1. INTRODUCTION

Doordarshan is the public television broadcaster of India and a division of

prasarabarathi a public service broadcaster nominated by the government of India. It

is one of the largest broadcasting organizations in the world in terms of the

infrastructure of studios and transmitters. Recently it has also started digital terrestrial

transmitters. On September 15 2009, Doordarshan celebrated its 50th anniversary.

Beginning:

Doordarshan had the modest beginning with the experimental telecast

starting in Delhi on 15 September 1959 with a small transmitter and a makeshift studio. The

regular daily transmission started as a part of all India radio. The television service was

extended to Bombay and Amritsar in 1972. Till 1975 seven Indian cities had television

service and Doordarshan remained the only television channel in India. Television services

were separated from radio in 1976. Each office of all India radio and Doordarshan were

placed under the management of two separate director generals in New Delhi. Finally,

Doordarshan as a national broadcaster came into existence.

Channels:

Presently, Doordarshan operates 19 channels-two all India channels-DD

national and DD news, 11 regional languages satellite channels (RLSC), four state networks

(SN), an international channel, a sports channel and two channels (DD-RS& DD-LS) for live

broadcast of parliamentary proceedings.

On DD national (DD-1), regional programs and local programs are

carried on time-sharing basis. DD-news channel, launched on 3 November 2003, which

1

replaced the DD-metro entertainment channel, provides 24-hour news service. The regional

languages satellite channels have two components- The regional service for the particular

state relayed by all terrestrial transmitters in the state and additional programs in the regional

language in prime time and non-prime time available only through cable operators. DD-

sports channel is exclusively devoted to the broadcasting of sporting events of national and

international importance. This is the only sports channels which telecast rural sports like

Kho- Kho , Kabaddi etc…. something which private broadcasters will not attempt to telecast

as it will not attract any revenues.

2

CHAPTER-2

BASIC TRANSMISSION SYSTEMS

   3

2. BASIC TRANSMISSION SYSTEMS

There are three types of Basic transmission Systems.

1. Cable transmission

2. Direct to home

3. Transmitter service

2.1 CABLE TRANSMISSION:

In addition to wireless transmission by broadcast stations, the cable

TV system provides a distribution system with co-axial cable. It is similar to a wired

telephone system but it is used for TV programs. The RF carrier signals ate supplied

so that a tuner can be used to select the desired channel cable TV has become very

popular because more channels are provided and strong signals can be supplied for

areas on which the antenna signal is not good enough cable television started as a

means by providing signals to communities that could not receive broadcast stations,

either because of distance or shadow areas in which the signal was too weak.

Today cable TV has developed far beyond that into huge systems that cover

huge areas; even for locations having food reception the reason is that cable TV does

not have the restriction of channel allocations for broadcasting. It offers up to 36

channels so many programs that not available on broadcast television reach the cable

operator via satellite transmission.

4

From the above figure the wire mainly contains three layers are core, cladding, sheath.

Sheath is top most layer which gives the protection from the losses, radiation can be

prevented by using the proper shielding.

Core is the inner most layer which plays a vital role in transmitting the

data and from the above it can be shown.

Cable channels:

Each cable channel is 6MHz wide for the AM picture signal and the

FM sound signal. However the cable signals are not radiated therefore, the

frequencies in between channels 6 and 7 can be used without interfacing with other

services. These mid band cable channels range from 88 to 176 MHz also all the low

band VHF channels (7 to 13) are used for cable TV. Those VHF channels not

assigned in a given area.

Cable distribution:

The head end provides the program signal for all channels. Local and distant

broadcasts are picked up by an antenna which is mounted on a very high tower, in

order to extend the line-of-sight distance.

The RF losses in co-axial cable are high especially in the 36 channel system that

operates in the cable TV super band in the distribution system the main line the trunk.

From the trunk branch lines extend out for groups of subscribes the line for each

subscriber is called a drop.

Power Supply:

1. Maximum demand/capacity: 30KW

2. Monthly average consumption : 6000 units

3. Monthly average expenditure’s 40000/month

2.2 DIRECT TO HOME (D.T.H):

Satellite TV, a direct to home (DTH) from the satellite through set-top

box that means there is no middle man (cable operator). So DTH puts an end to all the

problems like unreasonable charges, cable operator’s strike, power outages, not

getting your favorite channels and channels shifting their channel number position’s

5

WORKING OF DTH:

In DTH you receive the signal from satellite to a small dish antenna

installed at the roof top of your house. This signal is decoded by a set-top box which

is provided by the broadcaster and connects to the dish antenna directly with a cable.

The set-top box in turn connects to your TV. So you become the master of your

entertainment and watch the channel you wish and pay for only those channels which

you wish to watch.

Bands:

Frequency band up link down link

C- Band 6 GHz 4 GHz

X- Band 8 GHz 7 GHz

Ku-Band 14 GHz 11 GHz

Ka-Band 30 GHz 20 GHz

The above mentioned are the some of the bands which are useful in satellite

communications, military applications etc. the bands are mainly useful in the set the

Parameters of the channel for receiving.

Satellite transmission: C-Band:

Frequency band 4000 to 8000 MHz

Large sized dish required for reception

Useful to system providers / cable operators

Mainly used for contribution and distribution

Satellite transmission: Ku-Band:

Frequency band 12.5 to 18 GHz

Smaller dish (60 – 90 cms ) needed for reception

Most useful for DTH application

Coverage limited as compared to C-band due to narrow beam

Reception susceptible to failure during heavy rains

6

Now a days the present DTH services of different companies like Tatasky, airtel

Videocon, Sun dish follows the ku band transmission of their services. One of the

properties (increase in antenna gain) of higher frequency (Ku-band) in satellite

communication is that for a given power, it enables use of a smaller size antenna

compared to lower frequency (C-band). Due to this, Ku-band is preferred in DTH service,

which needs smaller size antenna in individual homes to facilitate ease of mounting etc.

Uplinked frequency from the satellite (geo satellite) is down linked using a parabolic

antenna which is used as a receiving antenna here (also called as dish antenna). The

parabolic antenna is micro wav antenna. The transmitting and receiving antennas for use

in the micro wave spectrum (1000-100,000 MHz) tend to be directive i.e. high gain and

narrow beam- width in both horizontal and vertical planes. As the frequency increases,

the wave length decreases and thus it becomes easier to construct an antenna system that

are large in terms of wave lengths, and which therefore can be made to have greater

directivity. The most important practical antenna in micro wave frequency range

parabolic reflector or paraboloid or micro wave dish.

A parabola may be defined as the locus of a point which moves in such way that its

distance from the fixed point( called focus) plus its distance from a straight line (called

directrix ) is constant. A parabola with focus F and vertex O is a two dimensional plane

curve. The equation of parabola curve in terms of its coordinate is given by y^2 = 4fx.

The open mouth (D) of the parabola is known as the aperture. The ratio of focal length to

aperture size (i.e. f/D) known as f over D ratio is an important characteristics of parabolic

reflector and its value usually varies between 0.25 to 0.50.

This implies that the entire wave thus, reaching at the aperture plane is in

phase. This shows that a wave front- a surface of constant phase-is created in the aperture

plane. Therefore, the rays are parallel to the parabolic axis, because rays are perpendicular

to a wave front. Since all the rays are in the phase, so a very strong and concentrated

beam radiation is there along the parabolic axis.

Alternatively, all the emanating from the source at focus and reflected by

parabola are traveling the same distance in same time in reaching the directrix and hence

they are in phase. The principle of equality of path length is maintained between all rays

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of two wave fronts. Putting in another way where there is path length difference between

the two rays cancellation action will take place. Hence the geometrical properties of

parabola provide excellent microwave reflectors that lead to the production of

concentrated beam of radiation.

In fact, parabola converts spherical wave front coming from the focus into a

plane wave front at the mouth of the parabola. The part of radiation from the focus which

is not striking the parabolic curve as spherical wave appears as minor lobes. Obviously

there is waste of power. This is minimized by partially shielding the source.

Further if a beam of parallel rays is incident on the parabolic surface, they will

be focused at a point i.e. Focus. This is in effect due to the principle of reciprocity

theorem already discussed which says that properties of antenna are independent whether

it is for transmission or reception. This parabolic reflector is directional for reception case

also as only rays coming perpendicular to directrix will be focused at the focus and not

others due to path length difference. Parallel rays are known as collimated.

A parabola is two dimensional plane curves. A practical reflector is a three

dimensional curved surface. Therefore a practical reflector is formed by rotating a

parabola about its axis. The surface so generated is known as paraboloid which often

known as microwave dish or parabolic reflector. Now a low noise block converter usually

known as LNB is used at the focus point of paraboloid to receive the down linked

frequency. The signal from LNB is received by the sophisticated receiving units that are

separately used for different frequencies they received.

Advantages of DTH TV:

1. Digital picture:

The picture quality in DTH is much better. The quality of

the picture is uniform across all channels.

2. Digital audio:

We get the stereo phonic sound. So if we have got a

home theatre, connect it to your set-top box we will get better sound effects.

8

3. Electronic program Guide (EGP):

It’s an on-screen guide that shows the program

schedule or listing of all channels. So we can find out what’s playing on any

channel. We can also set remainders for program’s we wish to watch and get

synopses of the program.

2.3 TRANSMITTER SERVICE:

1. High power transmitter (HPT):

Transmitter power 10KW

Distance covered by above transmitter is 60km-100km

Eg: located in Rajahmundry

2. Low power transmitter (LPT):

Transmitted power 100w-500w

Local area transmitter covers distance around 21kms

Eg: located in Kakinada

3. Very low power transmitter (VLPT):

Transmitted power – 10w

Distance covered is around 5-10Km

Eg: located in Yanam

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CHAPTER-3

BLOCK SCHEMATIC OF LPT

10

3. BLOCK SCHEMATIC OF LPT

FIG 3.1 : Block schematic of LPT

DG ROOM:

11

Work shop

Generator power supply

Receiver dish area

P.D.A

Monitoring transmitter input

Rack rack rack

Mast antenna

The generator generates 35KVA power supply.

Receiver Dish Area:

In receiver dish area parabolic dipole antennas (P.D.A) are used. The shape of the

dish mist be parabola because the parabola has specific focal point. When the information

from satellites through space is incident on parabolic dishes it reflects back and for parabolic

surfaces by the principle of foci, the rays incident on parabolic surfaces reflects back by the

cross the focal point. So that at focal point the receiver information by the dish is the exact

replica of transmitted information by the satellite.

P.D.A:

Passive receiver

It receives signal from satellite

If the size of the dish increases gain is also increases. So that receiving

capability increases.

MONITORING RACK INPUT RACK TRANSMITTER RACK

For case of understanding we can divide the functioning of input rack in to three blocks

1. Receiving section

2. Transmitting section

3. Mast and antenna

12

T.V

WAVE FROM MONITORING

DEMODULATION

RECEIVER-1

RECEIVER-2

RECEIVER-3

V.C.R

P.G.

SWITCHER

EXCITER

DRIVER AMPLIFIER

DIVIDER

COMBINER

RECEIVING SECTION:

P.D.A receives information from satellites which are located in geostationary orbits.

The following are the point lobe considered while placing P.D.A’s

Look angle

Azimuthally angle

Elevation angle

Latitudes and longitudes

PARKING ANGLE:

The angle at which the satellite placed in geostationary satellite is called

parking angle.

LOOK ANGLE:

The angle at which the P.D.A is placed on earth with respect to latitudes and

longitudes is called look angle.

To fix the look angle, azimuthally angle and elevation angle should be

fixed.

Azimuthally angle determines the look angle in horizontal direction.

Elevation angle determine the look angle in vertical direction.

Latitudes and longitudes steels about the situation of P.D.A in

geometrical plane

13

TRANSMITTER RACK:

Fig3.2: Transmitter Rack

1. Audio- Video switcher:

This unit performs the function of selecting one of the four sets audio and

video inputs. The video input levels to the unit are 0.5 – 1.5 Vp-p and +10dBm

respectively. This unit as an associated power supply to derive +15v, +5v and -15v

required for its sub units from 230V AC. One of the programme sources (video or

audio) can be selected using ‘PUSH’ button switches available on the front panel.

2. Exciter:

The audio and video outputs from audio-video switcher unit are fed to

exciter unit. The audio input is fed directly to the aural modulator while the video

signal is passed through a low pass filter before being fed to its respective modulator.

The audio is frequency modulated using 33.4MHz IF. While video signal is amplitude

modulated using 38.9 MHz IF. The modulated signals are combined and then up

converted to the desired transmitted channel frequency. The video output power level

after vestigial sideband filter and mixer is 10MW synchronous peak while audio is

1mW ALC (automatic level control) input is available on VSBF mixer unit which can

be fed from P.A stages to keep the overall transmitter power output constant. The

power supply need +16V and +28V for the unit is supplied by P.S.U.

14

AUDIO VIDEO

SWITCHER

EXCITER DRIVER AMPLIFIER

DIVIDER COMBINER

3. Driver Unit:

The up- convertor signal from the exciter is fed to an attenuator which is

placed at the front panel and adjusting the input levels suitably. The signal is

amplified using class A driver stages. The overall gain of the amplifier can be

adjusted by the front panel attenuator control to be about 33db.

The output of the amplifier is fed to the directional coupler where in

samples of transmitted and reflected power is obtained and fed to metering unit which

defects the signal and feds suitable voltage to a DC meter placed at the front panel.

The three position switch on the front panel selects the parameters to be monitored

viz. vision, power, aural power and reflected power. Readings are to be read with

black picture aural power indication is valid for black picture only.

A separate exhaust fan operating at 230V AC is provided for blowing off air in the

driver unit to control the temperature raise for operation of driver amplifier.

A portion of output power is taken to the back panel of the driver unit for monitoring

purposes. The front panel output constant called ‘Ale’ can be fed to the exciter ALC

in to the driver output constant at the set level. The availability of the input power

“28V” to the unit is indicated through a green L.E.D on the front panel ‘DC Check’

facility is provided to monitor currents of 4 stages of power amplifiers by patching a

‘chord’ meter on combiner /divider unit.

4. Power Amplifier Unit:

The power amplifier unit comprises of two similar 60W power amplifier

modules. The R.F power output from the driver unit is divided in to two parts using

the divider in the divider/combiner unit and fed to each 5.0W power amplifiers. Each

power amplifier is fed with power input which is amplified to SOW (Sync peak) by

four class A paralleled power amplifier stages with a gain of approx 6 & 10dB for

channel 9.10 & 11; 12 respectively.- this output is fed to a directional coupler for

obtaining samples of forward & reflected power (30 db coupling) for monitoring

purposes for the control unit. The control unit also obtains the temperature of heavy

sink assembly through a thermistor.

15

Separate power supply is made available for each power amplifier (28V, 20A).

The power supplies are placed at the bottom portion of chassis assembly. A DC

voltage proportional to current drawn by each of the transistor in power amplifier is

available from “bias unit “on DC check connector placed over the front panel. This

can be monitored on the current meter provided on divider combiner unit through

suitable patch cord provided separately.

There are two types of transmitters:

1. V.H.F transmitter

2. U.H.F. transmitter are discussed in the next chapters

.

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CHAPTER-4

VHF &UHF TRANSMITTERS

17

VHF TRANSMITTER

In this transmitter the frequency ranges from 224 MHz-231MHz.

Video signal

DRIVER

Fig: Block Diagram of Exciter unit

18

LPF VIDEO PROCESSOR

VISION MODULATOR

POWER CONTROLLER

VESTIGIAL SIDEBAND FILTER

CONTROL OSCILLATOR

IF OSCILLATOR

AURAL MODULATOR

AUDIO SIGNAL

EXCITER:

Exciter provides amplitude modulated visual drive of 10MW. Sync peak and a frequency

modulated all drive of 1mW required for the power amplifier stages of 100W TV transmitter

at the designated channel frequencies. It consists of the following individual units:

1. Video signal

2. Low pass filter

3. Video processor

4. Vision modulator

5. IF oscillator

6. Control oscillator

7. Aural modulator

8. Audio signal

9. Power combiner

10. Vestigial side band filter

11. Driver

12. +12V regulated power supply

Video signal:

The video signal is limited to 5 MHz by the low pass filter and group delay by its

corrected group delay introduced by it is corrected by the active group delay

equalizer.

Low pass filter:

The LPF is used to limit the video frequency to 5MHz only, and it attenuates the

video signal more than 20dB above 5.5MHz the group delay introduced by steep

falling characteristic at 5.5 MHz is corrected using 5-6 active group delay equalizer

LPF unit consists of single PCB consisting of a video amplifier section and clamp

pulse generator section.

Video Amplifier:

It amplifies the video signal to level sufficient to modulate the vision carrier in the

visual modulator unit. The video input to this unit is at level of 1Vp-p clamp pulse.

19

UHF TRANSMITTER

In this transmitter the frequency range is from 564-574MHz. it requires 500W

power. DD news is broadcasted in channel 33. This transmitter is manufactured by Bharat Electronics

(BEL)

BLOCK DIAGRAM OF UHF TRANSMITTER:

Video signal splitter & PA

FIG: Block Diagram of UHF Transmitter

Linearity corrector:

Linearity corrector operates in the UHF TV band of 470-600MHz and its function

is to correct the non-linearity’s that occur in power amplifiers operated in this band. Non linearity in

TV amplifiers are measured in terms of 3-tone IMD and differential gain . The linearity

corrector is a pre-distorter circuit that is placed ahead of the power amplifier and pre- corrects the

above mentioned distortion so as to reduce them at the power amplifier output.

Up- convertor:

The up-convertor unit combines modulated vision IF an aural IF signals and translates to

respective channels frequency suitable for transmission. The unit has in-built power supply.

The status and fault information are displayed on front panel of the unit.

Splitter:

The linearity corrector output is dividing into four equal amplitude and phase outputs to fed

four PA to get the required output power. To achieve this connection, a four way splitter by

20

Linearity corrector

Base band corrector

exciter Up convertor

terminating unused parts. The four ways splitter doesn’t have any achieve components for

isolation resistor. It is a micro strip circuit design based on Wilkinson’s power divided

principles.

Combiner:

The two way power combiner is a sub unit in the 500W transmitter there are such units. Two

way combiner is used to combine the outputs of four amplifiers. For the first level combining

pairs of amplifiers are combined output or pairs of amplifiers is combined in a second kevel

of combining resulting in 600W peak sync output power. All units are identical electrically

and mechanically and are interchangeable. It is based on the Wilkinson’s power combiner

principle. The combiner is realized as a micro strip line on a PCB substrate with a isolation

resistor for isolating all the ports.

21

CHAPTER-5

RECEIVING &TRANSMITTING SECTION

22

5.1 RECEIVING SECTION:

Fig: Block Diagram of Receiving Section

The parabolic dish antenna is metal structure with a shape of half circle, and apart

from that at a distance a feed arm is held with support in air to which a low noise amplifier in addition

to the low noise block convertor and the internal relay station there is a digital broadcast receiver in

for monitoring and later on re-transmission of the signal is done in the transmitting section.

5.2 TRANSMITTER SECTION

Fig: Block Diagram of Transmitter Section

23

Parabolic dish antenna

Low noise amplifier

Low noise block convertor

Digital video broadcast receiver

antenna

V1 AUDIO &

V2 VIDEO

V3 SWITCH

EXCITER

DRIVER AMPLIFIER

POWER AMPLIFIER

A1 A2 A3

CHAPTER-6

ANTENNA SECTION

6. ANTENNA SECTION

6.1 ANTENNA BASICS:24

What is an antenna?

An Antenna is a transducer which transmits or receives electromagnetic waves.

Or

An antenna is a metallic object which used to convert high frequency current into electro-

magnetic waves and vice versa.

What is radiation?

Antennas radiate electromagnetic waves radiation will result from the flow of high-

frequency current in a suitable circuit. This is predicted mathematically by the

Maxwell equations, which show that current flowing in a wire is accompanied by a

magnetic field around it. If the magnetic field is changing, as it does with alternating

current, an electric field will also be present. A proportion of the electric and

magnetic field is capable of leaving the current-carrying wire. How much of it leaves

the conductor depends on the relation of its length to the wavelength of the current.

Radiation pattern:

The radiation pattern of an antenna is a graphical representation of the radiation of the

antenna a function of direction. When the radiation is expressed as field strength E Volt

per meter, the radiation pattern is a field strength pattern. If the radiation pattern is

expressed is term of power per unit solid angle, the resultant pattern as power pattern. A

power pattern is a proportional to the square of the field strength pattern.

Formula for calculation of field strength:

Field Strength= 2.85 √P ht.hr/ʎd2 milli volt/meter

P=Transmitted Power in KW

Ht=height of transmitted antenna in meters

Hr=height of the receiving antenna

D=the distance from transmitting antenna in Meters/I- wave length of signal

Field Strength in DBV/m = 20 log (F.S in milli volt per meter)

25

Isotropic antenna:

An Isotropic antenna is a standard reference antenna radiating equally in all direction

so that its radiation pattern is spherical. This is very useful property and very easy to

visualize but practically such antenna does not exist.

Power density:

Power density of an antenna is defined as radiated power per unit area.

Directive gain:

Directive gain is defined in a particular direction, as the ratio of the power density

radiated in that direction by the antenna to the power density that would be radiated

by an isotropic antenna. If power densities are measured at the same distance & both

antenna radiate the same power.

Directive gain is a ratio of power density and is therefore a power ratio.

Directivity:

Directivity is defined as a maximum directive gain i.e. the gain in the direction of one

of the major lobes of radiation pattern compare to isotropic radiation.

Power gain:

It is the ratio of the power that must be radiated by an isotropic antenna to develop

certain field strength at a certain distance and divided by practical power.

The practical power is that power which must be fed to the directive antenna to

develop the same field strength at the same distance in its direction of maximum

radiation.

A = n D

A=Power Gain

D = Directivity (maximum directivity)

N = Antenna efficiency

=1 for loss less antenna

26

Polarization:

Polarization refers to the physical orientation of the radiated waves in space. Waves

are said to be polarized (actually linearly polarized) if they all have the same

alignment in space. In fact, it is a characteristic of most antennas that the radiation

they emit is linearly polarized. For example, a vertical antenna will radiate waves

whose electric vectors will be vertical and will remain so in free space.

Thus vertical antennas radiate vertically polarized waves, and similarly horizontal

antennas produce waves whose polarization is horizontal.

Circular polarization:

When an antenna produces vertically and horizontally polarized fields with equal

amplitude and with a phase difference of exactly 90 degrees, the resulting signal is

circularly polarized.

Band width:

It refers to the frequency range over which operation of antenna is satisfactory and is

generally taken between the half-power points.

The radiation pattern bandwidth is equal to the difference between the frequencies at

which the received power falls to one-half of maximum, in the direction of maximum

radiation.

Beam width:

The beam width of an antenna is the angular separation between the two half-power

points on the power density radiation pattern. It is also, of course, the angular separation

between the two 3-dB down points on the field strength radiation pattern of an antenna and

is illustrated in Figure.

Null filling:

There are three methods of introducing null fill in a panel array:

Mechanically tilting some panels downward.

Using a non-linear phase taper between bays.

Using an unequal power split between bays.

27

Since some energy is taken from the main beam to fill the null, the maximum gain of

the antenna system will be reduced; typically 0.5 to 1.5 dB, when null fills is

introduced.

Standing wave ratio:

The ratio of maximum current to minimum current along a transmission line is called

the standing-wave ratio, as is the ratio of maximum to minimum voltage, which is

equal to the current ratio. The SWR is a measure of the mismatch between the load

and the line, and is the first and most important quantity calculated for a particular

load.

The SWR is equal to unity when the load is perfectly matched. When the line is

terminated in a purely resistive load, the SWR is defined as

SWR = Z o / Rl

Where R l is the load resistance.

The higher the SWR, the greater the mismatch between the line and load, power loss

increase with SWR and so a low value of standing Wave-ratio is always sought.

Practical implications of SWR:

SWR has a number of implications that are directly applicable to broadcast use.

SWR is an indicator of reflected waves bouncing back and forth within the

transmission line, and as such, an increase in SWR corresponds to an increase in power in the

line beyond the actual transmitted power. This increased power will increase RF losses, as

increased voltage increases dielectric losses, and increased current increases resistive losses.

Matched impedances give ideal power transfer. Mismatched impedances give high SWR and

reduced power transfer. Higher power in the transmission line also leaks back into the line,

which causes it to heat up.

The higher voltages associated with a sufficiently high SWR could damage the transmitter

which have a lower tolerance for high voltages may automatically reduce output power to

prevent damage. The high voltages may also cause transmission line dielectric to break down

and/or burn.

VSWR measurements may be taken to ensure that a waveguide is

contiguous and has no leaks or sharp bends. If such bends or holes are present in the

28

waveguide surface, they may diminish the performance of transmitter and receiver equipment

strings. Arcing may occur if there is a hole, if transmitting at high power, usually 200 watts or

more. A very long run of coaxial cable especially at a frequency where the cable itself is loss

can appear to a radio as a matched load. The power coming back is in these cases, partially or

almost completely lost in the cable run.

How can we measure SWR?

We measure SWR in the form of VSWR. The VSWR may be measured by Site

Master available at all HPT’s.

The VSWR of antenna may be measured at 7-port patch panels. VSWR measurement

should be done for individual feeder cable and combined feeder cables.

The measurement should be done invariably once in quarter, if reflected power shown

on through line power meter is more than 1% of total output power of transmitter than

it is a serious concern.

VSWR measurement should be taken and reason of high reflected power should be

find out.

Yagi -uda antenna:

A Yagi-Uda antenna is an array consisting of a driven element and one or more

parasitic elements. They are arranged collinearly and close together, as shown in

Figure.

Since it is relatively unidirectional, as the radiation pattern shows and has a moderate

gain in the vicinity of 7dB, the Yagi-Uda antenna is used as an HF transmitting

antenna. It is also employed at higher frequencies, particularly as a VHF television

receiving antenna.

The Yagi-Uda antenna does not have high gain, but it is very compact, relatively broadband

because of the folded dipole used and has quite a good unidirectional radiation pattern. It has

one reflector and several directors which are either of equal length or decreasing slightly

away from the driven element

29

6.2 Details of antennas used in TV Transmission/Reception:

Single dipole antenna system:

Vertical polarization

Horizontal polarization dependent on tower structure

Quasi Omni HRP possible

Extremely cost effective

Dipoles may be stacked for higher gain / high transmission power applications.

Panel antenna system:

Minimum influence from tower

Full band operation

Flexible pattern shaping

High power application

We generally use turnstile antenna here for the purpose of high power transmission.

Turnstile antenna:

Turnstile antenna is generally used for television transmission. The turnstile antenna is the

earliest and most popular resonant antenna for VHF broadcasting. It is made up of four

batwing shaped elements mounted on a vertical pole in a manner resembling a turnstile. Four

30

batwings are in effect, two dipoles fed in quadrature phase. The azimuth field pattern is a

function of diameter of support mast. The pattern is usually within 10-15% of true circle.

It is made up of several layers, usually six for channel six for channels 2-6 and 12 for 7-13.

It is not suitable for side mounting, except for stand by applications in which coverage

degradation can be tolerated.

Features:

Omni directional horizontal polarization

Horizontal polarization

Suitable for mount

For the propagation the electrical energy is converted into electro-

magnetic wave. This is done by antenna section and the different types of propagation

are explained below as

1. Sky wave or Ionospheric wave propagation [between 2 to 30MHz]

The sky wanes are of practical importance for every long radio communications at

medium and high frequencies i.e. medium waves and short waves.

In this mode the EM waves transmitted by the transmitting antenna reach the receiving

antenna at very long distance away from transmitting antenna after the reflection from the

ionized region in the upper part of the atmosphere of the earth.

This part is called ionosphere and it is located above earth’s surface at about 70km to

400km height. The ionosphere acts as the reflecting surface and reflects the EM wave

back to the earth if the frequency is between 2 t0 30 MHz.

As the sky wave propagation is useful for the frequencies between 2MHz to 30MHz only

this mode of propagation is also called short wave propagation.

As the waves propagate due to the reflection by the ionosphere the mode of propagation

is also called ionosphere propagation using the sky wave propagation is also called

ionosphere propagation. Using the sky wave propagation a long distance point to point

communication is possible and hence it is also called point to point propagation or point

to point communication.

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2.Space wave propagation[above 30MHz]

When the frequency of the EM wave is between 30MHz to 300MHz the space

wave propagation mode is of importance. The EM waves in the space wave

propagation mode reach the receiving antenna either directly from the transmitting

antenna or after reflection from the atmosphere above the earth’s surface around

16km of height called troposphere.

Space wave consists of two components i.e. direct wave and indirect wave. The space

wave propagation is mainly used in VHF band as both previous modes namely ground

wave propagation and sky wave propagation both fail at very high frequencies.

3. Troposphere scatter propagation or forward scatter

propagation [above 30MHz i.e. UHF and micro wave range]:

The UHF and microwave signals are propagated beyond line of sight propagation

through the forward scattering in the troposphere regulations. This mode of

propagation is of practical significance at UHF and microwave frequency ranges.

This mode uses the properties of the troposphere. Hence it is also known as

troposphere scatter propagation. This type of scatter propagation also needs to the

ionosphere scatter propagation for frequencies in the lower range. Both ionosphere

scatter and troposphere scatter produce undesirable noise and fading which can be

taken with diversity reception.

4. Ground wave propagation- plane wave earth reflection:

When the transmitting and receiving antennas are elevated the useful

propagation can be achieved by means of the space wave propagation.

As the two antennas are within the line of sight of each other the propagation of such

space wave is also called line of sight propagation. Basically for the line of sight

propagation the resultant signal obtained is the combination of the space wave and the

surface wave. Where the VHF and UHF transmissions are different.

Here the antennas are of two types where the propagation of the signal is done. The

word mast means that a supporting structure.

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1. Self supporting mast:

It is a general broadcasting purpose antenna here the antenna is held at

height so that the transmission of the signals would be without any

obstacles. It is generally almost used in all media using sectors.

2. Guided wire mast:

The mast here is suspended from the ground and it is supported by some wires so

that it would with stand to the climatic conditions.

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CHAPTER-7

CONCLUSION

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CONCLUSION

We would like to conclude this training as a very great and enriching the experience

to learn about the low power TV transmitter.

The transmitter service involves great equipment that deals with monitoring section

exciting system and we learn about the above equipment of the Doordarshan relay centre and

it’s working.

We also learned about the procedure of transmission, reception. And strengthening of

the signal and retransmitting the signal into space for the broadcast around the range of

propagation.

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BIBILOGRAPHY

i) www.google.com

ii) http://www.northcountryradio.com/Kitpages/lptvx.htm

iii) http://www.ddindia.gov.in/Kendra/Delhi/Program+Column+3/delhi.htm

iv) http://www.ddindia.gov.in/

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