analysis and design, 2ndEdition, New York, Wiley, 1997Skolnik, M., Introduction to Radar Systems, New York,McGraw-Hill, 3rd Edition, 2001Mailloux, R. J., Phased Array Antenna Handbook, Norwood,Mass., Artech House, 1994Radar Antennas - 35PRH 6/18/02MIT Lincoln Laboratory