Antenna Fundamentals and its applications
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CHAPTER: 1
INTRODUCTION
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1. Introduction:-
1.1. Introduction to Doordarshan:-
In November, 1982 DD replay center with 100W transmitter was started. 28th
November, 1982 marked the start of DD1 10KW HPT with BEL transmitter. The
installation and commissioning of studio took place in may, 2000. Commissioning of
10kW (Thom cast) DD2 transmitter (DD news) was done on 3rd July, 2000.
January.2004 marked the starting of 30min local transmission (narrowcasting).
Replacement of DD1 BEL transmitter with 10kW ROHDE and SCHWARZ
transmitter was done on 18thjuly, 2005. Finally starting of additional 30 minutes local
transmission came into effect from 3rd September, 2007.
DD NATIONAL
DD NATIONAL is commissioned from 28th November, 1984. The primary coverage
area is 65 to 70 kilometers. The cost of transmitter is Rs. 1, 10, 87,854.
DD NEWS
DD NEWS is started from 3rd July, 2000. The coverage area is around 65 to 70
kilometers radial. The cost of transmitter is Rs. 1, 95, 01,167.
1.1.1. Studio:-
Finally the studio is established in May, 2000. The floor area is around 14mtrs. The
height of studio is 7.5mts. Three studio cameras, 16 channels for vision mixer and
eight channels for audio mixer are used. The total land area of DDK, Indore is 3.31
areas. In this total constructed area is 2364 square meter. The plot number is 114. The
state government provided the land for free on 28th March,1984. The latitude and
longitude is 22deg.42min.02sec and 75deg.52min.48sec respectively. The height
from mean sea level is 550m.
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1.1.2. Mast:-
The height of the tower is 150m. the year of erection is of the tower is 28th January,
1985. The height of platform is 40m, 75m, 104m, 125m. The antenna used is stacked
dipole (wideband,24 panels,6 panel per size). The other antenna used as AIR FM,
IGNOV FM (FM channel at heights 104meters).
1.2 Introduction to Antenna:-
An antenna is a device for converting electromagnetic radiation in space into
electrical currents in conductors or vice-versa, depending on whether it is being used
for receiving or for transmitting, respectively. Passive radio telescopes are receiving
antennas. It is usually easier to calculate the properties of transmitting antennas.
Fortunately, most characteristics of a transmitting antenna (e.g., its radiation pattern)
are unchanged when the antenna is used for receiving, so we often use the analysis of
a transmitting antenna to understand a receiving antenna used in radio
astronomy. First radio antenna was assembled in 1886 by Heinrich Hertz. He
developed a circuit resembling a radio system with end loaded dipoles as a
transmitting antenna while resonant square loop antenna as a receiving antenna
operating at one meter wavelength. The laboratory work done by Hertz was further
completed by Guglielmo Marconi and in 1901; he demonstrated world
communication of signal over long distances. Thus Hertz and Marconi are the
pioneers of antenna.
A RF antenna is defined as a component that facilitates the transfer of a guided wave
into, and the reception from, free space. In function, the antenna is essentially a
transducer that converts alternating currents into electromagnetic fields or vice versa.
The physical components that make up an antenna’s structure are called elements.
From a coat hanger to a tuned Yagi, there are literally hundreds of antenna styles and
variations that may be employed. Receive and transmit antennas are very alike in
characteristics and in many cases are virtual mirror images of each other. However, in
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many Part 15 applications it is advantageous to select different characteristics for the
transmitter and receiver antennas. For this reason, we will address each separately.
According to the strict definition of an antenna as a device for converting between
electromagnetic waves in space and currents in conductors, the only antennas in most
radio telescopes are half-wave dipoles and their relatives, quarter-wave ground-plane
verticals. The large parabolic reflector of a radio telescope serves only to focus plane
waves onto the feed antenna. [The term "feed" comes from radar antennas used for
transmitting; the "feed" feeds transmitter power to the main reflector. Receiving
antennas used in radio astronomy work the other way around, and the "feed" actually
collects radiation from the reflector.]
The Transmitter Antenna:-
The transmitter antenna allows RF energy to be efficiently radiated from the output
stage into free space. In many modular and discrete transmitter designs, the
transmitter’s output power is purposefully set higher than the legal limit. This allows
a designer to utilize an inefficient antenna to achieve size, cost, or cosmetic objectives
and still radiate the maximum allowed output power. Since gain is easily realized at
the transmitter, its antenna can generally be less efficient than the antenna used on the
receiver.
The Receiver Antenna:-
The receiving antenna intercepts the electromagnetic waves radiated from the
transmitting antenna. When these waves impinge upon the receiving antenna, they
induce a small voltage in it. This voltage causes a weak current to flow, which
contains the same frequency as the original current in the transmitting antenna. A
receiving antenna should capture as much of the intended signal as possible and as
little as possible of other off-frequency signals. Its maximum performance should be
at the frequency or in the band for which the receiver was designed. The efficiency of
the receiver’s antenna is critical to maximizing range performance. Unlike the
transmitter antenna, where legal operation may mandate a reduction in efficiency, the
receiver’s antenna should be optimized as much as is practical. Actual half-wave
dipoles, backed by small reflectors about quarter wavelength behind them to focus the
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dipole pattern in the antenna are almost unidirectional. There is a back lobe which can
be reduced by placing elements close to each other. The folded dipole element
resonates at a frequency of resonance but reflector resonates at a frequency lower
than resonant frequency while director resonates at a frequency greater than resonant
frequency.
1.2.1. Types of Antenna:-
Yagi-Uda Antenna:-
The Yagi-Uda antenna consists of folded dipole as driven element along with
reflector and one or more directors. The director and reflectors are straight conductors
which are called parasitic element. The directors are placed in front of driven
elements while the reflector is placed behind the driven element. The length of folded
dipole is half the wavelength while length of director is less than half the wavelength
and that of reflector is greater than half the wavelength. The radiation pattern of the
Yagi-Uda antenna is almost unidirectional. There is a back lobe which can be reduced
by placing elements close to each other. The folded dipole element resonates at a
frequency of resonance but reflector resonates at a frequency lower than resonant
frequency while director resonates at a frequency greater than resonant frequency
Advantages of Yagi-Uda Antenna:
1) It has excellent sensitivity.
2) Its front to back ratio is excellent.
3) It is useful as receiving antenna at high frequency for TV reception.
4) It has almost unidirectional pattern.
5) It is broadband antenna.
Lens Antenna:-
A lens antenna is an antenna consisting an electromagnetic lens with a feed. In other
words, it is a three dimensional electromagnetic device having refractive index n
other than unity. Its operation is similar to a glass lens used in optics. The lens
antenna can be used in transmitting mode and in receiving mode both. Advantages of
lens Antenna:
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1) In lens antenna, the rays are transmitted away from the feed system; hence the
aperture is not obstructed due to feed and feed support.
2) In lens antenna, as the wave enters from one side end leaves out from other end,
greater extent of wrapping and twisting is possible without disturbing electrical
path length.
3) Lens antenna can be used to feed at a point off the axis, so it is most extensive
used in the applications where beam is needed to be moved angularly with respect
to axis.
Turnstile Antenna:-
The turnstile Antenna is formed by placing two half wave dipoles perpendicular to
each other. These dipoles are excited such that the currents are equal in magnitude but
in phase quadrature. The radiation pattern produced is almost unidirectional. The
Turnstile antenna is useful to match 70 ohms dual coaxial line to increase directives
an array of turnstile antennas is used.
Long-wire Antenna:-
The antennas which operate between frequency ranges of 3-30 MHz are called High-
Frequency antennas .For the HF band, the wavelength ranges in 100 -10 meters the
HF antennas can be made in size comparable with the wavelength. The directional
properties can also be obtained for such antennas. In case of Low frequency band and
Mid frequency band, the wavelength is greater, the size of antenna becomes larger
and it becomes difficult to achieve highly directional system.
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CHAPTER: 2
DESCRIPTION ABOUT THE
INDUSTRIAL TRAINING
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2. Description about the industrial training:-
2.1 Theoretical considerations:-
2.1.1. Various Sections of Doordarshan:-
Studio
Mast
Antenna
Transmitters
Earth Station
CAR
ENG
Video Tape recorders
Computer Section
Non-linear editing
OB-VAN
DSNG
Video chain of OB VAN
Studio:-
Finally the studio is established in may,2000. The floor area is around 14mtrs. The
height of studio is 7.5mts. Three studio cameras, 16 channels for vision mixer and
eight channels for audio mixer are used. The total land area of DDK, Indore is 3.31
area. In this total constructed area is 2364 square meter. The plot number is 114. The
state government provided the land for free on 28th March,1984. The latitude and
longitude is 22deg.42min.02sec and 75deg.52min.48sec respectively. The height
from mean sea level is 550m.
Mast:-
The height of the tower is 150m. the year of erection is of the tower is 28th January,
1985. The height of platform is 40m, 75m, 104m, 125m. The antenna used is stacked
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dipole (wideband,24 panels,6 panel per size). The other antenna used as AIR FM,
IGNOV FM (FM channel at heights 104meters). An antenna is a device for
converting electromagnetic radiation in space into electrical currents in conductors or
vice-versa, depending on whether it is being used for receiving or for transmitting,
respectively. Passive radio telescopes are receiving antennas. It is usually easier to
calculate the properties of transmitting antennas. Fortunately, most characteristics of a
transmitting antenna (e.g., its radiation pattern) are unchanged when the antenna is
used for receiving, so we often use the analysis of a transmitting antenna to
understand a receiving antenna used in radio astronomy.
Transmitters DD-I & DD-II:-
The 10KW TV Transmitter NEC for DD-I is being radiated on 150 Meter Mast
having two panels and at present both panels are working satisfactory.
The 10 KW TV Transmitter NEC make for DD-II NEWS is being radiated on 10
KW power and the antenna is installed on same 150 meter mast of DDI. The
antenna for 10 KW power is under installation.
The two Numbers of Diesel Generators set 63KVA each has been provided for dd-I
and DD-II for standby power supply and both are working satisfactory.
For the reception of the programs for DD-I and DD-II NEWS Digital Receivers
(IRD) has been provided. Since this year we also received the KU band equipments
for monitoring as well as used for both the transmitters and the quality of signal is
very good.
The Details of 10 KW Transmitter of DDK is given below:
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Table 1. Technical Specifications of Transmitter for DD-I
Transmitter Make NEC-VHF
Transmitter type PCN 1610 SSPH/1
Band of Operation Band-III
Channel for operation CH-6(-)
Channel Frequency (vision) 182.25 M Hz
Channel Frequency (Audio) 187.75 M Hz
Date of Commission as HPT 30th August 1984
Date of Replacement of Transmitter 10th July 2003
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Table 2. Technical Specifications of Transmitter for DD-II
Transmitter Make NEC-UHF
Transmitter type PCN 1110 SSP/1
Channel for operation CH-28(-)
Channel Frequency (vision) 527.25 M Hz
Channel Frequency (Audio) 532.75 M Hz
Date of Commission as HPT 18th August 2002
CAR :-
CAR This stands for central apparatus room. Car is basically a channel or path
through which the video signal passes. All studio has its own CCU i.e. camera
control unit. PCR i.e. panel control room The no. or model of video console is PDS
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ENG:-
ENG stands for electronic news gathering. The purpose of eng is to gather news from
different outside locations. Cameras used in eng section are small and light weight.
These cameras are called camcorders.
Video Tape recorders:-
Video Tape recorders VTR room is provided at each studio center. It houses at least
two console type 1”videotape recorders (VTRs) and a few Broadcast standard
Videocassette recorders (VCRs). In these recorders, sound and video signals are
recorded simultaneously on the same tape.
Computer Section:-
Computer Section It is basically related to editing. There are two types of editing’s.
Linear editing. Non linear editing. Software used in non-linear editing. For news
editing Velocity 6.0 is used. For graphics MOV CG 2003 n Adobe Photoshop. For
program editing velocity 8.0 and FCP (final cut pro)
Non-linear editing:-
Non linear editing Problem with Linear Editing is sequential – first shot first Long
hours spent on rewinding of tapes , search of material Potential risk of damage to
original footage Difficult to insert a new shot in an edit Difficult to experiment with
Variations Quality loss more in analog; even with digital Limited Compositing,
effects, color correction Capability
NLE:-
NLE is video editing in digital format with standard computer based technology
Computer technology is harnessed in Random access, computational and
manipulation capability, multiple copies, intelligent search, sophisticated project and
media management tools, standard interfaces, and powerful display .
OB-VAN:-
OB-VAN It is known as outdoor broadcasting van and used for outer coverage.
Events like sports, functions are covered by this van. It consists of 8 cameras and 3
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external sources and 4 VTR. LSM-LIVE SLOW MOTIN MACHINE. It is used to
play the replays in slow motion. There are also pc racks, mixers racks.
DSNG:-
This is known as direct satellite news gathering. It works simultaneously with OB
van. It’s also called as mobile earth station. The dish can be aligned acc to the
requirement. Monitors are available in this van to check the telecast. High power ups
are also available in this van.
Video chain of OB VAN:-
Video chain of OB VAN Output from the switcher goes to stabilizing amplifier via
PP and VDAs. Output from the stab. Is further distributed to various destinations. It
may be noted that the use of VDAs helps to monitor the video signal at different
locations and the use of PP is very helpful for emergency arrangements during
breakdowns and trouble shooting. A separate monitoring bus is provided in CCU,
LCU and END CONTROL with sources as. END CONTROL also has a remote for
the adjustment of levels etc. in the STAB AMP unit. R out for the other sources is
similar to this and can be understood from the block schematic
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2.2 Material and methods:-
2.2.1 Some fundamentals of antenna:-
Radiation pattern
Radiation intensity
Directive gain and directivity
Power gain
Antenna beam width
Antenna bandwidth
Antenna input impedance
Effective aperture
Antenna temperature
Antenna polarization
Radiation pattern:-
Fig. 1 Radiation pattern
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The radiation pattern of an antenna is a plot of the relative field strength of the radio
waves emitted by the antenna at different angles. It is typically represented by a three
dimensional graph, or polar plots of the horizontal and vertical cross sections. The
pattern of an ideal isotropic antenna, which radiates equally in all directions, would
look like a sphere. Many non-directional antennas, such as monopoles and dipoles,
emit equal power in all horizontal directions, with the power dropping off at higher
and lower angles; this is called an omnidirectional pattern and when plotted looks like
a doughnut. The radiation of many antennas shows a pattern of maxima or "lobes" at
various angles, separated by "nulls", angles where the radiation falls to zero. This is
because the radio waves emitted by different parts of the antenna typically interfere,
causing maxima at angles where the radio waves arrive at distant points in phase, and
zero radiation at other angles where the radio waves arrive out of phase. In
a directional antenna designed to project radio waves in a particular direction, the
lobe in that direction is designed larger than the others and is called the "main lobe".
The other lobes usually represent unwanted radiation and are called "side lobes". The
axis through the main lobe is called the "principal axis" or "bore sight axis".
Radiation intensity:-
The radiation intensity is defined as power per unit solid angle. It is expressed in
W/Sr (watts/Steradian). A solid angle is a section of the surface of the imaginary
sphere around the antenna. Unlike power density, radiation intensity does not depend
on distance: because radiation intensity is defined as the power through a solid angle,
the decreasing power density over distance (i.e. over of the imaginary sphere
around the antenna) due to the inverse-square law is offset by the increasing area of
the solid angle due to the same law. Therefore, power density can be converted to
radiation intensity by multiplying it with .
Directive gain and directivity:-
The directive gain is defined as the ratio of power density to the average power
radiated. An isotropic antenna is the omnidirectional antenna. The meaning of the
omnidirectional antenna is the antenna acting as a point radiator which radiates
equally in all directions. Directive gain of isotropic antenna is unity. Directive gain
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can be defined as a measure of the concentration of the radiated power in a particular
direction. Directivity or maximum directive gain of an antenna is defined as the ratio
of maximum radiation intensity to its average radiation intensity.
Power gain:-
Gain is a parameter which measures the degree of directivity of the antenna's
radiation pattern. A high-gain antenna will preferentially radiate in a particular
direction. Specifically, the antenna gain, or power gain of an antenna is defined as the
ratio of the intensity (power per unit surface) radiated by the antenna in the direction
of its maximum output, at an arbitrary distance, divided by the intensity radiated at
the same distance by a hypothetical isotropic antenna. The gain of an antenna is a
passive phenomenon - power is not added by the antenna, but simply redistributed to
provide more radiated power in a certain direction than would be transmitted by an
isotropic antenna. An antenna designer must take into account the application for the
antenna when determining the gain. High-gain antennas have the advantage of longer
range and better signal quality, but must be aimed carefully in a particular direction.
Low-gain antennas have shorter range, but the orientation of the antenna is relatively
inconsequential. For example, a dish antenna on a spacecraft is a high-gain device
that must be pointed at the planet to be effective, whereas a typical Wi-Fi antenna in a
laptop computer is low-gain, and as long as the base station is within range, the
antenna can be in any orientation in space. It makes sense to improve horizontal range
at the expense of reception above or below the antenna. In practice, the half-wave
dipole is taken as a reference instead of the isotropic radiator. The gain is then given
in dBd (decibels over dipole). 0 dBd = 2.15 dBi. It is vital in expressing gain values
that the reference point be included. Failure to do so can lead to confusion and error.
Antenna beam width:-
Antenna beam width is the measure of the directivity of the antenna. The antenna
beam width is an angular width in degrees. It is measured on a radiation pattern on
major lobe. Antenna beam width is defined as the angular width in degrees between
the two points on a major lobe of a radiation pattern where the radiated power
decreases to half of its maximum value. The beam width is also called 3db beam
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width as reduction of power to half of its maximum value corresponds to a reduction
of power expressed in dB by 3db.
Antenna bandwidth:-
Although a resonant antenna has a purely resistive feed-point impedance at a
particular frequency, many (if not most) applications require using an antenna over a
range of frequencies. An antenna's bandwidth specifies the range of frequencies over
which its performance does not suffer due to a poor impedance match. Also in the
case of a Yagi-Uda array, the use of the antenna very far away from its design
frequency reduces the antenna's directivity, thus reducing the usable bandwidth
regardless of impedance matching. Except for the latter concern, the resonant
frequency of a resonant antenna can always be altered by adjusting a suitable
matching network. To do this efficiently one would require remotely adjusting a
matching network at the site of the antenna, since simply adjusting a matching
network at the transmitter (or receiver) would leave the transmission line with a
poor standing wave ratio. Instead, it is often desired to have an antenna whose
impedance does not vary so greatly over a certain bandwidth. It turns out that the
amount of reactance seen at the terminals of a resonant antenna when the frequency is
shifted, say, by 5%, depends very much on the diameter of the conductor used. A
long thin wire used as a half-wave dipole (or quarter wave monopole) will have a
reactance significantly greater than the resistive impedance it has at resonance,
leading to a poor match and generally unacceptable performance. Making the element
using a tube of a diameter perhaps 1/50 of its length, however, results in a reactance
at this altered frequency which is not so great, and a much less serious mismatch
which will only modestly damage the antenna's net performance. Thus rather thick
tubes are typically used for the solid elements of such antennas, including Yagi-Uda
arrays.
Antenna input impedance:-
As an electro-magnetic wave travels through the different parts of the antenna system
(radio, feed line, antenna, and free space) it may encounter differences in impedance
(E/H, V/I, etc.). At each interface, depending on the impedance match, some fraction
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of the wave's energy will reflect back to the source, forming a standing wave in the
feed line. The ratio of maximum power to minimum power in the wave can be
measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A
SWR of 1.5:1 is considered to be marginally acceptable in low power applications
where power loss is more critical, although an SWR as high as 6:1 may still be usable
with the right equipment. Minimizing impedance differences at each interface
(impedance matching) will reduce SWR and maximize power transfer through each
part of the antenna system. Complex impedance of an antenna is related to
the electrical length of the antenna at the wavelength in use. The impedance of an
antenna can be matched to the feed line and radio by adjusting the impedance of the
feed line, using the feed line as an impedance transformer. More commonly, the
impedance is adjusted at the load (see below) with an antenna tuner, a balun, a
matching transformer, matching networks composed of inductors and capacitors, or
matching sections such as the gamma match.
Effective aperture:-
The effective area or effective aperture of a receiving antenna expresses the portion of
the power of a passing electromagnetic wave which it delivers to its terminals,
expressed in terms of an equivalent area. For instance, if a radio wave passing a given
location has a flux of 1 pW / m2 (10−12 watts per square meter) and an antenna has an
effective area of 12 m2, then the antenna would deliver 12 pW of RF power to the
receiver (30 microvolts rms at 75 ohms). Since the receiving antenna is not equally
sensitive to signals received from all directions, the effective area is a function of the
direction to the source. Due to reciprocity (discussed above) the gain of an antenna
used for transmitting must be proportional to its effective area when used for
receiving. Consider an antenna with no loss, that is, one whose electrical efficiency is
100%. It can be shown that its effective area averaged over all directions must be
equal to λ2/4π, the wavelength squared divided by 4π. Gain is defined such that the
average gain over all directions for an antenna with 100% electrical efficiency is
equal to 1. Therefore the effective area Aeff in terms of the gain G in a given direction
is given by:
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For an antenna with an efficiency of less than 100%, both the effective area and gain
are reduced by that same amount. Therefore the above relationship between gain and
effective area still holds. These are thus two different ways of expressing the same
quantity. Aeff is especially convenient when computing the power that would be
received by an antenna of a specified gain, as illustrated by the above example.
Antenna temperature:-
If we replace a resistor by a lossless antenna of radiation resistance R in an any
anechoic chamber at temperature Tc, then under condition Tr =Tc, the noise power
per unit bandwidth remains same. Finally if we remove antenna form anechoic
chamber and kept pointing to the sky at temperature Ts then the noise power per unit
bandwidth remains unchanged if the temperature Ts and Tr are same. This condition
is studied under the assumption that the whole radiation pattern is at sky temperature
Ts. In this way, antenna can be used to measure distinct temperature and it is called
passive remote sensing. The antenna used for remote sensing is called radio telescope.
To measure the distant temperature, the antenna noise temperature is compared with
that of resistor at temperature Tr. For comparison of the two temperatures, the
antenna is connected to the receiver and then the resistor is connected to the receiver.
When there is no difference in the temperature, we get condition Ta=Ts=Tr. Thus the
noise temperature Ta of a losses antenna is equal to the sky temperature Ts and not
equal to the physical temperature. But this is contradictory to the resistor because it is
loss and hence its noise temperature is equal to the physical temperature. Hence for a
practical antenna used for the remote sensing, the noise power per unit bandwidth is
given by,
P=kTa w/Hz.
Antenna polarization:-
The polarization of an antenna is the orientation of the electric field (E-plane) of the
radio wave with respect to the Earth's surface and is determined by the physical
structure of the antenna and by its orientation. It has nothing in common with antenna
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directionality terms: "horizontal", "vertical", and "circular". Thus, a simple straight
wire antenna will have one polarization when mounted vertically, and a different
polarization when mounted horizontally. "Electromagnetic wave polarization
filters" are structures which can be employed to act directly on the electromagnetic
wave to filter out wave energy of an undesired polarization and to pass wave energy
of a desired polarization. Reflections generally affect polarization. For radio waves
the most important reflector is the ionosphere - signals which reflect from it will have
their polarization changed unpredictably. For signals which are reflected by the
ionosphere, polarization cannot be relied upon. For line-of-sight communications for
which polarization can be relied upon, it can make a large difference in signal quality
to have the transmitter and receiver using the same polarization; many tens of dB
differences are commonly seen and this is more than enough to make the difference
between reasonable communication and a broken link. Polarization is largely
predictable from antenna construction but, especially in directional antennas, the
polarization of side lobes can be quite different from that of the main propagation
lobe. For radio antennas, polarization corresponds to the orientation of the radiating
element in an antenna. A vertical omnidirectional Wi-Fi antenna will have vertical
polarization (the most common type). An exception is a class of elongated waveguide
antennas in which vertically placed antennas are horizontally polarized. Many
commercial antennas are marked as to the polarization of their emitted signals.
Polarization is the sum of the E-plane orientations over time projected onto an
imaginary plane perpendicular to the direction of motion of the radio wave. In the
most general case, polarization iselliptical, meaning that the polarization of the radio
waves varies over time. Two special cases are linear polarization (the ellipse
collapses into a line) and circular polarization (in which the two axes of the ellipse are
equal). In linear polarization the antenna compels the electric field of the emitted
radio wave to a particular orientation. Depending on the orientation of the antenna
mounting, the usual linear cases are horizontal and vertical polarization. In circular
polarization, the antenna continuously varies the electric field of the radio wave
through all possible values of its orientation with regard to the Earth's surface.
Circular polarizations, like elliptical ones, are classified as right-hand polarized or
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left-hand polarized using a "thumb in the direction of the propagation" rule. Optical
researchers use the same rule of thumb, but pointing it in the direction of the emitter,
not in the direction of propagation, and so are opposite to radio engineers' use.
2.3 Application of antenna:-
Military applications:-The high velocity aircrafts, space craft, missiles, rockets
require low profile, light weight antennas which can be conformably mounted to the
exterior surfaces of these vehicles. The micro strip antennas are best suited for above
application. Other areas where the micro strip antennas are used include application
areas such as missile guiding fusing telemetry, satellite communications radars,
altimeters, GPS etc.
Space applications:- The micro strip antennas are invariable used in the space
programs such as earth limb measurement Satellite, International sun earth explorer,
Shuttle Imaging radar, solar Mesospheric Explorer, Cosmic Background Explorer,
GEOSTAR, SEASAT and Mars Pathfinder. Out of these space program, the antennas
used for SEASAT and SIR-A, B, C series are all large panels consisting micro strip
arrays nearly 10m in length at L and C band frequencies. These arrays are apart
synthetic aperture radar used to perform earth remote sensing function.
Commercial applications:- The micro strip antenna are used commercially in
applications such as mobile Satellite communication, Direct Broadcast Satellite
services, Global positioning system, Aeronautical and Marine Radar and Earth
Remote sensing.
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CHAPTER: 3
METHODOLOGY ADOPTED
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3. Methodology Adopted:-
Transmission Lines
Antenna Working
Antenna Tuning
Antenna Matching
Reflector structure
Mount structure
Drive mechanism
Fixing the feed onto the antenna
Description of Transmitter
Driver
Power amplification
3.1 Transmission Lines:-
A transmission line is any medium whereby contained RF energy is transferred from
one place to another. Many times a transmission line is referred to as “a length of
shielded wire” or a “piece of coax”. While technically correct, such casual references
often indicate a lack of understanding and respect for the complex interaction of
resistance, capacitance, and inductance that is present in a transmission line.
The diameter and spacing of the conductors as well as the dielectric constant of the
materials surrounding and separating the conductors plays a critical role in
determining the transmission line’s properties. One of the most important of these
properties is called characteristic impedance. Characteristic impedance is the value in
ohms at which the voltage-to-current ratio is constant along the transmission line. All
Links modules are intended to be utilized with transmission lines having a
characteristic impedance of 50 ohms.
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In order to achieve the maximum transfer of RF energy from the transmission line
into the antenna, the characteristic impedance of the line and the antenna at frequency
should be as close as possible. When this is the case the transmission line and antenna
are said to be matched. When a transmission line is terminated into an antenna that
differs from its characteristic impedance, a mismatch will exist. This means that all of
the RF energy is not transferred from the transmission line into the antenna. The
energy that cannot be transferred into the antenna is reflected back on the
transmission line. Since this energy is not reflected into space, it represents a loss.
The ratio between the forward wave and the reflected wave is known as the Standing
Wave Ratio (SWR). The ratio between the sum of the forward voltage and the
reflected voltage is commonly called the Voltage Standing Wave Ratio (VSWR).
3.2 Antenna Working:-
The electric and magnetic fields radiated from an antenna form an electromagnetic
field. This field is responsible for the propagation and reception of RF energy. To
understand an antenna’s function properly, an in-depth review of voltage, current, and
magnetic theory would be required. Since this is not in keeping with the basic nature
of this application note, a simplistic overview will have to suffice. Assume for a
moment that a coaxial transmission line was stripped and the shield and center
conductor were bent at right angles to the line as illustrated. Presto, a basic antenna
called a half-wave dipole has just been formed.
3.3 Antenna Tuning:-
This is the process whereby the resonant point of an antenna is adjusted. In most
instances, this is accomplished by physically adjusting the antenna length. While
simple range tests can be used to blindly tune an antenna, a network analyzer is a
virtual necessity for serious characterization. In some cases external inductive or
capacitive components may be used to match and bring the antenna to resonance.
Such components can introduce loss. It should be
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remembered that match and resonance do not necessarily translate into effective
propagation.
3.4 Antenna Matching:-
Antenna resonance should not be confused with antenna impedance. The difference
between resonance and impedance is most easily
understood by considering the value of VSWR at its lowest point. The lowest point of
VSWR indicates the antenna is resonant, but the value of that low point is determined
by the quality of the match between the antenna and the transmission line it is
attached to. This point of attachment is called the feed point. The point of resonance
is largely determined by antenna length, but how is antenna impedance determined.
When an antenna is at resonance it presents a purely resistive load. This resistance is
made up of three factors. First, when considered only as a conductor, there is loss
through the real physical resistance of the antenna element. This is
Called ohmic resistance loss. The second and most important area of loss is through
radiation resistance (Rr). Since the real and leakage resistances are usually negligible,
we will focus on radiation resistance. As mentioned previously, radiation resistance is
a hypothetical concept that describes a fictional resistance that, if substituted in place
of the antenna, would dissipate the same power that the antenna radiates into free
space. The radiation resistance of an antenna varies along the length of the antenna
element but our concern is with the resistance at the feed point. The radiation
resistance increases as a conductor lengthens. In general, the radiation resistance for a
¼-wave vertical is about 37-ohms, for a ½-wave about 73-ohms.
3.5 Reflector structure:-
The 6.3 m diameter antenna is made up of 4 quarter segment. Each and every quarter
is made up of 10 segments fixed on five trusses. Panels which are fixed to the trusses
are made up of fine aluminum expanded mesh strengthened with the help of channel
sections and tee sections whose end share fixed to the backup structure. Trusses are
composed of aluminum square tubes and the welded back up made up of hub and 20
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trusses. The hubs and trusses are constructed in such a way that they constitute to the
high level of surface accuracy.
3.6 Mount structure:-
A simple tubular steel space frame makes up most of the mount structure. It allows
rotation about x-axis as well as y axis. The x axis drive rod is connected between the
top of the mounted structure and the concrete foundation. The y axis drive rod is
connected between the base of the x axis bearing mount and the reflector back up
structure on the left hand side as viewed from the rear of the antenna. The mount is
rigidly attached to the concrete base which is facing north such that it can survive
even in wind speeds up to 200 kmph.
3.7 Drive mechanism:-
It has a telescopic pipe arrangement and a screw rod within it along with drive
mechanism. It has a telescopic pipe arrangement and a screw rod within it along with
manual handle. There are mechanical angle indicators along the screw rod which
indicate the exact position and angle of the antenna with respect to both the axes.
3.8 Material:-
Most of the parts of the panel and antenna structure are made up of aluminum alloy
which has corrosion resistance and yield strength.
Fig. 2 Materials
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3.9 Finish:-
The reflector is treated in the following order before installation (A) Etch primer is
applied after caustic soda acid treatment (b) Painted with white matt paint. The mount
is treated with the following (a) A hot dip which galvanizes all steel parts (b) Etch
primer treatment (c) White enamel paint is applied as a last coating
3.10 Fixing the feed onto the antenna:-
The feed is supported by a set of four pipes called as a quadruped. It is fixed before
the whole antenna structure is hoisted, that is, it is fixed on the ground itself before
the whole antenna structure is fixed. Care should be taken that the feed is at the exact
focus of the reflector. A maximum tolerance of +3mm is allowed for the separation
between the actual focus and feed position. Also the feed entrances and cable output
ports are water proof Teflon sheet to prevent the entry of moisture into the
arrangement. The LNBC (Low Noise Block Converter) and cables are connected to
the feed output. The x-y adjustment is then done and fixed. The bolts are tightened
with care and the arrangement is set. Care should be taken while lifting and fixing of
the whole apparatus to prevent any damage. The Trivandrum station has the
following specifications which are used for following advantages like the
maintenance personnel of one type can work with the other type as well and spare
parts can be shared. All amplifiers are WB devices (170 to 230 MHz in B3 and 44
and 88 MHz in B1) and can operate in band 3 and band 1 of both sound and vision. In
the driver Audio and video I/P signals are connected to vision and sound IF signals.
These IF proceed prior to concession to RF output frequencies and amplified. The
attenuated 5 and 10 KW sideband pattern is obtained through the use of a lithium
neonate ground wave filter. Each amplifier is equipped with AGC. The driver also
consists of a vision synchronization detection circuit used to automatically switch the
transmitter on and off. Also the transmitter can be controlled locally and remotely.
All IF and RF interconnections use 50ohmcoaxial links to simplify maintenance. By
the use of redundant of the ampliform and power supplies, briefly can estimated
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reduced power levels in the event of a failure in several transistors amplifiers or a
power supply.
3.11 Description of Transmitter:-
The TX is in a single cabinet which the diplexer and filter assembly is associated. The
TX as discussed above has two drivers’ two RF amplification channels, power
supplies and associated co-ordination and control system, a diplexer and a RF filter.
All amps power supplies and their driver components are plug-in drawers and sub
assemblies are designed for easy access and removal. The main switch is designed for
use with all types of 3phiW/W with or without neutral 208V or 480V.
3.12 Driver:-
This subassembly is used to generate vision and sound signals corresponding to the
selected standard using input video and audio signals. This subassembly performs the
processing and conversion required to generate the filtered and vision and sound
signals in the selected RF band. The dent also provides phase and amplitude
corrections to ensure that the linearity specifications comply with various standards.
The driver acknowledges s the presence or absence of the video and audio signals that
are applied to the driver. The driver consists of plug-in mounted in a single PCB rack,
6 units high. Each driver has 5 modules connected to the mother board. Each can be
replaced separately without changing the entire assembly. Max Output power is
19ddBm for vision signals and 13dBm for sound signals. Local driver controls are on
the local freq and interface board. In the maintenance mode of the TX these controls
are active. The 2 drivers and associated passive resonance relays are directly
controlled by the control system. (Each driver has +_ 12V power supply).Each driver
has its own internal oscillator. However they can be made to work with an external
frequency synthesizer. In case of synthesizer failure the change into internal oscillator
takes place automatically. In this dual drive configuration the sys automatically
switches over to the reserve driver.
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3.13 Power amplification:-
The driver generates a low power vision RF signal and a low power sound RF signal.
They are applied to the vision and sound amplification chains consisting of identical
parallel wired high gain amplifier decreases.
Fig. 3 Power Amplification
These drivers are used for the 10Kw sys. They are distributed as follows each high
gain amplifier provides a power of 1600 W at peak .Three 2X300 W amplifiers
grouped by a empty system diagonal power in the high generator amplifier drawer
to1600W peak .A power supply distribution board. Each amplifier has its own
protection devices for
1. Power surge
2. SWR
3. Temperature rise the LCD screen provides control system monitoring and analysis.
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The amplifier drivers are provided by plug in high power supplies. (1 power supply
for 2 amplifiers.) These highly reliable units generate 50V with 120. The dish antenna
does one amplification by concentrating the signals at the focus. The LNB mounted
exactly at the focus amplifies this signal again. This signal cannot be sent through a
coaxial cable because of high frequency attenuation. So the LNB converts it to a
lower frequency between .950MHzto2.150MHz as that is the frequency required by
the IRD. The IRD used is a Scopus IRD. it has a demultiplexer, an MPEG-2 video
and audio decoder as well as data and VBI insertion functions. It can also handle high
seed and low speed data input functions. And has an on board DVB descrambling
with BISS mode1 and BISS-E support. It can be used to descramble Scopus CAS
5000 encryption system and a DSNG CA fixed key encryption system.
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CHAPTER: 4
EXPECTED OUTCOME OF
INDUSTRIAL TRAINING
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4. Expected Outcome of Industrial Training:-
4.1. Advantages:-
The most widely used antenna is parabolic reflector antenna. It provides following
advantages:
Increased overall gain.
It provides diversity reception.
It cancels out interference from a particular set of directions.
It steer the array so that it is most sensitive in a particular direction.
It determines the direction of arrival of the incoming signals.
To maximize the Signal to Interference plus Noise Ratio (SINR).
Low cross polarization.
Wide bandwidth.
Narrow beam width pattern
4.2 Disadvantages:-
Because of large size antenna, it can’t be used at low frequencies.
Main beam is pencil shaped and is surrounded by number of side lobes producing
electromagnetic interference.
At the edges of parabolic diffraction takes place resulting in side lobes. It
produces electromagnetic induction.
The feed antenna can’t be located at the focus exactly.
The parabolic is not illuminated uniformly and tapers at the edges leading to the
smaller capture area.
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5. CONCLUSION
I had a very knowledgeable experience during my industrial training. I have learnt
about different equipment like TV Transmitter, Studio equipment’s, Earth Station and
their associated equipment. There I studied about antenna, structure of antenna, their
radiation patterns, its characteristics, the advantages and disadvantages of many
instruments used for transmitting and receiving audio and video signals, way the
audio and video signals are transmitted over a distance of hundreds of thousands of
kilometers, how the noise interfere the signals and how it can be avoided. I learnt
about antenna working, antenna tuning, antenna matching, feed of antenna, antenna
used in transmitter and receiver, specific antennas used in Doordarshan-I and
Doordarshan-II, number of channels broadcasted by Doordarshan Kendra, Indore. I
came to know about the technique of displaying programs on the television. I had a
practical experience of how the artistes record their shows, type and numbers of
cameras used in studio, the audio and video signals are then synchronized, modulated
and finally telecasted to the house of thousands of people.
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6. Reference
1.en.wikipedia.org/wiki/Doordarshan
2. www.ddindia.gov.in/
3. Antenna and Wave Propagation by K.D. Prasad, Deepak Handa