low power tv transmitter document
TRANSCRIPT
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
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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.
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CHAPTER-2
BASIC TRANSMISSION SYSTEMS
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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.
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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
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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
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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.
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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
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3. BLOCK SCHEMATIC OF LPT
FIG 3.1 : Block schematic of LPT
DG ROOM:
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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
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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
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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.
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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.
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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
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VHF TRANSMITTER
In this transmitter the frequency ranges from 224 MHz-231MHz.
Video signal
DRIVER
Fig: Block Diagram of Exciter unit
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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.
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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
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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.
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CHAPTER-5
RECEIVING &TRANSMITTING SECTION
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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
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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)
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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
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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.
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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
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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
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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
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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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