final mini project

7/31/2019 Final Mini Project

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INTERNAL GUIDE HEAD OF THE DEPARTMENT

(CH.D.VISHNUPRIYA, B.TECH) (V.Prasanth, M.Tech)

ABSTRACT

Studied LOW POWER TV TRANSMITTER in Doordarshan Relay station. In this transmission

of signals are possible from cable TV and DTH service. Low

Power transmitter in Kakinada is very high frequency and ultra high frequency.

VHF transmitter is used within channel 11 for DD1.

UHF transmitter is used within channel 33 for DD news.

And here we are studying about three sections respectively

1. Receiving section

2. Transmitter section

3. Antenna section and some specifications


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1. INTRODUCTION

Doordarshan is a public television broadcaster of India and a division ofprasarabharathi, a public service broad caster nominated by government of India. It is one of the

largest broadcasting organizations in the world. Doordarshan had the modest beginning with the

experimental telecast starting in Delhi on 15th

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 Mumbai and Amritsar in 1972. Till 1975 seven Indian cities had

television service and Doordarshan 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. In 1992 there are

three high power television transmitters, 369 medium power transmitters, 76 low power station

and 23 transponders. Regular satellite transmission began in 1982.


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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.

CHAPTER-2

BASIC TRANSMISSION SYSTEMS

There are three types of Basic transmission Systems.

Cable transmission Direct to home Transmitter service

2.1CABLE 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


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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.

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


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between channels 6 and7 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:

Maximum demand/capacity: 30KW Monthly average consumption : 6000 units Monthly average expenditures 40000/month

CABLE TV NETARCHITECTURE


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2.2DIRECT 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 operators strike, power outages, not getting your

favorite channels and channels shifting their channel number positions

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.


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Bands:

Frequency band Uplink Downlink

C-Band 6 GHz 4 GHz

C-Band 8 GHz 7 GHz

C-Band 14 GHz 11 GHz

C-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 distributionSatellite transmission: Ku-Band:

Frequency band 12.5 to 18 GHz Smaller dish (6090 cms) needed for reception Most useful for DTH application


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Coverage limited as compared to C-band due to narrow beam Reception susceptible to failure during heavy rainsNow 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.


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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 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.


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Advantages of DTH TV:

Digital picture:The picture quality in DTH is much better. The quality of the picture is uniform

across all channels.

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.

Electronic program Guide (EGP):Its an on-screen guide that shows the program schedule or listing

of all channels. So we can find out whats playing on any channel. We can also

set remainders for programs we wish to watch and get synopses of the program.

2.3TRANSMITTER SERVICE:

High power transmitter (HPT):


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Transmitter power 10KW Distance covered by above transmitter is 60km-100km

E.g.: located in Rajahmundry

Low power transmitter (LPT): Transmitted power 100w-500w Local area transmitter covers distance around 21kms

E.g.: located in Kakinada

Very low power transmitter (VLPT): Transmitted power10w Distance covered is around 5-10Km

E.g.: located in Yanam

CHAPTER-3

BLOCK SCHEMATIC OF LPT

Generator power supply

Receiver dish area

P.D.A


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Work shop

Monitoring transmitter input

Rack rack rack

Mast antenna

FIG 3.1: Block schematic of LPT

DG ROOM:

The generator generates 35KVA power supply.

Receiver Dish Area:


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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 RACKINPUT RACKTRANSMITTER RACK

T.V

WAVE FROM MONITORING

DEMODULATION

SWITCHER

EXCITER

DRIVER AMPLIFIER

DIVIDER

COMBINER

RECEIVER-1

RECEIVER-2

RECEIVER-3

V.C.R

P.G.


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

Receiving section Transmitting section Mast and antenna

RECEIVING SECTION:

P.D.A receives information from satellites which are located in geostationary orbits. Thefollowing are the point lobe considered while placing P.D.As

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.


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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.

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 +28Vfor the unit is supplied by P.S.U.

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


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AUDIO SIGNAL

Fig: Block Diagram of Exciter unit

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:

Video signal Low pass filter Video processor Vision modulator IF oscillator


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Control oscillator Aural modulator Audio signal Power combiner Vestigial side band filter Driver +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.

Generator section:

The sync component of the input video signal is separated and differentiated

to produce a trigger pulse which operates as multivibrator.

Vision modulator:


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provided for monitoring the output signal for measuring the peak value of the output and for

frequency deviation. the unit output is applied to VHF or UHF mixer unit the aural modulator

unit consists mainly of the following sections in a simple PCB.

Audio amplifier:The balanced audio signal at a level of +10dbm from the studio is converted into unbalanced

signal by the audio transformer the output of the audio transformer is taken through a rotary

fader to the input of the audio amplifier.

b) Voltage controlled oscillator:

The VCO is a varactor tuned oscillator, the frequency of which can be varied manually by the

coil. Transistor forms the oscillator. The output of the oscillator is taken through a buffer

amplifier by two stages. The output of the one of the amplifier is fed to the mixer, is brought out

to the socket. The output level of the is about 0dbm.

c) Mixer:

The visual IF signal from the IF oscillator (38.9 MHz) and the aural IF signal from the VC0

(33.4 MHz) are injected in the base of the mixer transistor. Coil and capacitor are tuned to 33.4

MHz to provide isolation between visual IF port and VCO input port. The mixer output is at

505Mhz.the square signal is fed to the APC.

d) AUTOMATIC PHASE CONTROL:

The APC is a sample and hold circuit using a CMOS analog gate for sampling .The square wave

signal from divider chain of mixer is transformed into a triangular wave by means of transistors

and output is taken through source follower

E) POWER COMBINER UNIT:

Combiner unit is essentially a wide amplifier of 32c-42Mhz linearly combining the Vision IF and

sound IF signals , maintaining sufficient isolation between vision and sounds . It consists two

pads of 16db in the vision and sound inputs followed by a 6db hybrid resistance combiner.


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f) Driver unit:

The converted signal from the exciter is fed to coaxial attenuator which is placed at the front

panel for adjusting the input level suitably. The signal is amplified using class a driver stages. A

portion of output power to the back panel of the driver unit for monitoring purpose .the front

panel output called "ALC" can be fed to the exciter "ALC IN" to keep the driver output to

constant at the level.

+12V Regulated power supply:

The part of this power supply consists of a step-down transformer, a bridge rectifier and an

electrolytic capacitor for smoothing. The output voltage is maintained constant at 12V, 1V (both)

as said by the sensing potential meter.

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:

Up convertor

exciter

Base band corrector

Linearity corrector

Video signal splitter & PA


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FIG: Block Diagram of UHF Transmitter

10 KW UHF TRANSMITTER


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Linearity corrector:

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

linearitys 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-distortercircuit 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

terminating unused parts. The four ways splitter doesnt have any ach ieve components for

isolation resistor. It is a micro strip circuit design based on Wilkinsons 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 Wilkinsons 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.


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CHAPTER-5

RECEIVING &TRANSMITTING SECTION

5.1 RECEIVING SECTION:

Digital video broadcast receiver

Low noise block convertor

Low noise amplifier

Parabolic dish antenna

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.

antenna

5.2 TRANSMITTER SECTION

DRIVER AMPLIFIER

POWER AMPLIFIER


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EXCITER

V1 AUDIO &

V2 VIDEO

V3 SWITCH

A1 A2 A3

Fig: Block Diagram of Transmitter Section

CHAPTER-6

ANTENNA SECTION

6.1 ANTENNA BASICS:

What is an antenna?

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


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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 EVolt 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/d2 mille 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)

Isotropic antenna:


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An Isotropic antenna is a standard reference antenna radiating equally in all direction sothat 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 densityradiated 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 certainfield 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 developthe same field strength at the same distance in its direction of maximum radiation.

A= nD

A=Power Gain

D = Directivity (maximum directivity)

N = Antenna efficiency

=1 for loss less antenna


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Polarization:

Polarization refers to the physical orientation of the radiated waves in space. Waves aresaid 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 horizontalantennas 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 isgenerally taken between the half-power points.

The radiation pattern bandwidth is equal to the difference between the frequencies atwhich 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.


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Since some energy is taken from the main beam to fill the null, the maximum gain of theantenna 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 thestanding-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 isterminated in a purely resistive load, the SWR is defined as

SWR = Zo/Rl Where Rl 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 whichhave 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.


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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 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 Masteravailable at all HPTs.

The VSWR of antenna may be measured at 7-port patch panels. VSWR measurementshould be done for individual feeder cable and combined feeder cables.

The measurement should be done invariably once in quarter, if reflected power shown onthrough 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 parasiticelements. They are arranged collinearly and close together, as shown in Figure.

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


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6.2Details 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


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

communication.

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 earths 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 groundwave 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


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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.

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.

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

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 its

working.


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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.

final mini project

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