01-07-2014, 04:36 PM
ALL INDIA RADIO
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INTRODUCTION
ALL INDIA RADIO is an institute in the field of broadcasting and communication.As India’s National Broadcaster and also the premier public service broadcaster, All India Radio (AIR) has been serving to inform, educate and entertain the masses since its inception, truly living up to its motto – ‘BahujanHitaya : BahujanSukhaya’. One of the largest broadcasting organizations in the world in terms of the number of languages of broadcast, the spectrum of socio-economic and cultural diversity it serves. AIR had a network of six stations and a complement of 18 transmitters covering
10 percent area of the country. AIR’s home service comprises 376 stations today located across the country, reaching nearly 92% of the country’s area and 99.19 % of the total population. AIR originates programming in 23 languages and 146 dialects.
AIR Jodhpur is a major centre in Rajasthan in field of rural, cultural, and informative programs. It is broadcasting two channels:
1. Primary channel.
2. FM carrying VividhBharati Commercial Broadcast service.
It is having two separate transmitters for both channels.
OBJECTIVES OF AIR
To provide information, education and entertainment, for promoting the welfare and happiness of the masses (BahujanaHitayaBahujanaSukhaya), All India Radio strives to:-
• Uphold the unity of the country and the democratic values enshrined in the constitution.
• Present a fair and balanced flow of information of national, regional, local andinternational interest, including contrasting views, without advocating any opinion orideology of its own
ABOUT AIR JODHPUR
All India Radio, Jodhpur is the major transmitting station in the west area of Rajasthan.
• The 2X300 KW HMB-140 BEL transmitter operating at the medium wave frequency of 531 KHz uses a self-radiating guyed mast of 168 meters height.
• The programs from Jaipur are received through satellite Radio Networking.
• The transmitters covers fully or partially about 18 districts of Rajasthan which is about 160000 square km area and 260 lakhs population of the state of Rajasthan.
STUDIO TRANSMITTER LINK
A studio-transmitter link (or STL) sends a radio station’s signal from the broadcast studio to a radio transmitter in another location.
This is often necessary because the best locations for an antennaare on top of a mountain, where a much shorter tower is required, but where a studio is completely impractical. Even in flat regions, the center of the station's allowed coverage area may not be near the studio location or within a populated area where a transmitter would be frowned upon by the community, so the antenna must be placed several miles or kilometers away.Depending on the locations microwave link of 1489 MHz frequencyis used.
FM TRANSMITTER
INTRODUCTION
The VHF FM transmitter is designed to meet the performance standards required for mono and stereo sound broadcasting application over band-II channel frequency.The transmitter adopts the advance trend in design such as use of solid state VHF power amplifier, VHF exciter with synthesizer technique permitting fast frequency selection in 10 KHz
FEATURES
The system has following features:
• The system is fully solid state design conforming to the state of art technology in the field of sound broadcasting equipment.
• The VHF exciter equipped with stereo coder sends a frequency modulated mono or stereo signal in the frequency range 87.5MHz to 108MHz.
• Overall efficiency of the total transmitter is high to economize the powerrequirement
EXCITER
The exciter equipped with stereo coder sends a frequency modulated stereo signal in the frequency range 87.5 MHz to 108 MHz the transmitter frequency is crystal control and set in step of 10 KHz over the entire frequency range using the synthesizer
VHF POWER AMPLIFIER
The RF output power of the exciter is distributed in phase to the amplifier via powerdivider.RF power transistor used in the power amplifier assembly contain Beryllium Oxidematerial if not properly handled can cause serious hazard to health. Two amplifier of 1.5 KWpower each; supply power to dual power coupler with harmonic filter
VOLTAGE STABILIZER
A 3 phase voltage stabilizer is supplied along with the FM Transmitter system. This unit supplied very stable line voltage to transmitter rack and automatic dehydrator.
AUTOMATIC DEHYDRATOR
This unit is mainly used for pressuring the RF feeder cable and to dehydrate the moisture collected inside the feeder cable after sometime. Pressurization and dehydration of feeder cable helps to maintain the low VSWR and low RF feeder cable power loss
MEDIUM WAVE TRANSMITTER (300 KW)
The purpose of transmitter is to broadcast an audio frequency signal by amplitude modulation of carrier in the medium wave range.Use of transmitter is simple and entirely automated. The equipment, control system and transmit parameters are automatically control by
The system, operation and maintenance are control through a man machine interface. The transmitters are designed to ensure personal safety. The amplifier block in autonomous and self protected.
A.M. transmitter of any power in general will have aseparate RFand AF stages. In the conventional transmitters, Vacuum Tubes are used right from the first stage to the final stage and the preliminary stages are Solid State Devices
CRYSTAL OSCILLATOR
To oscillate at a consistent frequency, the crystal is kept in an oven. The temperature of the oven is maintained between 68 to 72oC and the corresponding indication is available in the meter panel. Crystal oven is heated by +12 V. One crystal oscillator with a standby has been provided. It gives an output of 5 V square wave which is required to drive the Transistor Power Amplifier. The crystal oscillator works between 3 MHz and 6 MHz for different carrier frequencies.Different capacitors are used to select different frequency ranges
RF POWER AMPLIFIER
CQK - 50, condensed vapour cooled tetrode valve is used as a PA stage. High level anode modulation is used, using a class B modulator stage. The screen of the PA tube is also modulated by a separate tap on modulation transformer. Plate load impedance of the PA stage is about 750 ohms and the output impedance is 120 ohms, and it is matched by L-C components
AF CHAIN OF TRANSMITTER
The AF stage supply the audio power required to amplitude modulate the final RF stage. The output of the AF stage is superimposed upon the DC voltage to the RF PA tube via modulation transformer.Anauxiliary winding in the modulation transformer, provides the AF voltage necessary to modulate the screen of the final stage. The modulator stage consists of two CQK-25 ceramic tetrode valves working in push pull class B configuration. The drive stages up to the grid of the modulator are fully transistorized.
12 AF PRE-CORRECTOR
Pre-amplifier output are fed to the AF Pre-correctors. As the final modulator valve in the AF is operating as class B, its gain will not be uniform for various levels of AF signal. That is the gain of the modulator will be low for low level, input, and high for high level AF input because of the operating characteristics of the vacuum tubes. Hence to compensate for the non-linear gain of the modulator. The pre-corrector amplifies the low level signal highly and high level signal with lowgain.Hum compensator is used to have a better signal to noise ratio.
AF DRIVER
Two AF drivers are used to drive the two modulator valves. The driver provides the necessary DC bias voltage and also AF signal sufficient to modulate 100%. The output of AF driver stage is formed by four transistor in series as it works with a high voltage of about -400 V. The transistors are protected with diodes and zener diodes against high voltages that may result due to internal tube flashovers. There is a potentiometer by which any clipping can be avoided such that the maximum modulation factor will not exceeded.
TRANSMISSION LINES
INTRODUCTION
R.F. energy of a transmitter is guided up to radiator (mast) by the propagation of transverse electro-magnetic waves along systems of parallel conductors called ‘Transmission lines or feeder lines’. The input energy is stored in the field of conductors and is propagated along the system at some finite velocity.
It is essential to keep the antenna at a distance from transmitter due to prevent
• Radiation hazard
• Pick up from antenna and consequent problem with transmitter circuit normally this distance is either on 50 V/m field strength contour or minimum half the wavelength at frequency of operation.
The feeder line should carry the power from the transmitter to Antenna with
• Minimum loss
• Minimum radiation.
TYPES OF FEEDER LINE
On basis of circuit, they are:
• Balanced lines: Where there are equal and opposite potential in both wires.
• Unbalanced lines: Here one wire is at high potential and the other side is at low potential
BASIC APPLICATIONS OF FEEDER LINE
• To guide energy from transmitter to antenna. In this mode energy move along the lines in a single travelling wave.
• For Storing energy in excess of that dissipated in load, in the form of standing waves.
LOSSES IN THE FEEDER LINES
There are four types of losses. They are:
• Copper Loss: It is due to the heating of conductor.
• Earth Loss: It arises due to imperfect earth conductivity.
• Insulation Loss: It is due to insulation loss and is minor in a well-designed system
CHOICE OF FEEDER LINE IMPEDENCE
When the feeder line impedance is chosen low, feeder current will be more, resulting increase in copper loss and earth loss. When feeder line impedance is high, feeder voltage will be high resulting in the use of higher voltage rating insulators. So the choice depends upon the availability of components and technology in use.
In AIR, following types of feeder lines are used.
• 230 ohm 6 wire (open wire) lines – for all old 100 KW as well as 10/20 KW.
• 60 ohm quasi coaxial feeder line - megawatt of Chinsuraha, Rajkot and Nagpur.
• 120 – ohm quasi coaxial feeder - all 300 KW and all 100/200 KW new version
MEDIUM WAVE ANTENNA
The signals from ATU are fed to Marconi type antenna. It is 168 m high and supported on insulators and guys. In AM transmitter full antenna is used for transmission. The signals propogates parallel to the ground. MW propogates along the surface of the earth. Because of the differaction the wavefront gradually tilts over. As the wave propogates over the earth it tilts over more and the increasing tilts causes greater short circuiting of the electric field component of the wave and hence field strength reduces and it will vanish at some distance
INTRODUCTION
When the electromagnetic waves in the medium wave (MW) range are directed towards the ionosphere, they are absorbed by the D-region during the day time and are reflected from the E layer during the night time, which may travel longer distances to cause interferences. The wave length of MW signals are very large, of the order of few hundred meter, and therefore the antenna cannot be mounted a few wavelengths above the earth to radiate as space waves. MW antenna, therefore, have to exist close to the surface of the earth and the radio waves from them have to travel close to the earth as ground waves. If the electric vector of such MW radiation is horizontal, they will be attenuated very fast with distance due to the proximity of the earth. MW antenna have to be placed vertically, so that they radiate vertically polarised signals.It is for this reason, all the MW antenna are installed vertically close to the ground. Directional antenna systems also exist in many All India Radio stations
NEED OF EARTH RADIALS
The MW propagates close to the earth as ground waves. The MW mast also is placed close to the ground. The electric field in the mast extends from the top to the ground. Current density of typical λ/2 tower being significant upto 0.37 to 0.4 wave length and current flows through the ground back to the mast. The electric field passes through the ground. The earth usually is not a perfect conductor and field may be attenuated. In order to improve the earth conductivity when it takes off from the mast the conductivity of the earth around the mast is artificially increased by burring about 120 radial copper wires. The radial wires are suitably brazed among them forming a mesh
[b]GUY SUPPORT FOR MW MAST[/b]
The guy wires are used at a number of levels depending upon the height of the tower, its cross section, the maximum wind velocity expected in that region etc. The guy wires have to be insulated from the mast so also the guys are broken into a number of small sections λ /10 or λ /12 separated by low loss, high mechanical strength insulators to minimize distortion of radiation pattern due to field induced in them. These insulators are shunted by suitable inductors to provide dc path for lightning discharges while at the same time blocking the MW energy from earthing. A high resistance shunt across the insulator is another method of allowing the static leaks. Some types of insulators have built in thyristers which provide low resistance to high static charge while presenting high resistance for low voltages. Ultraviolet detectors which is sensitive to arcs or spark overs may also be used to activate the protective devices in the transmitter.
Directional MW antenna, using more than one vertical mast exist in a number of stations like Jalandhar, Nagpur (National Channel) in the network. Special care must be taken to allow for proper bandwidth of the directional antenna system.They guy tensions are usually given in the completion report. It is necessary to measure the guy tensions as per AIR technical manual to ensure the verticality or absence of twist in the mast. Measurement of verticality and twist of the mast are also required to be carried out as per AIR Technical Manual. Loss of verticality will affect the range of the service due to earth's proximity.
COOLING ARRANGEMENT
INTRODUCTION
In modern A.M. transmitters power valves are used in the PA and modulator stages, which are condensed vapor cooled ceramic tetrodes. In the old generation transmitters, triodes are used in the PA, modulator and exciter stages. Both the tetrodes and triodes tubes are capable of being operated at high voltages (11 KV DC) and large anode current of the order of 50 A. They also draw large filament current of about 620 A at 24 volt CQK-350. Hence the tubes dissipate large amount of power which require effective cooling.
COOLING SYSTEM USED IN TRANSMITTER
In high power A.M. transmitter, lot of power is dissipated in the valve as the input power is not fully converted into output R.F. power due to the efficiency of the amplifier which never reaches 100%. Hence the valves have to be cooled. In addition filaments are drawing large current of the order of 210 A at 10 volt for CQK valve. Hence they also have to be cooled. The dissipated heat in the valves also circulates in the concerned cubicle and heat develops there. Hence some kind of cooling has to be provided to the transmitting equipment. Different types of cooling are used in AIR (Jodhpur) transmitter at present.
CONCLUSIONS
The training at radio broadcasting station indeed adds knowledge of wide concept of wireless communication especially for broadcasting purpose. The study of operation and maintenance of the studio consoles and the medium wave high power transmitter of the as well as FM low power transmitter help in relating theoretical concept of communication with the practical one.
Also, the measuring instruments used there give better understanding of the monitoring and control of the audio signal as well as modulated signal.
It was very satisfactory period of the training where I gained very useful overall training. It is also provided me an opportunity to developed myself as an engineer with the competitive edge