1. Radar System
INTRODUCTION
RADAR is a system, which uses radio waves to detect and fix the
position of distinct objects or targets. RADAR is not a signal instrument but an
electronic device which includes various techniques used for the purpose of detection
and location of distant object or targets. Radar is basically a means of gathering
information about distant objects or targets by sending electromagnetic waves (Radio
waves) at them and analying the echoes.
Radar was developed during second world war, by the scientist of
Great Britain, United states of America, Germany and France simultaneously.
Radar may be put to various uses both civilian and military. The main
civilian uses of Radar is an navigational aid. Various regions such as mountains,
aircrafts, aeroplanes, cities, rivers, oceans etc.
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2. Radar System
FUNDAMENTALS OF RADAR
Radar consist of a transmitter and a receivers, each connected to a
directional antenna. The transmitter is capable of sending out a large U.H.F. or micro
wave power through the antenna.
The receiver collects as much energy as possible as from the reflected
echoes in it's direction by the target and then treats and displays this information in a
suitable way.
The receiving antenna is very often the same as the transmitting
antenna. This is accomplished through a kind of Time Division Multiplexing
(T.D.M.) arrangements since the radio energy is often sent out in the from of short
pulse.
Objects such as aircrafts, ships, mountains buildings etc. Reflects
some of this energy back to the transmitter. The time it takes the radio wave to
complete it's two way journey indicates the distance between sender and target.
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3. Radar System
PRINCIPLE OF RADAR
The block diagram of pulsed radar system is shown in fig. Basically a
radar system consist of transmitter which transmit microwave power through a
highly directional antenna, a receiver which collects the echo signal received after
reflection from target, analyses this echo signal and displays it on radar screen.
It is possible to detect in height speed and direction of travel of target
by the time takes for the echo to come back after reflection to indicate the data.
Since, the same directional antenna has to be used for the transmitting
electromagnetic (EM) waves and for receiving it after reflection, there should be
switching system called duplexer. The duplexer should be automatic and disconnect
the unwanted hardware when operated in a particular direction.
Thus radar has five essential component viz. transmitter, receiver,
duplexer, display unit and highly directional antenna.
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4. Radar System
PULSED RADAR SYSTEM
The block diagram shows the arrangement of a high power pulsed
radar set. it consist of two basic transmitter section and receiver section.
TRANSMITTER SECTION :
It consists of trigger source, Modulator, out put tube, Anti
transmit receiver tube and Transmit receiver tube switch with antenna.
TRIGGER SOURCE :
It provided pulses of particular frequency to modulator and
establish the rate at which pulse are to be transmitted.
MODULATION :
It provides rectangular voltages pulse which are used as supply
voltage for o/p tube. There are two types of modulator used i.e. tine pulsing and
active switch modulator. Generally active switch modulator is used.
OUTPUT TUBE :
The output may be magnetron, Klystron or TWTA amplifier
depending on specific requirement. Magnetron is commonly used because it can
develop large power. Then o/p of this tube give to the ATR switch.
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5. Radar System
RECEIVER SECTION :
The function of receiver is just like it super heterodyne. It consist of
RF amplifier mixer, local oscillator, If amplifier, detector, video amplifier and
display unit.
DUPLEXER :
The information obtained from Antenna is feed to receiver section by
switching CKT called duplexer. it consist of TR and ATR switch. During
transmission ATR acts as short CKT and during reception TR act's as short CKT.
While ATR is open CKT.
RF AMPR :
It is generally low noise TWTA amplifier which amplifies the
echo signal received from target. The capacity of TWTA 250 kw and to effective
between 0.5 GH and95G Hz.
MIXER STAGE :
Mixer gets two signals from RF amplifier and local oscillator. The
frequency of local oscillator is greater than RF signal. The o/p from mixer is selected
at difference I frequency is about 30 MHZ.
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6. Radar System
LOCAL OSCILLATOR:
The local oscillator in Radar is reflex klystron with narrow band filter
and reduces noise. It belongs to klystron with narrow band filter and reduces noise. It
belongs to klystron family of microwave generator. It is one effective variable
frequency oscillator having range 1 GHz to 100 GHz.
Fig shows MTI Radar system compare a set echo received during
screen the echo in phase are cancelled out this applic to due to stationary object but
for moving target there is change in phase so they are not cancelled in less time. It
also the detection of moving target whose shoce are t100 times smaller than
stationary target.
Transmitted frequency in MTI system is a sum of o/p of two oscillator
produced in mix - The first is stable local oscillator and second is conetent oscillator.
Mixes 1 &2 are identical and both use same local oscillator which makes doppler
ship at intermediate frequency. The conetent oscillator is used to generate RF signals
as well as for a reference of phase detector and the Mix 2 do not produce change in
phase. So the transmitted and reference signal are lock in phase and sent to be
content the phase different between transmitted and receiving signal may be constant
for fixed target which vary for moving target to doppler ship and thus the position of
moving target is detected according to change in Reflected frequency in small time.
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7. Radar System
CONTINUOUS WAVE RADAR
IF AMPLIFIRES :
It is generally low noise transistorized amplifier operates between
30 MHz to 60 MHz.
DETECTOR :
Since the vacuum tube or transistor do not operates at microwave
frequency we use short key diode or crystal diode.
VIDEO AMPLIFIRES :
Video Amplifiers has same band width as that of if Amplifiers it
amplifies the detector o/p and then feed to display Unit.
DISPLAY UNIT:
The o/p of Radar Receiver is presented to the operator in the form the
mutual indication. Such as C.R.T which shows the position of target.
In this way pulse Radar system works.
A simple Doppler Radar is shown in fig. 5.1. It sends out continues
sine waves rather than pulse. It uses the Doppler effect to detect the frequency
change caused by a moving target and displays this as a relative velocity.
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8. Radar System
Because the transmission is continuous, the circular is used to
provided isolation between the transmitter and receiver. The isolation of atypical
circulator is of the order of 30 dB, so that some of the transmitted signal leaks into
the receiver. This is quite alright as the signal can be mixed in the detector with
returns from the target and the difference is the Doppler frequency. This frequency
being in the audio range hence the detector output can be amplified with an audio
amplifier before feeding it to frequency counter. The counter is a normal one, except
that its output is shown as Kilometers or Miles per hour, rather actual than actual
frequency in hertz.
The disadvantage of simple Doppler cw radar is that it is not sensitive.
It can be modified by using a local oscillator. The type of diode detector which has
been used to accommodate the high incoming frequency is not a good device at the
audio output frequency because to the modulation noise which exhibits at a low
frequency such a modified version of radar set is shown in fig. 5.2
A small portion of transmitter output is mixed with the O/P of a local
oscillator, and the sum is fed to the receiver mixer. This also receives the Doppler
shifted signal from its antenna and presents an output difference frequency that is
typically 30 MHz + or - the Doppler frequency AF . The output of this mixer is
amplified and demodulated again and the signal from second detector is Doppler
frequency. When its sign is lost then it is not possible to say the target is going or
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9. Radar System
approaching away. The overall receiver system is same as superhetrodyne. Extra
sensitivity is obtained by low noise.
A separate transmitting and receiving antennas are shown in Fig. but
this is not compulsion, instead of two antenna single antenna with circulator can be
used. Separate antennas increase the isolation between transmitter and receiver
section of radar, since there is no longer mixers there was in simpler Doppler radar
(see Fig. 5.1). Sue leakage is undesirable because it brings with it the hum an noise
from the transmitter and thus degrade the receive performance.
ADVANTAGES & APPLICATION OF CW RADAR
SYSTEM
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10. Radar System
ADVANTAGES:
1) CW radar gives accurate measurement of relative velocity of the target, using
low transmitted power .
2) Power consumption is very low unlike a pulse radar because of low transmitted
power.
3) CW doppler radar receiver is working all time and pulses loss does not occurs.
4) Simple circuitry and smaller size than that of pulsed radar system.
5) Stationary target cannot affect results will not yield doppler frequency.
6) A large range of target velocities can be measured quickly and accurately.
DISADVANTAGE OF CW DOPPLER RADAR.
1) It is limited in the maximum power it transmits and this places a limit on its
maximum range.
2) If a number of simultaneous target are presents, the doppler frequency will be
incorrect.
APPLICATION OF CW RADAR :
1) CW doppler radar is used in o/r craft navigation for speed measurement.
2) It is used in echo of climb meter for vertical take off planes.
3) It is used in radar speed meters used by police.
BIBLOGRAPHY
1. Electronic Communication System
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11. Radar System
- George Keneddy
2. Communication Electronic
- D. A. Deshpande
- N. D. Deshpande
- P. K. Rangole
3. Electronics Engineers Reference Book
- F. F. Mazda
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