Introduction to Radar Systems

Radar Antennas

MIT Lincoln Laboratory Radar Antennas - 1 PRH 6/18/02 Disclaimer of Endorsement and Liability

• The video courseware and accompanying viewgraphs presented on this server were prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, nor the Massachusetts Institute of Technology and its Lincoln Laboratory, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, products, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government, any agency thereof, or any of their contractors or subcontractors or the Massachusetts Institute of Technology and its Lincoln Laboratory.

• The views and opinions expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or any of their contractors or subcontractors MIT Lincoln Laboratory Radar Antennas - 2 PRH 6/18/02 Focus

Waveform Propagation Transmitter Medium Generator Signal Processor Target Pulse Doppler Cross Receiver A / D Section Compression Processing Antenna Main Computer Console / Tracking & Display Detection Parameter Estimation Recording

Track 2 2 Pt G λ σ G = Gain Radar S / N = This 3 4 Equation (4 π ) R k Ts Bn L Lecture Ae = Effective Area

T = System Noise s Radar Search Temperature Pav Ae ts σ Equation Radar S / N = Lecture 4 L = Losses Equation 4 π Ω R k Ts L

MIT Lincoln Laboratory Radar Antennas - 3 PRH 6/18/02 Antenna Definition

• “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. MIT Lincoln Laboratory Radar Antennas - 4 PRH 6/18/02 * IEEE Standard Definitions of Terms for Antennas (IEEE STD 145-1983) 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. Used with permission.

Courtesy of U. S. Navy. MIT Lincoln Laboratory Radar Antennas - 5 PRH 6/18/02 Outline

• Introduction

• Fundamental antenna concepts

• Reflector antennas

• Phased array antennas

• Summary

MIT Lincoln Laboratory Radar Antennas - 6 PRH 6/18/02 Radiation

1

0.5

Dipole* 0

Vertical Distance (m) -0.5

driven by * -1 oscillating 0 0.5 1 1.5 2 source Horizontal Distance (m) MIT Lincoln Laboratory Radar Antennas - 7 PRH 6/18/02 Antenna Gain

Isotropic Antenna Directional Antenna

G = Gain

Radiation Radiation Intensity Intensity .

• Same power is radiated • Radiation intensity is power density over sphere (watt/steradian) • Gain is radiation intensity over that of an isotropic source MIT Lincoln Laboratory Radar Antennas - 8 PRH 6/18/02 Antenna Pattern

• Pattern is a plot of gain versus angle • Dipole example

⎡ ⎛ π ⎞⎤ cos2 ⎜ cosθ ⎟ ⎢ 2 ⎥ G()θ = 643.1 ⎢ ⎝ ⎠⎥ ⎢ sin2 θ ⎥ ⎢ ⎥ ⎣ ⎦

Figure by MIT OCW. G = 1.64 = 2.15 dBi Polar Plot max Linear Plot 0 2 30 30 5 1.5 0 60 1 60 -5 0.5 -10 90 90 -15 -20 Gain (dBi) 120 120 -25 -30 150 150 -35 180 0 30 60 90 120 150 180 Theta θ (deg) Theta θ (deg) MIT Lincoln Laboratory Radar Antennas - 9 PRH 6/18/02 Antenna Pattern Characteristics

Figure by MIT OCW.

Gain: 24 dBi Aperture diameter D: 5 m Isotropic Sidelobe Level: 6 dBi Frequency: 300 MHz Sidelobe Level: 18 dB Wavelength: 1 m Half-Power Beamwidth: 12 deg

MIT Lincoln Laboratory Radar Antennas - 10 PRH 6/18/02 Effect of Aperture Size on Gain

Parabolic Reflector Antenna Gain vs Diameter 50 Frequency 45 Increases Feed D 40

35

30 Dish 25

πA4 20 Gain = e Effective λ2 Area Maximum Gain (dBi) 15 λ = 100 cm (300 MHz) 4πA λ = 30 cm (1 GHz) ≅ Rule of Thumb 10 λ = 10 cm (3 GHz) λ2 (Best Case)

2 5 ⎛πD ⎞ 1 2 3 4 5 6 7 8 9 10 = ⎜ ⎟ Aperture Diameter D (m) ⎝ λ ⎠ GainGain increasesincreases asas apertureaperture becomesbecomes electricallyelectrically largerlarger (diameter(diameter isis aa largerlarger numbernumber ofof wavelengths)wavelengths)

MIT Lincoln Laboratory Radar Antennas - 11 PRH 6/18/02 Reflector Comparison Kwajalein Missile Range Example ALTAIR 45.7 m diameter MMW 13.7 m diameter scale by 1/3

Operating frequency: 162 MHz (VHF) Operating frequency: 35 GHz (Ka) Wavelength λ: 1.85 m Wavelength λ: 0.0086 m Diameter electrical size: 25 λ Diameter electrical size: 1598 λ Gain: 34 dB Gain: 70 dB Beamwidth: 2.8 deg Beamwidth: 0.00076 deg

MIT Lincoln Laboratory Radar Antennas - 12 PRH 6/18/02 Polarization

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

Electromagnetic Wave Electric Field

Magnetic Field

Vertical Horizontal Linear Linear (with respect (with respect to Earth) E to Earth)

E (For over-water surveillance) (For air surveillance looking upward) MIT Lincoln Laboratory Radar Antennas - 13 PRH 6/18/02 Circular Polarization (CP)

• “Handed-ness” is defined by observation of electric field along propagation direction • Used for discrimination, polarization diversity, rain mitigation

Propagation Direction Into Paper

Electric Field Right-Hand (RHCP)

Left-Hand (LHCP) Figure by MIT OCW.

MIT Lincoln Laboratory Radar Antennas - 14 PRH 6/18/02 Circular Polarization (CP)

• “Handed-ness” is defined by observation of electric field along propagation direction • Used for discrimination, polarization diversity, rain mitigation

Propagation Direction Into Paper

Right-Hand (RHCP)

Left-Hand (LHCP) Figure by MIT OCW.

Electric Field MIT Lincoln Laboratory Radar Antennas - 15 PRH 6/18/02 Field Regions

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

< . D620R 3 λ > D2R 2 λ

• Energy is stored in vicinity of antenna • All power is radiated out • Near-field antenna quantities • Radiated wave is a plane wave – Input impedance • Far-field antenna quantities – Mutual coupling – Pattern – Gain and directivity – Polarization – Radar cross section (RCS)

R

Wave Propagation D Direction

Reactive Near-Field Wave Fronts Region

Radiating Near-Field Far-Field (Fraunhofer) (Fresnel) Region Region

MIT Lincoln Laboratory Radar Antennas - 16 PRH 6/18/02 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

feed Γ Incident Power Γ = 0 is Delivered Transmission Antenna to Antenna Line

All Incident Γ = 1 Power is Reflected Standing Wave

MIT Lincoln Laboratory Radar Antennas - 17 PRH 6/18/02 Outline

• Introduction

• Fundamental antenna concepts

• Reflector antennas

• Phased array antennas

• Summary

MIT Lincoln Laboratory Radar Antennas - 18 PRH 6/18/02 Parabolic Reflector Antenna

Parabolic Surface

Wavefront

Feed Antenna at Focus D F Beam Axis

Figure by MIT OCW.

• Design is a tradeoff between maximizing dish illumination and limiting spillover • Feed antenna choice is critical

MIT Lincoln Laboratory Radar Antennas - 19 PRH 6/18/02 Cassegrain Reflector Antenna

Figure by MIT OCW. Geometry of Ray Trace of Cassegrain Antenna Cassegrain Antenna

MIT Lincoln Laboratory Radar Antennas - 20 PRH 6/18/02 ALTAIR

Dual frequency

VHF Parabolic

UHF Cassegrain

FSS (Frequency Selective Surface) used for reflector

MIT Lincoln Laboratory Radar Antennas - 21 PRH 6/18/02 Outline

• Introduction

• Fundamental antenna concepts

• Reflector antennas

• Phased array antennas

• Summary

MIT Lincoln Laboratory Radar Antennas - 22 PRH 6/18/02 Arrays

• Multiple antennas combined to enhance radiation and shape pattern

Isotropic Array Array Phased Array Element

Σ Σ Σ Phase Combiner Shifter Response Response Response Response

Direction Direction Direction Direction

MIT Lincoln Laboratory Radar Antennas - 23 PRH 6/18/02 Two Antennas Radiating

1

0.5 Dipole 1* 0 Dipole 2*

Vertical Distance (m) -0.5

*driven by oscillating -1 sources 0 0.5 1 1.5 2 (in phase) Horizontal Distance (m) MIT Lincoln Laboratory Radar Antennas - 24 PRH 6/18/02 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.

MIT Lincoln Laboratory Radar Antennas - 25 PRH 6/18/02 Increasing Array Size by Adding Elements Linear Broadside Array Isotropic Elements λ/2 Separation No Phase Shifting Figure by MIT OCW.

N = 10 Elements N = 20 Elements N = 40 Elements

20 10 dBi 13 dBi 16 dBi

10

0

-10 Gain (dBi)

-20

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

• Gain ~ 2N(d / λ) for long broadside array MIT Lincoln Laboratory Radar Antennas - 26 PRH 6/18/02 Increasing Array Size by Separating Elements

• Linear Broadside Array

• N = 10 Isotropic Elements L = (N-1) d • No Phase Shifting Figure by MIT OCW.

d = λ/4 separation d = λ/2 separation d = λ separation 20 Grating 7 dBi 10 dBi Lobes 10 10 dBi

0

-10 Gain (dBi)

-20

-30 0 30 60 90 120 150 180 0 30 60 90 120 150 1800 30 60 90 120 150 180 Angle off Array (deg) Angle off Array (deg) Angle off Array (deg)

LimitLimit elementelement separationseparation toto dd << λλ toto preventprevent gratinggrating lobeslobes forfor broadsidebroadside arrayarray MIT Lincoln Laboratory Radar Antennas - 27 PRH 6/18/02 Increasing Array Size of Scanned Array by Separating Elements

• Linear Endfire Array

• N = 10 Isotropic Elements L = (N-1) d • Phase Shifted to Point Up Figure by MIT OCW. d = λ/4 separation d = λ/2 separation d = λ separation 20 10 dBi 10 dBi Grating 10 dBi Grating Lobe Lobes 10

0

Gain (dBi) -10

-20

-30 0 30 60 90 120 150 180 0 30 60 90 120 150 180 0 30 60 90 120 150 180 Angle off Array (deg) Angle off Array (deg) Angle off Array (deg) • No grating lobes for element separation d < λ / 2 • Gain ~ 4N(d / λ) ~ 4L /λfor long endfire array without grating lobes MIT Lincoln Laboratory Radar Antennas - 28 PRH 6/18/02 Linear Phased Array Scanned every 30 deg, N = 15, d = λ/4

Figure by MIT OCW.

ToTo scanscan overover allall spacespace withoutwithout gratinggrating lobes,lobes, keepkeep elementelement separationseparation dd << λλ // 22 MIT Lincoln Laboratory Radar Antennas - 29 PRH 6/18/02 Planar Arrays

Pattern No Scanning

Figure by MIT OCW. Figure by MIT OCW. • As scan to θo off broadside:

– Beamwidth broadens by 1/cosθo

– Directivity decreases by cosθo

ToTo scanscan overover allall spacespace withoutwithout gratinggrating lobes,lobes, keepkeep elementelement separationseparation inin bothboth directionsdirections << λλ // 22 MIT Lincoln Laboratory Radar Antennas - 30 PRH 6/18/02 Mutual Coupling

Drive Both Antennas • 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

Z Z – Can cause scan blindness where no power is radiated ~ ~ • Can limit scan volume and array bandwidth Antenna Antenna m n

But... mutual coupling can sometimes be exploited to achieve certain performance requirements MIT Lincoln Laboratory Radar Antennas - 31 PRH 6/18/02 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

MIT Lincoln Laboratory Radar Antennas - 32 PRH 6/18/02 Outline

• Introduction

• Fundamental antenna parameters

• Reflectors

• Phased arrays

• Summary

MIT Lincoln Laboratory Radar Antennas - 33 PRH 6/18/02 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

MIT Lincoln Laboratory Radar Antennas - 34 PRH 6/18/02 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

MIT Lincoln Laboratory Radar Antennas - 35 PRH 6/18/02 Increasing Array Size by Separating Elements

• Linear Broadside Array

• N = 10 Isotropic Elements L = (N-1) d • No Phase Shifting

d = λ/4 separation d = λ/2 separation d = λ separation 20 Grating 7 dBi 10 dBi Lobes 10 10 dBi

0

-10 Gain (dBi)

-20

-30 0 30 60 90 120 150 180 0 30 60 90 120 150 1800 30 60 90 120 150 180 Angle off Array (deg) Angle off Array (deg) Angle off Array (deg)

LimitLimit elementelement separationseparation toto dd << λλ toto preventprevent gratinggrating lobeslobes forfor broadsidebroadside arrayarray MIT Lincoln Laboratory Radar Antennas - 36 PRH 6/18/02 Linear Phased Array Scanned every 30 deg, N = 20, d = λ/4

Beam Pointing Direction

Broadside: No Scan Scan 30 deg Scan 60 deg Endfire: Scan 90 deg 20 10 dBi 10 dBi 10.3 dBi 13 dBi 10

0

Gain (dBi) -10

-20

-30 0 30 60 90 120 150 180 Angle off Array (deg) 10 deg beam 12 deg beam 22 deg beam 49 deg beam

ToTo scanscan overover allall spacespace withoutwithout gratinggrating lobes,lobes, keepkeep elementelement separationseparation dd << λλ // 22 MIT Lincoln Laboratory Radar Antennas - 37 PRH 6/18/02