Teori Radar Antena

Radar Systems Engineering Lecture 8 Antennas Part 1 - Basics and Mechanical Scanning Dr. Robert M. O’Donnell IEEE New Hampshire Section Guest Lecturer

IEEE New Hampshire Section Radar Systems Course 1 Antennas Part 1 1/1/2010

IEEE AES Society

Block Diagram of Radar System Transmitter

Propagation Medium Target Radar Cross Section

Power Amplifier

Waveform Generation

T/R Switch Antenna Receiver

Signal Processor Computer A/D Converter

Pulse Compression

Clutter Rejection (Doppler Filtering)

User Displays and Radar Control

General Purpose Computer

Tracking

Parameter Estimation

Thresholding

Detection

Data Recording Photo Image Courtesy of US Air Force Used with permission.

Radar Systems Course 2 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE 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

Track Radar Equation

Search Radar Equation

S / N=

S / N=

Pt G2 λ2 σ (4 π

)3

R4 k

Ts Bn L

Pav Ae ts σ 4 π Ω R4 k Ts L

G = Gain Ae = Effective Area Ts = System Noise Temperature L = Losses

This Lecture

Radar Equation Lecture

* IEEE Standard Definitions of Terms for Antennas (IEEE STD 145-1983) Radar Systems Course 3 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Radar Antennas Come in Many Sizes and Shapes Electronic Scanning Antenna

Mechanical Scanning Antenna

Hybrid Mechanical and Frequency Scanning Antenna Photo Courtesy of Northrop Grumman Used with Permission

Courtesy US Army

Courtesy of MIT Lincoln Laboratory, Used with permission Courtesy US Dept of Commerce

Mechanical Scanning Antenna Radar Systems Course 4 Antennas Part 1 1/1/2010

Photo Courtesy of Raytheon Used with Permission

Electronic Scanning Antenna

Photo Courtesy of ITT Corporation Used with Permission

Hybrid Mechanical and Frequency Scanning Antenna IEEE New Hampshire Section IEEE AES Society

Outline

Radar Systems Course 5 Antennas Part 1 1/1/2010

•

Introduction

•

Antenna Fundamentals

•

Reflector Antennas – Mechanical Scanning

•

Phased Array Antennas

•

Frequency Scanning of Antennas

•

Hybrid Methods of Scanning

•

Other Topics

Part One

Part Two

IEEE New Hampshire Section IEEE AES Society

Outline • •

Introduction Antenna Fundamentals – Basic Concepts – Field Regions Near and far field

– – – –

•

Radar Systems Course 6 Antennas Part 1 1/1/2010

Electromagnetic Field Equations Polarization Antenna Directivity and Gain Antenna Input Impedance

Reflector Antennas – Mechanical Scanning

IEEE New Hampshire Section IEEE AES Society

Tree of Antenna Types Antennas

End Fires

Loops

Polyrods

Dipoles

Folded Dipoles

Stubs

Curtains Curtains

Yagi-Udas

Twin Lines

Log Periodics

Vees Biconical

Adapted from Kraus, Reference 6 Radar Systems Course 7 Antennas Part 1 1/1/2010

Slots

Patches

Arrays

Helices

Conical Spirals

Apertures

Arrays

W8JKs

Lenses

Spirals

Reflectors

Horns

Flat

Parabolic

Corner

Long Wires Beverage

Rhombic

Radomes

Frequency Selective Surfaces

IEEE New Hampshire Section IEEE AES Society

Tree of Antenna Types Antennas

End Fires

Loops

Polyrods

Dipoles

Folded Dipoles

Stubs

Curtains Curtains

Yagi-Udas

Twin Lines

Log Periodics

Vees Biconical

Adapted from Kraus, Reference 6 Radar Systems Course 8 Antennas Part 1 1/1/2010

Slots

Patches

Arrays

Helices

Conical Spirals

Apertures

Arrays

W8JKs

Lenses

Spirals

Reflectors

Horns

Flat

Parabolic

Corner

Long Wires Beverage

Rhombic

Radomes

Frequency Selective Surfaces

IEEE New Hampshire Section IEEE AES Society

Generation of Electromagnetic Fields & Calculation Methodology •

Radiation mechanism – –

Radiation is created by an acceleration of charge or by a time-varying current Acceleration is caused by external forces Transient (pulse) Time-harmonic source (oscillating charge

•

EM wave is calculated by integrating source currents on antenna / target –

•

Electric currents on conductors or magnetic currents on apertures (transverse electric fields)

Source currents can be modeled and calculated using numerical techniques –

(e.g. Method of Moments, Finite Difference-Time Domain Methods) Electric Current on Wire Dipole

z b

a

a/2

y

a/2 x

Radar Systems Course 9 Antennas Part 1 1/1/2010

Electric Field Distribution (~ Magnetic Current) in an Aperture

λ/2 3λ/2

λ/4 λ 2λ

IEEE New Hampshire Section IEEE AES Society

Antenna and Radar Cross Section Analyses Use “Phasor Representation” Harmonic Time Variation is assumed :

[

r ~ E ( x, y , z; t ) = Re al E ( x, y , z ) e j ω t

Instantaneous Electric Field

Calculate Phasor : Instantaneous Harmonic Field is :

]

ej ω t

Phasor

~ ~ E ( x, y , z ) = eˆ E ( x, y , z ) e j α

r ~ E ( x, y , z; t ) = eˆ E ( x, y , z ) cos (ωt + α )

Any Time Variation can be Expressed as a Superposition of Harmonic Solutions by Fourier Analysis Radar Systems Course 10 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Outline • •

Introduction Antenna Fundamentals – Basic Concepts – Field Regions Near and far field

– – – –

•

Radar Systems Course 11 Antennas Part 1 1/1/2010

Electromagnetic Field Equations Polarization Antenna Directivity and Gain Antenna Input Impedance

Reflector Antennas – Mechanical Scanning

IEEE New Hampshire Section IEEE AES Society

Regions of Radiation Radiating Fields (Free Space)

Transmission Line / Waveguide

Transmitter

Antenna

Near Field (Spherical Wave)

Far Field (Plane Wave)

Adapted from Kraus, Reference 6 Radar Systems Course 12 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

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

Field Regions Reactive Near-Field Region

Far-field (Fraunhofer) Region

R > 2D 2 λ

R < 0.62 D λ 3

• •

Energy is stored in vicinity of antenna Near-field antenna Issues –

Input impedance

–

Mutual coupling

• • •

All power is radiated out Radiated wave is a plane wave Far-field EM wave properties –

Polarization

–

Antenna Gain (Directivity)

–

Antenna Pattern

–

Target Radar Cross Section (RCS)

r E

R

r H

D Reactive Near-Field Region Radiating Near-Field (Fresnel) Region Radar Systems Course 13 Antennas Part 1 1/1/2010

rˆ

Plane Wave Propagates Radially Out

Equiphase Wave Fronts

Far-Field (Fraunhofer) Region

Courtesy of MIT Lincoln Laboratory, Used with permission

Adapted from Balanis, Reference 1

IEEE New Hampshire Section IEEE AES Society

Far-Field EM Wave Properties • • •

In the far-field, a spherical wave can be approximated by a plane wave z

There are no radial field components in the far field

rˆ

φˆ

The electric and magnetic fields are given by:

θ r

r ff ro e − jkr E (r , θ, φ ) ≅ E ( θ, φ ) r

μo = 377 Ω η≡ εo

k = 2π λ

Standard Spherical Coordinate System

x

Electric Field

is the intrinsic impedance of free space

z

Magnetic Field

λ

is the wave propagation constant

x Radar Systems Course 14 Antennas Part 1 1/1/2010

y

φ

r ff ro r ff e − jkr 1 H (r , θ, φ ) ≅ H ( θ, φ ) = rˆ × E r η

where

θˆ

y IEEE New Hampshire Section IEEE AES Society

Outline • •

Introduction Antenna Fundamentals – Basic Concepts – Field Regions Near and far field

– – – –

•

Radar Systems Course 15 Antennas Part 1 1/1/2010

Electromagnetic Field Equations Polarization Antenna Directivity and Gain Antenna Input Impedance

Reflector Antennas – Mechanical Scanning

IEEE New Hampshire Section IEEE AES Society

Propagation in Free Space •

Plane wave, free space solution to Maxwell’s Equations: – No Sources – Vacuum – Non-conducting medium

→

→ → r E(r , t ) = Eoe j( k ⋅ r − ωt ) → → → r j( k ⋅ r − ω t ) B(r , t ) = B oe

•

Most electromagnetic waves are generated from localized sources and expand into free space as spherical wave.

•

In the far field, when the distance from the source great, they are well approximated by plane waves when they impinge upon a target and scatter energy back to the radar

Radar Systems Course 16 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Modes of Transmission For Electromagnetic Waves •

Transverse electromagnetic (TEM) mode – Magnetic and electric field vectors are transverse (perpendicular) to the direction of propagation, kˆ , and perpendicular to each other – Examples (coaxial transmission line and free space transmission, – TEM transmission lines have two parallel surfaces

•

r E

kˆ

Transverse electric r (TE) mode – Electric field, E , perpendicular to kˆ – No electric field in kˆ direction

•

TEM Mode

Transverse magnetic r (TM) mode – Magnetic field,H, perpendicular to kˆ

r H Used for Rectangular Waveguides

– No magnetic field in kˆ direction

•

Hybrid transmission modes

Radar Systems Course 17 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Pointing Vector – Power Density r • The Poynting Vector, S , is defined as: r r r S ≡ E x H (W/m2)

•

It is the power density (power per unit area) carried by an electromagnetic wave

r r • Since both E and H are functions of time, the average power density is of greater interest, and is given by: r r r* 1 S = Re E x H 2 For a plane wave in a lossless medium

(

•

r 1 r2 S = E ≡ WAV 2η Radar Systems Course 18 Antennas Part 1 1/1/2010

)

where η =

μo εo

IEEE New Hampshire Section IEEE AES Society

Radiation Intensity and Radiated Power •

Radiation Intensity = Power radiated per unit solid angle 2 r2 r E ( r , θ, φ ) U(θ, φ) ≅ r Wrad (θ, φ) = 2η r 2 2 r2 ⎡ r E θ ( r , θ, φ ) + E φ ( r , θ, φ ) ⎤ ≅ ⎥⎦ 2η ⎢⎣ ro 2 2 1 ⎡ ro E θ (r , θ, φ ) + E φ ( r , θ, φ ) ⎤ ≅ ⎥⎦ 2η ⎢⎣ 2

where

r ro e − jkr = far field electric field intensity E(r , θ, φ) = E (θ, φ) r E θ , E φ = far field electric field components and

•

(W/steradian)

η=

μo εo

Total Power Radiated

Prad =

2π π

∫ ∫ U(θ, φ) sin θ dθ dφ

(W)

0 0

Radar Systems Course 19 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Outline • •

Introduction Antenna Fundamentals – Basic Concepts – Field Regions Near and far field

– – – –

•

Radar Systems Course 20 Antennas Part 1 1/1/2010

Electromagnetic Field Equations Polarization Antenna Directivity and Gain Antenna Input Impedance

Reflector Antennas – Mechanical Scanning

IEEE New Hampshire Section IEEE AES Society

Antenna Polarization • • •

Defined by behavior of the electric field vector as it propagates in time as observed along the direction of radiation Circular used for weather mitigation Horizontal used in long range air search to obtain reinforcement of direct radiation by ground reflection E θ

rˆ Eφ

Eθ

Major Axis Minor Axis

Eφ

Eφ

Courtesy of MIT Lincoln Laboratory, Used with permission Radar Systems Course 21 Antennas Part 1 1/1/2010

– Linear

–Vertical or Horizontal –Circular Two components are equal in amplitude, and separated in phase by 90 deg Right-hand (RHCP) is CW above Left-hand (LHCP) is CCW above

– Elliptical

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Polarization •

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

Electromagnetic Wave

Electric Field Magnetic Field

Vertical Linear (with respect to Earth)

E

Horizontal Linear (with respect to Earth) E

(For over-water surveillance)

(For air surveillance looking upward)

Courtesy of MIT Lincoln Laboratory, Used with permission Radar Systems Course 22 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

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

rˆ Electric Field

Eθ Right-Hand (RHCP)

Eφ Left-Hand (LHCP) Figure by MIT OCW.

Eφ Radar Systems Course 23 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

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

rˆ Eθ Right-Hand (RHCP)

Eφ Left-Hand (LHCP) Figure by MIT OCW.

Eφ Radar Systems Course 24 Antennas Part 1 1/1/2010

Electric Field

IEEE New Hampshire Section IEEE AES Society

Outline • •

Introduction Antenna Fundamentals – Basic Concepts – Field Regions Near and far field

– – – –

•

Radar Systems Course 25 Antennas Part 1 1/1/2010

Electromagnetic Field Equations Polarization Antenna Directivity and Gain Antenna Input Impedance

Reflector Antennas – Mechanical Scanning

IEEE New Hampshire Section IEEE AES Society

Antenna Gain Gain = Radiation intensity of antenna in given direction over that of isotropic source Maximum Gain

G=

4 π A eff 4 π η A = 2 λ λ2

Gain (max)

Radiation Intensity from a Sphere

•

Difference between gain and directivity is antenna loss

•

“Rules of Thumb”

G=

26,000 (degrees) θ B and φ Bare the azimuth and elevation half power beamwidths θB φB

Radar Systems Course 26 Antennas Part 1 1/1/2010

G=

θB =

D LA 65 λ D

(degrees)

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Directivity & Gain •

Radiation Intensity = U( θ, φ ) = Power radiated / unit solid angle

•

Directivity = Radiation intensity of antenna in given direction over that of an isotropic source radiating same power

4 π U( θ, φ ) D(θ, φ) = Prad

•

Gain = Radiation intensity of antenna in given direction over that of isotropic source radiating available power

– –

•

(dimensionless)

Difference between gain and directivity is antenna loss Gain < Directivity 4 π U( θ, φ )

G ( θ, φ ) =

(dimensionless)

Pin

Maximum Gain = Radiation intensity of antenna at peak of beam

4 π A eff 4 π η A G= = 2 λ λ2 Radar Systems Course 27 Antennas Part 1 1/1/2010

A η

= Area of antenna aperture = Efficiency of antenna IEEE New Hampshire Section IEEE AES Society

Example – Half Wavelength Dipole θ 30

0

2

Go = 1.643 = 2.15 dBi

θ

0

30

60

60

1 0.5

90

90

120

120

Polar Plot 150

Gain (dBi)

1.5

-10

-20

150 180

Far Field

⎡ ⎛π ⎞⎤ cos⎜ cos θ ⎟ ⎥ ⎢ I e 2 ⎠⎥ ⎢ ⎝ E ff (θ ) = θˆ jη o 2π r ⎢ sin θ ⎥ ⎢⎣ ⎥⎦ − jkr

⎡ ⎛π ⎞⎤ cos cos θ ⎟⎥ ⎜ I o e − jkr ⎢ ⎝ 2 ⎠⎥ ff ˆ ⎢ H (θ ) = φ j 2π r ⎢ sin θ ⎥ ⎢ ⎥ ⎣ ⎦ Adapted from Balanis, Reference 1, pp182 - 184 Radar Systems Course 28 Antennas Part 1 1/1/2010

Linear Plot

-30 0

45

135 90 Theta θ (deg)

180

Radiation Intensity

⎡ 2⎛ π ⎞⎤ cos ⎜ cos θ ⎟ ⎥ ⎢ I ⎝2 ⎠⎥ U(θ ) = η o 2 ⎢ 2 8π ⎢ sin θ ⎥ ⎢⎣ ⎥⎦ 2

Radiated Power 2

I Prad = η o Cin (2π ) 8π 2π 1 − cos y Cin (2π ) = ∫ dy ≈ 2.435 y 0

From MIT OCW

θ = angle down from z-axis Gain / Pattern

⎡ 2⎛ π ⎞⎤ cos ⎜ cos θ ⎟ ⎥ ⎢ 4 π U(θ ) ⎝2 ⎠⎥ = 1.643 ⎢ G (θ ) = 2 Pin sin θ ⎢ ⎥ ⎢⎣ ⎥⎦ Go =

4 π U max = 1.643 Pin

Effective A = λ Do = 0.13λ2 e Area 4π 2

IEEE New Hampshire Section IEEE AES Society

Outline • •

Introduction Antenna Fundamentals – Basic Concepts – Field Regions Near and far field

– – – –

•

Radar Systems Course 29 Antennas Part 1 1/1/2010

Electromagnetic Field Equations Polarization Antenna Directivity and Gain Antenna Input Impedance

Reflector Antennas – Mechanical Scanning

IEEE New Hampshire Section IEEE AES Society

Antenna Input Impedance •

Antenna can be modeled as an impedance (ratio of voltage to current at feed port) – –

•

Antenna “resonant” when impedance purely real Microwave theory can be applied to equivalent circuit

Design antenna to maximize power transfer from transmission line – –

Reflection of incident power sets up standing wave on line Can result in arching under high power conditions

Voltage Reflection Coefficient

feed

Γ Transmission Line

VMax Standing Wave

Loss (Ohmic, Dielectric) Antenna

Radiation Resistance (Power Radiated)

Prad =

1 2 Io Rr 2

Reactance (Energy Stored)

VMin Courtesy of MIT Lincoln Laboratory, Used with permission

Radar Systems Course 30 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Antenna Input Impedance •

Antenna can be modeled as an impedance (ratio of voltage to current at feed port) – –

•

Antenna “resonant” when impedance purely real Microwave theory can be applied to equivalent circuit

Design antenna to maximize power transfer from transmission line – –

•

Reflection of incident power sets up standing wave on line Can result in arching under high power conditions

Usually a 2:1 VSWR is acceptable

VMax 1 + Γ = VSWR = VMin 1 − Γ

feed

Γ Transmission Line

Antenna

Γ =0

Voltage Standing Wave Ratio All Incident Power is Delivered to Antenna

VSWR = 1

VMax

Γ =1 Standing Wave

VMin

VSWR → ∞

All Incident Power is Reflected

Courtesy of MIT Lincoln Laboratory, Used with permission Radar Systems Course 31 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Outline •

Introduction

•

Antenna Fundamentals

•

Reflector Antennas – Mechanical Scanning – Basic Antenna (Reflector) Characteristics and Geometry – Spillover and Blockage – Aperture Illumination – Different Reflector Feeds and Reflector Geometries

Radar Systems Course 32 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Antenna Pattern Characteristics Antenna Gain vs. Angle

Parabolic Reflector Antenna Parabolic Surface

Antenna Feed at Focus Beam Axis

Antenna Gain (dBi)

Wavefront

D

Half Power (3 dB) Beamwidth

20 Sidelobe Level

10

0

Isotropic Sidelobe Level

- 10 -20 -90

Aperture diameter D = 5 m Frequency = 300 MHz Wavelength = 1 m

Radar Systems Course 33 Antennas Part 1 1/1/2010

-60

-30

0

30

60

90

Angle (degrees) Gain = 24 dBi Isotropic Sidelobe Level = 6 dBi Sidelobe Level = 18 dB Half-Power Beamwidth = 12 deg IEEE New Hampshire Section IEEE AES Society

Parabolic Reflector Antenna Parabolic Reflector Antenna

Wavefront

Antenna Feed at Focus

D

Beam Axis

Relative Gain (dB)

Parabolic Surface

Normalized Antenna Gain Pattern

Angle off Beam Axis (degrees)

•

Reflector antenna design involves a tradeoff between maximizing dish illumination while limiting spillover and blockage from feed and its support structure

•

Feed antenna choice is critical

Radar Systems Course 34 Antennas Part 1 1/1/2010

Figure By MIT OCW

IEEE New Hampshire Section IEEE AES Society

Effect of Aperture Size on Gain Parabolic Reflector Antenna

Gain vs Antenna Diameter

50

Parabolic Surface

Antenna Feed at Focus

D

Beam Axis

Gain

=

4 πA e λ2

4πA ≅ 2 λ

⎛ πD ⎞ =⎜ ⎟ λ ⎝ ⎠ Radar Systems Course 35 Antennas Part 1 1/1/2010

Maximum Gain (dBi)

Wavefront

30

Wavelength Decreases

20

λ = 10 cm (3 GHz) λ = 30 cm (1 GHz) λ = 100 cm (300 MHz)

10 Effective Area Rule of Thumb (Best Case)

2

40

1

3

5

7

9

Aperture Diameter D (m)

Gain increases as aperture becomes electrically larger (diameter is a larger number of wavelengths) IEEE New Hampshire Section IEEE AES Society

Effect of Aperture Size on Beamwidth Antenna Beamwidth vs. Diameter

Parabolic Reflector Antenna

Wavefront

Antenna Feed at Focus

D

Beam Axis

Beamwidth (deg) ≅

180λ πD

Half-Power Beamwidth (deg)

20 Parabolic Surface

λ = 100 cm (300 MHz) λ = 30 cm (1 GHz)

16

λ = 10 cm (3 GHz) 12

Wavelength Increases

8 4

0

1

3

5

7

9

Aperture Diameter D (m)

Beamwidth decreases as aperture becomes electrically larger (diameter larger number of wavelengths) Radar Systems Course 36 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Parabolic Reflector Antenna Parabolic Reflector Antenna Parabolic Surface Wavefront

•Feed can be dipole or openended waveguide (horn) •Feed structure reduces antenna efficiency

Antenna Feed at Focus

D

•Point source is evolves to plane wave (In the Far Field)

Beam Axis

Examples of Parabolic Antenna Feed Structure

Adapted from Skolnik, Reference 2 Radar Systems Course 37 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Different Types of Radar Beams

Fan Beam

Pencil Beam Courtesy of MIT Lincoln Laboratory Used with permission

Courtesy of MIT Lincoln Laboratory, Used with permission

Shaped Beam

Stacked Beam Courtesy of US Air Force

Radar Systems Course 38 Antennas Part 1 1/1/2010

Courtesy of Northrop Grumman Used with Permission

IEEE New Hampshire Section IEEE AES Society

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) Wavelength λ: 1.85 m

Operating frequency: 35 GHz (Ka) Wavelength λ: 0.0086 m

Diameter electrical size: 25 λ

Diameter electrical size: 1598 λ

Gain: 34 dB Beamwidth: 2.8 deg

Gain: 70 dB Beamwidth: 0.00076 deg Courtesy of MIT Lincoln Laboratory, Used with permission

Radar Systems Course 39 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Outline •

Introduction

•

Antenna Fundamentals

•

Reflector Antennas – Mechanical Scanning – Basic Antenna (Reflector) Characteristics and Geometry – Spillover and Blockage – Aperture Illumination – Different Reflector Feeds and Reflector Geometries

Radar Systems Course 40 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Antenna Spillover •

• • •

Even when the feed is at the exact focus of the parabolic reflector, a portion of the emitted energy at the edge of the beam will not impinge upon the reflector.

Spillover Region Feed Spillover Feed

This is called “beam spillover” Tapering the feed illumination can mitigate this effect

Antenna Mainlobe

Sidelobe

As will be seen, optimum antenna performance is a tradeoff between: – – –

Beam spillover Tapering of the aperture illumination Antenna gain Feed blockage

Radar Systems Course 41 Antennas Part 1 1/1/2010

Diffracted Region

Reflector

Adapted from Skolnik, Reference 5

IEEE New Hampshire Section IEEE AES Society

Relative Radiation Intensity (dB)

Effect of Aperture Blocking in a Parabolic Reflector Antenna Examples of Aperture Blockage

0 Pattern with No Blockage

Pattern with Blockage

Feed and its supports

-10 Shadow pattern

Masts onboard a ship

Blockage Pattern

FPS-16 Courtesy of US Air Force

-20

-30

Adapted from Skolnik, Reference 2

-20

-10

0

10

20

Angle (degrees)

The effect of aperture blockage can be approximated by: Antenna pattern of – Antenna pattern produced by undisturbed aperture shadow of the obstacle Radar Systems Course 42 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Relative Radiation Intensity (dB)

Effect of Aperture Blocking in a Parabolic Reflector Antenna Examples of Aperture Blockage

0 Pattern with No Blockage

Pattern with Blockage

Feed and its supports

-10 Shadow pattern

Masts onboard a ship

Blockage Pattern

TRADEX

-20

-30 -20

-10

0

10

20

Angle (degrees)

This procedure is possible because of the linearity of the Fourier transform that relates the antenna aperture illumination and the radiation pattern Courtesy of MIT Lincoln Laboratory, Used with permission Radar Systems Course 43 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Examples of Antenna Blockage Courtesy of US Navy

SPG-51

Courtesy of US Navy

SPS-49

USS Abraham Lincoln Courtesy of US Navy

Courtesy of NASA

SPS-48

P-15 Flatface

USS Theodore Roosevelt Radar Systems Course 44 Antennas Part 1 1/1/2010

Courtesy of US Air Force

NASA Tracking Radar IEEE New Hampshire Section IEEE AES Society

Outline •

Introduction

•

Antenna Fundamentals

•

Reflector Antennas – Mechanical Scanning – Basic Antenna (Reflector) Characteristics and Geometry – Spillover and Blockage – Aperture Illumination – Different Reflector Feeds and Reflector Geometries

Radar Systems Course 45 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Antenna Radiation Pattern from a Line Source x Line Source

E(φ )

a/2 z

•

φ

−(a/ 2)

y

E(φ ) =

z ⎛ ⎞ ( ) π φ A z exp j 2 sin ⎜ ⎟ dz ∫ λ ⎝ ⎠ −a / 2 a/ 2

•

The aperture Illumination, A (z ) , is the current a distance origin (0,0,0), along the z axis

•

Assumes E(φ ) is in the far field, a >> λ and R >> a 2 / λ

•

Note that the electric field is the Inverse Fourier Transform of the Aperture Illumination.

z from the

Adapted from Skolnik, Reference 1 Radar Systems Course 46 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Effect of Source Distribution on Antenna Pattern of a Line Source Cosine Aperture Distribution

Uniform Aperture Distribution

A(z ) = cos π (a / z )

A(z ) = 1 E(φ ) =

z ⎞ ⎛ exp j 2 sin π φ ⎟ dz ⎜ ∫ λ ⎠ ⎝ −a / 2 a/2

A 0 sin[π(a / λ ) sin φ] (π / λ )sin φ

=

E(φ ) =

E(φ ) =

π ⎡ sin(ψ + π / 2) sin(ψ − π / 2 ) ⎤ + ⎢ (ψ − π / 2) ⎥⎦ 4 ⎣ (ψ + π / 2 ) where

ψ = π(a / λ ) sin φ

sin[π(a / λ ) sin φ] π(a / λ ) sin φ

Radar Systems Course 47 Antennas Part 1 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Antenna Pattern of a Line Source

E(φ )

2

Relative Radiation Intensity (dB)

(with Uniform and Cosine Aperture Illumination) 0

Curves Normalized to 0 dB at Maximum

-10

-20

-30 -4π

-2π

Adapted from Skolnik, Reference 1

•

Uniform Illumination

Cosine Illumination

0

π(D / λ ) sin φ

2π

4π ~0.9 dB Loss

Weighting of Aperture Illumination – Increases Beamwidth - Lowers Sidelobes - Lowers Antenna Gain

Radar Systems Course 48 Antennas Part 1 1/1/2010

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Illumination of Two-Dimensional Apertures •

z

Calculation of this integral is non-trivial – Numerical techniques used

θ

Antenna Aperture in x−y plane

•E(θ, φ )

•

A(x, y ) = A x (x ) A y (y )

y φ

Field pattern separable, when aperture illumination separable

•

Problem reduces to two 1 dimensional calculations

x

E(θ, φ ) = ∫∫ A(x, y ) e[(2 πj / λ )sin θ (x cos φ + y sin φ )]dx dy Radar Systems Course 49 Antennas Part 1 1/1/2010

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Relative Radiation Intensity (dB)

Uniformly Illuminated Circular Aperture •

0

Field Intensity of circular aperture of radius a: a

E(θ ) = 2π ∫ A(r )J 0 [2π(r / λ ) sin φθ] r dr

-10

0

• -20

For uniform aperture illumination :

E(θ ) = 2π a 2 J 1 (ξ ) / ξ

-30 -10

• • •

where ξ = 2π(a / λ ) sin θ and -5

0

ξ = 2π(a / λ ) sin θ

5

10

J 1 (ξ ) = 1st order Bessel Function

Use cylindrical coordinates, field intensity independent of Half power beamwidth (degrees) = 58.5(λ / a ) , first sidelobe = - 17.5 dB Tapering of the aperture will broaden the beamwidth and lower the sidelobes Adapted from Skolnik, Reference 1

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Radiation Pattern Characteristics for Various Aperture Distributions Type of Distribution Uniform : Cosine: n=0 n=1 n=2 n=3

z <1

A(z ) = 1

Gain Relative to Uniform (dB) 1.0

A(z ) = cos n ( π z / 2)

Parabolic: Δ=1.0 Δ=0.8 Δ=0.5 Δ=0

Heavier Taper • Lowers sidelobes • Increases beamwidth • Lowers directivity

A(z ) = 1 − (1 − Δ ) z 2

Beamwidth Half-Power (dB) 51 λ/D

Intensity, 1st Sidelobe (dB below Maximum) 13.2

1.0 0.810 0.667 0.515

51 λ/D 69 λ/D 83 λ/D 95 λ/D

13.2 23 32 40

1.0 0.994 0.970 0.833

51 λ/D 53 λ/D 56 λ/D 66 λ/D

13.2 15.8 17.1 20.6

Triangular:

A(z ) = 1 − z

0.75

73 λ/D

26.4

Circular:

A(z ) = 1 − z 2

0.865

58.5 λ/D

17.6

0.33 + 0.66 cos 2 ( π z / 2)

0.88

63 λ/D

25.7

0.08 + 0.92 cos 2 ( π z / 2) (Hamming)

0.74

76.5 λ/D

42.8

Uniform distribution always has 13 dB sidelobe

Cosine-squared + pedestal

Adapted from Skolnik, Reference 1 Radar Systems Course 51 Antennas Part 1 1/1/2010

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Taper Efficiency, Spillover, Blockage, and Total Loss vs. Feed Pattern Edge Taper 0.0

Spillover

Loss (dB)

- 0.5

-1.0

Feed Blockage

Taper Efficiency Total Loss

-1.5

-2.0 -20.0

-17.5

-15.0

-12.5

Feed Pattern Edge Taper (dB)

-10.0

-7.5

-5.0

Adapted from Cooley in Skolnik, Reference 4

Reflector Design is a Tradeoff of Aperture Illumination (Taper) Efficiency, Spillover and Feed Blockage Radar Systems Course 52 Antennas Part 1 1/1/2010

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Outline •

Introduction

•

Antenna Fundamentals

•

Reflector Antennas – Mechanical Scanning – Basic Antenna (Reflector) Characteristics and Geometry – Spillover and Blockage – Aperture Illumination – Different Reflector Feeds and Reflector Geometries Feed Horns Cassegrain Reflector Geometry Different Shaped Beam Geometries Scanning Feed Reflectors

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Feed Horns for Reflector Antennas

Flared Pyramidal Horn

Compound Flared Multimode Horn

• • •

Flared Conical Horn

Finned Horn

Corrugated Conical Horn

Segmented Aperture Horn

Simple flared pyramidal (TE01) and conical (TE11) horns used for pencil beam, single mode applications Corrugated, compound, and finned horns are used in more complex applications –

Polarization diversity, ultra low sidelobes, high beam efficiency, etc.

Segmented horns are used for monopulse applications

Radar Systems Course 54 Antennas Part 1 1/1/2010

Adapted from Cooley in Skolnik, Reference 4

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Cassegrain Reflector Antenna

Geometry of Cassegrain Antenna Radar Systems Course 55 Antennas Part 1 1/1/2010

Ray Trace of Cassegrain Antenna

Figure by MIT OCW.

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Advantages of Cassegrain Feed •

Lower waveguide loss because feed is not at the focus of the paraboloid, but near the dish.

•

Antenna noise temperature is lower than with conventional feed at focus of the paraboloid – Length of waveguide from antenna feed to receiver is shorter – Sidelobe spillover from feed see colder sky rather than warmer earth

•

Good choice for monopulse tracking – Complex monopulse microwave plumbing may be placed behind reflector to avoid the effects of aperture blocking

Radar Systems Course 56 Antennas Part 1 1/1/2010

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ALTAIR- Example of Cassegrain Feed ALTAIR Antenna

ALTAIR Antenna Feed

Note size of man

Dual Frequency Radar

• • • •

Antenna size - 120 ft. VHF parabolic feed UHF Cassegrain feed Frequency Selective Surface (FSS) used for reflector at UHF

Radar Systems Course 57 Antennas Part 1 1/1/2010

•

This “saucer” is a dichroic FFS that is reflective at UHF and transparent at VHF. The “teacup” to its right is the cover for a five horn VHF feed, located at the antenna’s focal point.

•

The FSS sub-reflector is composed of two layers of crossed dipoles

Courtesy of MIT Lincoln Laboratory, Used with permission

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Antennas with Cosecant-Squared Pattern •

Air surveillance coverage of a simple fan beam is usually inadequate for aircraft targets at high altitude and short range – Simple fan beam radiates very little energy at high altitude

•

One technique - Use fan beam with shape proportional to the hh square of the cosecant of the elevation angle – Gain constant for a given altitude

•

Gain pattern: – G(θ) = G(θ1) csc2 θ / csc2 θ 1 for θ1< θ < θ 2

θ2

csc2 Beam

hmax.

θ1

Parabolic Fan Beam

R

– G(θ) ~ G(θ1) (2 - cot θ2)

Radar Systems Course 58 Antennas Part 1 1/1/2010

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Antenna Pattern with Cosecant-Squared Beam Shaping Ray Trace for csc2 Antenna Pattern

FAA ASR Radars Use csc2 Antenna Reflector Shaping Courtesy of US Dept of Commerce

Parabolic Reflector Csc2 Shaped Reflector Parabolic Reflector

ASR-9 Antenna

Radar Systems Course 59 Antennas Part 1 1/1/2010

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Patterns for Offset Feeds 0

f = 32 in.

94 in

θ

Feed horns 2.5 in. square

Relative Gain (dB)

Frequency = 3 GHz

10°

5°

15°

θ = 0°

20°

-10

-20

-30 -10

0

10

20

30

40

Angle (deg)

•

Notice that a vertical array of feeds results in a set of “stacked beams” – Can be used to measure height of target

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Example of Stacked Beam Antenna TPS-43 Radar Courtesy of US Air Force

TPS-43 Antenna Feed Courtesy of brewbooks

• •

Stacked beam surveillance radars can cost effectively measure height of target, while simultaneously performing the surveillance function This radar, which was developed in the 1970s, under went a number of antenna upgrade in the 1990s (TPS-70, TPS-75) –

Radar Systems Course 61 Antennas Part 1 1/1/2010

Antenna was replaced with a slotted waveguide array, which performs the same functions, and in addition has very low sidelobes IEEE New Hampshire Section IEEE AES Society

Example of Stacked Beam Antenna TPS-43 Radar Courtesy of US Air Force

• •

TPS-78 Antenna Courtesy of Northrop Grumman Used with Permission

Stacked beam surveillance radars can cost effectively measure height of target, while simultaneously performing the surveillance function This radar, which was developed in the 1970s, was replaced in the 1990s with a technologically modern version of the radar. –

Radar Systems Course 62 Antennas Part 1 1/1/2010

New antenna, a slotted waveguide array, has all of the same functionality as TPS-43 dish, but in addition, has very low antenna sidelobes IEEE New Hampshire Section IEEE AES Society

Scanning Feed Reflector Antennas

•

Scanning of the radar beam over a limited angle with a fixed reflector and a movable feed – Paraboloid antenna cannot be scanned too far without deterioration Gain of antenna, with f/D=.25, reduced to 80%when beam scanned 3 beamwidths off axis

– Wide angle scans in one dimension can be obtained with a parabolic torus configuration Beam is generated by moving feed along circle whose radius is 1/2 that of torus circle Scan angle limited to about 120 deg Economical way to rapidly scan beam of very large antennas over wide scan angles

– Organ pipe scanner Mechanically scan feed between many fixed feeds Radar Systems Course 63 Antennas Part 1 1/1/2010

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Examples of Scanning Feed Reflector Configuration Parabolic Torus Antenna

Organ Pipe Scanner Feed

The length of each waveguide is equal

R

Outputs to Antenna

f Horn From Transmitter

R = Radius of Torus f = Focal Length of Torus Radar Systems Course 64 Antennas Part 1 1/1/2010

The output feed horns of the organ pipe scanner are located along this arc IEEE New Hampshire Section IEEE AES Society

Radar Example – Organ Pipe Scanner

Courtesy of MIT Lincoln Laboratory, Used with permission Radar Systems Course 65 Antennas Part 1 1/1/2010

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Radar Example – Organ Pipe Scanner BMEWS Site, Clear, Alaska

Courtesy of MIT Lincoln Laboratory, Used with permission Radar Systems Course 66 Antennas Part 1 1/1/2010

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Summary – Part 1 •

Discussion of antenna parameters – Gain – Sidelobes – Beamwidth Variation with antenna aperture size and wavelength

– Polarization Horizontal, Vertical, Circular

•

Mechanical scanning antennas offer an inexpensive method of achieving radar beam agility –

•

Slow to moderate angular velocity and acceleration

Different types of mechanical scanning antennas – Parabolic reflectors – Cassegrain and offset feeds – Stacked beams

•

Antenna Issues – Aperture illumination – Antenna blockage and beam spillover

Radar Systems Course 67 Antennas Part 1 1/1/2010

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Homework Problems

•

From Skolnik, Reference 2 – Problem 2.20 – Problems 9.2, 9.4, 9.5, and 9.8

Radar Systems Course 68 Antennas Part 1 1/1/2010

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Outline

Radar Systems Course 69 Antennas Part 1 1/1/2010

•

Introduction

•

Antenna Fundamentals

•

Reflector Antennas – Mechanical Scanning

•

Phased Array Antennas

•

Frequency Scanning of Antennas

•

Hybrid Methods of Scanning

•

Other Topics IEEE New Hampshire Section IEEE AES Society

Acknowledgement • •

Dr. Pamela Evans Dr Alan J. Fenn

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References 1. Balanis, C. A., Antenna Theory: Analysis and Design, Wiley, New York, 3rd Ed., 2005 2. Skolnik, M., Introduction to Radar Systems, McGraw-Hill, New York, 3rd Ed., 2001 3. Mailloux, R. J., Phased Array Antenna Handbook, Artech House, Norwood, MA, 1994 4. Skolnik, M., Radar Handbook, McGraw-Hill, New York, 3rd Ed., 2008 5. Skolnik, M., Radar Handbook, McGraw-Hill, New York, 2nd Ed., 2008 6. Kraus, J.D. et. al., Antennas, McGraw-Hill, New York, 1993. 7. Ulaby, F. T. , Fundamentals of Applied Electromagnetics, 5th Ed., Pearson, Upper Saddle River, NJ, 2007

Radar Systems Course 71 Antennas Part 1 1/1/2010

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