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DR Sanjeev Kumar Mishra Lecture 17-20
RADAR Antennas
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Antenna:
• An antenna is
• an electromagnetic radiator,
• a sensor,
• a transducer and
• an impedance matching device
• For Radar Application, A directive antenna which concentrates the
energy into a narrow beam.
• Most popularly used antennas are: Parabolic Reflector Antennas
• Planar Phased Arrays
• Electronically steered Phased array
antennas
• A typical antenna beamwidth for the detection or tracking of aircraft
might be about 1 or 2°.
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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.”
E & H Fields surrounding an Antenna Antenna as a transition device
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R A D A R
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Where ZA : antenna impedance
RA : Antenna resistance
Rr : radiation resistance
RL :loss resistance (i.e. due to conduction & dielectric losses)
XA : equivalent antenna reactance
Transmission-line Thevenin equivalent of antenna in transmitting mode
AAA jXRZ
ALr jXRR )(
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ANTENNA PARAMETERS
• Circuit Parameters
• Input Impedance
• Radiation Resistance
• Antenna Noise Temperature
• Return Loss
• Impedance bandwidth
• Physical Quantities
• Size
• Weight
• Profile
• Shape
• Electromagnetic Parameters
• Field Pattern (Beam Area,
Directivity, Gain)
• Radiated power
• Efficiency
• Effective Length and effective area
• Polarization (LP/CP/EP)
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TYPES OF ANTENNA
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TYPES OF ANTENNA
• Structural classification:
• Wire Antennas
• Aperture Antennas
• Microstrip Antennas
• Array Antennas
• Reflector Antennas
• Frequency dependency classification:
• Frequency Dependent Antennas
• Frequency Independent Antennas
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Dipole antenna Circular (Square) loop antenna
Helix antenna
Wire Antennas
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Horn antennas
Aperture Antennas
Conical Horn antennas
Slotted Waveguide antennas
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Pyramidal Horn antennas
Radiation pattern of a antenna
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Rectangular patch antennas
Microstrip Antennas
Circular patch antennas
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Yagi-Uda antenna
Array Antennas
Microstrip array antenna Slotted waveguide array antenna
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Parabolic reflector antenna with front feed
Reflector Antennas
Parabolic reflector antenna with cassegrain feed
Corner reflector antenna
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Frequency independent antennas
Log periodic antenna Planar Log periodic slot antenna Log-spiral antenna
Discone antenna
Various versions of Biconical antennas – Infinite
Biconical antenna, Finite Biconical antenna, a cone
with finite ground, a cone with a stem and discone
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FUNDAMENTAL PARAMETERS OF ANTENNA
• Radiation pattern
• Radiation power density
• Radiation intensity
• Beamwidth
• Directivity
• Antenna efficiency
• Gain
• Bandwidth
• Group Delay
• Polarization
• Antenna impedance
• Antenna temperature
• Brightness Temperature
• Antenna Factor
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Radiation pattern
A graphical or mathematical representation of the radiation properties of
an antenna such as amplitude, phase, polarization etc as a function of
the angular space coordinates θ and Φ.
Polar pattern Linear pattern
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Directional radiation pattern Omni-directional radiation pattern
Radiation pattern
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• Same power is radiated
• Radiation intensity is power density over sphere (watt/steradian)
• Gain is radiation intensity over that of an isotropic source
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Field regions of an antenna
(a) Reactive near field region
(b) Radiating near field (Fresnel) region
(c) Far field (Fraunhofer) region
Field regions of an antenna
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]2/62.0[ 23 DRD
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Radiation power density
Where W = Radiation power density (W/ m2)
E = radiated electric field intensity (V/ m)
H = radiated magnetic field intensity (A/ m)
*Re2
1,, HEzyxWav
The time average Poynting vector (average power density) can be
written as
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It can be defined as “the ratio of the radiation intensity in a given direction
from the antenna to the radiation intensity averaged over all directions”
Directivity (D)/ Directive Gain
)](dim[log10)(
4
4/
10
0
ensionlessDdBD
P
U
P
U
U
UD
radrad
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)
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rrA
D21
0
44
Where, D = directivity (dimensionless)
ΩA = beam solid angle)
θ1r= HPBW in one plane (radian)
θ2r= HPBW in a plane at a right angle to other (radian)
If beamwidth in degrees, equation can be written as:
dddd
ddrr
D
2121
2
2121
0
412531804
)180
()180
(
44
For a planar arrays, a better approximation is
dd
D21
0
32400
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Radiation pattern for a particular paraboloid reflector antenna
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Antenna efficiency
dcr
dcr
ee
eeee
0
Where, e0 = total antenna efficiency (dimensionless)
ecd = antenna radiation efficiency (dimensionless)
: used to relate the gain and directivity
er = reflection (mismatch) efficiency (dimensionless)
ec = conduction efficiency (dimensionless)
ed = dielectric efficiency (dimensionless)
dcee )1(2
0
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CL
CL
ZZ
ZZ
Where, ZL= Antenna impedance
ZC = characteristic impedance
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Gain (G)/ Power Gain
)(dim
)(
,4Re ensionless
ourceisotropicsP
UlativeGain
in
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)
)(dim
,4
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ensionlessP
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U
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UGain
cd
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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:
dd
G21
20000
Where, θ1d = HPBW in one plane (degree)
θ2d= HPBW in a plane at a right angle to other (degree)
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Effective Aperture (Aeff)
2
4
effAG
2
4
Ae
Where, = wavelength
A= Physical area of the antenna
e = antenna aperture efficiency
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• 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
Antenna Input Impedance
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Bandwidth
LH ffABW
LH
LH
ff
ffFBW
2
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)
Fractional Bandwidth (FBW).
(2.1)
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)
Polarization is defined as “the electric field vector of an antenna oriented
in space as a function of time”.
Electromagnetic Wave
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(2.1)
The polarization of a radiated wave is the property of an electromagnetic
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.
There are three classifications of antenna polarization:
• Linear polarization,
• circular polarization and
• Elliptical polarization.
#Circular and linear polarizations are special cases of elliptical polarization
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S Y S T E M S (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
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Note : Both the PLF and pe lead to the same answers
Polarization Loss Factor
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• The antenna factor is defined as the ratio of the electric field strength
to the voltage V (units: V or µV) induced across the terminals of a
antenna.
• For an electric field antenna, the field strength is in units of V/m or
µV/m and the resulting antenna factor AF is in units of 1/m:
AF= Eincident/Vreceived
Antenna Factor
• 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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• Most popularly used antennas are:
• Parabolic Reflector Antennas
• Planar Phased Arrays
• Electronically steered Phased array antennas
RADAR ANTENNAS
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Radar Antenna Architecture Comparison
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Array Radar
Passive Array Radar Active Array Radar
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Active Phased Array Radar
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Digital Array Radar Architecture:
Digital on Receiver
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.
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Reference
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