R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

DR Sanjeev Kumar Mishra Lecture 17-20

RADAR Antennas

i

prt

t nELLLNSBFkT

GPR

10

3

2

max4

)/(4

2

R A D A R

S Y S T E M S

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°.

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

• 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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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 )(

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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)

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

TYPES OF ANTENNA

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

TYPES OF ANTENNA

• Structural classification:

• Wire Antennas

• Aperture Antennas

• Microstrip Antennas

• Array Antennas

• Reflector Antennas

• Frequency dependency classification:

• Frequency Dependent Antennas

• Frequency Independent Antennas

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Dipole antenna Circular (Square) loop antenna

Helix antenna

Wire Antennas

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Horn antennas

Aperture Antennas

Conical Horn antennas

Slotted Waveguide antennas

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Pyramidal Horn antennas

Radiation pattern of a antenna

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Rectangular patch antennas

Microstrip Antennas

Circular patch antennas

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Yagi-Uda antenna

Array Antennas

Microstrip array antenna Slotted waveguide array antenna

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Parabolic reflector antenna with front feed

Reflector Antennas

Parabolic reflector antenna with cassegrain feed

Corner reflector antenna

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Directional radiation pattern Omni-directional radiation pattern

Radiation pattern

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

• Same power is radiated

• Radiation intensity is power density over sphere (watt/steradian)

• Gain is radiation intensity over that of an isotropic source

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

]2/62.0[ 23 DRD

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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)

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Radiation pattern for a particular paraboloid reflector antenna

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

CL

CL

ZZ

ZZ

Where, ZL= Antenna impedance

ZC = characteristic impedance

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

)(

int

ensionlessP

U

poweracceptedinputtotal

ensityradiationGain

in

incdrad PeP

)(dim

,4

/

,4,4

ensionlessDeP

Ue

eP

U

P

UGain

cd

rad

cd

cdradin

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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)

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Effective Aperture (Aeff)

2

4

effAG

2

4

Ae

Where, = wavelength

A= Physical area of the antenna

e = antenna aperture efficiency

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

• 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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Polarization (2.1)

Polarization is defined as “the electric field vector of an antenna oriented

in space as a function of time”.

Electromagnetic Wave

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

(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

R A D A R

S Y S T E M S

R A D A R

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

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Polarisation states for a z-directed plane wave

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

Note : Both the PLF and pe lead to the same answers

Polarization Loss Factor

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

• 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)]

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

R A D A R

S Y S T E M S

• Most popularly used antennas are:

• Parabolic Reflector Antennas

• Planar Phased Arrays

• Electronically steered Phased array antennas

RADAR ANTENNAS

R A D A R

S Y S T E M S

Radar Antenna Architecture Comparison

R A D A R

S Y S T E M S

Array Radar

Passive Array Radar Active Array Radar

R A D A R

S Y S T E M S

Active Phased Array Radar

R A D A R

S Y S T E M S

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.

R A D A R

S Y S T E M S

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