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A
REPORT
ON
INDUSTRIAL TRAINING
ON
“DOPPLER WEATHER RADAR TRANSMITTER”
TAKEN AT
“INDIAN METEOROLOGICAL DEPARTMENT, NEW DELHI”
Submitted in partial fulfillment for the award of Degree of
Bachelor of Technology of Rajasthan Technical University, Kota
2015-16
Submitted to: Submitted by:
Mr. Alok Kumar Pawan Kumar meena Ms. Suman Godara (PGI/EC/12/065)(PTS Coordinators, ECE-B)
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERINGPOORNIMA GROUP OF INSTITUTIONS
SITAPURA, JAIPUR (Raj.)-302022
1
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
POORNIMA GROUP OF INSTITUTIONSSitapura, Jaipur Rajasthan-302022
CERTIFICATE
This is to certify that a practical training seminar report entitled “Doppler
Weather Radar Transmitter” taken at “Indian Meteorological
Department, New Delhi” is submitted by “Pawan Kumar Meena
(PGI/EC/12/065)”, student of Fourth Year VII Semester in Electronics and
Communication Engineering of Rajasthan Technical University, Kota during
the academic year 2015-16.The report has been found satisfactory and is
approved for submission.
Mr. Alok Kumar Mr. Bhanwar Veer Singh
Ms. Suman Godara (HOD, Deptt.of ECE)
(PTS Coordinators, ECE-B)
Dr. Rakesh Duggal
(Campus Director, PGI)
ACKNOWLEDGEMENT2
I feel profound happiness in forwarding this industrial training report as an image of sincere effort. It is
almost inevitable to ensure indebtedness to all who generously helped by sharing their valuable
experience & devoting their precious time with me, without whom this seminar report would have
never been accomplished.
First and foremost I extend my thanks & gratitude to the entire unit of “Indian meteorological
Department” along with “Shri B. Arul Malar Kannan”, whose guidance, teaching and invaluable
suggestions provided me a deep insight in my chosen field of technology, enhanced my knowledge and
support in widening my outlook towards the electronics and communication industry. I am also very
thankful to all engineers of the department for their kind support throughout the training.
I am highly intend to Dr. Rakesh Duggal (Campus Director,PGI), Mr. Bhanwar Veer Singh (HOD,
Deptt. Of ECE), Mr. Alok Kumar (Assistant Professor), Ms. Suman Godara (Assistant Professor)
for providing me the desired platform and deliver the learning in effective and efficient manner.
Pawan Kumar Meena
IV Year ECE-B
(PGI/EC/12/065)
3
PREFACE
TABLE OF CONTENT
4
Chapter 1: Introduction Page No
1.1Introduction…………………………………………………… 6-7
1.2Background of IMD
1.3Organization Structure
Chapter 2: IMD Infrastructure…………………………………………... 8-11
2.1Departmental Structure
2.2Weather Radar Network of India Meteorological Department
2.3Hardware and software of RADAR
Chapter 3: Doppler Weather Radar
3.1 Introduction………………………………………………… 12-12
3.2 Principles…………………………………………………… 13-15
3.2.1 Radar signal 3.2.2 Interacting mechanism 3.2.3 Radar equation 3.2.4 Doppler Effect 3.2.5 Polarization 3.3 TYPES OF Radar's ………………………………………… 15-15
3.4 Introduction to Vaisala DWR
3.5 DWR hardware
3.6 dual polarised DWR
3.7 Transmitter………………………………………………….. 16-16
3.7.1 Introduction:-
3.7.1.1 Technical and operating characteristics: 3.7 .2Block Diagram of Transmitter:-………………….. 16-21
3.7.2.1Tx module of Radar Processor
3.7.2.2 Exciter
3.7.2.3 Tx Amplifier
3.7.2.4 Modulator
3.7.2.5 Klystron5
3.7.2.6 Solenoid
3.7.3Associated Elements…………………………… 21-24
3.7.3.1 Wave guide
3.7.3.2 WG Pressurization Unit
3.7.3.3 Cooling subsystems
3.7.3.4 Arc detection
3.7.3.5 Stub tuner
3.7.3.6 Harmonic filters
3.7.3.7 Dehydrator
3.7.3.8Vac ion PS
3.7.4 Built In Test Equipments…………………………. 24-28
Chapter 4: Conclusion……………………………………………………. 29-30
4.1 Conclusion
4.2 Advantages
4.3 Limitations of Doppler Weather RADAR
Chapter 1
6
1.1 Introduction
The India Meteorological Department was established in 1875. It is the
National Meteorological Service of the country and the principal
government agency in all matters relating to meteorology, seismology
and allied subjects.
To detect and locate earthquakes and to evaluate seismicity country for development projects in
different parts of the Modern Meteorology.
1.2Background of the IMD:-Very early in the history of IMD, the importance of the publication of scientific results had been
recognised. Blanford introduced the publication of the "Memoirs of the IMD" and himself authored
several of them. His work on the rainfall of India is unsurpassable in clarity of thought and content. In
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To take meteorological observations and to provide current and forecast meteorological
information for optimum operation of weather-sensitive activities like agriculture, irrigation,
shipping, aviation, offshore oil explorations, etc.
To warn against severe weather phenomena like tropical cyclones, nor westers, dust storms,
heavy rains and snow, cold and heat waves, etc., which cause destruction of life and property.
To provide meteorological statistics required for agriculture, water resource management,
industries, oil exploration and other nation-building activities.
To conduct and promote research in meteorology and allied disciplines.
view of the importance of foreshadowing monsoon seasonal rainfall for the agricultural economy of
the country, Blanford initiated the system of Long Range Forecasting (LRF).
Branford had recognized the need for inducting young Indians in IMD and the first two Indians
LalaRuchi Ram Sahni (Father of Professor BirbalSahni) and LalaHemraj joined IMD in 1884 and 1886
respectively. The Indenisation of IMD was accelerated under Walker, soon after World War I, and
further boosted by Sir C.W.B. Normand (Director-General during 1928 to 1944). Normand was
succeeded by Dr. S.K. Banerji as the first Indian DGO in 1944. During these years, many Indian
scientists joined IMD and they took IMD to greater heights themselves in the post-independence era.
From a modest beginning in 1875, IMD has progressively expanded its infrastructure for
meteorological observations, communications, forecasting and weather services and it has achieved a
parallel scientific growth. IMD has always used contemporary technology. Later IMD became the first
organisation in India to have a message switching computer for supporting its global data exchange.
One of the first few electronic computers introduced in the country was provided to IMD for scientific
applications in meteorology.
8
9
1.3Organization Structure:-
Figure 1.1 IMD Structure
Chapter 2IMD Infrastructure
2.1Departmental Structure:-
Figure 2.1 IMD Departmental Structure
10
2.2Weather Radar Network of India Meteorological Department:-
Figure 2.2 IMD Radar Network
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Radars are used in IMD for detection of various weather systems like, thunderstorms, hailstorm and
tracking of cyclonic storms. They are also used in rainfall estimation and hail warning. Various
meteorological, hydrological and aviation products generated from Doppler weather radar data using
are extremely useful to the forecasters. Radars helps to estimate the storm’s intensity, location and in
forecasting its future path for safe navigation of aircrafts and ships.
IMD has upgraded the conventional radars in the observational network with Doppler radars
using digital technology. The first such radar procured from M/S Gematronik Gmbh, Germany was
installed at Chennai in 2002 followed by at Kolkata.
Indigenous efforts were also made to design and develop advanced Radar technology for cost
effectiveness and self reliance. Under an MOU with ISRO, DoS, one S-Band, Doppler weather radar
was designed, developed and installed at Sriharikota in 2004. Later which through ToT BEL started
supplying indegenious radars.
Under Modernization IMDs observational radar network has been upgradaed with 21 Doppler
Weather Radars (DWRs). Two of them are C-band Polarimetric DWRs installed at Delhi and Jaipur.
These Radars provide additional information of shape, size and classification of hydrometeors.
In second phase of Modernization plan of IMD, 30 DWRs are proposed to be procured and
installed at various locations throughout India. There is a separate plan to install 9 DWRs at hilly
region under the scheme “Integrated Himalayan Metrology Programme” for western and Central
Himalayas. One X-band DWR is under installation at Srinagar to cater the needs of Amarnath Yatra.
The existing Doppler weather radars have also been networked to provide data for numerical
weather prediction models for now casting. Composite Radar images are being generated centrally.
Data is also converted to various formats such as Net CDF, HDF5, and Opera BUFR.
Thus Radar network is playing significant role in the modernization of IMD’s observational
and forecasting systems.
2.3Hardware and software of RADAR :-
RF Oscillator Tubes: Magnetrons, Klystron, Thyratron are the popularly used tubes in
weather radars. The thyratrons are generally used as modulator switching devices in transmitters.
Magnetrons are used in conventional radars. After improved technology Magnetrons are used in
DWRs also. Power Klystrons are used in DWRs particularly to achieve high coherence between the
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transmitted and received pulses. Here the Klystrons are used as Amplifiers where the output power is
controlled by modulator circuits.
Wave guides: RF power is transmitted to the antenna using wave guides which are also
Known as travelling wave tubes. Wave guides are metal tubes with rectangular cross section,
madefromaluminium or gun metal. Where ever bends are seen L-bends and U-bends are used.
Flexiblewave guides are also used where-ever links are to be negotiated slightly, during installation.
Antenna and duplexers:A Radar antenna is generally a parabolic dish antenna that issensitive with high gain. It is generally
designed to generate beam of about 1 degree bandwidthfor generating high resolution data sets. The
same antenna is used for transmitting andreceiving the RF Signals. The switching is done by
duplexers. The duplexers allow the receiver tobe cut-off from antenna during transmission to safe
guard the receivers. Circulators are one typeof duplexers and if ferrites are used as core of these
circulators they are known as ferrite circulators.
Receivers: Receivers are divided into two types basically. RF Front end amplifiers are RF Booster amplifiers that
increase the signal strength of received energy. Mixer-amplifiers actuallymix the Received energy with
STALO frequencies and the generated Intermediate Frequency IFis used for further processing. IMD
generally use 10 MHz or 30 MHz as IF frequencies. Someradar uses two stages IF mixing.
Signal Processors:Signal processing is the most complicated of all radar hardware. It involves identifying the echo
properties from the received signals. Algorithms like Pulse pairalgorithms and Fast Fourier
Transformation (FFT) techniques are used for this. The basic output ofthe Receiver consists of
information on Amplitude of the received signal and the Phase of thesignal. From amplitude
information we deduce the intensity of the back-scattered signal and from
Phase information we deduce the radial velocity of the moving targets.
Software :
Vaisala has designed radar systems with the most advanced technology available today which
incorporates the Segment product line. To configure, calibrate and operate a complete weather radar
system we have developed a suite of software tools called IRIS. IRIS has been shipped as a product for
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over 20 years and is the most comprehensive, user-friendly, robust software package in the industry.
This one week course provides an in depth explanation of the IRISsoftware. The new IRIS Software
for radar is a comprehensive software package developed specially for the Penetrator IRIS family of
products. It features real-time Windows based radar data acquisition, data display and signal
processing modules. With Penetrator’s IRIS Software for Windows, it is easy to display process and
analyze data.
The IRIS Software stores several hours of radar data directly on disk for processing and analysis,
while simultaneously displaying raw radar waveforms. Complete automatic processing capability is
provided for bridge deck and pavement inspection applications including delimitation detection in
concrete decks and multi-layer thickness measurement on pavements.
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Chapter-3
RADAR
3.1 Introduction:-
Radar is an object-detection system that uses micro-waves to determine the range, altitude,
direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor
vehicles, weather formations, and terrain. The radar dish (or antenna) transmits pulses of radio waves
or microwaves that bounce off any object in their path. The object returns a tiny part of the wave's
energy to a dish or antenna that is usually located at the same site as the transmitter.
Radar was secretly developed by several nations before and during World War II. The term
radar was coined by united state navy in 1940. Navy as an acronym for Radio Detection and
Ranging. The term radar has since entered English and other languages as a common noun, losing all
capitalization.
Fig.:- 1.1 RADAR Pictures of Thunderstorm on 20-11-1957
The modern uses of radar are highly diverse, including air and terrestrial traffic control, radar
astronomy, air-defence systems, antimissile systems; marine radars to locate landmarks and other
ships; aircraft anti-collision systems; ocean surveillance systems, outer space surveillance
and rendezvous systems; meteorological precipitation monitoring; altimetry and flight control
systems; missile target locating systems; and ground-penetrating radar for geological observations.
High tech radar systems are associated with digital signal processing and are capable of extracting
15
useful information from very high noise levels. Other systems similar to radar make use of other parts
of the electromagnetic spectrum. One example is "LIDAR", which uses ultraviolet, visible, or near
infrared light from lasers rather than radio waves.
3.2 Principles:-
3.2.1 Radar signal
A radar system has a transmitter that emits radio waves called radar signals in predetermined
directions. When these come into contact with an object they are usually reflected or scattered in many
directions. The radar signals that are reflected back towards the transmitter are the desirable ones that
make radar work. If the object is moving either toward or away from the transmitter, there is a slight
equivalent change in the frequency of the radio waves, caused by the Doppler Effect.
Radar receivers are usually, but not always, in the same location as the transmitter. Although
the reflected radar signals captured by the receiving antenna are usually very weak, they can be
strengthened by electronic amplifiers. More sophisticated methods of signal are also used in order to
recover useful radar signals.
The weak absorption of radio waves by the medium through which it passes is what enables
radar sets to detect objects at relatively long ranges—ranges at which other electromagnetic
wavelengths, such as visible light, infrared light, and ultraviolet light, are too strongly attenuated. Such
weather phenomena as fog, clouds, rain, falling snow, and sleet that block visible light are usually
transparent to radio waves. Certain radio frequencies that are absorbed or scattered by water vapour,
raindrops, or atmospheric gases (especially oxygen) are avoided in designing radars, except when
their detection is intended.
3.2.2 Interacting mechanism
If electromagnetic waves travelling through one material meet another, having a very
different dielectric constant or diamagnetic constant, the waves will reflect or scatter from the
boundary between the materials. This means that object, will usually scatter the incident waves from
its surface. This phenomenon has enabled to the detection of aircraft and ships. Radar absorbing
material, containing resistive and sometimes magnetic substances, is used on military vehicles
16
to reduce radar reflection. This is the radio equivalent of painting something a dark colour so that it
cannot be seen by the eye at night.
Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave
and the shape of the target. If the wavelength is much shorter than the target's size, the wave will
bounce off in a way similar to the way light is reflected by a mirror. If the wavelength is much longer
than the size of the target, the scattering phenomenon dominates (Rayleigh). Low-frequency radar
technology is dependent on resonances for detection, but not identification, of targets. This is described
by Rayleigh scattering.
3.2.3 Radar equation
The power Pr received at the antenna is given by the equation:-
Pr=PtGt A rσ(4 π )2 R4
Where
Pt = transmitter power
Gt = gain of the transmitting antenna
Ar = effective aperture (area) of the receiving antenna (most of the time noted as Gr)
σ = radar cross section, or scattering coefficient, of the target
R = distance from the transmitter to the target
In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the
term Rt² Rr² can be replaced by R4, where R is the range.
However for weather radar as the backscattered is a volume target the equation gets modified with
Pr=c ZR2
17
This shows that the received power declines as the fourth power of the range, which means that the
received power from distant targets is relatively very small.
Additional filtering and pulse integration modifies the radar equation slightly for pulse-Doppler
radar performance, which can be used to increase detection range and reduce transmit power.
The equation above with F = 1 is a simplification for transmission in a vacuum without
interference. The propagation factor accounts for the effects of multipath and shadowing and depends
on the details of the environment. In a real-world situation, path loss effects should also be considered.
3.2.4 Doppler Effect
Frequency shift is caused by motion that changes the number of wavelengths between the
reflector and the radar. That can degrade or enhance radar performance depending upon how that
affects the detection process. As an example, Indication can interact with Doppler to produce signal
cancellation at certain radial velocities, which degrades performance.
Doppler measurement is reliable only if the sampling rate exceeds the Nyquist frequency for
the frequency shift produced by radial motion. As an example, Doppler weather radar with a pulse rate
of 2 kHz and transmit frequency of 1 GHz can reliably measure weather up to 150 m/s (340 mph), but
cannot reliably determine radial velocity of aircraft moving 1,000 m/s (2,200 mph).
3.2.5 Polarization
In all electromagnetic radiation, the electric field is perpendicular to the direction of
propagation, and this direction of the electric field is the polarization of the wave. In the transmitted
radar signal the polarization can be controlled for different effects. Radars use horizontal, vertical,
linear and circular polarization to detect different types of reflections. For example, circular
polarization is used to minimize the interference caused by rain.
C- Band Polari Metric Doppler Weather RADAR
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3.3Types of RADAR:-
There are many types of RADAR’s like Tracking Radar’s, Weather Radar’s Etc. Our main
focus is on C-band Pole metric Doppler Weather Radar.
Fig. 2.1 Frequency band
3.4 Introduction to Vaisala Doppler Weather Radar:-
19
Weather radar measures the scattering of microwaves used to determine the intensity of
precipitation. This Doppler Weather Radar is a dual polarization weather radar that operates on the C-
band RF radio frequency (5 cm, 5.6 GHz). It uses a coherent klystron transmitter as the source of
microwaves. As dual polarization radars send vertical and horizontal microwaves, they can measure
parameters needed in analysing the target shape and improving the quality of data. With sophisticated
data processing techniques, the targets can be identified as, for example, rain, hail, or snow. The data
generated by weather radar can be used for the following purposes and applications:-
Weather surveillance network operation
Severe weather monitoring
Hydrometric applications
Flood forecasting
Airport wind shear detection (LLWAS integration)
Hurricane/Typhoon tracking
Hail detection
Weather modification
Meteorological research
Launch support systems
3.5 DWR Hardware:-
Doppler Weather Radar consists of the following parts:
Antenna and pedestal
Site waveguides
Radar cabinet
Main distribution unit
Uninterruptible power source (UPS)
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Fig. 2.2 Doppler Weather Radar System Overview
An overview of Doppler Weather Radar is shown in Fig. 2.2.
Fig. 2.3 DWR Cabinet
1 = Waveguide dehydrator
2 = Power distribution unit (PDU)
3 = Radar receiver
4 = Radar Control Workstation (RCW) and console for local controlling
21
5 = Solenoid power supply
6 = HVPS power (high voltage power supply) for transmitter
7 = Cooling fan
8 = Ventilation ducts
9 = Klystron tube
10 = Solenoid magnet
11 =Oil tank for pulse assembly
3.6 Dual Polarization:-
DWR is based on Dual Polarization. Dual polarization radars can be set to transmit only
horizontally (H) or simultaneously vertically (V) and horizontally (STAR mode). The horizontal and
simultaneous modes are selected with a switch that directs the transmitter power either only to H
channel or to a power splitter that divides the power to both H and V channels. The H and V channels
of the waveguide structure contain circulator sand limiters for receiver protection. A dual channel
rotary joint is used in azimuth and two single channel rotary joints in elevation. At the antenna, the
waveguide channels are connected by an orthomode transducer (OMT) to the antenna feed.
On receive H and V echo signals are separated by the OMT and directed by the circulators through
limiters to the radar receiver that has separate channels for them. From the phase and amplitude values
of the H and V signals, several dual polarization parameters are calculated.
3.7 Transmitter
3.7.1 Introduction:-
For determination of range the radar transmits short duration high-power RF pulses of energy.
These transmitted pulses have also to be highly coherent to sense the Doppler shift. Thus the radar
transmitter is a vital component in determining the radar usability and mode of data acquiring.
3.7.1.1Technical and operating characteristics:
The transmitter must have the ability to generate the required mean RF power and the required
peak power22
•The transmitter must have a suitable RF bandwidth.
•The transmitter must have a high stability to meet signal processing requirements, and in
deciphering the velocity signatures through the measured phase shift
•The transmitter must be easily modulated to meet waveform design requirements.
•The transmitter must be efficient, reliable and easy to maintain and the life expectancy
And cost of the output device must be acceptable.
Fig. 3.1 Transmitter Parts
1 = Inlet duct for air cooling
2 = Centrifugal fan
3 = Klystron tube
4 = Solenoid magnet
5 = Oil tank, contains the pulse assembly
6 = Modulator assembly
7 = Air inlet for transmitter and solenoid magnet cooling
8 = Exhaust duct to the top of the cabinet
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9 = Arc detector
10 = Waveguide
11 = Air inlet for modulator cooling
3.7.2 Block Diagram of Transmitter:-
Fig. 3.2 Transmitter Block Diagram
3.7.2.1Tx module of Radar Processor: -
In a fully coherent solid state radar system, the concept of Band Pass Filter, Intrinsic Automatic
Gain Control, etc are brought in PCBs and fully controlled by a industrial PC. Thus the same
frequency source chain also serves a part of the processing during reception. The module
connected with the transmission path in often termed as Transmission Module. This has a 60MHz
stable frequency out mimicking the transmitted signal for internal analysis.
The RVP8 is a floating-point signal processor implemented in software on a Linux PC. It uses
an "RX" PCI card to receive time series samples of the IF signal sent from the IFD. Optionally
another "TX" PCI card is available to generate the transmit waveform for compressed pulses.
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3.7.2.2 Exciter: -
A radar exciter provides coherent frequency and timing relationships performed by a direct
digital synthesis (DDS), capable of creating high-resolution wideband waveforms for radar system.
Fig. 3.3 Block Diagram of Exciter
The exciter provides fully coherent receiver local oscillator signals at radar frequency band as well as
requisite, auxiliary high frequency clock signals. The exciter function is divided into an internal
frequency synthesizer and an up converter.
Exciter Specifications in DWR:-
3.7.2.3 Transmitter Amplifier:-
25
An amplifier, electronic amplifier or (informally) amp is an electronic device that increases
the power of a signal.
It does this by taking energy from a power supply and controlling the output to match the input
signal shape but with larger amplitude. In this sense, an amplifier modulates the output of the power
supply to make the output signal stronger than the input signal. An amplifier is effectively the opposite
of an attenuator: while an amplifier provides gain, an attenuator provides loss.
It also acts a buffer between the continuous wave sources to that of a pulsed amplifier
(Klystron) on the other side
3.7.2.4 Modulator:-
Radio frequency energy in radar is transmitted in short pulses with time durations that may
vary from 1 to 5 microseconds or more. If the transmitter is cut off before any reflected energy returns
from a target, the receiver can distinguish between the transmitted pulse and the reflected pulse. After
all reflections have returned, the transmitter can again be cut on and the process repeated. The receiver
output is applied to an indicator which measures the time interval between the transmission of energy
and its return as a reflection. Since the energy travels at a constant velocity, the time interval becomes
a measure of the distance travelled (RANGE). Since this method does not depend on the relative
frequency of the returned signal, or on the motion of the target, difficulties experienced in cw or fm
methods are not encountered.
The pulse modulation method is used in many military radar applications. Most radar
oscillators operate at pulse voltages between 5 and 70 kilovolts. They require currents of several
amperes during the actual pulse which places severe requirements on the modulator. The function of
the high-vacuum tube modulator is to act as a switch to turn a pulse ON and OFF at the transmitter in
response to a control signal. The best device for this purpose is one which requires the least signal
power for control and allows the transfer of power from the transmitter power source to the oscillator
with the least loss.
3.7.2.5 Klystron:-
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical
engineers Russell and Sigurd Varian, which is used as an amplifier for high radio frequencies,
from UHF up into the microwave range. Low-power klystrons are used as oscillators in
26
terrestrial microwave relay communications links, while high-power klystrons are used as output tubes
in UHF television transmitters, satellite communication, and radar transmitters, and to generate the
drive power for modern particle accelerators.
Fig.3.4 Single Cavity Klystron
In the Klystron, an electron beam interacts
with the radio waves as it passes through resonant cavities, metal boxes along the length of the tube.
The electron beam first passes through a cavity to which the input signal is applied. The energy of the
electron beam amplifies the signal, and the amplified signal is taken from a cavity at the other end of
the tube. The output signal can be coupled back into the input cavity to make an electronic oscillator to
generate radio waves. The gain of Klystrons can be high, 60 dB (one million) or more, with output
power up to tens of megawatts, but the bandwidth is narrow, usually a few percent although it can be
up to 10% in some devices.
How It Works:-
Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio
frequency power. A beam of electrons is produced by a thermionic cathode (a heated pellet of
low work function material), and accelerated by high-voltage electrodes (typically in the tens of kilo
volts). This beam is then passed through an input cavity resonator. RF energy is fed into the input
cavity at, or nears, its resonant frequency, creating standing waves, which produce an oscillating
voltage which acts on the electron beam.
The electric field causes the electrons to "bunch": electrons that pass through when the electric
field opposes their motion are slowed, while electrons which pass through when the electric field is in
the same direction are accelerated, causing the previously continuous electron beam to form bunches at
the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities.
27
The beam then passes through a "drift" tube in which the faster electrons catch up to the slower ones,
creating the "bunches", then through a "catcher" cavity. In the output "catcher" cavity, each bunch
enters the cavity at the time in the cycle when the electric field opposes the electrons' motion,
decelerating them. Thus the kinetic energy of the electrons is converted to potential energy of the field,
increasing the amplitude of the oscillations. The oscillations excited in the catcher cavity are coupled
out through a coaxial cable or waveguide. The spent electron beam, with reduced energy, is captured
by a collector electrode.
To make an oscillator, the output cavity can be coupled to the input cavity(s) with a coaxial
cable or waveguide. Positive feedback excites spontaneous oscillations at the resonant frequency of the
cavities.
Klystron Transmitter Specifications in DWR:-
3.7.2.6 Solenoid
Klystron is a linear field amplifier, i.e. the accelerated electric beam is affected with a parallel
magnetic field creating a bunching effect. As the klystron is used as an amplifier rather than an
oscillator, it becomes necessary to amplify without any noise amplification. Thus a highly current
stable coil in generating a constant magnetic field is a necessity. Normally electromagnet is used
28
surrounding the klystron tube. The resistance of the solenoid coil is very less <0.2ohm and handles a
current on the order of 20A, thus in continuous operation due to temperature rise, the solenoid PS has a
voltage range of 80-120V to compensate this.
The solenoid is also air cooled to keep it cool-enough to operate within limits.
3.7.2.7 Solenoid Power Supply
The solenoid power supply produces the direct current for the solenoid magnet that focuses the
beam current of the klystron tube. The solenoid power supply also monitors the current consumed.
3.7.3Associated Elements
3.7.3.1Wave guide:-
Waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints.
Principle of operation:-
Depending on the frequency, wave guides can be constructed from either conductive or
dielectric materials. Generally, the lower the frequency to be passed the larger the waveguide is. For
example the natural wave guide the earth forms given by the dimensions between the conductive
ionosphere and the ground as well as the circumference at the median altitude of the Earth are resonant
at 7.83 Hz. This is known as Schumann resonance. On the other hand, waveguides used in extremely
high frequency (EHF) communications can be less than a millimeter in width.
Hollow Metallic Rectangular Wave guides:-
This type of wave guide is used in Doppler Weather Radar. In the microwave region of the
electromagnetic spectrum, a wave guide normally consists of a hollow metallic conductor. These wave
guides can take the form of single conductors with or without a dielectric coating, e.g. the Goubau line
and helical wave guides. Hollow wave guides must be one-half wavelength or more in diameter in
order to support one or more transverse wave modes.
Wave guides may be filled with pressurized gas to inhibit arcing and prevent multiplication, allowing
higher power transmission. Conversely, wave guides may be required to be evacuated as part of
evacuated systems (e.g. electron beam systems).
29
Wave guides are almost exclusively made of metal and mostly rigid structures. There are
certain types of "corrugated" wave guides that have the ability to flex and bend but only used where
essential since they degrade propagation properties. Due to propagation of energy in mostly air or
space within the wave guide, it is one of the lowest loss transmission line types and highly preferred
for high frequency applications where most other types of transmission structures introduce large
losses. Due to the skin effect at high frequencies, electric current along the walls penetrates typically
only a few micrometres into the metal of the inner surface. Since this is where most of the resistive loss
occurs, it is important that the conductivity of interior surface be kept as high as possible. For this
reason, most waveguide interior surfaces are plated with copper, silver, or gold.
FIG. 3.5 Wave guide Structure
Voltage standing wave ratio (VSWR) measurements may be taken to ensure that a wave guide
is contiguous. Poor transmission through the wave guide may also occur as a result of moisture build
up which corrodes and degrades conductivity of the inner surfaces, which is crucial for low loss
propagation. For this reason, wave guides are nominally fitted with microwave windows at the outer
end that will not interfere with propagation but keep the elements out. Moisture can also cause fungus
build up or arcing in high power systems such as radio or radar transmitters.
Moisture in wave guides can typically be prevented with silica gel, a desiccant, or slight
pressurization of the wave guide cavities with dry nitrogen or argon. Desiccant silica gel canisters may
be attached with screw-on nibs and higher power systems will have pressurized tanks for maintaining
pressure including leakage monitors. Arcing may also occur if there is a whole, tear or bump in the
conducting walls, if transmitting at high power (usually 200 watts or more). Wave guide plumbing is
crucial for proper wave-guide performance. Voltage standing waves occur when impedance
mismatches in the wave guide cause energy to reflect back in the opposite direction of propagation. In
addition to limiting the effective transfer of energy, these reflections can cause higher voltages in the
wave guide and damage equipment.30
Wave Guide Specifications in DWR:-
3.7.3.2WG Pressurization Unit:-
Prevents arcing of radio frequency energy. Dry air is essential to prevent arcing of radio frequency
energy, which can damage the wave guide and radar power amplifier. The Wave guide Air Pressure
Unit is designed to provide a continuous supply of clean, dry compressed air that prevents arcing and
improves power handling duties on ground based wave-guide radar systems. Air is filtered and dried to
remove water vapor and potentially damaging particles down to 1 micron in size. Dual compressors
provide a fail-safe security function; in the event of a primary compressor failure, a secondary
compressor maintains pressure to ensure that wave guide system performance is maintained. The unit
interfaces with radar Built in Test Equipment (BITE) to indicate high temperature and low pressure
and also has local “normal operation” alarm neon’s on the fascia panel.
3.7.3.3Cooling subsystems:-
To solve the problems associated with air- and pure water-cooling systems, we use
commercially available insulating oil made from 100% synthetic hydrocarbon oils. The chemical
family designation for this type of oil is Poly alpha Olefin (PAO) Hydrocarbon. This type of oil is
biodegradable and has excellent dielectric properties (23kV/mm) and heat transfer characteristics.
Although the oil’s ability to transport heat is diminished in comparison to water, this oil is used
extensively by the transformer industry for retrofitting transformers filled with mineral-based dielectric
oils. Using a dielectric Oil as the cooling agent presents several advantages:
The entire collector assembly can be immersed in the same oil bath, eliminating the
need for insulating cooling hoses between collector stages and thereby simplifying the
design of the collector.
The oil provides long-term corrosion protection to the collector surfaces.
The cooling loop can utilize a standard pump, standard plumbing components, and
standard hydraulic oil filters.
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The oil is biodegradable and does not need to be treated as hazardous waste.
Dielectric oil is expected to last for the life of the tube, resulting in dramatically re-
duced cost of ownership.
3.7.3.4 Arc detection:-
To ensure proper functioning of the klystron, an arc detection unit is fitted at the output cavity,
based to its status; radar operation is either enabled or stopped.
3.7.3.5 Stub tuner:-
A stub or resonant stub is a length of transmission line or wave guide that is connected at one end
only. The free end of the stub is either left open-circuit or (always in the case of wave guides) short-
circuited. Neglecting transmission line losses, the input impedance of the stub is purely reactive; either
capacitive or inductive, depending on the electrical length of the stub, and on whether it is open or
short circuit. Stubs may thus function as capacitors, inductors and resonant circuits at radio
frequencies.
Stubs work by means of standing waves of radio waves along their length. Their reactive
properties are determined by their physical length in relation to the wavelength of the radio waves.
Therefore stubs are most commonly used in UHF or microwave circuits in which the wavelengths are
short enough that the stub is conveniently small. They are often used to replace discrete capacitors and
inductors, because at UHF and microwave frequencies lumped components perform poorly due to
parasitic reactants. Stubs are commonly used in antenna impedance matching circuits, frequency
selective filters, and resonant circuits for UHF electronic oscillators and RF amplifiers.
Stubs can be constructed with any type of transmission line: parallel conductor line (where they
are called Lecher lines), coaxial cable, strip line, wave guide, and dielectric wave guide. Stub circuits
can be designed using a Smith chart, a graphical tool which can determine what length line to use to
obtain a desired reactance. Stubs can be used to match load impedance to the transmission line
characteristic impedance. The stub is positioned a distance from the load. This distance is chosen so
that at that point the resistive part of the load impedance is made equal to the resistive part of the
characteristic impedance by impedance transformer action of the length of the main line. The length of
the stub is chosen so that it exactly cancels the reactive part of the presented impedance. That is, the
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stub is made capacitive or inductive according to whether the main line is presenting inductive or
capacitive impedance respectively. This is not the same as the actual impedance of the load since the
reactive part of the load impedance will be subject to impedance transformer action as well as the
resistive part. Matching stubs can be made adjustable so that matching can be corrected on test.
A single stub will only achieve a perfect match at one specific frequency. For wideband
matching several stubs may be used spaced along the main transmission line. The resulting structure is
filter-like and filter design techniques are applied. For instance, the matching network may be designed
as a Chebyshev filter but is optimized for impedance matching instead of pass band transmission. The
resulting transmission function of the network has a pass band ripple like the Chebyshev filter, but the
ripples never reach 0dB insertion loss at any point in the pass band, as they would do for the standard
filter.
3.7.3.6Harmonic filters:-
A harmonic filter is used to eliminate the harmonic distortion caused by RADAR. Harmonics are
currents and voltages that are continuous multiples of the fundamental frequency of 60 Hz such as 120
Hz (2nd harmonic) and 300 Hz (5th harmonic). Harmonic currents provide power that cannot be used
and also takes up electrical system capacity. Large quantities of harmonics can lead to malfunctioning
of the system that results in downtime and increase in operating costs. The second harmonic would
have a frequency of 120 Hz; the third harmonic would have a frequency of 180 Hz and so on.
Inside the Harmonic Filter
The harmonic filter is built using an array of capacitors, inductors, and resistors that deflect
harmonic currents to the ground. Each harmonic filter could contain many such elements, each of
which is used to deflect harmonics of a specific frequency.
In a harmonic radar system design, one of the most important components is the filter used to
remove the self-generated harmonics by the high-power transmitter power amplifier, which is usually
driven close to its 1-dB compression point. The obvious choice for this filter is a low-pass filter. The
low-pass filter will be required to attenuate stop band frequencies with 100 dB attenuation or more.
Due to the high degree of attenuation required, multiple low-pass filters will likely be required. Most
commercially available low-pass filters are reflective devices, which operate by reflecting the
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unwanted high frequencies. Cascading these reflective filters causes issues in attenuating stop band
frequencies. We show that frequency diplexers are more attractive in place of reflective low-pass filters
as they are able to terminate the stop band frequencies as opposed to reflecting them
Maximum Unambiguous Range:- following transmit time where they remain unconsidered since the radar equipment isn't
ready to receive during this time, or
Into the following reception time where they lead to measuring failures (ambiguous
returns).
In order to generate a detectable harmonic-radar response from an electronic device, the required
power-on-target is comparable to that observed directly below a cellular base station. Also, the signal
emitted from the target is often very weak. This weak signal must not be masked by harmonics
generated by the radar itself. Thus, high transmit power must be provided with high linearity for
detection of a nonlinear-radar target.
3.7.3.7Dehydrator:-
In weather radar, the dehydrator supplies pressurized, dry air into the waveguide to prevent
condensation or moisture from building up.
3.7.3.8Vacuum Ion PS:-
The radar klystron is a vacuum tube. However in the process of electron amplification through
bunching and acceleration, ions creation is not ruled out. To maintain an ion free environment, a
Vacuum ion PS is used. Whenever the ionic current reaches around 20micro Amperes, the vac ion
triggers a high voltage potential of the order of few kV drawing out the ions.
3.7.4 Built In Test Equipments:-
The Bitex Utility is the built in test equipment of the IRIS software and designed for the remote
monitoring of the RADAR components. In Vaisala Weather RADAR the monitoring of the RADAR
hardware operations is divided into four categories: transmitter, receiver, general and antenna. All
categories are accessed via the Bitex main screen. 34
In the Bitex main screen, the button colors indicate the following:
RED- Alarm
YELLOW- Warning
GREEN- normal operation
FIG.3.6 Bitex Main Screen
Transmitter Screen
The Transmitter screen opens when you click the transmitter button on the Bitex main
screen. In the transmitter screen you can monitor the transmitter operation, reset the transmitter
alarms, test the arc detector operation, and reset the arc detector alarm.
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FIG.3.7 Bitex Transmitter Screen
Klystron Transmitter Control Panels:-
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FIG.3.8 Bitex Klystron Faults Screen
Bitex General Screen
37
FIG. 3.9 Bitex General Screen
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All the BITE units and corresponding parameters are active buttons. On right clicking any of
these buttons in the sub panels, its corresponding histogram (as shown in the above figures) is
displayed, as a time series for ten minutes duration till that particular minute, in a new window. The
graphical display can be a valuable tool when assessing the frequency and endurance of faults. The
time scale for viewing the histogram is adjustable from 10 minutes to 96 hours using the Time Span
slider.
The y-axis scale for analog output is also adjustable with the Vertical Span slider. Each circle
represents the time a status packet was received. The graphical display may be printed to a printer or
file. A new log file is generated each day at midnight and saved in a file. An IRIS Setup question
allows the operator to choose how many days of antenna log files to retain at any given time. Keeping
old log files preserved is helpful for post analysis of problems.
Chapter 439
4.1 Conclusion:-
RADAR is used to find velocity, range and position of the object
Radar transmitter must have the ability to generate the required mean RF power and the
required peak power
The transmitter must have a suitable RF bandwidth.
The transmitter must have a high stability to meet signal processing requirements, and in
deciphering the velocity signatures through the measured phase shift
The transmitter must be easily modulated to meet waveform design requirements.
The transmitter must be efficient, reliable and easy to maintain and the life expectancy
And cost of the output device must be acceptable
We have illustrate the PPI display from a signal Doppler radar band indicate several important
feature of the mesoscale circulation in mesoscale convective system in real time in particular
descending rear inflow jet, the ascending front to rare flow lying just above and low level
mesoscale deference in the strigiform rain region indicating the presence of the mesoscale
downdraft are already urgently clearly evident in Doppler display from a signal radar the
characteristic pattern associated with these feature should be especially useful to meteorologist
and at future NEXRAD radar installation in interpretation of mesoscale storm structure and
behaviour.
4.2 Advantage of RADAR
It is that it provides superior penetration capability through any type of weather condition.
1.High quality of data
Doppler radars are used by various industries because of the high quality of data it provides.
Many countries for example rely on Doppler radars when studying weather patterns and
climate changes. Aside from the estimates in the amount of rainfall, Doppler radars are
also able to give other data like wind velocity which are also important for meteorologists.
2. Reliable weather forecasts
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The data produced by Doppler radars are also considered reliable enough in terms of
forecasting possible heavy rains, thunderstorms, and other types of extreme weather
patterns. This capability is a huge help to various local government units, emergency
service providers, and all the residents of a community that may be hit by a violent storm
for example. With Doppler radar data, enough warnings can be issued to people before the
actual storm will hit.
3. More accurate results
Doppler radars are also used in the aviation industry and provide accurate results in terms
of managing traffic in an airport for example. With many planes trying to land, take off, or
taxi on a busy airport, Doppler radars are able to help air traffic controllers give a smooth
flow of the airplanes and promote safety for all people involved.
Doppler radars are considered highly-sensitive and so maintaining them may also incur
some huge costs. But the benefits they provide totally outweigh maintenance costs which
are why many industries continue to use them for various needs.
4.3 Limitations of Doppler Weather RADAR:-
The radar though can detect wind it is only through the radial component, thus anything moving perpendicular to the beam is sensed as zero velocity.
If the Doppler radar happened to be single polarized Tx, the limitation of identifying the scattered distribution and size variations is not feasible.
Based to the selected operational frequency, the target delectability is also determined. The current radar cannot sense fog.
All weather radars are normally with pencil beam operation thus scan time from the lower to the upper limit takes appreciable limits for the cloud to get modified. Similarly the cone of silence and not scanned area also matters.
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