Radar System Design Chapter 6 Radar Antenna 6-1 Chapter 6: Radar Antenna Dr. Sheng-Chou Lin Radar System Design Basic Antenna Theory x sin x ------------ N 2 x sin 1 2 x sin -------------------------------- With the uniform amp. a nd phase array you will approximate (Continuous) (discrete)
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Radar System Design
Chapter 6Radar Antenna
6 - 1Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Basic Antenna Theory
xsinx
------------
N 2 x sin1 2 x sin
---------------------------------
With the uniform amp. andphase array you willapproximate
(Continuous)
(discrete)
6 - 2Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Antenna Pattern Regions•Near-Field (Reactive): Fields are predominantly reactive
• Inter-mediate Region (Fresnel): Radiated near field angulardependence is a function of distance from the antenna (i.e., thingsare still changing rapidly)
•Far-Field (Fraunhofer): Radiated far field angular dependence isindependent of distance R the region of interest
g r
0 /2 2D 2 /
D
reactive radiatedNear Field Radiated far Field
Near Field
0 /2 2D 2 /
D
reactive radiatedNear Field Radiated far Field
Near Field
0.62 D 2
6 - 3Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Far-Field Approach
0 /2 2D2/
D
reactive radiatedNear Field Radiated far Field
Near Field
0 /2 2D2/
D
reactive radiatedNear Field Radiated far Field
Near Field
R
R 2 D 2 4+
D 2
Pointtarget
Case: Receiving signal from point target
•Assume the point target radiates field
•The phase difference between points (a) and (b) due to signal from point target is
, where wave number . Assumption: .
•Using
e jkR– R
k r2 D 2
4------+ r–
8--- PB PA–=
16------= k 2
------= r D»
r 1 D 2
4r2---------+ r 1 D 2 8r2+ = r 1 D 2 8r2+ r–
16------ D 2 8r 2 16 r 2D 2 = = =
(a)
(b)
6 - 4Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Point Target Approach
0 /2 2D2/
D
reactive radiatedNear Field Radiated far FieldNear Field
0 /2 2D2/
D
reactive radiatedNear Field Radiated far FieldNear Field
d
Case: Transmitting with antenna that has constant phase across aperture if we
assume beam-width . Usually .
•At the criteria , , or
•We select , .
•If the target is sufficiently small with respect to spot size d then the wave incident on thetarget is approximately uniform in phase and amplitude
— A number of source aperture distribution have been developed so asto optimize with respect to one or more of these parameters
0-10 -5
3 dB beamwidth
major lobe
sidelobe
null
(minor lobe)
-3 dB
back lobe
•Uniform aperture produces the narrowest mainlobe obtainable with linear or constant phaseacross the aperture but at the expense of sidelobe levels (~ 13 dB first sidelobe)
•For he Dolph Chebychev distribution, the optimum pattern is defined asthe one that produce the narrowest beamwidth between first nulls (oneach of main lobe) with no sidelobe higher than a stipulated level.
6 - 6Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Rectangular Aperture (Uniform)
Rectangular Aperture Antenna Pattern
6 - 7Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Circular Aperture (Uniform)
Circular Aperture Antenna Pattern
6 - 8Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
E-Plane v.s. H-Plane
Rectangular Aperture Circular Aperture
6 - 9Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Some Common RF Decibel UnitsOnly one number is needed if the other is a known, standardvalue:
•dBm = dB referred to 1 mW
•dBW = dB referred to 1 W
•dBi = dB referred to an isotropic source
•dBd = dB referred to a dipole = dBi - 2.15 dB
dBm 10 log P1 mW-------------------
=
dBW 10 log P1 W-------------
=
6 - 10Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Free-space Path Loss
gainPrPt------- G1G2
4d------------
2
G1G2c
4df---------------
2
G1G23 810
4d 1 310 f 1 610 --------------------------------------------------------------
2
= = = =
loss(dB) 32.44 20 log d 20 log f G1 dB – G2 dB –++=
Path Gain
for d in km, f in MHz
Path Loss = 1 / (Pr/Pt)
distance d
antenna 1 antenna 2
P rP t G1 G2
frequency f or wavelength
6 - 11Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Antenna Gain
Pr sin d d0
0
2
=
• = power accepted by antenna
• = power radiated by antenna
• = radiation efficiency =
Po
Pr
Pr Po
= Radiation intensity
= average radiation intensity
:Directivityrelative
to isotropic antenna
: Gain
avg Pr 4=
D avg
-------------------------- Pr 4
---------------------------= =
G D Po 4
---------------------------= =
P R R
---------------------------------------------
R 2-------------------------- G
Po4R 2------------= = =
Po
Pr
D G
6 - 12Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Antenna Aperture
: Effective Area
: Antenna efficiency
: Physical area of antenna’s aperture
: the peak or maximum value of
: Peak gain of antenna
: Standard directivity
: product of several factors
: aperture illumination efficiency, whichreduce the gain of antenna. Loss in gainresulting from tapering the aperturedistribution to produce sidelobs lower thanthose achievable from a uniform illumination
Ae 2
4------G =
aAeA-----=
A
Ae Ae
G a4A2
--------- aGo= =
Go4A2
---------=
a i123=
i
Half-power beamwidth BW of an antennais related to the beamwidth constant
•Weighting Distribution: A complex analysis todesign feed with a proper weighting based on
- Edge Taper
- Beamwidth of feeder
- Space Attenuation
G aGo= a
BW D----
Scanning Techniques
•Scan entire antenna
•Scan feed within reflector
6 - 34Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Parabolic Reflector Antenna Design(1)
Weighting Design rules
•Sidelobe level: Select H-plane edge taper from Fig. 6.11 necessary to obtain desired sidelobelevel. (Edge taper is determined by beamwidth of feeder and space attenuation)
•Space attenuation: Given f/D, find intercepted angle from Fig. 6.12. In order to determine spaceattenuation.
6 - 35Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
Parabolic Reflector Antenna Design (2)
•Using Fig. 6-13 to find space attenuations Fig. 17.17 todetermine 10dB beamwidth of feed horn
, given intercepted angle .
•Use (degree), for E-plane,
(degree), for H-plane
- : pattern width in degrees at -10dB level.
BW10dB 210dB=
BW10dB 88 B= B 2.5
BW10dB 31 79+ A= A 3
BW10dB
B: E-plane horn apertureA: H-plane horn aperture
H
EA
B
6 - 36Chapter 6: Radar Antenna Dr. Sheng-Chou Lin
Radar System Design
An Example•Design the feed horn for an antenna with f/D = 0.7 using the
technique describe above. It is required that the edge illuminationshould be 20dB down. Assume a wavelength of 3cm.
• total feed angle = 80o, space attenuation 1dB.
• , ,
•E-plane, , .
•H-plane, , .
•From Fig. 6-11, first sidelobe = -40dB for Taylor distribution