IEEE New Hampshire Section Radar Systems Course 1 Antennas Part 2 1/1/2010 IEEE AES Society Radar Systems Engineering Lecture 9 Antennas Part 2 - Electronic Scanning and Hybrid Techniques Dr. Robert M. O’Donnell IEEE New Hampshire Section Guest Lecturer
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IEEE New Hampshire SectionRadar Systems Course 1Antennas Part 2 1/1/2010 IEEE AES Society
Radar Systems EngineeringLecture 9Antennas
Part 2 - Electronic Scanning and Hybrid Techniques
Dr. Robert M. O’DonnellIEEE New Hampshire Section
Guest Lecturer
Radar Systems Course 2Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Block Diagram of Radar System
Transmitter
WaveformGeneration
PowerAmplifier
T / RSwitch
Antenna
PropagationMedium
TargetRadarCross
Section
Photo ImageCourtesy of US Air ForceUsed with permission.
PulseCompressionReceiver Clutter Rejection
(Doppler Filtering)A / D
Converter
General Purpose Computer
Tracking
DataRecording
ParameterEstimation Detection
Signal Processor Computer
Thresholding
User Displays and Radar Control
Radar Systems Course 3Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Antenna Functions and the Radar Equation
• “Means for radiating or receiving radio waves”*– A radiated electromagnetic wave consists of electric and
magnetic fields which jointly satisfy Maxwell’s Equations• Direct microwave radiation in desired directions, suppress
in others• Designed for optimum gain (directivity) and minimum loss
of energy during transmit or receive
Pt G2 λ2 σ
(4 π )3 R4 k Ts Bn LS / N =
TrackRadar
Equation
Pav Ae ts σ
4 π Ω R4 k Ts LS / N =
SearchRadar
Equation
G = Gain
Ae = Effective Area
Ts = System NoiseTemperature
L = Losses
ThisLecture
RadarEquationLecture
* IEEE Standard Definitions of Terms for Antennas (IEEE STD 145-1983)
Radar Systems Course 4Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Radar Antennas Come in Many Sizes and Shapes
Mechanical ScanningAntenna Hybrid Mechanical and Frequency
Scanning AntennaElectronic Scanning
Antenna
Hybrid Mechanical and FrequencyScanning Antenna
Electronic ScanningAntenna
Mechanical ScanningAntenna
PhotoCourtesy
of ITT CorporationUsed withPermission
Courtesy of RaytheonUsed with Permission
Photo Courtesy of Northrop GrummanUsed with Permission
Courtesy US Dept of Commerce
Courtesy US Army Courtesy of MIT Lincoln LaboratoryUsed with Permission
Radar Systems Course 5Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Outline
• Introduction
• Antenna Fundamentals
• Reflector Antennas – Mechanical Scanning
• Phased Array Antennas– Linear and planar arrays– Grating lobes– Phase shifters and array feeds – Array feed architectures
• Frequency Scanning of Antennas
• Hybrid Methods of Scanning
• Other Topics
PartOne
PartTwo
Radar Systems Course 6Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
• Multiple antennas combined to enhance radiation and shape pattern
Arrays
Array Phased ArrayIsotropicElement
PhaseShifter
Σ
CombinerΣ Σ
Direction
Res
pons
e
Direction
Res
pons
e
Direction
Res
pons
e
Direction
Res
pons
e
Array
Courtesy of MIT Lincoln LaboratoryUsed with Permission
Radar Systems Course 7Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Two Antennas Radiating
Horizontal Distance (m)
Vert
ical
Dis
tanc
e (m
)
0 0.5 1 1.5 2-1
0
-0.5
0.5
1
Dipole1*
Dipole2*
*driven by oscillatingsources
(in phase)Courtesy of MIT Lincoln LaboratoryUsed with Permission
Radar Systems Course 8Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Array Beamforming (Beam Collimation)
Broadside Beam Scan To 30 deg
• Want fields to interfere constructively (add) in desired directions, and interfere destructively (cancel) in the remaining space
Courtesy of MIT Lincoln LaboratoryUsed with Permission
Radar Systems Course 9Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Controls for an N Element Array
• Geometrical configuration– Linear, rectangular,
triangular, etc
• Number of elements
• Element separation
• Excitation phase shifts
• Excitation amplitudes
• Pattern of individual elements
– Dipole, monopole, etc.
Array FactorAntenna Element
ElementNumber
ElementExcitation
1
4
3
2
NNj
N ea φ
3j3 ea φ
4j4 ea φ
2j2 ea φ
1j1 ea φ
N
D
na
nφ
D
ScanAngle
Courtesy of MIT Lincoln LaboratoryUsed with Permission
Radar Systems Course 10Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
The “Array Factor”
• The “Array Factor” AF, is the normalized radiation pattern of an array of isotropic point-source elements
rrkjN
1n
jn
nn eea),(AF ⋅
=
φ∑=φθr
Position Vector
Excitation njna e Φ
Source Element n:
nnn1 zzyyxxr ++=r
Observation Angles (θ,φ):Observation Vector
θ+φθ+φθ= coszsinsinycossinxr
Free-SpacePropagation Constant c
f22k π=
λπ
=
NIsotropicSources
Observe at (θ, φ)
1rr
Nrr2rr 3rr
r
z
y
x
Radar Systems Course 11Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Array Factor for N Element Linear Array
∑∑∑−
=
θψ−
=
β+θ⋅
=
φ− ===φθ −−
1N
0n
)(nj1N
0n
)cosdk(njrrkjN
1n
j1n eAeAeea),(AF 1n1n
rd
θ
4321 1N − N
Where : and,
It is assumed that:
Phase progression is linear, is real.
Using the identity:
β+θ=θψ cosdk)(
nnjj a,ee n βφ =
The array is uniformly excited Aan =
1c1cc
N1N
0n
n
−−
=∑−
=
( )( )2/SinN
2/Nsin),(AFψψ
=φθThe Normalized Array Factor becomes :
Main Beam Location
0cosdk =β+θ=ψ
( ) π±=β+θ=ψ mcosdk
21
2
Radar Systems Course 12Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Properties of N Element Linear Array
• Major lobes and sidelobes– Mainlobe narrows as increases– No. of sidelobes increases as increases– Width of major lobe = – Height of sidelobes decreases as increases
• Changing will steer the peak of the beam to a desired– Beam direction varies from to– varies from to
• Condition for no grating lobes being visible:
N
N
NN/2π
β
ocos11d
θ+<
λ
π0oθ=θ
β+− kdψ β+kd
oθ = angle off broadside
Note how is defined.θ
Radar Systems Course 13Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
-20 0 20 40 60 80
Array and Element Factors
• Total Pattern = Element Factor X Array Factor
• Element Factor
• Array Factor Adapted fromFrank in Skolnik
Reference 2
Ten Element Linear Array – Scanned to 60 °
Angle (degrees)
Total Pattern
Element Factor
Array Factor
Rel
ativ
e Fi
eld
Stre
ngth
0.2
0.8
0.6
1.0
0.4
2/λ
ElementSpacing
)(E)(E)(E ae θ×θ=θ
θ=θ cos)(Ee
( )( )( )866.0sin2/sin10
866.0sin5sin)(Ea −θπ−θπ
=θ
0.707
Radar Systems Course 14Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Array Gain and the Array Factor
ArrayFactorGain
ArrayGain(dBi)
Array FactorGain(dBi)
ElementGain(dBi)
= +
Individual Array Elements are Assumed to Be Isolated
The Overall Array Gain is the Product of the Element Gain and the Array Factor Gain
RAD
2
AF P),(AF4
),(Gφθπ
=φθ
φθθφθ= ∫ ∫π π
ddsin),(AFP2
0 0
2RAD
Radar Systems Course 15Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Homework Problem – Three Element Array
• Student Problem:
– Calculate the normalized array factor for an array of 3 isotropic radiating elements. They are located along the x-axis (center one at the origin) and spaced apart. Relevant information is 2 and 3 viewgraphs back.
– Use the results of this calculation and the information in viewgraph 28 of “Antennas Part 1’ to calculate the radiation pattern of a linear array of three dipole, apart on the x-axis.
2/λ
2/λ
Radar Systems Course 16Antennas Part 2 1/1/2010
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Increasing Array Size byAdding Elements
• Gain ~ 2N(d / λ) for long broadside array
N = 10 Elements N = 20 Elements
Gai
n (d
Bi)
Angle off Broadside (deg)
Linear Broadside ArrayIsotropic Elements
Element Separation d = λ/2No Phase Shifting
-90 -60 -30 0 30 60 90-30
-20
-10
0
10
20
N = 40 Elements
10 dBi 13 dBi 16 dBi
Angle off broadside (deg)
Figure by MIT OCW.
-90 -60 -30 0 30 60 90Angle off broadside (deg)
-90 -60 -30 0 30 60 90
Courtesy of MIT Lincoln LaboratoryUsed with Permission
Radar Systems Course 17Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Increasing Broadside Array Size bySeparating Elements
Limit element separation to d < λ to preventgrating lobes for broadside array
Limit element separation to d < λ to preventgrating lobes for broadside array
d = λ/4 separation d = λ/2 separation d = λ separation
• The most prevalent cause of bandwidth limitation in phased array radars is the use of phase shifters, rather than time delay devices, to steer the beam
– Time shifting is not frequency dependent, but phase shifting is.
8192 phase shifters (in a plane) take the place of the dielectric lens. The spherical wave of microwave radiation is phase shifted appropriately to form a beam and point it in the desired direction
MOTR Space Fed Lens Antenna
Courtesy of Lockheed MartinUsed with Permission
Courtesy of Lockheed MartinUsed with Permission
Radar Systems Course 53Antennas Part 2 1/1/2010
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Examples: Space Fed - Lens Array Radars
Patriot Radar MPQ-53S-300 “30N6E” X-Band Fire Control Radar*
• * NATO designation “Flap Lid” – SA-10• Radar is component of Russian S-300 Air Defense System
Courtesy of MDA
Courtesy of L. Corey, see Reference 7
Radar Systems Course 54Antennas Part 2 1/1/2010
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Example of Space Fed - Reflectarray Antenna
S-300 “64N6E” S-Band Surveillance Radar*
• * NATO designation “Big Bird” – SA-12• Radar is component of Russian S-300 Air Defense System
• Radar system has two reflectarray antennas in a “back-to-back”configuration.
• The antenna rotates mechanically in azimuth; and scans electronically in azimuth and elevation
Radar System and Transporter Radar Antenna
Courtesy of Martin RosenkrantzUsed with Permission
Courtesy of Wikimedia / ajkol
Radar Systems Course 55Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Two Examples of Constrained Feeds(Parallel and Series)
• Parallel (Corporate) Feed– A cascade of power splitters, in parallel, are used to create a tree like structure– A separate control signal is needed for each phase shifter in the parallel feed
design• Series Feed
– For end fed series feeds, the position of the beam will vary with frequency– The center series fed feed does not have this problem– Since phase shifts are the same in the series feed arraignment, only one control
signal is needed to steer the beam• Insertion losses with the series fed design are less than those with the
parallel feed
Parallel (Corporate) Feed
2/1 PowerSplitter
2/1 PowerSplitter
2/1 PowerSplitter
φ φ2
MicrowavePower φ
End Fed - Series Feed
φ φ φ
φ3 φ4
Radar Systems Course 56Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Outline
• Introduction
• Antenna Fundamentals
• Reflector Antennas – Mechanical Scanning
• Phased Array Antennas
• Frequency Scanning of Antennas
• Hybrid Methods of Scanning
• Other Topics
Radar Systems Course 57Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Frequency Scanned Arrays
• Beam steering in one dimension has been implemented by changing frequency of radar
• For beam excursion , wavelength change is given by:
• If = 45°, 30% bandwidth required for , 7% for
λπ=π=φ /L2v/Lf2
The phase difference between 2 adjacent elements is
where L = length of line connecting adjacent elements and v is the velocity of propagation
L Serpentineor
“Snake” Feed”
Main BeamDirection
1θ
TerminationStub
AntennaElements
D
( ) 1o sinL/D2 θλ=λΔ1θ±
1θ 5L/D = 20L/D =Adapted from Skolnik, Reference 1
Radar Systems Course 58Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Example of Frequency Scanned Array
• The above folded waveguide feed is known as a snake feed or serpentine feed.
• This configuration has been used to scan a pencil beam in elevation, with mechanical rotation providing the azimuth scan.
• The frequency scan technique is well suited to scanning a beam or a number of beams in a single angle coordinate.
Planar Array Frequency Scan Antenna
Adapted from Skolnik, Reference 1
Radar Systems Course 59Antennas Part 2 1/1/2010
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Examples of Frequency Scanned Antennas
SPS-52
SerpentineFeed
SPS-48E
Courtesy of US NavyCourtesy of ITT CorporationUsed with Permission
SerpentineFeed
Radar Systems Course 60Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Outline
• Introduction
• Antenna Fundamentals
• Reflector Antennas – Mechanical Scanning
• Phased Array Antennas
• Frequency Scanning of Antennas
• Example of Hybrid Method of Scanning
• Other Topics
Radar Systems Course 61Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
ARSR-4 Antenna and Array Feed
• Joint US Air Force / FAA long range L-Band surveillance radar with stressing requirements
– Target height measurement capability– Low azimuth sidelobes (-35 dB peak)– All weather capability (Linear and Circular Polarization)
• Antenna design process enabled with significant use of CAD and ray tracing
ARSR-4 Antenna ARSR-4 Array Feed
OffsetArrayFeed
10 elevation Beams
Array Size17 x 12 ft
23 rowsand
34 columnsof elements
Courtesy of Northrop GrummanUsed with Permission
Courtesy of Frank SandersUsed with Permission
Radar Systems Course 62Antennas Part 2 1/1/2010
IEEE New Hampshire SectionIEEE AES Society
Phased Arrays vs Reflectors vs. Hybrids
• Phased arrays provide beam agility and flexibility – Effective radar resource management (multi-function capability) – Near simultaneous tracks over wide field of view– Ability to perform adaptive pattern control
• Phased arrays are significantly more expensive than reflectors for same power-aperture
– Need for 360 deg coverage may require 3 or 4 filled array faces– Larger component costs– Longer design time
• Hybrid Antennas – Often an excellent compromise solution– ARSR-4 is a good example array technology with lower cost
reflector technology– ~ 2 to 1 cost advantage over planar array, while providing
Courtesy of MIT Lincoln LaboratoryUsed with Permission
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Antenna Stabilization Issues
• Servomechanisms are used to control the angular position of radar antennas so as to compensate automatically for changes in angular position of the vehicle carrying the antenna
• Stabilization requires the use of gyroscopes , GPS, or a combination, to measure the position of the antenna relative to its “earth” level position
• Radars which scan electronically can compensate for platform motion by appropriately altering the beam steering commands in the radar’s computer system
Radar Systems Course 66Antennas Part 2 1/1/2010
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Radomes
• Sheltering structure used to protect radar antennas from adverseweather conditions
– Wind, rain, salt spray
• Metal space frame techniques often used for large antennas– Typical loss 0.5 dB
• Inflatable radomes also used – Less loss, more maintenance, flexing in wind
ALCOR COBRA GEMINI
MMW
Courtesy of US Navy
Courtesy of MIT Lincoln LaboratoryUsed with Permission
Courtesy of MIT Lincoln LaboratoryUsed with Permission
Radar Systems Course 67Antennas Part 2 1/1/2010
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Summary
• Enabling technologies for Phased Array radar development– Ferrite phase shifters (switching times ~ few microseconds)– Low cost MMIC T/R modules
• Attributes of Phased Array Radars– Inertia-less, rapid, beam steering– Multiple Independent beams– Adaptive processing– Time shared multi-function capability– Significantly higher cost than other alternatives
• Often, other antenna technologies can offer cost effective alternatives to more costly active phased array designs
– Lens or reflect arrays– Reflectors with small array feeds, etc.– Mechanically rotated frequency scanned arrays
Radar Systems Course 68Antennas Part 2 1/1/2010
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Acknowledgements
• Dr. Pamela R. Evans• Dr. Alan J. Fenn
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References
1. Skolnik, M., Introduction to Radar Systems, New York, McGraw-Hill, 3rd Edition, 2001
2. Skolnik, M., Radar Handbook, New York, McGraw-Hill, 3rd Edition, 2008
3. Balanis, C.A., Antenna Theory: Analysis and Design, 3nd Edition,New York, Wiley, 2005.
4. Kraus, J.D. and Marhefka, R. J., Antennas for all Applications, 3nd
Edition, New York, McGraw-Hill, 2002.5. Hansen, R. C., Microwave Scanning Antennas, California,