Introduction to Radar Systems 2002 Radar Antennas · Radar Antennas - 1 PRH 6/18/02 MIT Lincoln Laboratory Introduction to Radar Systems Radar Antennas. Radar Antennas - 2 PRH 6/18/02

Radar Antennas - 1PRH 6/18/02

MIT Lincoln Laboratory

Introduction to Radar Systems

Radar Antennas



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Focus

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

Antenna

PropagationMedium

TargetCross

Section

Transmitter

PulseCompression

Recording

Receiver

Tracking &ParameterEstimation

Console /Display

DopplerProcessingA / D

WaveformGenerator

Detection

Signal Processor

Main Computer



Antenna Definition

* IEEE Standard Definitions of Terms for Antennas (IEEE STD 145-1983)

• “Means for radiating or receiving radio waves”*– A radiated electromagnetic wave consists of electric and

magnetic fields which jointly satisfy Maxwell’s Equations

• Transitional structure between guiding device and free space

Figure by MIT OCW.



Antenna Characteristics

• Accentuates radiation in some directions, suppresses in others• Designed for both directionality and maximum energy transfer

Courtesy of Raytheon. Used with permission.

Courtesy of Raytheon. Courtesy of Raytheon. Used with permission.Used with permission.

Courtesy of U. S. Navy.



Outline

• Introduction

• Fundamental antenna concepts

• Reflector antennas

• Phased array antennas

• Summary



Radiation

Horizontal Distance (m)

Dipole*

Vert

ical

Dis

tanc

e (m

)

*driven by oscillating

source0 0.5 1 1.5 2

-1

0

-0.5

0.5

1

Presenter�

Presentation Notes�

The first is radiation. How is radiation caused? Well, it is caused by a time-varying current, or an acceleration/deceleration of charge. I hook up a voltage source to my antenna. If I drive the source then I can get the electrons on the metal of the antenna to accelerate and decelerate. This is what causes the radiation! Let’s take an example of a simple dipole (hold it up) which is perhaps the simplest antenna that can be made. It consists of a single wire a half-wavelength long. I can take an oscillating voltage source and hook it up to my dipole. As the source oscillates, it causes electrons on the dipole to accelerate and decelerate which creates radiation (start movie). Point out: Electromagnetic waves propagating with time Similar to dropping a stone in a lake to create water waves. Spherical wavefronts - the separation between the wavefronts is determined by the frequency of the oscillating source (slower oscillation, separated more). Unlike water waves, the power is not radiated out in all directions equally. Some is concentrated broadside to the dipole, shown in red. The dipole is a directional antenna. The enhancement in power in this region is called gain which we’ll discuss next.�



Antenna Gain

Isotropic Antenna

G = Gain

.

Directional Antenna

• Same power is radiated• Radiation intensity is power density over sphere (watt/steradian)• Gain is radiation intensity over that of an isotropic source

RadiationIntensity

RadiationIntensity



Antenna Pattern

• Pattern is a plot of gain versus angle

• Dipole example

0 30 60 90 120 150 180-35

-30

-25

-20

-15

-10

-5

0

5

Gai

n (d

Bi)

Theta θ

(deg)

0.5

1

1.5

2

60

120

30

150

0

180

30

150

60

120

90 90

Theta θ

(deg)

Polar Plot Linear Plot

( )⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛

=θ

θπ

θ 2

2

sin

cos2

cos643.1 G

Gmax = 1.64 = 2.15 dBiFigure by MIT OCW.

Presenter�


Antenna pattern is simply a plot of gain versus angle. There are different ways to plot: 3D first, then polar, then linear. Then, explain the meaning of units of gain, or dBi.�



Aperture diameter D: 5 mFrequency: 300 MHzWavelength: 1 m

Gain: 24 dBiIsotropic Sidelobe Level: 6 dBi Sidelobe Level: 18 dB Half-Power Beamwidth: 12 deg

Antenna Pattern Characteristics

Figure by MIT OCW.



Effect of Aperture Size on Gain

Parabolic ReflectorAntenna

DFeed

Dish

Gain vs Diameter

Gain

1 2 3 4 5 6 7 8 9 105

10

15

20

25

30

35

40

45

50

Max

imum

Gai

n (d

Bi)

λ

= 100 cm (300 MHz)λ

= 30 cm (1 GHz)λ

= 10 cm (3 GHz)

Aperture Diameter D (m)

FrequencyIncreases

EffectiveArea

Rule of Thumb(Best Case)

2

⎟⎠⎞

⎜⎝⎛=

λπD

24λπA

≅

2eA4

λπ

=

Gain increases as aperture becomes electrically larger(diameter is a larger number of wavelengths)

Gain increases as aperture becomes electrically larger(diameter is a larger number of wavelengths)



Reflector Comparison Kwajalein Missile Range Example

Operating frequency: 162 MHz (VHF)Wavelength λ: 1.85 mDiameter electrical size: 25 λGain: 34 dBBeamwidth: 2.8 deg

Operating frequency: 35 GHz (Ka)Wavelength λ: 0.0086 mDiameter electrical size: 1598 λGain: 70 dBBeamwidth: 0.00076 deg

ALTAIR45.7 m diameter

MMW13.7 m diameter

scale by1/3



Polarization

E

HorizontalLinear

(with respectto Earth)

• Defined by behavior of the electric field vector as it propagates in time

(For air surveillance looking upward)

E

(For over-water surveillance)

VerticalLinear

(with respectto Earth)

ElectromagneticWave Electric Field

Magnetic Field

Presenter�


Polarization is another important radar property. -Definition -Applications... H: air surveillance to ping off horizontal wings V: better for propagation (multi-path) problem. Give example. Lead out: Horizontal and linear polarization can be combined to form circular polarization...�



Circular Polarization (CP)

• “Handed-ness” is defined by observation of electric field along propagation direction

• Used for discrimination, polarization diversity, rain mitigation

Right-Hand(RHCP)

Propagation DirectionInto Paper

Left-Hand(LHCP)

Figure by MIT OCW.

ElectricField

Presenter�


Again, you are watching the position of the electric field with time, which in the case of circular polarization, is cork-screwing as it propagates forward. Go through the exercise of defining LHCP vs. RHCP and actually show them the right-hand rule with your hand. Talk about the application of discrimination (spheres).�



Circular Polarization (CP)

• “Handed-ness” is defined by observation of electric field along propagation direction

• Used for discrimination, polarization diversity, rain mitigation

Right-Hand(RHCP)

Propagation DirectionInto Paper

Left-Hand(LHCP)

Electric Field

Figure by MIT OCW.

Presenter�


Again, you are watching the position of the electric field with time, which in the case of circular polarization, is cork-screwing as it propagates forward. Go through the exercise of defining LHCP vs. RHCP and actually show them the right-hand rule with your hand. Talk about the application of discrimination (spheres).�



Field Regions

• All power is radiated out • Radiated wave is a plane wave• Far-field antenna quantities

– Pattern– Gain and directivity– Polarization– Radar cross section (RCS)

• Energy is stored in vicinity of antenna• Near-field antenna quantities

– Input impedance– Mutual coupling

λ3D620R .<

Reactive Near-Field Region Far-field (Fraunhofer) Region

λ2D2R >

Far-Field (Fraunhofer)Region

D

R

Reactive Near-FieldRegion

Radiating Near-Field(Fresnel) Region

Wave Fronts

Wave PropagationDirection



Antenna Input Impedance

• Antenna can be modeled as an impedance– Ratio of voltage to current at feed port

• Design antenna to maximize power transfer from transmission line– Reflection of incident power sets up standing wave

• Input impedance usually defines antenna bandwidth

TransmissionLine

Antenna

Γ

Standing Wave

feed

Incident Poweris Deliveredto Antenna

All IncidentPower isReflected

Γ

= 0

Γ

= 1



Outline

• Introduction




• Summary



Parabolic Reflector Antenna

• Design is a tradeoff between maximizing dish illumination and limiting spillover

• Feed antenna choice is critical

Feed Antennaat Focus

Beam Axis

Parabolic Surface

Wavefront

FD

Figure by MIT OCW.



Cassegrain Reflector Antenna

Ray Trace of Cassegrain Antenna

Geometry of Cassegrain Antenna

Figure by MIT OCW.



ALTAIR

Dual frequency

VHF Parabolic

UHF Cassegrain

FSS (Frequency Selective Surface) used for reflector



Outline

• Introduction




• Summary



• 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



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)



Array Controls

• Geometrical configuration– Linear, rectangular,

triangular, circular grids

• Element separation

• Phase shifts

• Excitation amplitudes– For sidelobe control

• Pattern of individual elements

– Isotropic, dipoles, etc.



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 Array (deg)

Linear Broadside ArrayIsotropic Elements

λ/2 SeparationNo Phase Shifting

0 30 60 90 120 150 1800 30 60 90 120 150 1800 30 60 90 120 150 180-30

-20

-10

0

10

20

N = 40 Elements

10 dBi 13 dBi 16 dBi

Angle off Array (deg) Angle off Array (deg)

Figure by MIT OCW.



Increasing Array Size bySeparating Elements

Limit element separation to d < λ

to preventgrating lobes for broadside array



d = λ/4 separation d = λ/2 separation d = λ

separation

• Linear Broadside Array• N = 10 Isotropic Elements• No Phase Shifting

Angle off Array (deg) Angle off Array (deg) Angle off Array (deg)0 30 60 90 120 150 180

Gai

n (d

Bi)

GratingLobes

0 30 60 90 120 150 180-30

-20

-10

0

10

20

0 30 60 90 120 150 180

10 dBi7 dBi

10 dBi

L = (N-1) d

Figure by MIT OCW.

Presenter�


(Explain problem of grating lobes... you get the increase in gain, but you have target position ambiguity!) Lead out: Let’s scan the linear array to the edge of its space, straight up, and repeat this test.�



Increasing Array Size of Scanned Arrayby Separating Elements

• No grating lobes for element separation d < λ / 2• Gain ~ 4N(d / λ) ~ 4L / λ for long endfire array without grating lobes

• Linear Endfire Array• N = 10 Isotropic Elements• Phase Shifted to Point Up

0 30 60 90 120 150 1800 30 60 90 120 150 180

Gai

n (d

Bi)

GratingLobe

GratingLobes

Angle off Array (deg) Angle off Array (deg) Angle off Array (deg)0 30 60 90 120 150 180-30

-20

-10

0

10

20d = λ/4 separation d = λ/2 separation d = λ

separation

10 dBi 10 dBi 10 dBi

Figure by MIT OCW.

L = (N-1) d

Presenter�


Grating lobes pop up earlier!! Emphasize half-wavelength spacing. Now, what if I scanned between those two limits?�



Linear Phased ArrayScanned every 30 deg, N = 15, d = λ/4

To scan over all space without grating lobes, keep element separation d < λ / 2


Figure by MIT OCW.



Planar Arrays

• As scan to θo off broadside:– Beamwidth broadens by 1/cosθo

– Directivity decreases by cosθo

To scan over all space without grating lobes, keep element separation in both directions < λ / 2

To scan over all space without grating lobes, keep element separation in both directions < λ / 2

PatternNo Scanning

Figure by MIT OCW.Figure by MIT OCW.



Mutual Coupling

• Effect of one element on another– Near-field quantity– Makes input impedance dependent on

scan angle

• Can greatly complicate array design– Hard to deliver power to antennas for all

scan angles– Can cause scan blindness where no

power is radiated

• Can limit scan volume and array bandwidth

Z Z

~

Antennam

Antennan

~

Drive Both Antennas

But... mutual coupling can sometimes be exploitedto achieve certain performance requirements



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 view

• 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



Outline

• Introduction

• Fundamental antenna parameters

• Reflectors

• Phased arrays

• Summary



Summary

• Fundamental antenna parameters and array topics have been discussed

– Radiation– Gain, pattern, sidelobes, beamwidth– Polarization– Far field– Input impedance– Array beamforming– Array mutual coupling

• Reflector antennas offer a relatively inexpensive method of achieving 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 antennas



References

• Balanis, C. A., Antenna Theory: Analysis and design, 2nd

Edition, New York, Wiley, 1997 • Skolnik, M., Introduction to Radar Systems, New York,

McGraw-Hill, 3rd Edition, 2001• Mailloux, R. J., Phased Array Antenna Handbook, Norwood,

Mass., Artech House, 1994



Increasing Array Size bySeparating Elements





d = λ/4 separation d = λ/2 separation d = λ

separation

• Linear Broadside Array• N = 10 Isotropic Elements• No Phase Shifting

Angle off Array (deg) Angle off Array (deg) Angle off Array (deg)0 30 60 90 120 150 180

Gai

n (d

Bi)

GratingLobes

0 30 60 90 120 150 180-30

-20

-10

0

10

20

0 30 60 90 120 150 180

10 dBi7 dBi

10 dBi

L = (N-1) d

Presenter�


(Explain problem of grating lobes... you get the increase in gain, but you have target position ambiguity!) Lead out: Let’s scan the linear array to the edge of its space, straight up, and repeat this test.�



0 30 60 90 120 150 180-30

-20

-10

0

10

20

Beam PointingDirection

Linear Phased ArrayScanned every 30 deg, N = 20, d = λ/4

Angle off Array (deg)

Gai

n (d

Bi)



Broadside: No Scan Scan 30 deg Scan 60 deg Endfire: Scan 90 deg

10 deg beam 12 deg beam 22 deg beam 49 deg beam

10 dBi 10.3 dBi10 dBi 13 dBi

Presenter�


Take away is very important.�

Documents

Introduction to Radar Systems 2002 Radar Antennas · Radar Antennas - 1 PRH 6/18/02 MIT Lincoln Laboratory Introduction to Radar Systems Radar Antennas. Radar Antennas - 2 PRH 6/18/02