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  1. 1. A Seminar Report on 2*100 KW MEDIUM WAVES TRANSMITTER At All India Radio Ajmer Submitted In partial fulfillment For the award of the Degree of Bachelor of Technology In Department of Electronics and Communication Engineering Submitted To: Submitted By: Sanjeev Yadav Manish Kumar Sharma Assistant Professor 10EEAEC041 Engineering College Ajmer Department of Electronics and Communication Engineering GOVT ENGINEERING COLLEGE AJMER September 2013
  2. 2. i ACKNOWLEDGEMENT It was not possible to prepare this training report without the assistance and guidance of other people. On the very outset of this report, I would like to extend my sincere and heartfelt obligation towards all the personages who have helped me in this endeavor. I am ineffably indebted to our principal, training and placement officer and teachers who infused me with the spirit to work upon challenging field which has its inception in such a time when there is a dire need for new orientation. I am thankful to the management of All India Radio, Ajmer for allowing us to take training in their concern. I am greatly indebted to our supervision and guide Mr. K C Bhati who conceived detailed and superior guidance throughout our training. Without their ungrudging cooperation it would not have been possible to complete this training. Last but not the least; our efforts could never meet this success without the blessing of God and our families. We can never think of repaying their affection, care and encouragement without which it would have been difficult to reach the shores. Manish Kumar Sharma 10EEAEC041 VII SEM
  3. 3. ii ABSTRACT Prasar Bharti is a statutory autonomous body established under the Prasar Bharti Act and came into existence on 23.11.1997. It is the Public Service Broadcaster of the country. All India Radio (AIR), is the radio broadcaster of India and a division of Prasar Bharti (Broadcasting Cooperation of India), it is the sister service of Prasar Bharti’sDoordarshan. Report includes the description of all the departments of All India Radio, Ajmer. First block includes the power supply arrangements of Akashvani. AIR Ajmer derives power supply of 11kVA from two sources (one at a time):- 1. Madar Feeder 2. Gagvana Feeder However a Diesel Generator is also present as an alternative. It includes the thyristor rectifiers generally used for AC to DC conversion. Next section was the cooling section. It included the transmitter cooling and Air conditioning system. Water being used in transmitter cooling is deionized and purified in water purifier plants. AIR Ajmer has two 100kW transmitters connected in parallel. Combined output is of 200kW. Each transmitter section includes the RF and AF chains and the circuits for removing harmonics. Audio signal is obtained from Jaipur studio through C-Band. S-Band is used for reception of short wave signals transmitted from Delhi and other capital stations. The most important one was the mast/antenna along with the antenna tuning unit. ATU is used for impedance matching purpose. Antenna used here is of BEL HMB 120. Its physical height is of 200 m while electrical height is 248m. It is a self-radiating antenna. We dealt with the basics of Satellite Communication and antennas.
  4. 4. iii TABLE OF CONTENTS CHAPTER CONTENTS PAGE No. No. Cover page Certificate Acknowledgement i Abstract ii Table of Contents iii-vi List of Figures vii-viii List of Tables ix 1. ALL INDIA RADIO: AN INTRODUCTION 1-5 1.1 Introduction 1 1.2 Prasar Bharti 2 1.3 Infrastructure 2 1.4 High Power Transmitters 3 1.5 ATU, Mast and Aerial Field 3 1.6 Land 3 1.7 Power Supply 3 1.8 Three tier broadcasting 4 1.9 Broadcast Coverage 5 2. POWER SUPPLY 6-9 2.1 Potential Transformer(PT) 6 2.2 Current Transformer(CT) 6
  5. 5. iv 2.3 Circuit Breaker 7 2.3.1 Air Circuit Breaker 7
  6. 6. v 2.3.2 Oil circuit breaker 8 2.4 Line Isolator 9 2.5 Power Supply Arrangement Description 9 3. MODULATION 11-14 3.1 Modulation 11 3.2 Need for Modulation 11 3.3 Types of Modulation 11 3.3.1 Amplitude Modulation 11 3.3.2 Angle Modulation 12 3.3.3 Phase Modulation 14 4. SATELLITE COMMUNICATION 15-19 4.1 Satellites 15 4.2 Radio Networking Terminal 16 4.2.1 Outdoor unit 16 4.2.2 Indoor unit 19 5. MEDIUM WAVE TRANSMITTER 20-24 5.1 Introduction 20 5.2 Radio frequency chain 20 5.2.1 Crystal Oscillator 20 5.2.2 Transistor Power Amplifier 20 5.2.3 RF Driver 21 5.2.4 Various Pin Voltages 21 5.2.5 RF Power Amplifier 21 5.2.6 PA output circuit 22 5.3 AF Stage 22 5.3.1 High Pass Filter 23 5.3.2 AF Pre-amplifier 23 5.3.3 AF Pre-Corrector 23
  7. 7. vi 5.3.4 AF Driver 23 5.3.5 AF Final Stage 23 5.4 Transmitter Salient Features 24 6. PARALLEL OPERATION OF TRANSMITTERS 25-27 6.1 Need for Parallel Operation 25 6.2 Requirements 25 6.3 Procedure for Tuning and Combining 27 7. TRANSMISSION LINES 28-31 7.1 Introduction 28 7.2 Basic Transmission Lines 28 7.3 Types of Feeder Lines 29 7.3.1 Open Wire Feeder Lines 29 7.4 Choice of Feeder Line Impedance 30 7.4.1 Measurement Of Characteristic Impedance, Zo 30 7.5 Power Transmission Capacity of Feeder Line 31 8. COOLING TECHNIQUES IN TRANSMITTER 32-36 8.1 Introduction 32 8.2 Cathodes 32 8.3 Screen Voltage 32 8.4 Cooling System Used in Transmitter 32 8.4.1 Air Cooling 33 8.4.2 Vapour cooling 33 8.4.2 Condensed Vapor Cooling in HMB-140BEL 100 34 kW MW XTR 9. ANTENNA TUNING UNIT 37-41 9.1 Introduction 37
  8. 8. vii 9.2 L Networks 37 9.3 Pi Networks 38 9.4 T Network 40 10. PRINCIPLES OF ANTENNA SYSTEM 42-50 10.1 Introduction 42 10.2 The Function of an Antenna 42 10.2.1 How an Aerial Works 43 10.3 Basic Aerials 44 10.3.1 Hertz Aerial 44 10.3.2 Marconi Aerial 45 10.4 Antenna Radiation Resistance 46 10.5 Radiation Efficiency 47 10.5.1 Field Regions 47 10.5.2 Isotropic Antenna 47 10.6 Radiation Pattern (Polar Diagram) 47 10.7 Half Power (3 dB) Beam Width 48 10.8 Polarizations 48 10.9 Aperture of an Antenna 'A' 49 10.10 Snapshot 50 11. EARTHING SYSTEM 51-56 11.1 Earthing 51 11.2 Methods of Earthing 51 11.3 Measurement of Earth Resistance 51 REFERENCES 57
  9. 9. viii
  10. 10. ix LIST OF FIGURE FIGURE No. TITLE PAGE No. 1.1 AIR Logo 1 1.2 Prasar Bharti 2 1.3 Three Tier System 4 2.1. Current and Potential Transformer 6 2.2 Air circuit Breaker 8 2.3 Oil Circuit Breaker 8 2.4 OCB & ACB 9 2.5 Transformer Setup 10 3.1 Amplitude Modulation 11 3.2 Frequency Modulation 12 3.3 Envelope detector 14 4.1 Satellite Communication 16 4.2 Block Diagram of S-Band RN Receive Terminal (RNT) 18 4.3 Snapshot of PDA 19 5.1 Block Diagram of RF Chain (HMB-140) 20 5.2 PA Output Circuit (HMB-140) 22 5.3 AF Stage (HMB-140) 22 6.1 Bridged “T” Networks 26 6.2 Series to Parallel conversion 26 7.1 Zo of a Feeder Line 29 7.2 Open wire transmission line 29 7.3 Transmission Lines On a Tower 31 8.1 Vapour Cooling System 34 8.2 Water Flow Circuit 100 kW MW XTR 35 8.3 Cooling Pipes 37 9.1 Phase lag (Rin> RL) 38 9.2 Phase lag (RL>Rin) 38 9.3 Phase lead (Rin> RL) 38 9.4 Phase lead (RL>Rin) 38
  11. 11. x 9.5 Pi Network 39 9.6 T Network 41 10.1 The Aerial Converts Electrical Power Into Electro-magnetic 42 Waves 10.2 Half Wave Dipole Aerial 43 10.3 Electric Field Surrounding an Aerial 43 10.4 Magnetic Field Surrounding an Aerial 44 10.5 Basic Aerials 45 10.6 MW Antenna Isolated from ground 46 10.7 Antenna Radiation Pattern Top View and Side View 48 10.8 Antenna and Guy Wire Insulator 50 11.1 Typical Arrangement of Pipe Earthing 53 11.2 Typical Arrangement of Plate Earthing 54 11.3 Testing of Earth Electrode Resistance 55 11.4 Earth Tester 56
  12. 12. xi LIST OF TABLES TABLE No. TITLE PAGE No. 5.1 Transmitter Salient Features 24
  13. 13. xii
  14. 14. 1 Chapter.1 ALL INDIA RADIO: AN INTRODUCTION 1.1 INTRODUCTION All India Radio (AIR) ,one of the largest radio networks in the world is a division of Prasar Bharti (Broadcasting Cooperation of India) currently working under the Chairmanship of Smt. MRINAL PANDEY and Shri JAWAHAR SIRCAR as its Chief Executive Officer Established in 1936, today it is the sister service of Prasar Bharti’s Doordarshan, the national television broadcaster. All India Radio is also known as Akashvani. The head quarter is at the Akashvani Bhavan, New Delhi. AIR today has a network of 237 broadcasting centers with 149 medium frequency (MW), 54 high frequency (SW) and 177 FM transmitters. The coverage is 91.85% of the area, serving 99.18% of the people in the largest democracy of the world. AIR covers 24 Languages and 146 dialects in home services. In External services, it covers 27 languages; 17 national and 10 foreign languages Fig.1.1 AIR Logo
  15. 15. 2 1.2 PRASAR BHARTI Prasar Bharti has mainly two mainly two branches- Fig.1.2 Prasar Bharti 1.3 INFRASTRUCTURE All India Radio came to be known as Akashvani from 1957. From a meager 18 Transmitters in 1947 AIR acquired 46 by the end of 1st plan, 59 by 2nd plan, and 110 by the end of 8th plan. Total number of transmitters has gone up to 337, consisting of 144 MW, 54 SW and 139 FM transmitters. The number of radio stations went up to from 6 in 1947 to 215 by Dec-2004.AIR took over radio station being run by native states since British days such as Akashvani Mysore, Hyderabad radio, Radio Kashmir etc. PRASAR BHARTI ALL INDIA RADIO DOORDARSHAN
  16. 16. 3 1.4 HIGH POWER TRANSMITTERS These stations are equipped with short wave/ medium wave transmitters together with extensive aerial system to serve the external, home and news services of All India Radio. The main function of these centers is to transmit the programs produced at nearby studios and also from Delhi studios. 1.5 ATU MAST AND AERIAL There is a 200 m height ECIL makes Self Radiating Guyed Mast. There are six numbers of Guys on different height segments of this mast. The feeder lines, ATU, mast and aerial field are being maintained in proper condition. There is a permanent security wall with fencing around the Transmitters Complex site .However, the Ariel field is separately fenced with security wall. 1.6 LAND The Transmitters Land comprising of 140 bigha (approx. 23, 86,560sq.ft) is on lease from Government for an amount of Rs.140/-annum. The land is on Jaipur-Ajmer national highway. 1.7 POWER SUPPLY There are 2 numbers of 11KV Overhead HT Feeders from Rajasthan state Electricity Board, namely Madar feeder and Gagwana feeder. These 11 KV overhead feeders comes to the G.O.SWITCH,LOCATED NEAR THE MAIN GATE OF Transmitting complex the HT metering panel is also located near this, and these are maintained by RSEB.From G.O.Switch the HT Power Supply comes to the 2 independent 11 KV OCB’s through underground cables. From these OCB’s the HT power supply is connected to the 2 nos.750KVA capacity HT transformers through independent isolators. Here it may be mentioned that at one time anyone OCB with the corresponding isolator can be connected to the 1HT transformer.
  17. 17. 4 1.8 THREE-TIER BROADCASTING SYSTEM AIR has a three-tier system of broadcasting. These three levels of programmes are the National, Regional and Local each having distinct audiences. Fig.1.3 Three Tier System National programmes are broadcast from Delhi for relay by the Capital, Regional and Local Radio Stations. Some of these are the National Programme of Talks and Features in Hindi and English, the National Programmes of Drama and Music. • The National Channel of All India Radio located in Delhi broadcasts programmes which are heard on Medium Wave and also on Short Wave. Started on 18th May 1988, this channel works as a night service from 6.50 PM to 6.10 AM the next morning. Broadcasting in Hindi, Urdu and English, the programme composition of the channel has been designed to make it representative of the cultural mosaic and ethos of the country. • The Regional Stations in different States form the middle tier of broadcasting. They originate programmes in the regional languages and dialects. Regional Channels are located in the major linguistic-cultural region of every state. 116 Regional Channels are spread over 29 states & 6 Union Territories including the North-Eastern Service at Shillong that projects the vibrant cultural heritage of the North-Eastern region of this country. The Regional Channels, broadcast largely on the Medium Wave frequency, follow a composite programme pattern comprising of music - classical, light, folk and film, News and Current Affairs, Radio plays, features, Farm and Home programmes, programmes on Health & Family Welfare and programmes for Woman, Children etc. Three-tier Broadcasting System National Regional Local
  18. 18. 5 • Local Radio is relatively a newer concept of broadcasting in India. Local radio stations serve small communities, showcase local culture and broadcast area specific programmes for the benefit of the community. The transmission is in the FM mode. The programming is flexible and spontaneous and the stations function as the mouth piece of the local community. At present there are 86 Local Stations spread across the country. 1.9 BROADCAST COVERAGE a) By area 91.42% b) By Population 99.13% Chapter.2 POWER SUPPLY
  19. 19. 6 There are two 11KV Overhead HT Feeders from Rajasthan state Electricity Board, namely Madar feeder and Gagwana feeder. Various terms related to the Power Supply Arrangement are: 2.1 POTENTIAL TRANSFORMER (PT) The PT has large numbers of turns in primary and a small number of turns in secondary. It is connected across the line. Potential transformer is also used to step down from rated voltage/1.732 to 110volts/1.732. It is connected between phase and neutral. 2.2 CURRENT TRANSFORMER (CT) Current transformer is used for measuring and protection purpose. It is used to step down from rated current (40 A) to 5A. It is connected in series across conductors. The CT has a single turn primary and some few numbers of secondary turns, and it is ALWAYS kept short circuited. Using two CTs and PTs measurement of power in a three phase system can be obtained correctly. Fig.2.1 Current and Potential Transformer 2.3 CIRCUIT BREAKER: The devices used for making and breaking an electrical circuit under some predetermined condition are called circuit breakers. The functions of a circuit breaker are as follows:
  20. 20. 7 1. It must close on and carry full load currents for long period. 2. It must open automatically to disconnect the load, on over load under predetermined condition. 3. It must rapidly interrupt the heavy current, which may flow under a short circuit condition in any part of the system. 4. The circuit breaker must be capable of withstanding the effect of arcing at its contact and the thermal conditions, which arise due to flow of current. All circuit breakers consist essentially of pairs of matting contacts, each pair comprising fixed and moving elements. Under normal conditions, these elements are in contact and carrying full load current; but on receipt of a tripping signal initiated by hand or protective gear, the circuit will be interrupted. At the start of the separation, an arc will be established which is required to be extinguished as early as possible. Generally, we come across two types of circuit breakers at medium and high voltage, for indoor application. They are called Oil Circuit Breaker (OCB) and Air Circuit Breaker (ACB). 2.3.1 Air circuit breakers An air circuit breaker is that kind of circuit breaker which operates in air at atmospheric pressure. In air circuit breakers, the arc exists in the mixture of nitrogen, oxygen and metallic vapor and the successful arc interruption takes place due to cooling by diffusion. Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Often used for main power distribution in large industrial plant.
  21. 21. 8 Fig.2.2 Air circuit Breaker 2.3.2 Oil circuit breaker– In oil circuit breaker the fixed contact and moving contact are immerged inside the insulating oil. Whenever there is a separation of current carrying contacts in the oil, the arc is initialized at the moment of separation of contacts, and due to this arc the oil is vaporized and decomposed in mostly hydrogen gas and ultimately creates a hydrogen bubble around the arc. This highly compressed gas bubble around the arc prevents restriking of the arc after current reaches zero crossing of the cycle. Fig.2.3 Oil Circuit Breaker
  22. 22. 9 Fig.2.4 OCB & ACB 2.4 LINE ISOLATOR: The line isolator is used to isolate any RF (common mode) that may have worked its way back to the station i.e. Keep that RF off of the outside of the transmission line and delivered to the antenna where it belongs. 2.5 POWER SUPPLY ARRANGEMENT DESCRIPTION: 1. From the two incoming feeder lines, one of them is selected using the G.O.Switch. 2. The incoming high voltage (11 KV) is stepped down to 110 V using a Potential Transformer (PT). 3. The Current Transformer further reduces the current from 40 A to 5 A. 4. Further the two incoming feeder lines are interlocked mechanically using Kassel key. Oil Circuit Breakers are used in it.
  23. 23. 10 Fig.2.5 Transformer Setup Chapter.3 MODULATION
  24. 24. 11 3.1 Modulation Modulation is a process of superimposing information on a carrier by varying one of its parameters (amplitude, frequency or phase). 3.2 Need for Modulation • Modulating the signal over higher frequency can reduce antenna size. • To differentiate among transmissions (stations) • Maximum to minimum frequency ratio can be reduced to minimum by modulating the signal on a high frequency. • 3.3 Types of Modulation In general, there are three types of modulation: a) Amplitude Modulation b) Angle Modulation c) Pulse Modulation 3.3.1 Amplitude Modulation If the amplitude of the carrier is varied in accordance with the amplitude of the modulating signal (information), it is called amplitude modulation. This modulation has been shown in fig Fig. 3.1 Amplitude Modulation Bandwidth BW (fc fm) (fc fm) 2fm
  25. 25. 12 Angle Modulation Variation of the angle of carrier signal with time results in angle modulation. It is of two types a) Frequency Modulation b) Phase Modulation 3.3.2 Frequency Modulation If the frequency of the carrier is varied in accordance with the amplitude of the modulating signal (information), it is called frequency modulation. This has been shown in figure 3.2. Power This holds true for any type of a modulating signal and for any value of modulation index. 1 Maximum frequency corresponding to positve peak amplitude of modulation signal Fig . 3.2 Frequency Modulation Minimum frequency corresponding to negative peak amplitude of mod. signal Carrier freq. f c Amplitude A c Modulating Signal freq. f m amplitude A m Freq. Modulated signal Vc( )t Vm(t ) V(t)
  26. 26. 13 Band width of FM signal Band width can be defined as that frequency range which accommodates the carrier and the closest side bands contributing at least 98% of the total signal power. (Carson rule). BW = 2 (mf + 1) fm = 2 (mffm + fm) = 2 ( f + fm) Detection of AM Signal There are many types of detectors for detecting various form of AM signals. One such detector is Envelope Detector, which is used for detection of DSB - FC signal. The operation of this detector is discussed below: Envelope Detector The circuit diagram of envelope detector along with wave form is shown in Fig. 14 (a) and (b). During positive half cycle of the input, capacitor C charges to almost the peak value of the half cycle. When the level of input voltage starts falling, the diode D is reversed biased by the voltage on the capacitor. The capacitor discharges through the resistance R with a time constant RC. The discharging of the capacitor continues, till, in the next positive part of the carrier cycle, the input voltage exceeds the capacitor voltage. Then, the diode conducts again and the whole process repeats. Thus, in each carrier cycle, the capacitor charges to nearly the peak value of the cycle. The practical circuit will be somewhat different from the circuit of fig. 3.3. pc A2 2 c Power of the unmodulated carrier Where, Ac = Amplitude of the carrier Am = Amplitude of the modulating signal mf f fm f f Modulation Index fm fm Frequency deviation Frequency of the modulating signal
  27. 27. 14 D Output (a) Fig. 3.3 Envelope Detector 3.3.3 Phase Modulation If the Phase of the carrier is varied in accordance with the amplitude of the modulating signal (information), it is called phase modulation. Chapter .4 SATELLITE COMMUNICATION There are various sources of programs in Akashvani Ajmer. 1. S-Band 2. C-Band 3. DTH RC AM Input Diode Current Output Voltage t t (b)
  28. 28. 15 Programs are recorded in Jaipur studio. These recorded programs were previously transmitted through Telephone lines. But these days S-Band and C-Band are generally used for transmission. 4.1 Satellites: Satellites are basically reflectors in sky. Satellite Communication is the outcome of the desire of man to achieve the concept of global village. Penetration of frequencies beyond 30 Mega Hertz through ionosphere force people to think that if an object (Reflector) could be placed in the space above ionosphere then it could be possible to use complete spectrum for communication purpose. 1st satellite named as “Sputnik -1” was launched in 1957 by USSR from Bikonour Cosmodromme in Kazakhstan. It was a low orbit passive satellite. It could only reflect the signals mechanically and could not receive, amplify, or change the frequency before transmission. In 1962 Telstar was launched from Cape Carnival by USA. It was also a low orbit satellite but active satellite. Due to following advantages satellite communication is generally used: - This is only means which can provide multi access two way communication. Within the coverage area, it is possible to establish one way or two way communication between any two points. - Satellites are capable of handling very high bandwidth. Normally any satellite can accommodate about 500 MHz in C Band. For example the bandwidth of INSAT-I is 480 MHz in C Band and 80 MHz in S Band. INSAT-II has a bandwidth of 720 MHz in C Band and 80 MHz in S Band. - It is possible to provide large coverage using satellite. For example geostationary satellite can cover about 42% of earth surface using global beam. - It is easy and quicker to establish new satellite link using SNG terminal or VSAT terminal from any point to any other point as compared to any other means.
  29. 29. 16 Fig.4.1 Satellite Communication 4.2 Radio Networking Terminal The various All India Radio stations spread throughout the nation are required to relay certain programs which are originating from Delhi. Similarly there are certain programs which are originating from capital stations are relayed by the other stations in that region. In order to link Delhi and capital stations with other AIR stations, RN through INSAT is not only cost effective but also provide the good technical quality as compared to DOT lines and SW linkage. Thus RNT acts as the ground terminal for satellite signal reception. The block diagram of S-band RN terminal is shown in figure 4.1. The C-Band RNT has mainly two units: 1. Outdoor unit 2. Indoor unit. 4.2.1 Outdoor unit : Outdoor unit has mainly two components. a. PDA b. LNBC a. Parabolic Dish Antenna : PDA, 1.6 m Chicken mesh, is used to receive the downlink RF. The antenna assembly collects and concentrates RF transmitted signals that are produced by communication satellite (INSAT 3C, 74’E) and converts them to an electronic signal. PDA is installed such that it receives maximum signal. It’s azimuthal (Az) and elevation angles (EL) are given by following formulas as viewed to INSAT 3C.
  30. 30. 17 EL = tan-1 ( cosD.cos ǿ - r/R ) ( 1 – (cosD.cos ǿ)2 )1/2 AZ = 180˚± tan-1 (tanD/sinǿ) Where, D = λr – λs in degrees λ r = longitude of the given λ s = longitude of the satellite r = Radius of earth = 6367 kms R = Radius of synchronous orbit = 42,165 kms ǿ = latitude of given site Coordinates of HPT Ajmer Latitude (ǿ) = 26˚31’07’’ Longitude (λr) = 74˚43’00’’ Longitude (λs) = 74˚ E EL = 59˚ AZ = 181.61˚
  31. 31. 18 Fig. 4.2 Block Diagram of S-Band RN Receive Terminal (RNT) The RF signals gathered by the antenna (PDA) are forwarded on the feed horn, which collects the signals. The output of feed horn is then directed to the LNB down converter, which provides the initial amplification of the C-Band downlink signals and converts the C-Band signals to LBand. The output of LNB, down converter is routed to the IFL ( Interfaciality – Link ) cable (RG – 11)
  32. 32. 19 Fig 4.3 Snapshot of PDA 4.2.2 Indoor unit : Indoor unit has mainly satellite digital audio receivers. The ABR202 receive L-Band RF and process it. After processing this satellite receiver provides analog L-R audio channels and digital audio output also. The receiver is also connected with computer by data port. This receiver chain is remotely controlled through computer command. It has mainly three indication in front panel, power, sync, enable. If the power is ok green indication will be displayed and if EB is greater than 7 sync will be locked and if EB is less than 4, receiver will not respond to the system. Chapter .5 MEDIUM WAVE TRANSMITTER
  33. 33. 20 5.1 Introduction A.M. Transmitter of any power in general will have a separate HF and 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. 100 kW HMB 140 Medium Wave transmitter 5.2 Radio frequency chain RF circuits consist of a crystal oscillator, transistor power amplifier, RF. Driver and Power Amplifier of 100 kW HMB 140 MW transmitter are shown in Fig. 5.1 Fig.5.1 Block Diagram of RF Chain (HMB-140) 5.2.1 Crystal Oscillator To oscillate at a consistent frequency, the crystal is kept in a oven. The temperature of the oven is maintained between 68 to 72o C 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. 5.2.2 Transistor Power Amplifier Oscillator output is fed to the transistor Power amplifier (TRPA). It gives an output of 12 Watt across 75 ohms. It works on + 20 V DC, derived from a separate rectifier and regulator. For different operating frequencies, different output filters are selected. (Low Pass Filter). 5.2.3 RF Driver A 4-1000 A tetrode is used as a driver which operates under class AB condition, without drawing any grid current. About 7 to 10 Watts, of power is fed to the grid of the driver through75: 800
  34. 34. 21 ohms RF Transformer, which provides proper impedance matching to the TRPA output and also provides the necessary grid voltage swing to the driver tube. 5.2.4 Various Pin Voltages The cathode of the driver : - 600 V. Control grid : - 650 V. Screen grid : - 100 V. Plate Voltage : +1900 V Because the cathode is at -600 V, the effective grid to cathode bias voltage (fixed) is -50V and the effective plate voltage is 2500 V. The driver develops a peak grid voltage of 800 to 900 V at the grid of PA and PA grid current of about 0.3 A to 0.4 Amps. The required wave form for operating the PA as class -D operation is also developed at the output of the driver by mixing about 20% third harmonic with the fundamental which is the operating frequency of the transmitter. 5.2.5 RF Power Amplifier CQK - 50, condensed vapor 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. 5.2.6 PA output circuit Fig. 5.2 PA Output Circuit (HMB-140) The L-C combination of the output circuit provides the following:
  35. 35. 22 1. The required load impedance for Class D operation that is there should be third harmonic impedance in addition to the fundamental impedance. 2. Matches the plate impedance of 750 ohms to the feeder impedance of 120 ohms at the operating frequency. 3. Filters all the 2nd and 3rd harmonic before the feeder. 5.3 AF Stage Fig. 5.3 AF Stage (HMB-140) The AF stage supply the audio power required to amplitude modulates the final RF stage. The output of the AF stage is superimposed upon the DC voltage to the RF PA tube via modulation transformer. An Auxiliary winding in the modulation transformer, provides the AF voltage necessary to modulate the screen of the final stage. The modulator stage consists of two CQK25 ceramic tetrode valves working in push pull class B configuration as shown in figure 5.3. 5.3.1 High Pass Filter The audio input from the speech rack is fed to active High Pass Filter. It cuts off all frequencies below 60 Hz. Its main function is to suppress the switching transistors from the audio input. This also has the audio attenuator and audio muting relay which will not allow AF to further stage till RF is about 70 kW of power. 5.3.2 AF Pre-amplifier
  36. 36. 23 The output of the High Pass Filter is fed to the AF Pre-amplifier, one for each balanced audio line. Signal from the negative feedback network from the secondary of the modulation transformer and the signals from the compensator also are fed to this unit. 5.3.3 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 low gain. Hum compensator is used to have a better signal to noise ratio. 5.3.4 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 transistors 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 exceed. 5.3.5 AF Final Stage AF final stage is equipped with ceramic tetrodes CQK-25. Filament current of this tube is about 210 Amps. at 10V. The filament transformers are of special leakage reactance type and their short circuit current is limited to about 2 to 3 times the normal load current. Hence the filament surge current at the time of switching on will not exceed the maximum limit. 5.4 TRANSMITTER SALIENT FEATURES:- Table no. 5.1 S. No Property Description 1 Make/ Type BEL HMB 140
  37. 37. 24 2 Radiated Power 2x100 KW 3 Frequency 603Khz 4 Wavelength 497.51 meters 5 MAST height 200 meters 6 Electrical Height 248 meters 7 MAST base Impedance 778+j50.30HMS 8 Site Area 2386565, 140 BIGHA 9 Transmission Hours 05:55 – 09.30 12:00-15:00 17:00-23:10 10 Coverage AJMER-KOTA-JHALAWAR-UJJAIN 256Kms. AJMER-CHITTOR-UDAIPUR- BANSWARA 223Kms AJMER-SIKAR-CHURU-BIKANER 197Kms AJMER-JAIPUR-ALWAR-DELHI 215Kms. AJMER-PALI-JODHPUR-POKRAN 221Kms. 11 Power Suppy Two 11kv overhead feeder from MADAR and GAGWANA 12 Standby Power Supply 2*400KVA 3ph Alternator Kirloskar Chapter.6 PARALLEL OPERATION OF TRANSMITTERS 6.1 Need for Parallel Operation At times it may not be possible to get the required power from the single transmitter for the required coverage of the broadcast service. In such conditions, it is essential to combine two or more transmitters to get the required power. Besides combined operations also facilitate operation
  38. 38. 25 of single transmitter in case of failure of one transmitter thereby achieving reliability of the service. 6.2 Requirements Like parallel operation of alternators/generators there are three conditions to be satisfied for parallel operation of two transmitters. They are • Frequency of the transmitter should be the same. • The phase of the signal of the transmitters at combiner should be the same. • The power levels of both the transmitters should be such that the amplitude at the combiner is equal. In order to meet the first condition, it is possible to use one frequency source for both the transmitters. Hence if there is any drift in the frequency, it will be common to both the transmitters. The phase of the signal of the transmitters depends upon the tuning stages which employ active amplifiers. Such a network used for combining shall be such that it should • Should offer equal load impedance to both the transmitters. • Shall be able to continue the operation even if one of the transmitter goes off the air. • Shall facilitate to dissipate the unbalanced power flowing through the combiner network. The most common network which is used is a bridged “T” network. The figure 1 shows such a network. It has four reactive networks. Two capacitive and two inductive and all are having impedance’s equal to that of feeder line. These impedances can be interchanged as shown in figure 6.1 (a) & (b). The bridged arm also has got one resistive load equal to feeder impedance. This shall take care of the unbalances in the network.
  39. 39. 26 Fig. 6.1 Bridged “T” Networks The break-up of impedances indicates how this network offers proper impedances to the transmitters at all possible conditions. Impedances can be converted into a parallel impedance and vice versa using the following formulae: - Rp Rs 1 Xs/Rs 2 Xp Xs 1 Rs/ Xs 2 , Rs Rp 1 2 , 1 Rp/ Xp 1 Xs Xp 1 Xp/Rp 2 With the help of these equations we can break up the impedances as shown in figure 2. There are two cases of possibilities. They are I) both transmitters are working in combined mode ii) One of (a) (b) Fig. 6.2 Series to Parallel conversion
  40. 40. 27 the transmitter failed in combined mode. In the previous case we see that both transmitters get proper load. And in the later condition, the transmitter in working condition gets proper load, but half the power is lost in combiner reject load. 6.3 Procedure for tuning and combining • Tune each arm of the network for impedance equal to load impedance. • Connect the network and terminate the load impedance. • Measure the load impedance offered at each of the transmitter. It should be equal to load impedance. If not adjust the reactance. • Open and short-circuit the output point of one of the transmitter (in off condition) and measure the load impedance at the other transmitter. It should not change. • Now put on the transmitters with a single oscillator source. • If there is unbalance try to adjust with the phase control of oscillator for minimum unbalance. • If the unbalance still persists try to adjust the power levels of the transmitters either by HT or AVR variations. • Modulate the transmitter slowly to see whether there is unbalance. If so check the audio phase to each of the transmitter.
  41. 41. 28 Chapter.7 TRANSMISSION LINES 7.1 INTRODUCTION R.F. Energy of a transmitter is guided up to radiator (mast) by the propagation of Trans-verse 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 The feeder line should carry the power from the transmitter to Antenna with • Minimum loss • Minimum radiation. 7.2 BASIC TRANSMISSION LINES There are three types of transmission lines used at RF. They are: (i) Open wire feeder lines (ii) Co-axial feeder lines (iii) Wave guides Characteristics Impedance of Feeder Lines Characteristics impedance (Zo) is defined as the input impedance of an infinite line. A transmission line can be represented as having R, L, and C.
  42. 42. 29 Fig. 7.1 Zo of a Feeder Line The inductance, resistance, capacitance and conductance of the line determine the characteristics impedance. The characteristic impedance is given by the following basic formula Zo R j L G j C At higher frequencies R&G becomes negligiblewith respect to reactances of L &C. There fore L Zo C 7.3 TYPES OF FEEDER LINES 1. 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. 2. Structurally there are two basic forms : (I) Open wire line (ii) Enclosed line. 7.3.1 Open wire feeder lines Z0 276log2S/d d S
  43. 43. 30 Fig.7.2 open wire transmission line In MW band, normally the feeder lines used are unbalanced and has following characteristics: 6 wires- 230 Ohms 16 Wires- 120 Ohms 24 Wires- 60 Ohms In SW, normally the balanced feeder lines are used. The impedances are 300 Ohms- 4 wire 600 Ohms- 2 wire 7.4 CHOICE OF FEEDER LINE IMPEDANCE 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. 120 ohm feeder line is now standardized for modern transmitters. 7.4.1 Measurement Of Characteristic Impedance, Zo Zo of a feeder line is given by the relation Zo Zoc.Zsc Zoc = Open circuit Impedance, measured at input by keeping the feeder line end open. Zsc = Short circuit Impedance, measured at input by keeping the feeder line end short. Generally Zoc&Zsc are either capacitive or inductive depending upon the length of feeder line as multiple of /4. .
  44. 44. 31 7.5 POWER TRANSMISSION CAPACITY OF FEEDER LINE The power handling capacity of a line depends upon : • Numbers of live wires used in parallel. • The charge density per unit surface of the wire. • Maximum allowable potential gradient to avoid flashover, and corona etc. Fig.7.3 Transmission Lines On a Tower
  45. 45. 32 Chapter.8 COOLING TECHNIQUES IN TRANSMITTER 8.1 Introduction In modern A.M. transmitters power valves are used in the power amplifiers (PA)and modulator stages, which are condensed vapour 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 Amps. They also draw large filament current of about 620 Amps at 24 volt CQK-350. Hence the tubes dissipate large amount of power which require effective cooling. The CQK series of transmitting tubes are tetrodes, specially designed for transmitters and power amplifiers used in broadcasting. Slowly insert the tube in the connection head so that sudden impact is avoided. If the dead weight of the tube is not sufficiently to overcome contact resistance in the connection head, apply gentle pressure. 8.2 Cathodes All the ceramic tetrodes used in AIR transmitters have directly heated thoriated tungsten cathode. The filament voltage should not vary beyond +5% of the rated filament voltage. The filament voltage must always be measured at the concentric contact rings using sub-standard volt meter. The cathode cum filament has only a very small resistance when cold. Hence the filament voltage is applied and increased smoothly as per the design of the transmitter. 8.3 Screen Voltage The screen grid current can become dangerously high, even at normal screen grid voltage, when the anode voltage is lower than that of the screen grid. Hence the screen grid supply will be switched ON only when the anode voltage has become about 40%. 8.4 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 RF 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 Amps at 10 volt for CQK valve. Hence they also have to be cooled. Hence some kind
  46. 46. 33 of cooling has to be provided to the transmitting equipment. Different types of cooling are used in AIR transmitter at present. a) Air cooling b) Vapour cooling system c) Condensed vapour cooling 8.4.1 Air Cooling At present forced air cooling is used in AIR transmitters. A blower sucks the air through an Air filter and a guided duct system and the forced air is passed on to the required transmitting tubes. There has to be minimum air flow to cool the valves. Hence there will be an air operated Air Flow Switch (Relay) AFR : the AFR will close only when sufficient amount of air has been built up with the blower. Otherwise, AFR will not close and filament cannot be switched on. Sometimes, if the filter is not cleaned, sufficient air may not go out of the blower. Hence the blower needs periodical cleaning. Caution: Do not by pass the AFR on any account. In case the AFR has failed but the blower is throwing the required amount of air, the AFR may be shorted to bring the transmitter on air. But AFR should be replaced at the earliest. At times if the 3 phase supply is phase reversed the blower may be running. But air may not go to the valves and AFR will not close. On such rare occasions, through Anemometer or by a paper flop the direction and the amount of air can be checked and suitable steps can be taken to rectify the phase reversed. At present, transmitting tubes like BEL 3000, BEL 6000, BEL 25000 etc. are forced air cooled types of valves used in AIR transmitters. 8.4.2 Vapour Cooling System This system is used in 100 kW BEL Transmitters. For very high power valves and efficient cooling, air cooling is not sufficient. Hence some of the valves like BEL 15000, BEL 75000 etc. are cooled by vapour cooling. (Hence called Vaptron). Here the principle of heat required to convert water into steam at its boiling point is used (Latent heat of steam). The valves are kept in a in-tight water container filtered and de-ionized water. This water has high resistivity and comes in contact with anode. The water containers called "Boilers" are provided with inlet and outlet pipes.
  47. 47. 34 Fig.8.1 Vapour Cooling System The main features of this vapour cooling system are as follows: • This system is based on closed cycle operation and hence does not require large amount of water. • The cooling efficiency is high. The amount of water flow required for water-cooling system is about 2000 grams per minute to absorb 1 kW of heat whereas in this vapour cooling system the amount of water flow required is only 20 grams per minute to absorb the same heat. • As water flows in a closed loop contamination due to atmospheric dust and dirt is eliminated and hence the steam and water pipes are free of any deposition. The maintenance of this system becomes easier and requires cleaning of anodes and associated piping assemblies only periodically at long intervals. • Water level is maintained at the required normal level by the level monitor and water level control mechanism. The system switches off the transmitter and gives as visual warning when the water level goes down behind the empty level. 8.4.3 Condensed Vapour Cooling in HMB-140BEL 100 kW MW XTR: In BEL/BBC solid state transmitter of 100 KW/300 kW MW and 50 KW/100KW/500KW SW transmitters, condensed vapour cooling is used for the PA and modulator valves. Here a circulation of fast flowing stream of de-mineralized water is used. A high velocity water flows through the valve
  48. 48. 35 jacket and transforms into vapour due to the dissipation of power in anodes. The tubes are fitted with a specially formed anode which sits in a cylindrical cooler. Due to the fast flow of water, the vapour is condensed to water as soon as they are formed. Hence the cooling efficiency is much higher. The temperature of water coming from the transmitter can theoretically reach about 900 C, but in practice, it is desired to about 700 C in normal modulation. The demineralized water is pumped by pumps (one in circuit and one as standby) from the water tank to the PA and modulator tubes through the water piping. Fig.8.2 Water Flow Circuit 100 kW HMB 140 The water flow rate is monitored by three flow switches at the outlet side of each tube. For PA tube, the flow switch is set at 37.5 liters/minute and for modulator valve; it is set at 11.5 liters/minute. The temperature of the water in the pipes is monitored by 3 thermostats one at the outlet of each tube. These thermostats are set at an operating temperature of 900 C, the transmitter will trip
  49. 49. 36 automatically to standby condition and WTR (Water Temperature High) indication comes up in indicator panel. Fig.8.3 Cooling Pipes Chapter.9 ANTENNA TUNING UNIT
  50. 50. 37 9.1 Introduction Antenna Tuning Unit (ATU) is to match the feeder line impedance to the mast impedance of MW Transmitters for maximum transmission of power. So ATU is located between the mast base and the feeder line and is very close to the mast base. Commonly “Feeder Unit” which is located in the aerial field, houses the ATU. Generally the mast impedance (aerial impedance) is obtained in a complex form i.e. the real part (resistive) and the imaginary part (reactive) component. When the mast impedance is expressed in polar form then negative angle indicates the mast is capacitive and positive angle indicates the mast is inductive. Whether the mast impedance is inductive or capacitive depends on the height of the mast in terms of wave length ( ). If the height is less than /4, it will be capacitive and inductive if more than /4. This can be measured with impedance bridges. Design of ‘L’ ‘Networks 9.2 ‘L’ Networks L networks consists of two elements of reactance. It could be inductance and capacitance in any combination L-C, L-L, C-L, C-C. The network can be in the shape of ‘L’ or inverted ‘L’ depending on impedance at the input and at output. If inductive reactance comes in series arm it is a lagging network and when it is capacitive it is leading. i. Phase lag (Rin> RL) X L RL (Rin RL ) Rin RL RL X C Rin Rin RL Fig. 9.1Phase lag (Rin> RL) ii. Phase lag (RL>Rin) XL XC
  51. 51. 38 X L Rin (RL Rin ) Rin X C RL RL Rin Fig.9.2 Phase lag (RL>Rin) iii. Phase lead (Rin> RL) RL X L Rin Rin RL Rin X C RL (Rin RL ) Fig. 9.3 Phase lead (Rin> RL) iv. Phase lead (RL>Rin) Rin X L RL RL Rin Rin RL X C Rin (RL Rin ) Fig. 9.4Phase lead (RL>Rin) ‘L’ networks are simple to implement and component losses are less. Phase angle introduced Rin . Therefore phase control is not possible with this network. is equal to Cos RL 9.3 Pi Networks PI Networks can be designed in number of ways like By assuming phase shift By assuming the Q factor By splitting the network as two “L” networks Rin RL XL XC RLXL XC XL XC
  52. 52. 39 By assuming Phase shift Find the ratio n = R1/R2 Assume the phase angle from 0 to 180 Fig. 9.5 Pi Network Then n Cos a = n.Sin 1 c = n .Cos n .Sin Then XA = R2/a XB = R2/b XC = R2/c In the above case for all phase angles the XB shall be positive and hence inductive , XA shall be negative and hence capacitive. Whereas XC could be positive/negative depending on phase angle selected. For some values of phase angle Pi network becomes L network. nsin b = 1 R1 Xc R2 XB XA
  53. 53. 40 By Assuming XB For Stability XB 2 R1 . R2 Then , XXCA((XXBA XXCB)) R1 /R2 XA = - R1.XB R1 R1.R2 XB2 XC = - R2.XB R2 R1.R2 XB2 Pi networks are flexible and can be designed for any phase shift. They can act as very good fitters to suppress harmonics. 9.4 T Network In some typical cases where “L” network is not possible it may be necessary to design “T” networks. This can be designed as follows. X1 X2 X3 R1 R2
  54. 54. 41 Fig. 9.6 T Network T Network between R1 and R2 is possibile only if X3 2 R1R2 X1 = R1 c, X2 = R1a , X3 =R1 b n Cos Where a = nsin b = - 1 n.Sin 1 n .Cos c = n .Sin = Phase shift and n = R1/R2 Some times it is possible to design “T” networks with the load reactance as the X2 arm of the above network. Therefore there will be only two components in the matching networks , means it is an “L” network. ‘T’ network can precisely control phase shifts and can tune wide range of impedances.
  55. 55. 42 Chapter.10 PRINCIPLES OF ANTENNA SYSTEM 10.1 Introduction Antenna is usually a metallic device (as a rod or a wire) used for radiating or receiving electromagnetic waves. The radio frequency power developed at the final stage of a transmitter is delivered through cables/feeders, without themselves consuming any power to the transmitting antenna. This travels in the free space in the form of radio waves (electromagnetic waves). The receiving antenna picks up the radio waves and delivers useful signal at the input of a receiver for reception of signals. The transmitting and receiving antennae are reciprocal in the sense, any characteristics of the antenna in general applies equally to both. 10.2 The Function of an Antenna The purpose of a transmitting aerial is to convert the power delivered by the transmission line into a wave called an "Electro-magnetic wave." This electromagnetic wave is then radiated through space. All aerials work on the same principle-the aerial current generates an electromagnetic field, which leaves the aerial and radiates outwards as an electro-magnetic wave. Fig.10.1 The Aerial Converts Electrical Power Into Electro-magnetic Waves
  56. 56. 43 10.2.1 How an Aerial Works If the wires of an open-ended transmission line are bent back at right angles to the line, at a point one quarter wavelength back from the open end, a simple aerial is formed which is called either a "half-wave dipole," a "doublet," or a "Hertz aerial." The voltage and current distribution on this aerial are the same as on the original transmission line. Fig. 10.2 Half Wave Dipole Aerial Observe that the standing waves of voltage and current indicate that the aerial ends are points of maximum voltage and minimum current; whereas the center of the aerial is a point of maximum current and minimum voltage. The voltage difference between the two wires of an aerial also generates an electrical field like a charged capacitor which has a pattern and direction that you can see in the diagram (fig.3). Fig. 10.3 Electric Field Surrounding an Aerial Besides this electrical field, there is also a magnetic field, which is generated by the aerial current. The plane of this magnetic field is at right angles to the direction of the current flow; and therefore
  57. 57. 44 is at right angles to the aerial (see below). The electrical and magnetic fields are therefore at right angles to each other. Fig. 10.4 Magnetic Field Surrounding an Aerial These electrical and magnetic fields alternate about the aerial building up reaching a peak collapsing and building up again in the opposite direction, at the same frequency as the aerial current. In the process of building up and collapsing a portion of these fields escape from the aerial and become the electro-magnetic waves which radiate through space, conveying the transmitted intelligence to distant receivers. 10.3 Basic aerials: 10.3.1 Hertz aerial: The half wave dipole or Hertz aerial is used for Short Wave (SW) frequencies in All India Radio network. This can be used at other high frequency also. Since at high frequency the overall length becomes smaller. It can be made of metal tubing, which makes it self supporting. 10.3.2 Marconi aerial:
  58. 58. 45 Another basic aerial is a vertical quarter-wave grounded aerial sometimes called a "Marconi aerial." If one of the arms of a hertz aerial is removed, and the wire which went to that element is earthed or grounded, the result is a Marconi aerial. When a Marconi aerial is used the earth directly beneath the aerial must be a good electrical conductor. To give good conductivity, the earth is simulated by placing metal rods or wire mesh at the base of an aerial. This arrangement is called a "counterpoise earth." Since a quarterwavelength dipole aerial is only half as long, it is often preferred at low frequencies (large wavelength). In is used in medium wave frequencies in All India Radio. Fig.10.5 Basic Aerials
  59. 59. 46 Fig. 10.6 MW Antenna Isolated from ground 10.4 Antenna Radiation Resistance The input impedance ’Zin’ of an antenna is the ratio of voltage to current at its input terminals where the power is fed to the antenna. Zin = Ra+jXa Ra = Resistive part of impedance Xa = Reactive part of impedance Ra = Rr+Ri Rr = Radiation resistance of the antenna Ri = Ohmic loss resistance of the antenna. It is through the mechanism of radiation resistance, power is transferred from the guided wave at antenna input to the free space wave.
  60. 60. 47 The reactive part of the input impedance is due to the storage of electric magnetic field (capacitive and inductive reactance) in the near field of the antenna. The net reactive impedance of the antenna can be matched with the conjugate impedance of the source driving the antenna. Radiation Resistance is a fictitious term. It is equivalent of resistance which would dissipate the same amount of power as being radiated by the antenna when fed with the same amount of power. 10.5 Radiation Efficiency The radiation efficiency determines the effective transfer of power from the input to free space, and given by R Radiation Efficiency = r Ri Rr 10.5.1 Field Regions D3 a) Reactive near field (induction field) up to a distance 0.62 2D2 b) Radiating near field (Fresnel field) beyond near field up to a distance of . c) Far field (Fraunhofer field) beyond Fresnel field where 'D' is the largest dimension of the antenna. 10.5.2 Isotropic Antenna It is an imaginary (non-existent) point (dimensionless) antenna which radiates equally with unity gain in all directions in three dimensional planes. 10.6 Radiation Pattern (Polar Diagram) Graphical representation of the directional radiation properties of the antenna as a function of space coordinates in three dimensions is called the radiation pattern. Such a representation will be usually very complicated to interpret. It is usual practice to represent the same in two
  61. 61. 48 dimensions for both horizontal and vertical planes. The length of vector from the centre or the reference point is proportional to the power gain in that direction Fig.10.7 Antenna radiation pattern 10.7 Half Power (3 dB) Beam Width The angle between the two directions in which the radiation intensity is one half (3 dB below) the maximum value of the beam. Bandwidth of Antenna The range of frequencies within which the performance of the antenna with respect to certain characteristic (such as input impedance, pattern, beam width, polarization, side lobe level, beam direction, gain)conforms to a specified standard. More commonly in broadcasting the characteristics of importance are gain and input impedance. 10.8 Polarizations The plane containing the electric vector in the electromagnetic wave describes the polarization of the radiated wave. Ideally maximum signal is coupled if the antennae (both transmitting and receiving) are oriented in the plane of polarization of the electro -magnetic wave. A vertical
  62. 62. 49 radiator radiates/picks up vertically polarized wave, horizontal radiator radiates/picks up horizontally polarized wave. There are number of well defined polarizations such as horizontal (HP), vertical (VP), slant (+ 45o (SP), circular (left or right) (LCP, RCP), dual (DP), mixed (MP), elliptical (left or right) (LEP/REP) etc. HP : The electric vector is in horizontal plane. TV broadcasting in India use horizontal polarization. VP : The electric vector is in the vertical plane. The self-radiating MW masts of AIR radiate VP waves. Electric supply undertakings use vertical polarization for their VHF communications. CP: The electric vector in circular polarization rotates in a circular motion. They may be considered as the resultant of equal amplitude of vertical and horizontal polarized components combined in phase quadrature (90o ). The polarization is said to be right or left circular polarized (RCP or LCP) depending on the rotation of electric vector of the propagating wave clockwise or anti clockwise respectively, as seen from the transmitting point or by an observer with his back to the transmitter. INSAT down link signals are left hand circularly polarized. 10.9 Aperture of an Antenna 'A' This term usually relates only to receiving antenna. Aperture (or effective area) of a receiving antenna is the ratio of power delivered to the load (connected to the Antenna) to the incident power density. A G 2 4 where G is the gain with respect to the isotropic antenna.
  63. 63. 50 10.10 SNAPSHOTS Fig.10.8 Antenna and guy wire insulator
  64. 64. 51 Chapter.11 EARTHING SYSTEM 11.1 Earthing Earthing is the connection of electrical equipment and wiring systems to the earth by a wire or other conductor. The primary purpose of grounding is to reduce the risk of serious electric shock from current leaking into uninsulated metal parts of an appliance, power tool, or other electrical device. In a properly grounded system, such leaking current (called fault current) is carried away harmlessly. Grounding is also used in manufacturing industries to prevent accumulation of hazardous static electrical charges. The function of earthing is two fold 1. It is for ensuring that no current carrying conductor rises to a potential with respect to general mass of earth than its designed insulation. 2. It is for the safety of the human beings from the electric shocks. 11.2 Methods of Earthing There are two popular methods of earthing : i) Pipe Earthing (Fig. 1) ii) Plate Earthing (Fig. 2) 11.3 Measurement of Earth Resistance The determination of resistance between the earthing electrode and the surrounding ground is of utmost importance. The resistance measurement is made by the potential fall method. The resistance area of an earth electrode is the area of soil around the electrode within which a voltage gradient measurable with commercial instrument exists. In Fig. 11.3 E is the earth electrode under test and A is an auxiliary earth electrode positioned so that two resistance areas do not overlap. B is a second auxiliary electrode placed half way between E and A.
  65. 65. 52 An alternating current of steady value is passed through the earth path from E to A and the voltage drop between E and B is measured. Then earth resistance R = V/I V = Voltage drop between E and B I = Current through the earth path. To ensure that resistance areas do not overlap, the auxiliary electrode B is moved to positions B1& B2 respectively. If the resistance values determined are of approximately same magnitude in all the three cases, the mean of the three readings can be taken as the earth resistance of the earth electrode. Otherwise the auxiliary electrode A must be driven in at a point further away from E and the above test repeated until a group of three readings is obtained which are in good agreement. The use of alternating current source is necessary to eliminate electrolysis effects. The test can be performed with current at power frequency from a double wound transformer by means of a voltmeter and an ammeter or by means of an earth tester. The earth tester is a special type of meggar which sends AC through earth and DC through the measuring instruments. It has got four terminals P1, C1, P2 and C2 outside. The terminals P1, and C1 are shorted to form a common terminal which is connected to the earth electrode under test. The other two terminals C2 and P2 are connected to the auxiliary electrodes A and B respectively . For measurement of earth resistance two electrodes A and B are driven into the ground at a distance of 25 metres and 12.5 metres respectively from earth electrode E under test. The megger is placed on a horizontal firm stand free from surrounding magnetic field. The range switch is set to a suitable scale. The handle is then turned in proper direction at a slightly higher speed than rated one and the reading on the scale is noted. The three readings are taken for different distance. If they are practically the same, it is good otherwise average of these readings is taken as earth resistance.
  66. 66. 53 Fig.11.1 Typical Arrangement of Pipe Earthing
  67. 67. 54 Fig.11.2 Typical Arrangement of Plate Earthing
  68. 68. 55 Fig. 11.3 Testing of Earth Electrode Resistance
  69. 69. 56 Fig.11.4 Earth Tester REFERENCES [1] [2] [3] [4] [5]
  70. 70. 57 [6] Electronic Communication Systems by George Kennedy [7] Electronic Communication systems in advance by Wayne Tomasi [8] Taub’s Principles of communication by Herbert Taub, Donald L Schilling [9]