RADAR RF SOURCES
RADAR RF SOURCES
INTRODUCTION TO THE
WHAT IS RADAR?
1. RADAR Stands for
2. RADAR can operate in:
Darkness Haze Fog Rain or Snow
AIRPORT RADAR SYSTEM
RADAR BLOCK DIAGRAM
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
2. Sources of Carrier Wave:
2. Traveling Wave Tube (TWT)
Functions of Receiver:
1. Detects the received signal
2. Amplifies the returned signal.
In order to provide the greatest range, the receiver must have a high
signal-to-noise ratio (S/N).
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
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.
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.
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
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.
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
PLAN POSITION INDICATOR (PPI)
REAL AVIATION DISPLAY
TRUE & RELATIVE BEARINGS
DETERMINATION OF SIGNAL STRENGTH
In Search RADAR
maximum echo is
• 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
• Refers to the ability of a radar system to
distinguish targets at the same range but
• Degree of range bearing depends on:
1. Radar beam width
2. Range of targets
1. Navigational aids and surveillance of
enemy aircrafts in military applications
2. Air traffic control as primary and
3. Weather radars
4. Law enforcement as radar speed meters
5. Games for measuring speed of balls, etc.
RADAR FREQUENCIES (1)
Coastal radar systems, over-the-horizon
radar (OTH) radars
< 300 MHz
Applied retrospectively to early radar
Very long range, ground penetrating;
Very long range (e.g. ballistic missile early
warning), ground penetrating, foliage
RADAR FREQUENCIES (2)
Long range air traffic control
Moderate range surveillance,
Terminal air traffic control, longrange weather, marine radar;
Satellite transponders;; weather;
long range tracking
Missile guidance, marine radar,
mapping and ground
surveillance; in the USA the
narrow range 10.525 GHz
±25 MHz is used
forairport radar; short range
'L' for 'long'
'S' for 'short'
A compromise (hence
'C') between X and S
Named X band
frequency was a
secret during WW2
RADAR FREQUENCIES (3)
K band (hence 'u')
From German kur
z, meaning 'short'
above K band
High-resolution, also used for satellite transponders
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.
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.
IEEE STANDARD RADAR
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.
RADAR RADIO FREQUENCY
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
• The voltage gain of a TWT can be of the order
of 70 decibels
THE TRAVELLING WAVE TUBE
A traveling-wave tube (TWT) used to
amplify microwave signals to high power, usually in an
electronic assembly known as a traveling-wave tube
The bandwidth of a broadband TWT can be as high as
one octave, although tuned (narrowband) versions
Operating frequencies range from 300 MHz to 50 GHz.
The voltage gain of a TWT can be of the order of
• 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,
• 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
• They are especially suited to airborne
applications, where their small size and low
weight are valuable.
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).
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.
TWT: SLOWING OF SIGNAL
1. Slowing is usually
achieved using the helix
electrode, which is simply
a spiral of wire around the
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
4. The condition for
equal slow-wave and
velocities is therefore
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
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.
TYPICAL POWER SUPPLY OF A TWT
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 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
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
LINE-TYPE MODULATOR (1)
A line-type modulator contains a gas tube and a delay line
as the energy storage element.
150V +ve Trigger
LINE-TYPE MODULATOR (2)
150V +ve Trigger
CHARGING CURVE OF THE PFN IN A LINE TYPE
The inductance of the
charging coil offers a large
inductive resistance to the
current and builds up a
strong magnetic field.