RADAR
Antennas
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22


M 4
PG
S
t
R max  
nEi 
3
S
4  kT0 BF (S / N )1 Lt Lr L p



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Antenna:
• An antenna is
•
•
an electromagnetic radiator,
•
a sensor,
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a transducer and
•
an impedance matching device
For Radar Application, A directive antenna which concentrates the
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energy into a narrow beam.
Y
• Most popularly used antennas are: Parabolic Reflector Antennas
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• Planar Phased Arrays
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E
• Electronically steered Phased array
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antennas
S • A typical antenna beamwidth for the detection or tracking of aircraft
might be about 1 or 2°.
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A
D
D
A
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• An antenna is defined by Webster’s Dictionary as “a usually metallic
device (as a rod or wire) for radiating or receiving radio waves.”
• The IEEE Standard Definitions [IEEE Std 145–1983]: Antenna (or
aerial) “a means for radiating or receiving radio waves.”
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E & H Fields surrounding an Antenna
Antenna as a transition device
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Transmission-line Thevenin equivalent of antenna in transmitting mode
Z A  RA  jX
A
 (Rr  RL ) jX A
Where ZA : antenna impedance
RA : Antenna resistance
Rr : radiation resistance
RL :loss resistance (i.e. due to conduction & dielectric losses)
XA : equivalent antenna reactance
ANTENNA PARAMETERS
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Circuit Parameters
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Input Impedance
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Radiation Resistance
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Antenna Noise Temperature
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Return Loss
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Impedance bandwidth
Physical Quantities
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Electromagnetic Parameters
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Field
Pattern
(Beam
Area,
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Size
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Weight
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Profile
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Radiated power
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Shape
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Efficiency
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Effective Length and effective area
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Polarization (LP/CP/EP)
Directivity, Gain)
TYPES OF ANTENNA
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TYPES OF ANTENNA
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Structural classification:
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Wire Antennas
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ApertureAntennas
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MicrostripAntennas
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Array Antennas
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Reflector Antennas
Frequency dependency classification:
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Frequency Dependent Antennas
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Frequency IndependentAntennas
Wire Antennas
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Dipole antenna
Circular (Square) loop antenna
Helix antenna
Aperture Antennas
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Horn antennas
Conical Horn antennas
Slotted Waveguide antennas
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Pyramidal Horn antennas
Radiation pattern of a antenna
Microstrip Antennas
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Rectangular patch antennas
Circular patch antennas
Array Antennas
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Yagi-Uda antenna
Slotted waveguide array antenna
Microstrip array antenna
Reflector Antennas
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Parabolic reflector antenna with front feed
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Parabolic reflector antenna with cassegrain feed
Corner reflector antenna
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Frequency independent antennas
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Log periodic antenna
Discone antenna
Planar Log periodic slot antenna
Log-spiral antenna
Various versions of Biconical antennas – Infinite
Biconical antenna, Finite Biconical antenna, a cone
with finite ground, a cone with a stem and discone
FUNDAMENTAL PARAMETERS OF ANTENNA
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• Radiation pattern
• Radiation power density
• Radiation intensity
• Beamwidth
• Directivity
• Antenna efficiency
• Gain
• Bandwidth
• Group Delay
• Polarization
• Antenna impedance
• Antenna temperature
• Brightness Temperature
•
Antenna Factor
Radiation pattern
R A graphical or mathematical representation of the radiation properties of
A an antenna such as amplitude, phase, polarization etc as a function of
D
the angular space coordinates θ and Φ.
A
R
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Polar pattern
Linear pattern
Radiation pattern
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Directional radiation pattern
Omni-directional radiation pattern
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M • Same power is radiated
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S • Radiation intensity is power density over sphere (watt/steradian)
•
Gain is radiation intensity over that of an isotropic source
Field regions of an antenna
R (a) Reactive near field region
A (b) Radiating near field (Fresnel) region
D (c) Far field (Fraunhofer) region
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Field regions of an antenna
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[0.62 D3 /   R  2D 2  ]
Radiation power density
R The time average Poynting vector (average power density) can be
A written as W x, y, z   1 ReE  H * 
av
2
D
Where W = Radiation power density (W/ m2)
A
E = radiated electric field intensity (V/ m)
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H = radiated magnetic field intensity (A/ m)
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Directivity (D)/ Directive Gain
R It can be defined as “the ratio of the radiation intensity in a given direction
A from the antenna to the radiation intensity averaged over all directions”
D
U
4U
U
A

D

U 0 Prad / 4
Prad
R
D(dB)  10 log10[D(dim ensionless)]
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Where, D = directivity (dimensionless)
U = radiation intensity (W/ unit solid angle)
U0= radiation intensity of isotropic source (W/ unit solid angle)
Prad= total radiated power (W)
4
4
R
D0 

 A 1r 2r
A
D Where, D = directivity (dimensionless)
A
ΩA = beam solid angle)
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θ1r= HPBW in one plane (radian)
θ2r= HPBW in a plane at a right angle to other (radian)
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YS If beamwidth in degrees, equation can be written as:
4 4
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D0 

1r2r  (  ) (  )
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T
1d
2d
180
180
T
E
2
180
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4
41253




M
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1d2d
1d2d
S For a planar arrays, a better approximation is
 
D0 
32400
1d 2d
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D
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Radiation pattern for a particular paraboloid reflector antenna
Antenna efficiency
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R
e0  ereced
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2
YS  e r e c d

e

(1

)e
0
cd
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S
S Where, e0 = total antenna efficiency(dimensionless)
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T
E
ecd = antenna radiation efficiency(dimensionless)
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: used to relate the gain and directivity
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er = reflection (mismatch) efficiency (dimensionless)
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ec = conduction efficiency (dimensionless)
ed = dielectric efficiency (dimensionless)
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Z L  ZC
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
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Z L Z C
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E
Where, ZL= Antenna impedance
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ZC = characteristic impedance
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Gain (G)/ Power Gain
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radiation intensity
Gain 
total  input(accepted ) power

4U , 
(dim ensionless)
Pin
4U ,  4U , 
 Gain 

Pin
Prad / ecd
Prad  ecd Pin
4U , 
 e cd D(dim ensionless)
 ecd
Prad
4U , 
RelativeGain 
(dim ensionless)
Pin (isotropicsource)
Where, D = directivity (dimensionless)
U = radiation intensity (W/ unit solid angle)
Pin= total input power (W)
Prad= total radiated power (W)
ecd= antenna radiation efficiency (dimensionless)
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A
D
A
R
The relationship between the gain and the beamwidth of an antenna
depends on the distribution of current across the aperture.
For a "typical" reflector antenna the following expression is sometimes
used:
G
20000
1d2d
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YS Where, θ1d = HPBW in one plane (degree)
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θ2d= HPBW in a plane at a right angle to other (degree)
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Effective Aperture (Aeff)
R
4Aeff
4 e A
G

A
2
2
D
A Where, = wavelength
R
A= Physical area of the antenna
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e = antenna aperture efficiency
Antenna Input Impedance
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Antenna can be modeled as an impedance
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Design antenna to maximize power transfer from transmission line
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Ratio of voltage to current at feed port
Reflection of incident power sets up standing wave
Input impedance usually defines antenna bandwidth
Bandwidth
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(2.1)
Bandwidth of the antenna is defined as the range of frequencies within
which the performance of the antenna provides desired characteristics.
•Generally, Impedance BW when S11  -10dB [VSWR 
2]
The frequency bandwidth of an antenna can be expressed
Absolute Bandwidth (ABW)
ABW  f H  f L
Fractional Bandwidth (FBW). FBW  2 f H  f L
fH  fL
Where, fH and fL denote the upper edge and the lower edge of the antenna
bandwidth, respectively.
For broadband antennas, the bandwidth can also be expressed as the
ratio of the upper to the lower frequencies, where the antenna performance
is acceptable
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Polarization
(2.1)
R Polarization is defined as “the electric field vector of an antenna oriented
A in space as a function of time”.
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A
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Electromagnetic Wave
(2.1)
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The polarization of a radiated wave is the property of an electromagnetic
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There are three classifications of antenna polarization:
wave describing the time varying direction and relative magnitude of the
electric-field vector at a fixed location in space, and the sense in which it
is traced, as observed along the direction of propagation.
• Linear polarization,
• circular polarization and
• Elliptical polarization.
#Circular and linear polarizations are special cases of elliptical polarization
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(a) Rotation of plane electromagnetic wave and
(b)its polarization ellipse at z =0 as a function
of time
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Polarisation states for a z-directed plane wave
Polarization Loss Factor
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Note : Both the PLF and p lead to the sameanswers
Antenna Factor
R
• The antenna factor is defined as the ratio of the electric field strength
A
to the voltage V (units: V or µV) induced across the terminals of a
D
antenna.
A
R • For an electric field antenna, the field strength is in units of V/m or
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µV/m and the resulting antenna factor AF is in units of 1/m:
AF= Eincident/Vreceived
•
In a 50 Ω system, the antenna factor is related to the antenna gain G and the
wavelength λ via: AF= [9.73/ (λ*G1/2)]
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RADAR ANTENNAS
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Most popularly used antennas are:
•
Parabolic Reflector Antennas
•
Planar Phased Arrays
•
Electronically steered Phased array antennas
Radar Antenna Architecture Comparison
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A
D
A
R
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Y
S
T
E
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S
Array Radar
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A
D
A
R
S
Y
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T
E
M
S
Passive Array Radar
Active Array Radar
Active Phased Array Radar
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D
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Y
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Digital Array Radar Architecture:
Digital on Receiver
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Each active analog T/R module is followed by an A/D for immediate digitization
Multiple received beams are formed digitally by the digital beam-former.
Reference
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1. C A Balanis, Antenna Theory and Design, 3rd Edition, Wiley, 2005.
2. G Kumar and K P Ray, Broadband Microstrip Antenna, Arctech
Publication, 2003.
3. R K Shevgaonkar, Electromagnetic Waves, 2006