RADAR RF SOURCES

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Transcript RADAR RF SOURCES

INTRODUCTION TO THE
RADAR
ELC 451L
1
WHAT IS RADAR?
1. RADAR Stands for
Radio
Detection
And
Ranging
2. RADAR can operate in:
Darkness Haze Fog Rain or Snow
2
AIRPORT RADAR SYSTEM
3
RADAR SYSTEM
4
RADAR SPECIFICATIONS
5
RADAR BLOCK DIAGRAM
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TRANSMITTER
1. Functions:
1. Creates the radio wave to be transmitted
2. Conditions the wave to form the pulse train.
3. Amplifies the signal to a high power level to provide adequate
range.
2. Sources of Carrier Wave:
1. Klystron,
2. Traveling Wave Tube (TWT)
3. Magnetron.
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RECEIVER
Functions of Receiver:
1. Detects the received signal
2. Amplifies the returned signal.
Basic Characteristic:
In order to provide the greatest range, the receiver must have a high
signal-to-noise ratio (S/N).
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POWER SUPPLY
Function of Power Supply:
Provides the electrical power for all the components.
Power Requirement by Transmitter:
1. The transmitter is the largest consumer of power is , which may require
several kW of average power.
2. The actually power transmitted in the pulse may be much greater than 1
kW.
3. Usually only average amount of power consumed is provided , not the high
power level during the actual pulse transmission.
4. Energy is often stored in a capacitor bank, during the rest time.
5. The stored energy then can be put into the pulse when transmitted,
increasing the peak power.
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SYNCHRONIZER
Function of the Synchronizer:
1. Regulates that rate at which pulses are sent (i.e. sets PRF)
2. Resets the timing clock for range determination for each pulse.
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DUPLEXER SWITCH
Purpose of Duplexer:
1. To protect the receiver from the high power output
of the transmitter.
2. A duplexer is not required if the transmitted power
is low.
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ANTENNA
Function of Antenna:
1. Transmit pulses
2. Focus the energy into a well-defined beam.
3. Keep track of its own orientation by using a
synchro-transmitter or phased array system.
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DISPLAY
Types of Radar Displays:
1. A-Scan (Amplitude vs Time) – No information on direction of target
2. Plan Position Indicator – Displayed in the same relative direction as
the antenna.
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A-SCAN
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PLAN POSITION INDICATOR (PPI)
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REAL AVIATION DISPLAY
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RADAR REFERENCE
COORDINATES
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RANGE
t
R  c
2
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TRUE & RELATIVE BEARINGS
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DETERMINATION OF SIGNAL STRENGTH
In Search RADAR
Systems where
the antenna
moves
continuously,
maximum echo is
determined by
the detection
circuitry.
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RANGE RESOLUTION
• Refers to the ability of a Radar system to
distinguish between two or more targets on the
same bearing but different ranges.
• A well designed radar system, will all other
factors at maximum efficiency should be able to
distinguish targets separated by ½ of the Pulse
Width (PW), i.e
RRES
PW
 c
2
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BEARING RESOLUTION
• Refers to the ability of a radar system to
distinguish targets at the same range but
different bearings.
• Degree of range bearing depends on:
1. Radar beam width
2. Range of targets
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RADAR APPLICATIONS
1. Navigational aids and surveillance of
enemy aircrafts in military applications
2. Air traffic control as primary and
secondary radars
3. Weather radars
4. Law enforcement as radar speed meters
5. Games for measuring speed of balls, etc.
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RADAR FREQUENCIES (1)
BAND
NAME
FREQUENCY
RANGE
WAVELENGT
H RANGE
NOTES
HF
High
Frequenc
y'
3–30 MHz
10–100 m
Coastal radar systems, over-the-horizon
radar (OTH) radars
P
'P' for
'previous',
< 300 MHz
1 m+
Applied retrospectively to early radar
systems
VHF
Very High
Frequenc
y'
30–300 MHz
1–10 m
Very long range, ground penetrating;
0.3–1 m
Very long range (e.g. ballistic missile early
warning), ground penetrating, foliage
penetrating;
UHF
Ultra High
Frequenc
y
300–1000 MHz
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RADAR FREQUENCIES (2)
NAME
FREQUENCY
WAVELENGTH
APPLICATION
1–2 GHz
15–30 cm
Long range air traffic control
and surveillance
2–4 GHz
7.5–15 cm
Moderate range surveillance,
Terminal air traffic control, longrange weather, marine radar;
4–8 GHz
3.75–7.5 cm
Satellite transponders;; weather;
long range tracking
2.5–3.75 cm
Missile guidance, marine radar,
weather, medium-resolution
mapping and ground
surveillance; in the USA the
narrow range 10.525 GHz
±25 MHz is used
forairport radar; short range
tracking..
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L
'L' for 'long'
S
'S' for 'short'
C
A compromise (hence
'C') between X and S
bands
X
Named X band
because the
frequency was a
secret during WW2
8–12 GHz
RADAR FREQUENCIES (3)
BAND
Ku
Frequency under
K band (hence 'u')
K
From German kur
z, meaning 'short'
Ka
Frequency just
above K band
(hence 'a')
FREQUENC
Y
WAVELE
NGTH
12–18 GHz
1.67–
2.5 cm
High-resolution, also used for satellite transponders
18–24 GHz
1.11–
1.67 cm
Limited use due to absorption by water vapour, so Ku and
Ka were used instead for surveillance. K-band is used for
detecting clouds by meteorologists, and by police for
detecting speeding motorists. K-band radar guns operate at
24.150 ± 0.100 GHz.
24–40 GHz
0.75–
1.11 cm
Mapping, short range, airport surveillance; Photo radar, used
to trigger cameras which take pictures of license plates of
cars running red lights, operates at 34.300 ± 0.100 GHz.
APPLICATION
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IEEE STANDARD RADAR
FREQUENCIES
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HALF-POWER POINTS
1. When all half-power points are connected to the antenna by a
curve, the curve is called the Antenna Beam Width.
2. Two targets at the same range must be separated by at least one
beam width so as to be distinguished from one another.
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RADAR RADIO FREQUENCY
GENERATORS/AMPLIFIERS
ELC 544E
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TRAVELLING WAVE TUBE
• A traveling-wave tube (TWT) used to
amplify microwave signals to high power, usually
in an electronic assembly known as a travelingwave tube amplifier (TWTA).
• The bandwidth of a broadband TWT can be as
high as one octave, although tuned
(narrowband) versions exist.
• Operating frequencies range from 300 MHz to
50 GHz.
• The voltage gain of a TWT can be of the order
of 70 decibels
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THE TRAVELLING WAVE TUBE
1.
2.
3.
4.
A traveling-wave tube (TWT) used to
amplify microwave signals to high power, usually in an
electronic assembly known as a traveling-wave tube
amplifier (TWTA).
The bandwidth of a broadband TWT can be as high as
one octave, although tuned (narrowband) versions
exist.
Operating frequencies range from 300 MHz to 50 GHz.
The voltage gain of a TWT can be of the order of
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70 decibels
TWT- HISTORY
• In December 1943 the first tube gave again of
about 8 dB at a 9.1 cm wavelength, with a
13 dB noise figure. The work was later
transferred to the Clarendon Laboratory,
Oxford.
• Much of the mathematical analysis of TWT
operation was developed by John R. Pierce,
of Bell Labs.
• Nowadays, TWTS are by far the most widelyused of microwave tubes, and are employed
extensively in communication and radar
systems.
• They are especially suited to airborne
applications, where their small size and low
weight are valuable.
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TWT – THEORY (1)
•
Electrons from a heated cathode are
accelerated towards the anode, which is
held at a high positive potential with
respect to the cathode, and a proportion
pass through a hole in the anode to
produce the beam.
•
To achieve good focussing by this
method requires a very large
magnetic field, which can mean a
bulky, heavy magnet. The
arrangement usually employed is
called periodic permanent magnet
(PPM) focussing, in which a number
of toroidal permanent magnets of
alternating polarity is arranged
along the tube.
This arrangement reduces
enormously the required weight of
magnet (under ideal conditions by a
factor 1/N2; where N is the number
of magnets used).
•
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TWT – THEORY (2)
• The velocity, v, of an electron beam is given by:
• An anode voltage of 5 kV gives an electron velocity of 4.2 x 107 m/s.
The signal would normally travel at c, the velocity of light (3x108 m/s),
which is much faster than any 'reasonable' electron beam.
• If the signal can be slowed down to the same velocity as the electron
beam, it is possible to obtain amplification of the signal by virtue of its
interaction with the beam.
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TWT: SLOWING OF SIGNAL
1. Slowing is usually
achieved using the helix
electrode, which is simply
a spiral of wire around the
electron beam.
2. Without the helix, the
signal would travel at a
velocity c. With the helix,
the axial signal velocity is
approximately c x (p
/2πa) where a, p are as
shown on the left.
3. By proper selection of a
and p, the signal can be
slowed.
4. The condition for
equal slow-wave and
electron-beam
velocities is therefore
approximately:
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TWT AMPLIFICATION
1. The interaction between the beam and the slow wave takes the
form of 'velocity modulation' of the beam (i.e some electrons are
accelerated and some retarded) forming electron bunches within
the beam.
2. The beam current becomes modulated by the RF signal, and the
bunches react with the RF fields associated with the slow wave
travelling down the helix, resulting in a net transfer of energy
from the beam to the signal, and consequent amplification.
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TYPICAL POWER SUPPLY OF A TWT
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KLYSTRON AMPLIFIER
1. Used as amplifiers at microwave and radio frequencies to
produce both low-power reference signals
for radar receivers and to produce high-power carrier 38
waves for communications.
RADAR MODULATORS
• Radar modulators Control the following:
1. the RF energy to be produced
2. The Repetition frequency
3. Shape of the pulse.
• There are two types of radar modulators:
1. Anode Modulator
2. Grid Modulator
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BLOCK DIAGRAM OF RADAR MODULATOR
Purpose of the Charging Impedance:
1. To restrict the rate at which energy is delivered to the storage devices
2. To Prevent the dissipation of energy through the Source during the
discharge period.
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LINE-TYPE MODULATOR (1)
A line-type modulator contains a gas tube and a delay line
as the energy storage element.
2Vs
Vs
Charging Current
DC
Shield
Delay Line
Oscillating Device
e.g. Magnetron
150V +ve Trigger
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LINE-TYPE MODULATOR (2)
2Vs
Vs
Charging Current
DC
Shield
Delay Line
Oscillating Device
e.g. Magnetron
150V +ve Trigger
42
CHARGING CURVE OF THE PFN IN A LINE TYPE
MODULATOR
Uo
The inductance of the
charging coil offers a large
inductive resistance to the
current and builds up a
strong magnetic field.
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