PREFACE

Practical training is an important constituent of any curriculum and Bachelor of Technology

is no exception to this general rule. Practical training helps the student in getting acquainted

with the manner in which his knowledge is being practically used outside the institution and

this is normally different from what he has learned in the books. As one switches from the

process of learning to that of implementation of his concepts, he finds an abrupt change.

This is exactly why summer training session during the B.Tech. curriculum becomes

all the more important. Summer training is prescribed for the student of Technical Colleges

as a part of the four year degree course of engineering by the AICTE. We are required to

undergo summer training for a period of 30 days after the completion of the 3rd year.

There are various technological advances in the field of electronics and

communication that the world at large is witnessing. The vaccum tubes have been replaced

by small ICs and the transmission medium of wires and cables which have high dielectric and

tangent losses has been replaced by negligible lossy medium like the optical fibers. The wired

communication has been replaced by wireless communication and even in wireless

communication the analog means of communication is getting replaced by digital means of

communication for higher accuracy and bit transfer rate and for less susceptibility towards

noise.

This training report describes in detail the training after the 3rd year

session, which I completed successfully at the All India Radio, Jaipur which is the oldest

government organization in the field of media and broadcasting. This report also gives the

information about the organization and its working along with the vocational training

undertaken during the training period.

All India Radio provided me an exposure in the field of communication for

the first time. It was an honor for me to meet AIR professionals. They taught me about how

the organization works and strives to reach the maximum of the population, through its

various intricate and complicated technologies of communication. We were made aware of

the technologies which are used in AIR for the purpose of broadcasting and communication

and of the importance of radio in today‟s world which has still hooked the country into one

unit in spite of the popularity of television and internet among the people.

ACKNOWLEDGEMENT

Exchange of ideas generates the new objects to work in a better way. This report is based on

the ideas exchanged among the students and the trainers at AIR, Jaipur and the study

materials provided by the AIR professionals.

I owe my sincere thanks to Mr. Banerjee, Mr. R.K. Chaturvedi, Mr. Manohar Lal Sharma,

trainers at All India Radio, Jaipur for providing me their continuous motivation for the

project and whole hearted support in this endeavour, which proved vital for the successful

completion of summer training ’10. Without their expert help and guidance it was difficult to

develop this report.

I would like to thank Mr. H.K.Patsaria (Assistant Engineer) and Mr. ( HR Manager) for

accepting my application of summer internship in their esteem organization and also

considering my name eligible for the merit list.

I would also like to express my gratitude to Mr. Sudhanshu Mathur (HOD- ECE) and other

faculties whose kind inspiration and invaluable guidance encouraged me from time to time

and helped me in the successful completion of the summer practical training report. I am also

thankful to all my colleagues for their co-operation and support.

Debojyoti Majumdar

(Vll Semester, E.C.E)

COMPANY PROFLE

All India Radio‟s history dates back to the early 1920s when radio broadcasting started in

India. The first programme was broadcasted in 1923 by the Radio Club of Bombay. This was

followed by setting up of Broadcasting Services in 1927 with two privately owned

transmitters at Bombay and Calcutta. The British Government then took over the radio

transmission in India and under its banner it came to be known as The Indian Broadcasting

Service. It was all changed to All India Radio (AIR) in 1936 with clear objective to inform,

educate and entertain the masses which after independence came to be known as Akashvani

from 1957.

There are at present 231 radio stations in the country. Each of these radio stations functions

as the sub-ordinate office of All India Radio. The main function of these centres is to transmit

the programmes produced at nearby local CBS or other studios and also from New Delhi

studio.

Since its inception All India Radio has become one of the largest broadcasting network in the

world serving the ever increasing population of India. At the time of independence there were

only 6 radio stations and 18 transmitters, which covered 11% population and 2.5% of total

land area of the country. Till December,2007 the network comprises of 231 radio stations and

373 transmitters which provide radio coverage to 99.14% of the population and reaches

91.79% area of the country, a total of 384 channels and transmits in 24 different languages

and dialects. In spite of recent penetration by other media such as Cable TV, AIR remains the

most common means of gaining access to information and entertainment, as the radio

receivers are relatively cheap and affordable.

In Rajasthan itself there are 23 transmitters and 5 CBS radio stations and 17 non-CBS

stations serving the population of the state of Rajasthan. The main high power transmitter of

50KW is stationed at Ajmer because Ajmer is located at the centre of Rajasthan, from where

the programmes are broadcasted to all other parts of Rajasthan. The Jaipur CBS station has

got its transmitter of 2KW stationed at Vaishali Nagar, Jaipur, which is used to transmit

programmes from the Jaipur CBS station to the Ajmer CBS station so that the programme

can be transmitted to all other parts of Rajasthan. For FM transmission, there are 21 FM

transmitters of 1KW in Jaipur situated at a distance of 40km from each other.

ORGANISATIONAL SET UP

The Directorate General, All India Radio functions under the Prasar Bharati. The Prasar

Bharati Board functions at the apex level ensuring formulation and implementation of the

policies of the organisation and fulfillment of the mandate in terms of the Prasar Bharati. Act,

1990. The Executive Member functions as a Chief Executive Officer (CEO) of the

Corporation subject to the control and supervision of the Board. The CEO, the Member

(Finance) and the Member (Personnel) perform their functions from Prasar Bharati

headquarters at 2nd Floor, PTI Building - Parliament Street, New Delhi-110001.

All important policy matters relating to Finance, Administration and Personnel are submitted

to CEO and the Board through the Member (Finance) and Member (Personnel) as required,

for the purpose of advice, implementation of proposals and decisions thereon. Officers from

different streams working in the Prasar Bharati Secretariat assist the CEO, Member (Finance)

and Member (Personnel) in integrating action, operations, plans and policy implementation as

well as to look after the budget, accounts and general financial matters of the Corporation.

Prasar Bharati also has a unified vigilance set up at the headquarters, headed by a Chief

Vigilance Officer.

The Director General of All India Radio is headed by the Director General. He functions in

close association with the Member (Finance) and Member (Personnel) and the CEO in

carrying out the day to day affairs of AIR. In AIR there are broadly five different Wings

responsible for distinct activities viz, Programme, Engineering, Administration, Finance and

News.

PROGRAMME WING

The Director General is assisted by Deputy Directors General in the Headquarters and

Deputy Directors General in the regions for a better supervision of the stations. The

Headquarters of the Regional DDGs are situated at Kolkata (ER) Mumbai and Ahmedabad

(WR), Lucknow (CR-I), Bhopal (CR-II), Guwahati (NER), Chennai SR-I), Bangaluru (SR-

II), Delhi (NR-I) and Chandigarh (NR-II).

ENGINEERING WING

In respect of technical matters of All India Radio, The Director General is assisted by the

Engineer-in-Chief and Chief Engineers posted in the headquarters and the zonal Chief

Engineers. In addition, there is a Planning and Development Unit in the Headquarters to

assist the Director General in respect of Development Plan Scheme of All India Radio. In

respect of Civil Construction activities, the Director General is assisted by the Civil

Construction Wing, which is headed by a Chief Engineer. CCW also caters to the needs of

Doordarshan.

ADMINISTRATIVE WING

A Dy. Director General (Administration) assists the Director General on all matters of

administration while Dy. Director General (Programme) assists DG in administration of

Programme personnel. A Director looks after the Engineering Administration of All India

Radio, while another Director (Admin. & Finance) assists DG in matters of administration

and finance.

SECURITY WING

The Director General is assisted by a Deputy Director General (Security), Asstt. Director

General (Security) and a Dy. Director (Security) on matters connected with the security and

safety of AIR installations, transmitters, studios, offices etc.

AUDIENCE RESEARCH WING

There is a Director, Audience Research to assist the Director General in carrying out surveys

of audience research on the programmes broadcast by various station of All India Radio.

ACTIVITIES OF SUBORDINATE OFFICES OF AIR

There are a number of subordinate offices of All India Radio performing distinct functions.

Broad activities, in brief, are given below.

NEWS SERVICES DIVISION

News Services Division works round the clock and broadcasts over 500 news bulletins both

in the home and external services. The bulletins are in Indian and Foreign languages. It is

headed by Director General, News Service. There are 44 regional News Units. The bulletins

vary from region to region according to news interest.

EXTERNAL SERVICE DIVISION

As a 'Voice of the Nation', External Services Division of All India Radio has been India's

"Authentic Window to the World". With growing importance of India in the world, an

increasingly important role is envisaged for External Broadcast for times to come. External

Services Division of All India Radio broadcasts in 16 foreign and 11 Indian languages for

approximately 72 hours in a day covering more than 100 countries.

TRANSCRIPTION & PROGRAMME EXCHANGE SERVICE

This service looks after exchange of programmes among the stations, building and

maintenance of sound archives and commercial release of prestigious recordings of music

maestros.

RESEARCH DEPARTMENT

The functions of the Research Department include Research and Development of equipment

required by AIR and Doordarshan, investigation and studies relating to AIR and

Doordarshan, development of prototype models of R&D equipment for limited use, field

trials in the network of AIR and Doordarshan.

CENTRAL STORE OFFICE

The Central Store Office located at New Delhi performs functions relating to procurement,

stocking and distribution of engineering stores required for the maintenance of technical

equipment at All India Radio Stations.

STAFF TRAINING INSTITUTE (PROGRAMME)

The Staff Training Institute (Programme) started with Directorate since 1948 has presently

two main branches functioning from Kingsway Camp, Delhi and Bhubaneshwar. It imparts

in-service training to programme personnel and administrative staff and induction course for

the newly recruited staff and short duration refresher courses. It conducts examinations for

administrative staff. In addition, at present five Regional Training Institutes at Hyderabad,

Shillong, Lucknow, Ahmedabad and Thiruvananthapuram are working.

1 INTRODUCTION

Communication is a term which means a process of transferring information from one place

to the other. That place can be anywhere within the earth. But the earth atmosphere and the

other surrounding objects are a lossy medium which absorbs the signals that is being

transmitted with the help of the transmitter. As a result of this the signal that is received by

the receiver is of very weak strength. Further more noise is also added in a considerable

amount to the signal. In order to reduce the addition of noise to the signal and the signal

could be transmitted to a longer distance from antenna of adequate width various modulation

schemes have been employed which are discussed below.

1.1 Modulation

Modulation is a process of superimposing information on a carrier by varying one of its

parameters (amplitude, frequency or phase).

1.2 Need for Modulation

Modulating the signal over high frequencies can reduce antenna size.

To differentiate among transmissions (stations).

Maximum to minimum frequency ratio can be reduced to minimum by modulating the

signal over high frequency.

1.3 Types of Modulation

In general there are two types of modulation:

a) Amplitude Modulation

b) Angle Modulation

1.31 Amplitude Modulation

If the amplitude of the carrier is varied in accordance with the amplitude of the baseband

signal, it is called amplitude modulation. This modulation is shown in figure 1.1. We can see

this on the screen of oscilloscope.

1.32 Spectrum of AM Signal

The spectrum of AM signal is shown in figure 1.2. If fm = modulating frequency fc =

Carrier frequency. The Spectrum of AM signal will be as below:

Power in the carrier,

Pc = (Ac / 2 ½)

2 = (Ac)

2 / 2

Power in sideband,

Plsb = Pusb = (ma2 Ac

2)/8

Power in upper sideband (Pusb) = Power in lower sideband (Plsb)

Hence total power, Pt = Pc + Pusb + Plsb = Ac2 {1+ (ma

2)/2} / 2 = Pc {1+ (ma

2)/2}

Where,

Ac = Amplitude of the carrier

Am = Amplitude of the modulating signal

ma = Modulation index = Am/Ac

Bandwidth,

BW = (fc + fm) – (fc + fm) = 2fm

1.33 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

1.33.1 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 given

below.

1.33.2 Spectrum of FM signal with sinusoidal modulation

A frequency modulated signal has a large number of frequency components even when the

modulating signal is a single frequency signal. The adjacent frequency components are

spaced just fm apart. These components lie on both sides of the carrier and are symmetrically

placed about it. The amplitudes of the corresponding component are equal. The components,

with frequencies (fc + fm) and (fc - fm) are called the first order side bands. The amplitude of

each of the first order side bands is AJ1 (mf). The components with frequencies (fc - nfm) and

(fc - nfm), where n is an integer, are called the nth order side bands. The amplitude of carrier is

AJ0 (mf). The relative amplitudes of various side bands, therefore, depend upon the index of

modulation alone, the amplitude of a particular side bands being equal to the Bessel function

of the corresponding order. The spectrum is shown in figure given below.

From the above we find that the process of frequency modulation results in a reduction of

carrier amplitude and generation of an infinite number of side bands. Generally, a limited

number of side bands closer to the carrier frequency will have significant amplitudes.

Power

Pc = Ac2/2 = Power of the unmodulated carrier

This holds true for any type of modulating signal and for any value of modulation index.

Where, Ac = Amplitude of the carrier

Am = Amplitude of the modulating signal

mf = df / fm = Modulation index

df = Frequency deviation

fm = Frequency of the modulating signal

Bandwidth 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(mf fm + fm) = 2(df + fm)

1.33.3 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. This modulation has got minimum use.

2. MW TRANSMITTER

All India Radio uses various MW transmitters in its network. They are from 1 kW to 500 kW

power. Various power and make of transmitters used are:

1. 1 kW MW Transmitter – BEL, Harris, BE

2. 10 kW/20 kW Transmitter – BEL, Harris

3. 100/200 kW BEL – HMB 140.

4. 100 kW – Thales (Fully Solid state)

5. 200 kW/300 kW – Thales (Fully Solid State)

6. 300 kW – BBC – Tube Version.

7. 500 kW BBC/Russian Transmitter.

2.1 10 KW MW TRANSMITTER (HMB 163)

2.11 SALIENT FEATURES

Identical type of valves both for PA and Modulators.

Except in the final stage, all other stages are solid state operated.

Valves are ceramic metal tetrode permits full range operation upto 110 MHz.

PA is in Class C amplitude modulated which is one of the oldest and yet most popular

modulation technique used in India and elsewhere in the world.

Microprocessor controlled system with self diagnostic facility.

Efficiency is better than 50%.

A compact and modular system with everything in a single cabinet.

2.12 TECHNICAL SPECIFICATION

1. Emission : Double side band broadcasting on MW.

2. Rated Carrier output kW : 10 kW however it can go upto 15 kW (max.)

3. Output Impedance : 230 ohm for open line and 50 ohm for cable

feed type.

4. AF response : -1.5 dB

5. Distortion : Less than 4%

6. Noise Level : -60 dB

7. Audio Input Level : 0 dbm for 100% modulation

8. Audio Input impedance : 600 ohm bal

9. Power consumption : 20 KVA on carrier

22 KVA on 40% modulation

30 KVA on 100% modulation

2.2 100 KW HMB 140 MEDIUM WAVE TRANSMITTER

AIR has 52 transmitters of this type and is the back bone of MW service. So let us discuss

this transmitter in detail.

RF circuits consists of a crystal oscillator, transistor power amplifier, RF. Driver and Power

Amplifier of 100 kW HMB 140 MW transmitter are shown in Fig. 7.

Block Diagram of RF Chain (HMB-140)

2.21 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 stand by 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. In addition, variable capacitor is used for varying the frequency of the crystal within

a few cycles. The oscillator frequency is divided by 2, 4, or 8 which is selected by jumpering

the appropriate terminals. The oscillator Unit gives 3 outputs, one each for RF output, RF

Monitoring and RF output indication.

2.22 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).

2.23 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

through a 75 : 800 ohms RF Transformer, which provides proper impedance matching to the

TRPA output and also provides the necessary grid voltage swing to the driver tube.

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.

2.24 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. 11 kV DC, the HT voltage is connected to the plate of the PA valves through

the secondary of the modulation transformer and RF chokes : hence the AF signal is super

imposed on the DC for the PA plate.

2.25 PA OUTPUT CIRCUIT

PA Output Circuit (HMB-140)

The L-C combination of the output circuit provides the following :

1. Proper load impedance.

2. Matches the plate impedance of 750 ohms to the feeder impedance of 120 ohms/ 230

ohms at the operating frequency.

3. Filters all the 2nd and 3rd harmonic before the feeder.

2.26 AF STAGE

AF Stage (HMB-140)

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. 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 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.

2.26.1 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. This also has the audio attenuator and audio muting relay.

2.26.2 AF PRE-AMPLIFIER

The output of the High Pass Filter is fed to the AF Pre-amplifier, one for each balanced audio

line. Signal from the negative feed back network from the secondary of the modulation

transformer and the signals from the compensator also are fed to this unit.

2.26.3 AF PRE-CORRECTOR

This card corrects the audio for the non-linearity of modulator tubes. It uses an op-amp and

switching circuits which will distort the audio for different audio levels, opposite to that of

tube distortion.

1.26.4 AF DRIVER

2 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

set such that the maximum modulation factor will not exceeded.

2.26.5 AF FINAL STAGE

AF final stage is equipped with two ceramic tetrodes CQK-25. Filament current of this tube

is about 210 Amps. at 10V. They are working in fresh pull class „B‟ mode, through

modulation transformer.

A varistor at the screen or spark gaps across the grid are to prevent over voltages. As the

modulator valve is condensed vapour cooled tetrodes, deionised water is used for cooling.

The valve required about 11.5 litres/min. of water. Two water flow switches WF1 and WF2

in the water lines of each of the valves protect against low or no water flow. Thermostats

WT1 and WT2 in each water line provide protection against excessive water temp. by

tripping the transmitter up to stand-by if the temperature of the water exceeds 70o C.

Special high power varistor is provided across the secondary winding of the modulation

transformer to prevent transformer over voltages.

Power Supply in 100 kW HMB 140 MW Transmitter

1. HT -11 kV PA & Modulator : thyristor controlled for smooth variation of HT

2. 800 V Power Supply : Screen voltage to PA valve.

3. 1070 V : Screen voltage to modulate valve.

4. 1900 V : Plate voltage to RF Driver

5. - 650 V : (i) Grid Bias to PA Modulator & RF

Driver

(ii) A tap on -650 V provides -600 V supply to the

cathode of RF Driver

(iii) -100 V for the screen of RF Driver.

6. Thycon Unit : + 12 V DC and - 12 V DC

7. Audio Unit : + 24 V and + 10V.

8. Reflectometer : + 15 V & - 15 V

9. Control Circuits :

VDDB + 15 V - Logic circuits.

VDDC + 12 V - Relays

VDDD + 15 V for indication lamps.

VDDE - 15 V - Comparator.

10. Main supply to transmitter 415 V. 3 Phase 50 Hertz.

Earthing switch operated by a handle from the front of the rack has been provided in the filter

tank. The main HT terminal and also the live ends of the filter condensers C201 to C 210

have been brought to the earthing switch. In addition all the MT voltage (- 650, 800, 1070,

1900) are also brought to the earthing switch. The 11 kV point is discharged initially through

a resistor R - 543 before it is grounded. The earthing switch is interlocked to the main

transmitter by micro switches S 302, S 303 and S 304. In addition, a key interlock system is

provided to prevent accidental contact with high voltages.

2.27 CONTROL AND INTERLOCK SYSTEMS IN TRANSMITTER

Control and interlocking circuits of the transmitter are to perform four major functions :-

1. Ensure correct switching sequence.

2. Safety of the equipment.

3. Safety of the operating personnel.

4. Indication of the status of the transmitter.

In the following paragraph the details regarding the above aspects are dealt briefly :

1. Switching Sequence of Transmitter

a) Ventilation.

b) Filament

c) Grid Bias/Medium Tension

d) High Tension.

a. Ventilation : All the transmitters handle large amount of power. Basically the

transmitters convert power from AC main's to Radio Frequency and Audio Frequency

energy. The conversion process always result in some loss. The loss in energy is

dissipated in the form of heat. The dissipated energy has to be carried away by a

suitable medium to keep the raise in temperature of the transmitting equipment

within limits. Hence, in order to ensure that the heat generated by the equipment is

carried away as soon as it is generated the ventilation equipment need to be switched

on first. Normally the cooling provided in a transmitter could be classified on the

following lines :

Cooling for the tube filaments.

Cooling for the tube Anodes.

General cooling of the cubics.

Cooling for coils, condensers, Resistors etc.

The cooling equipments comprise of blowers, pumps and heat exchangers. Another

important consideration is that during the switching off sequence the cooling

equipments should run a little longer to carry away the heat generated in the

equipments. This is ensured by providing a time delay for the switch off of the

cooling equipment. Normal time delay is of the order of 3 to 6 Minutes.

The water flow and the air flow provided by the cooling equipments to the various

equipments are monitored by means of air flow and water flow switches. In case of

failure of water or air flow, these switches provide necessary commands for tripping

the transmitter.

b. Filaments : All the transmitters invariably employ tubes in their drive and final

stages of RF amplifiers and sub modulator and modular stages of AF amplifiers.

After ventilation equipments are switched on and requisite air and water flow

established, the filament of the tubes can be switched on. While switching on

filament of the tube, the control and interlocking circuits have to take care of the

following points.

The cold resistance of the filament is very low and hence application of full filament

voltage in one strike would result in enormous filament current and may damage the tube

filament.

The emission from the tubes depend upon the temperature of the filament. Generally

it takes some time for the filament to reach a steady temperature after it is switched

on. This aspect is taken care of by providing a time delay of 3 to 5 minutes between

the filament switching on and the next sequence namely bias switching on.

c. Bias And Medium Tension : The control grid of the tube has to be given the

necessary negative bias voltage before its anode voltage can be applied. Hence, after

the application of full filament voltage and after the lapse of necessary delay for the

filament temperature to become stable bias voltage can be switched on. Along with

bias generally anode and screen voltages of intermediate stages and driver stages are

also switched on. Application of bias and medium tension makes available very high

voltages for the various transmitter equipment. Hence, to ensure the safety of the

personnel various interlocks and safety are checked by control circuit before putting

on MT.

2.28 CONNECTION OF LOAD (ANTENNA/DUMMY LOAD)

After the application of ventilation, filament and bias the anode voltage can be switched

on. But before the anode voltage can be increased the interlocking circuit is to ensure that

the load of the transmitter namely antenna or dummy load is connected to the transmitter.

Application of Screen Voltage : In the case of tetrode tubes, the screen voltage to

the tube should not be applied before the application of anode voltage to keep the

screen current low. This is taken care of by an interlocking provision that the screen

voltage is applied only after the anode voltage reach a certain pre-determined value

well above the normal screen voltage.

Release of Audio frequency : The application of AF signal to the AF stage in the

absence of carrier power would result in the operation of modulation transformer with

no load connected. This is not desirable. Therefore, the AF signal should be applied

to the Audio frequency stages only when the RF power amplifier is delivering the

nominal power. Normally AF frequency signal to the AF stage is released only when

the carrier power is approximately 80% of the normal power.

3 FM TRANSMITTER

There is too much over-crowding in the AM broadcast bands and shrinkage in the night-time

service area due to fading, interference, etc. FM broadcasting offers several advantages over

AM such as uniform day and night coverage, good quality listening and suppression of noise,

interference, etc. All India Radio has gone in for FM broadcasting using modern FM

transmitters incorporating state-of-art technology.

The configurations of the transmitters being used in the network are :

3 kW Transmitter

2 x 3 kW Transmitter

5 kW Transmitter

2 x 5 kW Transmitter

SALIENT FEATURES OF BEL/GCEL FM TRANSMITTERS

1. Completely solid state.

2. Forced air cooled with the help of rack-integrated blowers.

3. Parallel operation of two transmitters in passive exciter standby mode.

4. Mono or stereo broadcasting

5. Additional information such as SCA signals and radio traffic signals (RDS) can

also be transmitted.

6. Local/Remote operation

7. Each transmitter has been provided with a separate power supply.

8. Transmitter frequency is crystal controlled and can be set in steps of 10 kHz using

a synthesizer.

3.1 MODERN FM TRANSMITTER

Simplified block diagram of a Modern FM Transmitter is given below. The left and right

channel of audio signal are fed to stereo coder for stereo encoding. This stereo encoded

signal or mono signal (either left or right channel audio) is fed to VHF oscillator and

modulator. The FM modulated output is amplified by a wide band power amplifier and then

fed to Antenna for transmission.

Voltage controlled oscillator (VCO) is used as VHF oscillator and modulator. To stabilize its

frequency a portion of FM modulated signal is fed to a programmable divider, which divides

the frequency by a factor „N‟ to get 10 kHz frequency at the input of a phase and frequency

comparator (phase detector). The factor „N‟ is automatically selected when we set the station

carrier frequency. The other input of phase detector is a reference signal of 10 kHz generated

by a crystal oscillator of 10 MHz and divided by a divider (1/1000). The output of phase

detector is an error voltage, which is fed to VCO for correction of its frequency through

rectifier and low pass filter.

Block Diagram of Modern FM Transmitter

3.2 2 X 3 KW FM TRANSMITTER

Simplified block diagram of a 2 x 3 kW FM transmitter is shown in Fig.2. 2 x 3 kW

Transmitter setup, which is more common, consists of two 3 kW transmitters, designated as

transmitters A and B, whose output powers are combined with the help of a combining unit.

Maximum of two transmitters can be housed in a single rack along with two Exciter units.

Transmitter A is provided with a switch-on-control unit (GS 033A1) which, with the help of

the Adapter plug-in-unit (KA 033A1), also ensures the parallel operation of transmitter B.

Combining unit is housed in a separate rack.

Block Diagram Of 2x3 Kw Fm Transmitter

Low-level modulation of VHF oscillator is carried out at the carrier frequency in the Exciter

type SU 115. The carrier frequency can be selected in 10 kHz steps with the help of BCD

switches in the synthesizer. The exciter drives four 1.5 kW VHF amplifier, which is a basic

module in the transmitter. Two such amplifiers are connected in parallel to get 3 kW power.

The transmitter is forced air-cooled with the help of a blower. A standby blower has also

been provided which is automatically selected when the pre-selected blower fails. Both the

blowers can be run if the ambient temperature exceeds 40oC.

Power stages are protected against mismatch (VSWR > 1.5) or excessive heat sink

temperature by automatic reduction of power with the help of control circuit. Electronic

voltage regulator has not been provided for the DC supplies of power amplifiers but a more

efficient system of stabilization in the AC side has been provided. This is known as AC-

switch over. Transmitter operates in the passive exciter standby mode with help of switch-

on-control unit. When the pre-selected exciter fails, standby exciter is automatically selected.

Reverse switch over, however, is not possible.

A simplified block diagram of a 2 x 5 kW FM Transmitter is also given below.

RF Block Schematic of 2x5 kW FM Transmitter

Ref.Drg.No.:-STI(T)557,(DC309)

3.3 EXCITER (SU 115)

The Exciter (SU115) is, basically, a self-contained full-fledged low power FM Transmitter.

It has the capability of transmitting mono or stereo signals as well as additional information

such as traffic radio, SCA (Subsidiary Channel Authorisation) and RDS (Radio Data System)

signals. It can give three output powers of 30 mW, 1 W or 10 W by means of internal links

and switches. The output power is stabilized and is not affected by mismatch (VSWR > 1.5),

temperature and AC supply fluctuations. Power of the transmitter is automatically reduced in

the event of mismatch. The 10 W output stage is a separate module that can be inserted

between 1 W stage and the low pass harmonics filter. This stage is fed from a switching

power supply which also handles part of the RF output power control and the AC supply

stabilizations. In AIR set up this 10 W unit is included as an integral part of the Exciter.

This unit processes the incoming audio signals both for mono and stereo transmissions. In

case of stereo transmission, the incoming L and R channel signals are processed in the stereo

coder circuit to yield a stereo base band signal with 19 kHz pilot tone for modulating the

carrier signal. It also has a multiplexer wherein the coded RDS and SCA signals are

multiplexed with the normal stereo signal on the modulating base band. The encoders for

RDS and SCA applications are external to the transmitter and have to be provided separately

as and when needed.

3.31 FREQUENCY GENERATION, CONTROL AND MODULATION

The transmitter frequency is generated and carrier is modulated in the Synthesiser module

within the Exciter. The carrier frequency is stabilized with reference to the 10 MHz

frequency from a crystal oscillator using PLL and programmable dividers. The operating

frequency of the transmitter can be selected internally by means of BCD switches or

externally by remote control. The output of these switches generates the desired number by

which the programmable divider should divide the VCO frequency (which lies between 87.5

to 108 MHz) to get a 10 kHz signal to be compared with the reference frequency. The

stablised carrier frequency is modulated with the modulating base band consisting of the

audio (mono and stereo), RDS and SCA signals. The Varactor diodes are used in the

synthesizer to generate as well as modulate the carrier frequency.

3.32 SWITCH-ON CONTROL UNIT (TYPE GS 033 A1)

The switch-on-control unit can be termed as the “brain” and controls the working of the

transmitter „A‟. It performs the following main functions:

1. It controls the switching ON and OFF sequence of RF power amplifiers, rack blower

and RF carrier enable in the exciter.

2. Indicates the switching and the operating status of the system through LEDs.

3. Provides automatic switch over operation of the exciter in the passive exciter standby

mode in which either of the two exciters can be selected to operate as the main unit.

4. It provides a reference voltage source for the output regulators in the RF amplifiers.

5. It is used for adjusting the output power of the transmitter.

6. It evaluates the fault signals provided by individual units and generates an overall sum

fault signal which is indicated by an LED on the front panel. The fault is also stored

in the defective unit and displayed on its front panel.

3.33 ADAPTER UNIT (KA 033A1)

Adapter Unit is a passive unit which controls transmitter B for its parallel operation with

transmitter A in active standby mode. The control signals from the Switch-on control unit are

extended to transmitter B via this Adapter unit. If this unit is not in position the transmitter B

can not be energized.

3.34 1.5 KW VHF AMPLIFIER (VU 315)

This amplifier is the basic power module in the transmitter. It has a broad band design so that

no tuning is required for operation over the entire FM Broadcast band. RF power transistors

of its output stages are of plug in type which are easy to replace and no adjustments are

required after replacement. Each power amplifier gives an output of 1.5 kW. Depending on

the required configuration of the transmitter, output of several such amplifiers is combined to

get the desired output power of the transmitter. For instance, for a 3 kW set-up two power

amplifiers are used whereas for a 2 x 3 kW set-up, 4 such amplifiers are needed. The

simplified block diagram of 1.5 kW Power Amplifier is given below.

Block Diagram of 1.5 kW Amplifier VU 315

Ref. Drg.No.:-STI(T)444(DC196)

This amplifier requires an input power of 2.5 to 3 W and consists of a driver stage (output 30

W) followed by a pre-amplifier stage (120 W). The amplification from 120 W to 1500 W in

the final stage is achieved with the help of eight 200 W stages. Each 200 W stage consists of

two output transistors (TP 9383, SD1460 or FM 150) operating in parallel. These RF

transistors operate in wide band Class C mode and are fitted to the PCB by means of large

gold plated spring contacts to obviate the need for soldering. The output of all these stages is

combined via coupling networks to give the final output of 1.5 kW. A monitor in each

amplifier controls the power of the driver stage depending on the reference voltage produced

by the switch-on-control unit. Since this reference voltage is the same for all the VHF

amplifiers being used, all of them will have the same output power.

Each amplifier has a meter for indicating the forward and reflected voltages and transistor

currents. Also a fault is signaled if the heat sink temperature or the VSWR exceed the

prescribed limits. In both cases, the amplifier power is automatically reduced to protect the

transistors.

3.35 POWER SUPPLY SYSTEM

The FM transmitter requires 3-phase power connection though all the circuits, except the

power amplifiers, need only single phase supply for their operation. An AVR of 50 kVA

capacity has been provided for this purpose.

Power consumption of the transmitters under various configurations is as follows :

Frequency of Power Consumption

operation 3 kW 5 kW 2 x 3 kW 2 x 5 kW

87.5 to 100 MHz 5100 W 8500 W 10200 W 17000 W

100 to 108 MHz 5320 W 8860 W 10640 W 17720 W

These figures do not include the power consumption of blowers which is 200 W for each

blower.

For each transmitter, there is a separate power distribution panel (mounted on the lower

portion on the front of the rack). Both the distribution panels A&B are identical except for

the difference that the LEDs, fuses and relays pertaining to switching circuit of blowers and

absorber are mounted on the „A‟ panel.

Power Reduction in case of Amplifier or Transistor Failure

When an amplifier module or a push-pull output stage in an amplifier module fails due to

failure of any one transistor, the output gets reduced according to the following formula. :

P = Po { (m-n)/m}2

Where

Po = nominal power

P = reduced power available at the antenna

M = total number of amplifier modules or of push

Pull output stages in circuit

N = number of faulty amplifiers or push pull output stages.

The power consumed in the absorber resistors is calculated according to the formula :

Pabsorber = Po – Pn

Where Pn is the faulty partial power, which in case of failure of an entire amplifier module

equals 1250 W.

If power reduction occurs due to failure of one or more VHF amplifiers, the transmitter

should be switched off immediately and the working transmitter should be selected on the

antenna using the U-links on the Combining unit.

4 COOLING TECHNIQUES IN TRANSMITTER

In modern A.M. transmitters power valves are used in the PA and modulator stages, which

are condensed vapour cooled ceramic tetrodes. In the old generation transmittes, 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 Anps at 24 volt CQK-350. Hence

the tubes dissipate large amount of power which require effective cooling.

The CQK series of transmitting tubes are tetrode specially designed for transmitters and

power amplifiers used in broadcasting.

The tube is installed vertically with the heating connections at the bottom. Handle and

transport the tube with utmost care : vibrations and external impacts can cause invisible

damage. Avoid sudden movement. 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. The ceramic parts must be

always kept clean. If necessary, they should be cleaned with alcohol or acetone but no

circumstances should they be rubbed with emery paper.

The contact surfaces are coated with a heat resistant lubricant film, which does not attack

silver. Electrical connections and connection head are provided with contact rings for all

electrodes including the anode. The connection head is stationary. It supports and locate the

tube, which can be inserted into the connection head only in a certain position. This position

is determined by the guide groove on the anode.

4.1 CATHODES

All the ceramic tetrodes used in AIR transmitters are 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. In some transmitters, the filament voltage is applied in steps. In some

transmitter, the design of the filament transformer is such, it will restrict the surge current to 3

to 4 times the normal steady current.

4.2 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% or so of the anode

voltage.

4.3 TEMPERATURE

A separate air cooling has been arranged to control the temperature of the ceramic cylinder

and all metal ceramic seals in addition to the condensed vapour cooling.

There must not be any high frequency on the supply leads. To ensure this filament RF by

pass condensers are provided.

4.4 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 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 Amps 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 transmitter at present.

a) Air cooling

b) Vapour cooling

c) Condensed vapour cooling

a) 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.

b) 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 an 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.

Block Diagram of Vapour-Cooling System

The inlet pipes are interconnected at the bottom to keep the water level same in all the boiler

and the outlet pipes are joined at the top which provides passage of the steam to the

condensing equipment known as Heat Exchanger.

The heat produced at the anode of the vacuum tubes is absorbed by water and gets converted

into steam. The steam thus produced goes up through the glass-tube to the steam pipe and to

the heat exchanger mounted on top of the transmitter room. The condenser is made of copper

tubes. This is a mono-block fine tube moulded from copper in extrusion moulding system

and has high terminal conductivity. Steam flows inside the tube and cool air is forced outside

through the fins and the action of heat exchange takes place. Now the steam is condensed to

water and the cooled water flows down through the water pipe due to gravity back to the

boiler.

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.

The capacity of the heat exchanger is 150 kW where the inlet air temperature is 50o C max

for 100 kW HMB-105 BEL MW Transmitter. To cool the heat exchanger a 3 phase propeller

fan type is used. The outgoing water is about 40L/hour. The temperature of incoming steam

is about 100 C and the outgoing water temperature is 100. There are two propeller fan to

cool the fins of the heat exchanger. Any one of them can be selected through a change over

switch.

The amount of water required in each boiler is about 27 litres. As the water inside the boiler

is in direct contact with the anodes of the vacuum tubes where the high tension voltage is

present, the specific resistance of water should be high so that the steam pipes and water

pipes are free of any dangerous voltages.

To obtain good quality of water for the purpose of using as a cooling medium in the

transmitter, a de-ionizing equipment with a filter unit or distilled water plant is used to

prepare the distilled water at transmitter. A conductivity tester is used to check the quality of

water in terms of resistivity. If the water resistivity is less than 200 kilo ohm, the water

should be changed.

There is water level monitor to check the level of water. If the level of water is less than the

prescribed level, the transmitter will trip. There is a provision to add water and drain out the

old water.

Pressure equalizing line has been provided in the system. Through this line the high pressure

formed over the tubes is distributed equally to the level monitoring system. Without this line,

the higher pressure on the vapour side of the system would depress the water level in the

boiler, relative levels would be determined by the boiling rate that is the valve dissipation and

the function of the control box would be utilized.

c) 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 jaket 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 cylinderical 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 70

0 C in

normal programme modulation.

Filaments of the tubes are cooled by forced air by means of a high pressure blower. It also

cools the R.F. driver valves, the third harmonic and second harmonic suppression coils.

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. At the inlet/outlet of

each tube, a double ball valve is provided to facilitate shutting off water supply when the

valve is required to be changed. Except for changing the valve, this should be kept in open

condition always. (Lever in the horizontal position).

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 litres/minute and for modulator valve, it is set at 11.5

litres/minute. When the water flow comes below this value, the transmitter will be tripped up

to filament & pump will also be switched off. Off condition through the control electronics

and WFF (Water Flow Failure) fault indication comes ON in the fault indicator panel.

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 automatically to standby condition and WTR (Water Temperature High) indication

comes up in indicator panel. Until the water temperature comes down to normal temperature,

it will not be possible to put ON MT or HT.

The hot water from the valves returns to the tank through a water to air heat exchanger where

it is cooled (100 C to 15

0 C) by blower in the heat exchanger. To ensure good quality of the

water, a part of water is fed through a pressure reducing valve to de-ionizer for regeneration.

A conductivity tester to check the quality of water in terms of conductivity is also available

with the transmitter.

A filter is provided before the pump for filtering the water. The actual filter sleeve can be

removed with the screw plug whenever required for cleaning. Manometers are provided for

indicating the pressures at the inlet and outlet water lines of tubes.

The water level switch senses the level of water and gives indication WLL (Water Level

Low) when the water level in the water tank becomes low. The level switch is a pressure

switch with a stainless steel pressure element. The switching point can be set with the help of

the knob provided in the switch.

More or less the same system is used in the transmitter where the transmitting tubes are of

CQK-25, 50, 350, 650 etc.

5 LIMITERS

Limiting amplifiers are used in transmitter. During a transmission, there may be peaks

occasionally which may overload the modulator valves and trip the transmitter. The

occasional peaks may be for a short duration and it can not be controlled manually.

Modulation of the transmitter also has to be kept very high of the order of 70% or more on

average. To take care of all these things, limiting amplifiers are used in transmitter.

5.1 AM LIMITER (ME 277 DX)

In AIR Limiting Amplifiers of Meltron make ME 277 DX Limiter are being used in large

numbers. Many of the stations may also be using Limiting Amplifiers of BEL make. The

principle of working and the modules used in both the Limiting Amplifiers are same. In the

Meltron Limiter, integrated circuits (mostly operational amplifiers) are used. Whereas the

same function is realised in BEL Limiter using transistors.

Over driving the audio stage of transmitter is protected by the Limiting Amplifier by

employing a combination of pre-delay and feed back control. The audio signal is delayed by

approximately 300 micro second during which time necessary control signal is calculated and

set. The limiter (both Meltron and BEL) uses the combination of compressor and expander

circuits, which increases the audio level (density) as is necessary for extending broadcast in

fringe area.

BROADCAST SPECIFICATIONS OF ME 277 DX LIMITER

Inputs : Input Impedance 600 Ohms

Nominal Input level - 20 dBm to + 15 dBm

Maximum Input level + 24 dBm (12.29 V approx.)

Outputs : Balanced floating impedance maximum 40 ohms.

Nominal output level - 20 dBm to + 15 dBm

Frequency Response 30 Hz to 15 kHz + 0.3 dB

Static Settings :

Limiter threshold At internal nominal level

Compressor Ratio 2 to 6 adjustable

Compressor Gain 0 to 18 dB adjustable

Expander (switchable) - 45 to -65 dB adjustable

Expander ratio 2 fixed

Dynamic Settings :

Attack time : Less than 100 micro secs. Imperceptible due to 300 micro secs delay of the

signal.

Release time : Automatic, programme dependent, switchable to manual and can be adjusted

from 0.5 to 7 seconds.

Signal to Noise Ratio :

Gain = 0 Expander ON-76 dB

Gain = 18 Expander ON-36 dB

Gain = 18 Expander OFF-59 dB

Harmonic Distortion : Less than or equal to 0.2%.

5.11 BRIEF WORKING OF THE LIMITING AMPLIFIER

Meltron Limiter ME 277 DX is a control amplifier. It consists of a limiter, a compressor and

expander. The peaks of the audio signal at the input of the transmitter is prevented for

exceeding a predetermined amplitude and Low level signals are amplified.

The audio signal is controlled using a multiplier signal is weighted by a factor 'g' such that the

output signal corresponds to the desired function.

X

g

INPUT OUTPUT

gx

x

When g = 1, the output = input level

g greater than 1, input is amplified at the output

g less than 1, input is reduced at the output

The factor g depends upon the program signal and is determined in the amplification

computer. In the DX Limiter the Gain Control factor 'g' is obtained with a combination of

signal delay of about 0.3 milli second and feed back control. As shown in fig. 2, the input

signal is delayed before it reaches the multiplier. The amplification factor g is determined

during the delay period.

X

g

AUXILARY MULTIPLIER

X

MULTIPLIER

OUT

GAIN COMPUTER

DELAY

IN

Block Diagram

The amplification factor g itself is ascertained using a feed back regulation system which

consists of a GAIN COMPUTER and Auxiliary multiplier. The auxiliary multiplier and the

multiplier employ identical circuitry, so that g serves directly as a factor for both.

The delay time 0.3 milli sec is brief enough not to disturb comparative listening between the

input signal and output signal, but sufficiently long to allow the control process to have been

completed when the input signal emerges from the delay network, independent of over

driving at the input and practically imperceptible to ear.

5.2 STATIC CHARACTERISTICS

As indicated previously the DX-Limiter includes a limiter, compressor and expander.

Limiter

The limiter prevents all signals from exceeding the pre determined threshold, while signals at

power levels are not affected. The limiter responds to peak amplitudes. Short signal peaks

that are not indicated by conventional level meters like VU meter because of the time

constants employed will nevertheless lead to signal limiting. The limiter will thus protect the

transmitting tubes against arcing.

Compressor

The compressor is to increase the loudness of the modulation signal. This enable broadcasting

coverage for instance to be extended without increasing transmitter power. The compressor

function is by employing a non constant amplification characteristics at medium level. The

closer the level approaches the limiting threshold, the more amplification is reduced.

Expander

An expander is linked directly to the compressor. Low level signals down to level

immediately above the noise level are expanded. As a result the signal level may be boosted

without raising the noise level. Undesirable increase in noise are there by prevented during

pauses in the modulation.Performance of the ME limiting amplifier can be explained first

with reference to its static operating curves.

The amplification factor 1 (0 dB) increases at an angle of 45o up to the limiter threshold and

the output level is equal to the input level. In parallel to this line is the line depicting the

maximum amplification 18 dB to which the compressor can be adjusted. This is termed as

compressor gain. In addition to the gain, the compressor is further characterised by the ratio.

Any setting between 2 and 6 (2:1 and 6:1) may be made. If the compressor ratio is 6:1 a 6 dB

increase in the input level will produce only 1 dB increase in the output levels. The circuit of

the compressor designed such that the operating curve is rounded at the top, advancing

smoothly to the limiter region without an abrupt change.

The second region of non-constant amplification is created by the expander, which is

effective in the lower range of signal. The change in amplification is designed as an

expansion ratio : the expander in the ME 277 DX Limiter exhibits a fixed ratio of 1:2 i.e. if

the input level increases by 1 dB, the output level is increased by 2 dB. Signals are expanded

in an adjustable range such that the threshold of the expander is generally set above the noise

level so that only the signal information is amplified. Increases in the noise during pauses in

the modulation are thus effectively prevented.

Ref.Drg.No:-STI(T)446,(DC198)

5.3 DYNAMIC CHARACTERISTICS

Among the dynamic characteristics, the release performance of a particular interest. All

transient events are completed within the delay time and not perceived an unpleasant.

The ME 277 DX Limiter is characterised by a three segment release curve. It begins with a

holding phase (1) of 40 mili sec to prevent the limiter from reacting a new to every half-wave

of a low-frequency signal, a process which would inevitably cause higher distortion,

immediately thereafter, a rapid release phase (2) follows, which is hardly perceived by the ear

and prevents excessive level and loudness losses. The original amplification is restored,

however, only after a very long, practically inaudible phase (3).

The additional feature of the DX-Limiter is that the release performance is programme

dependent, i.e. controlled from the signal characteristics. Very brief over driving in an

otherwise low signal level will be followed by rapid release, where as an extended high-level

signal with occasionally over-driven peaks will cause a release phase of very long duration.

In this manner, for instance, a short signal impulse will not reduce the level for an in

appropriate interval, while dreaded "pumping" effects will be prevented to a great extent for

signals frequently exceeding the threshold.

Normal Setting of the ME 277 DX Limiter as recommended by DG:AIR :

Expander thresholds - 50 dB

Compressor ratio 2:1

Compressor Gain 18 dB

Nominal input level - 6 dBm

Nominal output level + 6 dBm

Threshold of limiting - 6 dBm

The +6 dBm output of limiting amplifier will be followed by a 6 dB multiple pad for feeding

0 dBm input to the transmitter for100% modulation.

For limiting amplifier alignment, 1 kHz is to be fed from the audio console of control room

through the normal programme chain. The level of the audio console at control room should

be adjusted such that the VU meter indicate + 3VU. Now the limiting amplifier input will be

adjusted at threshold of limiting and the output adjusted to 95% modulation. During normal

progamme, the VU meter will peak to zero and as such the peak programme level the input of

limiter will always be 3 dB less than the threshold of limiting amplifier.

The above alignment once made, may not be disturbed thereafter.

5.4 FM LIMITER (EMT 266 TX)

Limiters are commonly employed at the programme input of FM Transmitters to protect

against over-deviation. Accurate control of the programme level by a suitable method is

absolutely essential for maintaining the peak deviation below the permitted maximum value.

Usually the level control proceeds the pre-emphasis. If we consider a reference deviation of

+ 40 kHz corresponding to 0 dB at 500 Hz., we have a margin of only 5.5 dB. But the pre-

emphasis circuit that follows, boosts the signal up to 13.5 dB at 15 kHz thereby exceeding the

permissible margin by about 8 dB. If the time constant of level meter is taken into account,

the increase would be still higher. Without limitation or peak clipping this would lead to

intolerable peak deviations. In case the programme level control is affected after the pre-

emphasis, over modulation can be avoided, but the average deviation would be reduced

unnecessarily.

It has thus been realised that the conventional limiter together with the constant pre-emphasis

of 50 microseconds cannot provide any protection. In conventional limiting amplifiers gain

variations affect all sound signals equally i.e. they are not frequency dependent. Tests have

shown that the shortcomings of conventional limiters can be avoided by employment of

frequency selective form of limiter.

As long as the resulting output signal is much below the nominal level, standard 50 micro

second pre-emphasis is employed. When high frequency components of the programme

signals exceed the prescribed limits, pre-emphasis is reduced momentarily to avoid

overloading. Principle of operation of variable or adaptive pre-emphasis followed in the New

Transient Limiter is illustrated in fig. 4. Here pre-emphasis time constant is reduced by

shifting the start of the boost sufficiently towards higher frequencies until the established

threshold can be maintained. The variable time constant is implemented by a quite simple

circuit. The signal is reproduced linearly and over a separate path, with a 6 dB per octave

increasing gain characteristics, and the results are added. The level of the portion with

increasing gain is controlled by a multiplier and thereby determines the pre-emphasis corner

frequency when both portions are summed together.

In AIR VU meters are normally used for aligning and monitoring of levels of programme

chain. The signal levels read on the VU meter are not necessarily the true signal levels of the

bus they are monitoring. Since the VU meter is an average reading meter, the peaks touching

0VU on VU meter may be actually 6 to 8 dB higher. This is very important factor, which is

normally over looked while deciding the threshold point. Till peak reading meters are

introduced in the department, sufficient headroom is necessary to accommodate the margin of

peak readings. Thus, in order to arrive at proper line up settings, implications arising from

various options require due consideration.

Multiplier

Adaptive

Pre-emphasis

Output Time Delay

Gain Computer

Input

Aux.

(a) Block Diagram of ME 266 X

Transmitter Limiter

Multiplier

Outp

ut le

vel

Input level

(b) Variable Pre-emphasis (c) Static operating curve of the ME 266 X Transient diameter

20

10

0

-10

-20

-30

dB

20 dB 10 0 -10 -20 -30

30

20

0

10

-10

-20

dB

20K Hz 2K

P

200

f

20

6 MEDIUM WAVE ANTENNA

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 metres, 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. However vertical wire antenna, inverted 'L' type antenna, top loaded

antenna and umbrella antenna are at a few All India Radio stations. Directional antenna

systems also exist in many All India Radio stations.

6.1 SELF RADIATING MW MAST ANTENNAS

They are broadly of two types :

Mast isolated from ground and fed at its base.

Grounded mast fed at a suitable point along its height

As most of the All India Radio MW towers are of the first category, only they are discussed

here.

The first consideration of such mast is its height in terms of the wave length. What is the

optimum height ? Obviously the main considerations are economy consistent with maximum

coverage and minimum high angle radiation (sky wave).

The relative characteristics of mast height 30o to 225

o (electrical lengths) are given below :

Electrical height in

degrees

Height in wave length

()

Field strength at one

mile V/m

Polar

pattern

30 1/12 186 Please

60 1/6 189 See

90 1/4 196 Fig. 12

135 3/8 214

180 1/2 242

190 0.53 254

225 5/8 276

From the above analysis, it may be seen that as the height of the MW mast increases, the

field strength at one mile increases (range of the transmitter increases) and is maximum for

225o (5/8) of electrical length of the antenna. Examination of the polar

MW Antenna isolated from ground Ref. Drg.No:-STI(T)851,(DC603)

pattern shows that as the height increases, the high angle radiation decreases and the

horizontal gain increases. However at 5/8 height, the presence of side lobe will contribute

high angle radiation and therefore sky waves. Therefore electrical length of 190o (0.53)

would look optimum from the points of view of maximum range, high horizontal directivity

and maximum suppression of high angle radiation. 190o antenna is known as 'Antifading'

broadcast antenna as it eliminates the sky wave interference fading beyond the ground wave

range during night.

The height of the MW tower also will have to be coordinated with the civil aviation

authorities from the point of view of nearness of the airport. Should this require reduction in

actual physical height top loading technique can be adopted. This increases the current

distribution in the vertical portion of the radiator, thereby increases the efficiency of

radiators.

However in special cases such as the AIR's National Channel at Nagpur, the stress is

particularly for the night time service, to provide more sky wave average for which two short

antenna of 60m height (0.3) fed suitably are used.

The MW self supporting mast antenna could be excited in 3 different methods.

The first method requires an insulator at the base of the mast. The second method is called

shunt feed and the third top feed. The comparatively low voltage at the base and top of the

mast antenna, simplifies the operating condition of the insulators and enables to

accommodate a larger power into the mast antenna, than the wire antenna.

Shunt feed, earthed mast overcomes the difficulties of installing and maintaining masts

placed on insulators. The feed line is usually connected to the mast at a height equal to 1/5 to

1/10 the height of the mast.

Polar Patterns

The top fed antenna is fed by means of a coaxial line (or wires vertically forced inside the

body of the mast). The advantages are

More uniform current distribution compared to the base feed.

Absence of supporting insulator

High radiation resistance and high efficiency

6.2 TOP LOADED ANTENNA

It is possible to simulate higher electrical length of the MW antenna for any physically

smaller MW antenna by top loading. A large capacitance disc (insulated from the mast, and

series resonated by an inductance connected across the insulator at the top of short mast

effectively increases the electrical length of the mast.

Another alternative is to use a number of wires in the form of umbrella emanating from the

top of the radiator and secured via insulated rope to the ground . This is particularly valuable

for thin masts. One such umbrella antenna is installed in Nagarcoil; and some other stations

of AIR.

Umbrella Antenna

6.3 'T' AND 'L' ANTENNA

'T' and 'L' antenna find application in broadcasting. AIR have used such types of MW

antenna in the network. This may perhaps be very handy to rig up one for emergency

arrangements. The antenna is secured on two high (100 to 250m) mast (wood or metal),

spaced 100-250 m apart.

The antenna consists of two to sixteen wires spaced 1 to 1.5 m apart. The copper wires are

usually 5 to 8 mm in diameter. The supporting towers may be secured by several tiers of

guys in which insulators are inserted. The antenna down leads directly connect the radio

transmitter. There may not be any need for feeder lines if suitably structured.

The disadvantages are :

Need for two or more masts

Distortion of directional diagram caused by the influence of supporting cables.

The voltage at the base and at the end of wire antenna is very high compared to the mast

antenna,

25M 'T' Antenna

Inverted 'L' Antenna

6.4 NEED FOR EARTH RADIALS

The MW propogates 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 /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 of about 0.4 long (usually 10

swg) at 4 to 12 inches deep. The radial wires are suitably brazed among them forming a

mesh.

6.5 MATCHING THE MW ANTENNA

The MW Power Amplifier output has to be matched to the feeder line which again is to be

matched to the antenna impedance usually by a PI/T/L- network in the Antenna tunning unit

located close to the base of the mast for perfect match. The impedance of the mast at the feed

points can be measured by an impedance bridge VIM. Usually the individual component

values of the PI/T/L-matching networks could be computed using transmitter manufacturer's

information booklet.

6.6 GUY SUPPORT FOR MW MAST

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 /10

or /12 separated by low loss, high mechanical strength insulators to minimise distortion of

radiation pattern due to field induced in them. These insulators are shunted by suitable

inductors to provide d.c. 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

Jullandar, 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

7 FM ANTENNA AND FEEDER CABLE SYSTEM

The Antenna system for FM Transmitters consists of 3 main sub-systems, namely :

a) Supporting tower

b) Main antenna

c) Feeder Cable

7.1 TOWER

A tower of good height is required for mounting the FM antenna since the coverage of the

transmitter is proportional to the height of the tower. For a 100 m height, the coverage is

about 60 km. Wherever new towers were to be provided, generally they are of 100 m height

since beyond this height, there is steep rise in their prices because of excessive wind load on

the top of the tower. At some places existing towers of Doordarshan have also been utilized

for mounting the FM antenna. Provision has also been made on the AIR towers for top

mounting of TV antenna below FM antenna (Aperture for Band III).

7.2 ANTENNA

The main requirements of the antenna to be used for FM transmitters are :

- Wide-band usage from 88 to 108 MHz range.

- Omni-directional horizontal pattern of field strength.

- Circular polarization for better reception.

- High gain for both vertical and horizontal signals.

- Two degrees beam tilt below horizontal

- Sturdy design for maintenance-free service.

Further, depending on the type of tower available for mounting the requirement is for two

types of antenna. The first type is to be mounted on a small cross-section AIR Tower. For

which a pole type FM antenna has been selected. For mounting on the existing TV towers, a

panel type antenna has been used. The cross section of the TV tower at the AIR aperture is

2.4 x 2.4 m. the pole type antenna is quite economical as compared to panel type antenna,

but it can not be used on large area towers. For our requirement, the antennae supplied by

M/s. SIRA have been found suitable.

7.21 POLE TYPE ANTENNA

The pole type antenna is mounted on one of the four faces of the tower. This system will

give a field pattern within a range of 3 dB. The antenna is mounted in such a direction in

which it is required to enhance the signal. The important parameters for this antenna are :

Weight 200 Kg. (for 6 dipoles).

VSWR 1.4 : 1

Gain 5 dB

Rating of each dipole 5 kW

The other important features are :

Very low power radiation towards Transmitter building.

Spacing between dipoles is 2.6 m and all the dipoles are mounted one above

the other on the same face.

Lengths of feed cables of dipoles will be different and has been calculated to give

a beam tilt of 2o below horizontal.

The feed point of the antenna is looking towards ground so as to avoid

deterioration of the insulating flange. This flange consists of high density PVC.

The life of this is expected to be about 7 to 10 years.

The distance of the feeding strip is 240 mm from edge and this should not be

disturbed. All the six dipoles are mounted on a 100 mm dia Pole. This pole is

supported by the main tower.

The antenna is fed through a power divider which divides total power into 6

outlets for feeding the 6 dipoles. The power divider is mounted on a different face

of the tower.

The main feeder cables, power divider branch feeder cables, and dipoles are of

hollow construction to enable pressurization of the system.

The antenna can handle two channels with diplexing.

Suitable terminations are supplied for terminating the output of power divider in

case of failure of any dipole.

7.22 PANEL TYPE ANTENNA

The panel type antenna is to be used on TV tower. Doordarshan have provided an

aperture for FM antenna on their towers. The size of this section is 2.4 x 2.4 mtrs. and its

height is different at different places. The antenna system envisaged for FM broadcasting

consists of a total of 16 panels. For omni-directional pattern 4 panels are mounted on

each side of the tower. Ladders for mounting these panels have already been provided on

the four sides of the tower.

Each panel consists of :

Reflector panel

Two numbers of bent horizontal dipoles and

Two numbers of vertical dipoles

The capacity of each dipole is 2.5 kW. Therefore, each panel is able to transmit 10 kW

power. The reflector panels are constructed of GI bars whereas the dipoles are made out

of steel tubes. Since each panel consists of 4 dipoles, there are a total of 64 dipoles for all

the 16 panels. Therefore the power divider has 64 outlets to feed each of the dipoles. The

power divider will be mounted inside the tower. This antenna gives an omni-directional

pattern when the panels are mounted on all the four faces.

7.3 FEEDER CABLE

For connecting the output power of the transmitter to the dipoles through the power

divider, a 3” dia feeder cable has been used.

This cable is of hollow type construction and has to be handled very carefully. From the

building to the base of the tower, the cable is laid on horizontal cable tray. Along with

the tower this is fixed on the cable rack provided for this purpose. The cable is clamped

at every 1.5 m and the minimum radius of bending of this cable is about 1 m. The cable

has been provided with two numbers of EIA flange connectors of 3 1/8” size on both

ends. Both the connectors are of gas-stop type. The cable connector on the antenna end

i.e. on top of the tower is made gas-through before hoisting. This is achieved by drilling a

hole through the Teflon insulator inside the connector. A dummy hole (drilled only half

way) is already provided by the manufacturer for this purpose.

The weight of the cable is about 2.7 kg per meter and the power handling capacity is about 27

kW. Since enough safety margin has been provided in the power handling capacity, no

standby cable has been provided. This cable can be used later for two transmitters by

diplexing. The attenuation loss of the cable is about 0.44 dB per 100 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 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.

7.31 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. This is

determined wholly by the geometry of its cross section. A transmission line can be

represented as having R, L,C.

Zo of a Feeder Line

The inductance, resistance, capacitance and conductance of the line determine the

characteristics impedance. This is shown in the figure 1. G is the conductance of the line.

The characteristic impedance is given by the following basic formula

C

LZ

foreThere.C&Lofcestanreactorespectw ithnegligiblebecomesG&RsfrequenciehigherAt

CjG

LjRZ

o

o

The characteristics impedance can be lowered or increased depending on some specific

requirement by varying the above two parameters.

a) To obtain a lower Zo than designed, follow as under :

Increase conductor size maintaining the same Conductor to conductor

distance.

Decrease distance between conductor for same conductor size.

Increase no. of wires in each side.

Parallel two or more feeders

Connect lumped shunt capacitors across the line at equal distances.

b) To increase the impedance opposite of above is done.

7.32 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.

Open wire feeder lines

dSZ /2log2760

S

d

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

3. 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.

7.33 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.

Radiation Loss : It is due to irregularity and usually very small for well

designed lines.

Copper Loss

R.F. Wave travels along the exterior of a conductor due to skin effect. The conductor gets

heated up resulting in losses in the feeder line. It can be reduced by increasing the radius

of conductor and also by using more no. of wires in parallel. It is directly proportional to

square root of frequency, so higher the frequency, more the losses.

Earth Loss

In unbalanced open wire lines there is division of charges between ground wires and that

induced in the earth under feeder lines resulting in part of the return current in the ground.

The rotation of return current in the grounded wires and to the total current in live wires

decides earth losses. It can be reduced by laying two nos. of copper wires in the ground

through out the length of feeder wire line and by increasing the height.

Radiation Loss

One cause of radiation from open lines is from the vertical connections at the ends.

Decreasing the height can reduce it, but if height is decreased, the ground losses will increase.

So best way-out is to use better shielding of high potential wires by using greater number of

ground wires.

7.34 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.

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.

120 ohm feeder line is now standardised for modern transmitters.

7.35 230 OHMS COPPER WIRE FEEDER LINE

This type of feeder line is most popular and has been used in all old installations of

10/20/100 kW/MW XTRs. There are total 6 wire (8 SWG, app 4.064 mm). Two inner are

on high potential and four outer are ground conductors

7.36 QUASI COAXIAL FEEDER LINE

In this category of line normally there are two designs :

In which there are 8 inner wires and 8 outer wires each of 8 SWG. This has been

used in all 100/200/300 kW XTRs.

In which there are 12 inner conductors of 6 mm dia and 16 screen conductors of 8

SWG and this has been used at 1000 kW Nagpur, Calcutta (Chinsuraha) and

Rajkot.

Measurement Of Characteristic Impedance, Zo

Zo of a feeder line is given by the relation

scoco Z.ZZ

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.

Zoc & Zsc can be measured with VIM or RF bridge by keeping the line open and shorting

high potential wire (inner) with ground wire (outer) at other end.

Another method utilises the fact that when the feeder line is terminated by its characteristic

impedance, its input impedance is equal to the characteristic impedance. Input impedance is

measured for various termination. The characteristic impedance is equal to that termination

for which input impedance is same as the termination itself.

Pmeter length. The cable and the antenna system should be fed with dry air by means of

a dehydrator provided with the transmitter.

7.37 POWER HANDLING CAPACITY OF FEEDER LINES

The power handling capacity of a line depends upon :

Nos. 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.

Power handling capacity of a line (120 ) is calculated as below : RF Current carrying

capacity of copper conductor x dia of live conductor in inches = 76.2 x .1574 = 10 Amp.

For 8 wires total current is = 10 x 8 = 80 Amp.

Therefore, power handling capacity = I2 x R = (80)

2 x 120 = 768 kW say, 760 kW.

In practice, the maximum voltage that a line can handle/withstand with out flashover etc. is

80% of the D.C. Value of max. voltage.

7.38 PRECAUTION WHILE ERECTING A FEEDER LINE

Bends should be gradual and free of any sharp corners (preferably of 120o or so).

The exact and equal length of wires should be used at bends. To keep the length

same is more important than to maintain the equal spacing as it increases the

series inductance of line.

The poles should be placed at equal distance and symmetrically (app. 15 mtr),

Splitting joints should be smooth and free of any irregularity.

The height above ground should be uniform otherwise ground return current will

differ, varying the earth losses.

8 SATELLITES

Today satellites are used for communication for the following reasons as given below:

1. Multiple access capability, i.e. point-to-point, point-to-multipoint or multipoint to

multipoint connectivity,

2. Distribution capability (a particular case of point-to-multipoint transmission), including t v

programme broadcasting, data distribution e.g. for business services, internet wideband

services, etc., flexibility for change in traffic and in network architecture and also ease of

operation and putting into service.

8.1 SATELLITE SERVICES 1. FIXED SATELLITE SERVICE (FSS):

According to radio regulations (rr no. s1.21), the FSS is a radio communication service

between given positions on the earth's surface when one or more satellites are used. These

stations located at given positions on the earth's surface are called Earth Stations of the FSS.

The given position may be a specified fixed point or any fixed point within specified areas.

Stations located on board the satellites, mainly consisting of the satellite transponders and

associated antennas, are called Space Stations of the FSS. All links between the transmitting

Earth Station and receiving Earth Station are effected through a single satellite. These links

are comprised of two parts,

an Uplink between the transmitting Earth Station and the satellite

a Downlink between the satellite and the receiving Earth Station

Links between two earth stations could use two or more satellites directly interconnected

without an intermediate earth station. The satellite-to-satellite links are called inter-satellite

service (ISS).

2. MOBILE SATELLITE SERVICE (MSS):

According to radio regulations (rr no. s1.25), this is a radio communication service between

mobile earth stations and one or more space stations.This includes

Maritime Mobile Satellite Service (MMSS)

Land Mobile Satellite Service (lMSS)

Aeronautical Mobile Satellite Service (AMSS)

3. BROADCASTING SATELLITE SERVICE (BSS):

This is a radio communication service in which signals transmitted or retransmitted by space

stations are intended for direct reception by the general public using very small receiving

antennas (tvros). The satellites implemented for the BSS are often called direct broadcast

satellites (DBSS). The direct reception shall encompass both individual reception (DTH) and

community reception (CATV and SMATV).

8.2 Geostationary Satellite Orbit (GSO):

Most communication satellites with a few exceptions describe a circular orbit on the

equatorial plane at an altitude of about 36,000 km, resulting in a 24 hour period of revolution

round the centre of the earth. They are thus synchronous with the earth's rotation and appear

relatively motionless in relation to a reference point on the earth's surface. They are

consequently located on the geostationary-satellite orbit (GSO). This characteristics enables

the satellite to provide permanent coverage of a given area, which simplifies the design of

earth stations, since they are no longer required to track satellites moving at considerable

angular velocities and makes more efficient use of radio spectrum and orbital resources.

8.21 COVERAGE:

Satellite links allow for communication between any points on the earth's surface, without

any intermediate infrastructure and under conditions (technical, cost, etc.) which are

independent of the geographical distance between these points, provided they are located

within the satellite coverage area. In the case of GSO satellite, the points to be served must be

situated, not only in the region of the earth, visible from the satellite, but also within the

geographical areas covered by the beams of the satellite antennas: these areas arecalled the

coverage areas of the communication satellite system.

"Round an axis of revolution passing through the centre of the earth and the satellite, this

area falls within a cone with the satellite at its apex and apex angle of 17.3˚ which also

corresponds to a cone with the centre of the earth at its apex and an apex angle of 152.7˚.

These conditions describe an area from which the satellite is visible from the earth stations at

an angle of at least 5˚ above the horizontal direction."

8.22 PROPAGATION DELAY

An important feature of the satellite link is the propagation delay. In the case of GSO

systems, owing to the distance of the geostationary satellite from the earth, the propagation

time between two stations via the satellite can reach approximately 275 milliseconds.

8.3 MULTIPLE ACCESS

Multiple access is the ability for several earth stations to transmit their respective carriers

simultaneously to the same satellite transponder. This feature allows any earth station located

in the corresponding coverage area to receive carriers originated by several earth stations

through a single satellite transponder. Conversely, a carrier transmitted by one station into a

given transponder can be received by any earth station located in the corresponding coverage

area. This enables a transmitting earth station to group several carriers into a single

destination.

8.31 FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)

In FDMA, each earth station is allocated a specific frequency (with the necessary bandwidth)

for the emission of a carrier forming part of a multiple-destination multiplex. Each of the

corresponding station must receive this carrier and extract the channels intended for it from

the baseband. FDMA is often associated with frequency division multiplexing (FDM).

However, FDMA can also be combined with other types of multiplexing and modulation,

especially time division multiplexing (TDM) with digital modulation, which generally uses

phase shift keying (PSK).

8.32 TIME DIVISION MULTIPLE ACCESS (TDMA)

In TDMA, each station is periodically allocated, on the same carrier and within a "frame", a

period of time (a burst), during which it emits a digital signal forming part of a multiple-

destination multiplex. Each corresponding station receives this burst and extracts from it the

digital channels intended for it. multiplexing associated with TDMA is effected by TDM, the

telephone (or data transmission) channels being themselves coded digitally (e.g. PCM). The

carrier is generally phase shift keyed by the digital signal.

8.33 CODE DIVISION MULTIPLE ACCESS (CDMA):

In CDMA, the transmitted signals are not discreminated by their frequency assignment (as in

FDMA) nor by their time slot (as in TDMA), but by a characteristic code which is

superposed on the information signal. Multiple access can also result from various

combinations of FDMA and/or TDMA and/or CDMA and can be performed or changed in

the satellite by on board processing (OBP).

8.4 TRANSPONDER:

Basically, satellite transponders receive, amplify, frequency translate and retransmit various

types of communication signals. Therefore, a transponder is the series of interconnected

units which forms a single communication channel between the receive and transmit antennas

in a communication satellite.

8.5 Altitude and Orbit Control System (AOCS):

This subsystem consits of rocket motors that are used to move the satellite back to the correct

orbit when external forces cause it to drift off station and gas jets or inertial devices that

control the attitude of the spacecrafts. Control of the attitude of a spacecraft is necessary so

that the antennas, which often have narrow beams, are pointed correctly at the earth.

8.51 ALTITUDE CONTROL SYSTEM:

There are two ways to make a spacecraft stable when it is in orbit and weightless. The entire

body of the spacecraft can be rotated at 30 to 100 rpm to provide a powerful gyroscopic

action, which maintains the spin axis in the same direction, such spacecrafts are called

spinners. Alternatively, three momentum wheels can be mounted on three mutually

orthogonal axes of the spacecraft to provide stability. The momentum wheel is usually a

solid disk driven by a motor, rotating at high speed within a sealed, evacuated housing.

Increasing the speed of the wheel increases its angular momentum, which causes the

spacecraft to proceed in the opposite direction, according to the principle of conservation of

angular momentum. With three momentum wheels, rotation of the spacecraft about each axis

can be commanded from earth by increasing or decreasing the appropriate momentum wheel

speed. This is called three-axis stabilization.

Summary of frequency bands used in the FSS for GSO

Frequency Bands (GHz) Typical Utilization Current

Denomination

Up path

(Bandwidth)

Down path

(Bandwidth)

6/4 (c-band)

5.725– 6.275

(550 MHz)

3.4– 3.95

(550 MHz)

National satellites {(Russia : Statsionar

and Express International (Inter-

Sputnik)

5.850– 6.425

(575 MHz)

3.625 – 4.2

(575 MHz)

International and domestic satellites.

At the present the most widely used

bands : Intelsat.

National satellites : Westar, Satcom and

Comstar (USA), Anik (Canada), STW

and Chinasat (China), Palapa

(Indonesia), Telecom (France), n-star,

(Japan)

6.725–7.025

(300 MHz)

4.5– 4.8

(300 MHz)

National satellites

8/7

(x-band)

7.925– 8.425

(500 MHz)

7.25– 7.75

(500 MHz)

Governmental and Military satellites.

13/11

(ku-band)

12.75– 13.25

(500 MHz)

10.7– 10.95

11.2 - 11.45

(500 MHz)

National satellites

13-14 /11-12

(ku-band)

13.75– 14.5

(750 MHz)

10.95– 11.2

11.45– 11.7

12.5 - 12.75

(1000 MHz)

International and domestic satellites in

region 1 and 3. Intelsat, Eutelsat,

(Russia) Eutelsat, Telecom 2 (France),

DFS Kopernikus (Germany), Hispasat –

1 (Spain)

10.95– 11.2

11.45– 11.7

12.5– 12.75

(750 MHz)

International and domestic satellites in

region

2. Intelsat, Anik b and c (Canada), g-

star (United States), Hispasat-1 (Spain)

18/12 17.3 - 18.1

(800 MHz)

bss bands Feeder link for BSS plan

30/20

(ka-band)

27.5– 30.0

(2500 MHz)

17.7– 20.2

(2500 MHz)

International and National satellites,

various projects under study (Europe,

United States, Japan), n-star (Japan),

Intalsat (Italy)

9 ANALOG/DIGITAL RADIO NETWORKING RECEIVE TERMINAL

The analog/digital radio networking receive terminal is used to receive satellite radio signal

to feed the relay transmitter or for monitoring purpose. The basic system consists of a

parabolic dish antenna with feed, outdoor units (LNBC) and indoor units consisting of:

1. LNBC C/O Unit

2. Two Analog Receiver and two Digital receivers with low loss RF cable connecting

outdoor units to indoor units.

The integrated system is designed to satisfy the overall performance of Radio Networking

Receive Terminal, essentially the end results with an interconnecting low loss RF cable of

length 50m between outdoor and indoor units. Also separate 3 core cable assembly of 50m

lengths is connected between indoor units for LNBC change over. Biasing to LNBC is

supplied through RF cable.

The Analog/Digital Radio Networking Receive Terminal receives the signal in frequency

range 2.5 to 2.7 GHz (S Band) through 3.66m P.D.A. This signal is converted to 950 to 1150

MHz in the Low Noise Block Converter (LNBC). The down converted L Band signal is fed

to LNBC change over unit through 50m long low loss cable. The LNBC change over unit

provides biasing to the selected LNBC through the same RF cable. LNBC change over

monitors the health of selected LNBC and in case of failure change over to standby LNBC is

done through control cable (3 core). L Band RF signal received by LNBC changeover unit is

divided into four by four way splitter. L Band outputs from four-way splitter are connected to

two Analog and two Digital receivers through F to N low loss of 2m lengths.

The Analog and Digital receivers further down convert and demodulates these signals to

provide rebroadcast quality audio output.

9.1 COMPOSITION OF RADIO NETWORKING RECEIVE

TERMAINAL

The Analog/Digital Radio Networking Receive Terminal consists of following subunits:

1. 3.66m PDA

2. LNBC

3. LNBC change over

4. Coaxial cable

5. Cable assembly-3 core

6. Digital Audio Receiver with L Band input

7. Analog SCPC receiver with L Band input

8. Cable assembly N (M) to F (M)

TECHNICAL SPECIFICATION

Specification of Antenna

Type and Size : PDA (Perforated aluminum alloy)

3.66m Dia.

Receive Band : S - Band

Feed : Prime Focus

Polarization : LHCP

Frequency Band : 2.5-2.7 GHz

Gain : > 36.5 dBi (Nominal) at 2.5 GHz

Radiation/Receive Pattern : As per CCIR Rec. 580-1

Type of Mount : Elevation over Azimuth

Antenna Drive : Manual

Pointing Accuracy : Better than +/- 0.5 deg.

Steerability : Elevation : 30-80 deg.

Azimuth :+/- 30 deg.

V.S.W.R : 1.3 Max.

Feed Return Loss : 15 dB

Wind Speed : Operational : 75 Kmph

Survival : 160 Kmph

Axial Ratio : 3.5 dB

Focal length/Dia : 0.4

Feed Impedance : 50 0

Specification Of LNBC

Input Frequency : S-Band (2.5-2.7 GHz)

Output Frequency : L-Band (950-1150 MHz)

Noise Temperature : < 45 K

Conversion Gain : >50 dB

Gain Flatness : < 2 dB p-p in any 40 MHz Band

Gain Variation : < 2 dB p-p

Minimum Image Rejection : -60 dB

Input Return Loss : 12 dB

Output Return Loss : 12 dB

Spurious/Harmonics : -50 dBc

LO Stability : Better than +/- 2 ppm

Phase Noise

1 KHz : -60 dBc

100 KHz : -80 dBc

Specification of Analog Receiver

Input Frequency : 950-1450 MHz

C/No Threshold : 64 dBHz

Input Impedance : 75

Transponder Tuning : 1-24

I.F Frequency Tuning : 50-90 MHz

Tuning Steps : 10 KHz

Demodulation : FM

De- Emphasis : 75 sec

Audio Expanding : 1:2

Line Output : -10 to +9 dBm

Audio Distortion : < 1%

Audio Frequency Response : 30 Hz-15 KHz

S/N : 65 dB

Audio Output Impedance : 600

Specification of Digital Receiver

Input Frequency : 950-1450 MHz

Input Level : -30 to –70 dBm

Input Impedance : 75

Frequency Resolution : < 25 KHz

De-modulation : QPSK

Carrier Lock Range : Better than +/- 1.0 MHz

FEC De-coding : Rate ½ viterbi

Mode : Mono, dual Mono, Joint Stereo

Receiver Eb/No (10-5

BER) : 6.5dBHz (Threshold)

Audio Coding/Compression : ISO MPEG-1 Layer II

Audio Program Channel Rate : 128, 192, 256 Kbps

Audio Output Level : +9 dBm (max) adjustable

Audio Distortion : < 0.5 % ( at +9dBm O/P)

Frequency Response : +/- 1.0 dB (20 Hz – 20 KHz)

S/N : Better than 75 dB

Cross talk : Better than 75 dB

Audio Output Channel : L, R, L+R

Audio Output Impedance : 600

System level specification of Analog/Digital Radio Networking Receive Terminal

S-Band

size : 3.66 m

G/T : 11.7 dB/K

Polarization : LHCP

C-Band

size : 6.1m

G/T : 24 dB/K

Polarization : Linear H/V

System Characteristic

Mode : SCPC

RN Channel Bandwidth : 200 KHz

Sat EIRP EOC/RN carrier : -120 dBm

Analog System

Modulation : Companded FM

Base Band : 10/15 KHz

Compression : 2:1

Pre-Emphasis : 75 sec

Peak Deviation : +/- 75 KHz at + 9 dBm

ED suppression : 40 dBc

Digital System

Modulation : QPSK

FEC : 1/2 Convolution coding

Base Band compression : ISO MPEG-1 Layer II

MUSICAM

selectable rates

Base Band : 20Hz - 20 KHz

Mode : Stereo, Joint Stereo,

Mono, Voice/Data Channel

Insat System Details

CXS Band Transponder S1 S2

Uplink Frequency : 5870 5910

Downlink Frequency : 2570 2610

Polarization : Uplink : linear H/V

Downlink : LHCP

Sat EIRP : 42 dBw

S.F.D. : -90 dBw/m2

CXC Band Transponder

Uplink Frequency Band : 5950-6390

Downlink Frequency Band : 3725-4165

Polarization : Uplink : linear H/V

Downlink : linear H/V

Sat EIRP : 38 dBw

S.F.D : -90 dBw/m2

Brief Description

The LNBC housing (S-Band) consists of two S-Band LNBC assemblies connected in (1+1)

standby mode for redundancy. Two RF switches have been used to make the signal path

through main or standby LNBC assembly.

The LNBC receives the RF signal in the frequency range of 2.5-2.7 GHz from the feed and

down converts the signal into L-Band range of 950-1150 MHz which is then fed to the

satellite interface unit(LNBC Changeover Unit) through a 50m length RF cable.

LNBC changeover unit contains following modules

1. LNBC Changeover Logic

2. Four-way Splitter

3. AC/DC converter

BLOCK SCHEMATIC OF S-BAND ANALOG/DIGITAL R N TERMINAL

BLOCK SCHEMATIC OF LNBC HOUSING

RF IN

BPF

DC SUPPLY TO ACTIVE MODULES

IF OUT

LNA MIXER

REF TCXO PLO

BPF IF

AMP. BIAS TEE

SIMPLIFIED BLOCK DIAGRAM OF S-BAND LNBC

SCHEMATIC OF 4-WAY DIVIDER

9.2 SSC 1100 L-2 SCPC RECEIVER

It is Single channel Per Carrier FM receiver which is frequency agile over a 500 MHz band

from 950 MHz to 1450 MHz. It can select any receive frequency within this range on 10 KHz

steps. Frequency selection is made via front panel thumb wheel switches. The L-band input

signal to it should have a frequency stability of 20 KHz.

The L-band to IF converter is comprised of three oscillator/mixer combination sub-stages as

well as tracking filter for image rejection. When a carrier is selected using the thumb wheel

switches a set of EPROMs read the switch condition and adjusts the synthesized oscillators

accordingly. This results in an IF that directly represents the programming frequency desired.

The IF is then passed to the programme audio demodulator section. In programme audio

demodulator the selected receive carrier is demodulated, processed and expanded to its

original form. The rear panel contains the facilities for balanced audio output, signal strength

indication and AFC voltage output.

RECEIVER SPECIFICATION

Input Frequency L - band (950 – 1450 MHz)

Carrier Selection 52-88 MHz (10 KHz Steps)

Noise Bandwidth 200 kHz

Companding 2:1, single band

Emphasis 75 sec

Audio Output (nominal) +9dBm Max@ 75kHz deviation

0dBm Max@ 44.7 kHz deviation

@ 600 ohms Bal.Front Panel Adjustable

Frequency Response 1 dB Max.

Audio Response 30 Hz – 15 kHz

Input Connector Type F Female

LNB Power +18 VDC Present on Input Connector Centre

Conductor (300ma max)

THD+N 1 % @ 1 kHz

C/No 68 dB.Hz, operational 64 dB.Hz, threshold,

impulse noise @ one click per second.

Signal to Noise Ratio >75dB@ 75kHz deviation

AC Power 220 VAC @ 50 Hz

CONCLUSION

This vocational training was really a very educative and enlightening experience for

me and my objective of getting exposure to the industrial and the corporate world was

successfully accomplished. I got the opportunity to spend time with the professionals

and learn and get exposure in the governmental organization which is broadly in the

field of communication and broadcasting. I got the opportunity to see various

instruments used in the field of communication like the various antennas, power

supply equipments, the cooling arrangements, microwave devices, various digital

switching circuits, the transmitter, the receiver, etc. and learened how the theoretical

knowledge that is being imparted to us during the curriculum of engineering is applied

in the industries.

In the end, I would once again thank all the people who made such kind of training

possible for me.

REFERENCE

Basic Course in Communication: AIR Study Material

Radio Transmission: AIR Study Material

Antenna and Feeder System: AIR Study Material

FM Transmitter Specifications Handbook: BEL Publication

Handbook on FM Transmitters: GCEL Publication

Cooling Systems: AIR Study Material

Cooling System Technical Specifications Handbook: Voltas Publication

Earthing Systems: AIR Study Material

Studio Acoustics: AIR Study Material

Wireless Communication: Theodore S.Rappaport