Mahakal Institute of Technology and Science

2011-2012

Vocational Training Report

Submitted to

Rajiv Gandhi Proudyogiki Vishwavidhyalaya

Bhopal (M.P)

Towards Partial Fulfillment for the Award of

Bachelor of Engineering

In

Electronics & Communication Engineering

At

Doordarshan Relay Centre

Ujjain

Guided by: Submitted by:

Pankaj Pandit Megha Omshree

Doordarshan Relay Centre Ujjain

2011-2012

Certificate

Training in Electronics Department by MEGHA OMSHREE is a satisfactory account of the bona fide work done under our guidance is recommended towards partial fulfillment for the award of the Bachelor of Engineering (Electronics & Communication Engineering) degree from Mahakal Institute of Technology & Science, Ujjain.

Guide:

Date

ACKNOWLEDGEMENT

I am highly grateful to Senior Assistant Engineer Mr. P.S Sisodiya for giving me an opportunity to do my training at Doordarshan Relay Centre.

I would like to express my gratitude to Mr. Pankaj Pandit for his valuable guidance and support.

I would also like to thank all staff members for maintaining a creative atmosphere and offering timely help throughout the training.

Megha Omshree

Doordarshan

Doordarshan (literally Distant Show) is an Indian public service broadcaster, a division of Prasar Bharati. It is one of the largest broadcasting organizations in India 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. The DD provides television, radio, online and mobile services throughout metropolitan and regional India, as well as overseas through the Indian Network and Radio India.

Logo of Doordarshan

Type : Broadcast, Radio, Television network and online

Country : India

Availability : Nationwide

Founded : By Government of India in 1959

Motto : Satyam Shivam Sundaram

Headquarters : New Delhi, India

Owner : Prasar Bharti

Launch Date : 15 September 1959

Former names : All India Radio

Beginning

Doordarshan had a 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 in 1965 as a part of All India Radio. The television service was extended to Bombay (now Mumbai) and Amritsar in 1972. Up until 1975, only seven Indian cities had a television service and Doordarshan remained the sole provider of television in India. Television services were separated from radio in April 1 1976. Each office of All India Radio and Doordarshan were

placed under the management of two separate Director Generals in New Delhi. Finally, in 1982, Doordarshan as a National Broadcaster came into existence. KRISHI DARSHAN was first programme telecast on Doordarshan. It commenced on January 26, 1967 and is one of the longest running programs on Indian television.

Nationwide transmission

National telecasts were introduced in 1982. In the same year, color TV was introduced in the Indian market with the live telecast of the Independence Day speech by then prime minister Indira Gandhi on 15 August 1982, followed by the 1982 Asian Games which were held in Delhi. Now more than 90 percent of the Indian population can receive Doordarshan (DD National) programmes through a network of nearly 1,400 terrestrial transmitters. There are about 46 Doordarshan studios producing TV programmes today.

Indian National Satellite SystemINSAT or the Indian National Satellite System is a series of multipurpose Geo-stationary satellites launched by ISRO to satisfy the telecommunications, broadcasting, meteorology, and search and rescue operations. Commissioned in 1983, INSAT is the largest domestic communication system in the Asia Pacific Region. It is a joint venture of the Department of Space, Department of Telecommunications, India Meteorological Department, All India Radio and Doordarshan. The overall coordination and management of INSAT system rests with the Secretary-level INSAT Coordination Committee.

INSAT satellites provide transponders in various bands (C, S, Extended C and Ku) to serve the television and communication needs of India. Some of the satellites also have the Very High Resolution Radiometer (VHRR), CCD cameras for metrological imaging.

The Indian National Satellite (INSAT) system was commissioned with the launch of INSAT-1B in August 1983 (INSAT-1A, the first satellite was launched in April 1982 but could not fulfill the mission). INSAT system ushered in a revolution in India’s television and radio broadcasting, telecommunications andmeteorological sectors. It enabled the rapid expansion of TV and modern telecommunication facilities to even the remote areas and off-shore islands. Together, the system provides transponders in C, Extended C and Ku bands for a variety of communication services. Some of the INSATs also carry instruments for meteorological observation and data relay for providing meteorological services. KALPANA-1 is an exclusive meteorological satellite. The satellites are monitored and controlled by Master Control Facilities that exist in Hassan and Bhopal.

LOGARITHMIC UNITS

Bel and Decibel

Bel is defined as the logarithm to the base 10 of the ratio of the change in power level for audio measurement.

Bel = log10

P2

P1

Where P1 and P2 are the powers being compared.In practice, the unit Bel was found to be high. Hence the unit decibel was defined which is equal to one tenth of a Bel.

Decibel = 10log10

P2

P1

dB is used only to indicate Gain or Loss in a system like amplifier or attenuator respectively.Reference levelsThe dB may be used to indicate absolute power provided that the reference level is known. Without a reference level power expressed in dB is meaningless.

A reference level of 1 milliwatt is widely used and accepted internationally.Using this as reference level a power of 1 watt may be specified as :

a) + 30 dB (Reference level 1 milli watt)b) + 30 dB ( 0 dB = 1 MW)c) + 30 dBm (dBm indicates a power expressed in dB with a reference level of 1 mW).

In Broadcasting, 1 Watt is generally expressed as +30 dBm.

STANDARD REFERENCE LEVELS USED IN BROADCASTING1. dBm : Power in dB up or down with respect to 1 milli-watt of power that is 0.774 volt

across 600 ohms.It is used in broadcasting industries, and 1 milli watt of power is taken as reference level. 0 dBm means the power measured is 1 milli-watt across 600 ohms.

-45 dBm means the power measured is 45 dB below the reference level of 0 dBm that is 3.162 x 10-5 milliwatt across 600 ohms or 4.3 millivolt across 600 ohms.

2. dBu : 0.7746 volts is taken as the reference level and the voltage can be measured across any impedance.It need not be measured always across 600 ohms. Note the difference is only philosophical.The dBu unit has exactly the same magnitude of voltage as in dBm if the measurement is made across a 600 ohms circuits. Otherwise it will have different values.In AIR and DD this unit is used in Meltron/Keltron Audio Consoles.+8 dBu means the voltage is 1.946 volts or 8 dB above the reference level of 0.7746 volt.

-4 dBu means the measured voltage is 4 dB down with reference to 0.7746 volt or the measured voltage is 0.4887 volt.The impedance in both the cases may be any value or 600 ohms.

The output of a monitoring amplifier is 8 watts.It can be expressed as +39 dBu. Similarly a 10 watt output can be stated as +40 dBu.

3. dBw : 1 watt of power is taken as reference power. It is used when the amount of power involved is high.The EIRP of transmitter used in satellite communication is usually expressed in dBw.

4. dBv : When the reference level is taken as 1 micro volt the unit is called as dBv.5. DBv/m : when reference level is taken as 1 micro volt per meter, the unit is called dB

v/m. This unit is used in field strength measurements.

6. dBk : When the reference level is taken as 1 kilowatt (kW) the unit is called dBk. This

unit is used in high power calculation. Any level that is above 1 kW is expressed as +(x)

dBk and any level that is below 1 kW is expressed as –(x) dBk.

To convert power in watts to dBk, use the following formula.

dBk = 10 (log P-3) where P is in watts.

Modulation

Modulation is a process of superimposing information on a carrier by varying one of its parameters (amplitude, frequency or phase).

Need for Modulation

Antenna size can be reduced by modulating the signal over higher frequency. To differentiate among transmissions (stations) Maximum to minimum frequency ratio can be reduced to minimum by modulating the signal

on a high frequency.

Types of ModulationIn general, there are three types of modulation:

a) Amplitude Modulation b) Angle Modulationc) Pulse Modulation

Amplitude Modulation

If the amplitude of the carrier is varied in accordance with the amplitude of the modulating signal (information), it is called amplitude modulation.

Variation of AM Signals

DSB - FC : Double sidebands with full carrier. This is used in MW and SW Transmitters.

DSB - SC : Double sidebands with suppressed carrier. This method is used for transmission of chroma signals in TV and stereo signal in FM transmitter.

VSB : Vestigial sideband. This method utilises one side band (usually USB) with carrier and a portion of other sideband. This is used for picture (video) transmission in television.

SSB : Single Side band : In this method only one side band (without carrier) is utilised for transmission. There is considerable saving in power and bandwidth. But as the carrier is not transmitted it becomes difficult to recover the signal at the receiver end. Hence the receiver circuit is complex. The use of this method is restricted to special purpose only, such as military communications.

ISB : Independent side band : In this method each side band carries a different message and hence they are independent of each other. A reduced carrier is also inserted so as to facilitate an easy detection. This method is used in Telephone system.

Angle ModulationVariation of the angle of carrier signal with time results in angle modulation. It is of two types:

a) Frequency Modulation b) Phase Modulation

Frequency ModulationIf the frequency of the carrier is varied in accordance with the amplitude of the modulating signal, it is called frequency modulation.

Phase Modulation

If the Phase of the carrier is varied in accordance with the amplitude of the modulating signal (information), it is called phase modulation.

DIGITAL MODULATION

Criteria of Digital Transmission Systems

Digital communication system falls into 3 categories in their design.They are Bandwidth efficient,Cost efficient and power efficient.These three criteria are applicable in different environments.Radio spectrum is no more a luxury,And for broadcasters the digital system should be able to deliver within the BW available.COFDM is a type of transmission to meet these challenges.

ASK (Amplitude Shift Keying)

The simplest forms of band pass data modulation is ASK. Here the symbols are represented by discrete amplitudes of fixed frequency. Digital data is nothing but bits of 0 and 1 .To represent 0 and 1 the carrier is turned on or off. Hence this is also called as On-Off keying (OOK). Alternately we can use 2 amplitudes to represent 0 & 1.

Bandwidth and efficiency

The modulated ASK carrier becomes

Cos mt x Cos ct = 0.5 Cos (c - m)t + 0.5 Cos (c + m) t

Hence we can see that there are two symmetrical components in the modulated spectrum. So the bandwidth will be twice that of occupied base band stream. This is similar to analog amplitude modulation.

BW efficiency is defined as the ability of the modulation scheme to send number of bits/cycle/sec. The number of bits transmitted per second is called bit rate. And the number of symbols/sec is symbol rate. BW efficiency is the ratio of bit rate to symbol rate.

Bandwidth efficiency of ASK =1 bit/sec./Hz.

Detection

Detection can be coherent or non-coherent. A simple envelope detector can achieve Non coherent detection as shown above.

Frequency Shift Keying (FSK)

Here the frequency is switched from one frequency to another to represent 2 symbols. The modulator switches between two carriers of different frequencies to represent 2 symbols. This is shown in the diagram.

Alternatively a VCO (Voltage controlled oscillator) can be used as source for frequency shift keying. If the frequency shift can be minimised to 90 degrees phase shift , the same is known as Minimum shift keying.

Detection of FSK

A simplest way of detection of FSK is shown below. Here the modulated signal is filtered by 2 filters f1 & f2 which represents two different symbol states. Each filtered signal is detected and fed to Comparator, which reconstructs the bit stream.

Coherent FSK detection : Here the incoming modulated signal is mixed with two frequencies equal to the symbol carrier frequencies. Further it is filtered and sent to Comparator, which recovers the digital signal.

Advantages of FSK

- Not amplitude sensitive- No accurate frequency control is required, as only absolute change is essential.

Disadvantages of FSK- Bandwidth efficiency is poorer than PSK- Bit/symbol error rate is poorer than PSK

Phase Shift Keying (PSK)Here the phase of the carrier is changed by 180o absolutely to indicate another symbol. The receiver watches for the changes in phase of incoming signal to recover the symbol.

If the instant of phase change is indicated by a symbol it is known as differentially coherent PSK. The spectral occupancy is similar to ASK.

Detection

Only Coherent detection is possible in PSK. The figure gives the blocks of PSK detection.

QPSK Modulator (Quadrature phase shift keying)This is the most commonly used modulation scheme in digital satellite communications. Sometimes this is known as 4 QUAM (Quadrature amplitude modulation) as there are 4 states of symbols - one in each quadrant.

In this carrier is modulated with 4 phase states 0o, 90o, 180o, 270o or 45o, 135o, 225o, 315o. This is called QPSK. Due to orthogonality the information can be sent twice the speed of BPSK. Data is split into 2 streams and filtered before modulated orthogonal. The coding employed is known as gray coding as the symbol change is only 1 bit in successive symbols. In QPSK we can send 2 bits /symbol as shown in the constellation diagram.

Gray Coding

0 00 11 1

1 0

Bandwidth efficiency = 2 bits/sec./Hz

Sampling and Quantization

DIGITAL SIGNALS

A digital audio signal carries information about the sound wave in the form of a series of samples of the analogue signal taken at regular intervals.

Samples of Analog Signal

Digital Signal (PCM)

This is known as a Pulse Code Modulation or PCM signal. It is a very simple signal, having only two states: e.g. 0V or 5V. Because there are only two possible states, the meaning of the

signal is not altered in any way by moderate amounts of distortion or added noise as shown in Fig. 6 & 7.

Sampling ProcessThe square wave consists of the fundamental frequency plus all its odd harmonics (the lowest of these being 60 kHz in this case), it will therefore be possible to filter the signal to remove these unwanted harmonics and leave a perfect 20 kHz sine wave as shown in Fig.8 (c). This shows that filters play a very vital role in digital signal processing.

Fig. 16 A Circuit for Sampling of Analogue Signal

QUANTISING

The sampled audio signal is not in any way a digital signal – it is still as vulnerable to degradation as analog audio. Every sample can still have an infinite number of values. We must replace this infinite number by a finite number of values, each identified by a binary number.

Fig. 17 Sampled Signal on a Binary Scale

Each sample is given the value of the nearest quantising level and any fraction is ignored.As a result, each sample will be in error by up to half a quantising interval, plus or minus.This error is an unavoidable result of conversion to a digital signal and is known as quantising error.The smaller we make each quantising interval, the smaller the quantising error will be.A defined number of bits making up a binary number is known as a word. (A word of eight bits is given the special name of byte – common in computers ). Some different word-lengths and the maximum number they can count up to are shown below :

No.of bits Maximum count

3 74 158 25510 102312 409513 819114 1638315 3276516 6553518 26214320 1048575

The output of a Digital to Analogue converter is a ‘staircase’ signal as shown below :

Fig. 21 The Output of a Digital to Analog Converter

The steps represent the high ‘image’ frequencies. To return to the analog signal we need only to filter these off.This is done by the ‘anti-image’ or reconstruction’ filter.

PAL

PAL, short for Phase Alternating Line, is an analog television colour encoding system used in broadcast television systems in many countries

Colour television has the constraint of compatibility and reverse compatibility with the monochrome television system which makes it slightly complicated. Compatibility means that when colour TV signal is radiated the monochrome TV sets should also display Black & White pictures. This is achieved by sending Y as monochrome information along with the chroma signal. Y is obtained by mixing R,G & B as per the well known equation :

Y = 0.3 R + 0.59 G + 0.11 B

Reverse compatibility means that when Black & White TV signal is radiated the colour TV sets should display the Black & White pictures.

In view of the above the colour TV system should have :

a) Same line and field standards as that of existing monochrome.b) The same bandwidth as that of the existing monochrome system.c) The monochrome information in the Luminance signal along with colour signal.If we transmit Y, R & B and derive G then :

Since ,Y = 0.3R + 0.59G + 0.11 B

G = 1.7Y - 0.51 R - 0.19 B

In such a case what happens with a colour TV set when we transmit black and white signal. R and B are zero, but G gun gets 1.7 Y. The net result is black & white pictures on a colour TV screen appear as Green pictures. So reverse compatibility is not achieved.

Colour Difference Signals

To achieve reverse compatibility, when we transmit Y, R-Y and B-Y instead of Y, R & B, we do not take G-Y as this will always be much lower than R-Y and B-Y and hence will needs more amplification and will cause more noise into the system. G-Y can be derived electronically in the TV receiver.

G = 1.7 Y - 0.51 R - 0.19 B

So, G-Y = -0.51 (R-Y) - 0.19 (B-Y)

Thus,colour difference signals fulfill the compatibility and reverse compatibility. Because in this case the colour difference signals are zero if the original signal is monochrome (i.e. R = B = G)

Weighted Colour Difference Signals

Resultant of the two vectors of modulated R-Y and B-Y has to be added with Y to get a CCVS signal. If we allow this we find peak excursions going up to 1.79 in case of saturated yellow and similarly for some other cases it may go even below the black level. This amplitude is considered too great for transmission over equipment used also for monochrome, and for convenience the chrominance information is reduced in amplitude such that for saturated yellow the peak excursion is limited to 1.33 only. This requires reduction in all the colour vectors by a suitable weighing factor which is as per below :

87.7% of modulated R-Y, called V signal, V = 0.877 (R-Y)

49.3% of modulated B-Y, called U signal, U = 0.493 (B-Y)

Therefore weighted C = ± √U2 + V 2

PAL broadcast systems

PAL B PAL GTransmission Bands VHF UHFFields 50 50Lines 625 625Active Lines 576 576Channel Bandwidth 7 MHz 8 MHzVideo Bandwidth 5 MHz 5MHzSound carrier 5.5 MHz 5.5 MHz

Skin effectSkin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current. At 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth becomes much smaller. Increased AC resistance due to the skin effect can be mitigated by using specially woven litz wire. Because the interior of a large conductor carries so little of the current, tubular conductors such as pipe can be used to save weight and cost.

Skin depth is due to the circulating eddy currents (arising from a changing H field) cancelling the current flow in the center of a conductor and reinforcing it in the skin

Skin depth principle is used in transmission of RF signal from one direction and DC current from other direction simultaneously. Due to this the DC current travels from the centre of the conductor while the RF signal which is at high frequency travels from the oter surface i.e. the skin of the conductor.

Negative ModulationThe increase in picture brightness causes reduction in carrier amplitude i.e. the carrier amplitude will be maximum corresponding to sync tip and minimum corresponding to peak white.

In television though positive modulation was adopted in initial stages, negative modulation is generally adopted (PAL’B uses negative modulation) now a days, as there are certain advantages over positive modulation.

Advantages of Negative Modulation

i) Impulse noise peaks appear only in black region in negative modulation. This black noise is less objectionable compared to noise in white picture region.

ii) Best linearity can be maintained for picture region and any non-linearity affects only sync which can be corrected easily.

iii) The efficiency of the transmitter is better as the peak power is radiated during sync duration only (which is about 12% of total line duration).

iv) The peak level representing the blanking or sync level may be maintained constant, thereby providing a reference for AGC in the receivers.

v) In negative modulation, the peak power is radiated during the sync-tip. As such even in case of fringe area reception, picture locking is ensured, and derivation of inter carrier is also ensured.

Vestigial Side Band Transmission

It was not considered feasible to suppress one complete side band in the case of TV signal as most of the energy is contained in lower frequencies and these frequencies contain the most important information of the picture.If these frequencies are removed, it causes objectionable phase distortion at these frequencies which will affect picture quality.Thus as a compromise only a part of lower side band is suppressed while taking full advantage of the fact that:

i) Visual disturbance due to phase errors are severe and unacceptable where large picture areas are concerned (i.e. at LF) but

ii) Phase errors become difficult to see on small details (i.e. in HF region) in the picture. Thus low modulating frequencies must minimize phase distortion where as high frequencies are tolerant of phase distortions as they are very difficult to see.

The radiated signal thus contains full upper side band together with carrier and the vestige (remaining part) of the partially suppressed LSB. The lower side band contains frequencies up to 0.75 MHz with a slope of 0.5 MHz so that the final cut off is at 1.25 MHz.

Fig. 1 Response for VSB receptionPower Supplies

DC POWER SUPPLY

The Linear Power Supply System

230 V AC supply is fed to an isolation transformer which steps down the voltage to the required low level. Here the rating of the transformer depends on the current requirement of the load. Therefore, the transformer is normally bulky.

Linear Power Supply System

Merits

1. Very low output noise.

2. Very low ripple.Demerits

1. Because of the bulky transformer the power supply unit is usually bulky.

2. Relatively narrow input voltage range. Normally + 10%.

3. Very low output hold up time about 1 milli sec.

4. Low efficiency about 40 to 50%.

5. Heat dissipation is more.

Switch Mode Power Supply System (SMPS)

A switched-mode power supply (switching-mode power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains

power, to a load, such as a personal computer, while converting voltage and current characteristics. An SMPS is usually employed to efficiently provide a regulated output voltage,

typically at a level different from the input voltage.

Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions (which minimizes wasted energy). Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time. In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight

Fig.Switch Mode Power Supply

We know from the transformer theory that

E = 4.44 x f x B x A x N

Or E f x B x A

Where E=Voltage , f=Frequency , B x A=Flux density , N=No. of turns.

Because of the inherent limitations, the flux density cannot be increased in power transformer above 0.97 wb/m so if the frequency is increased the transformer size can be brought down and hence the weight. This is what has been done in SMPS.

The 230 V AC is fed directly to the rectifier and filter through Radio Frequency interference/electromagnetic interference filter. This dc voltage then chopped by a switching transistor or FET. This chopped high frequency AC is then applied to a power transformer for stepping it down. This stepped down voltage is further rectified and filtered and given to the load.

A sample of the output voltage is taken and compared with a reference voltage. The error voltage is then given to the base of the switching transistor for increased or decreased switching action.

Normally, the frequency of operation is between 10 kHz to 60 kHz from no load to full load. Even frequencies up to 1 MHz are now available in market.

Principle of SMPS

Because of higher frequencies and to reduce the core loss ferrite is normally used as the core. It can also be seen that the switching element is acting like switch (ON and OFF) and hence the heat dissipation will be low.

The merits and demerits of a SMPS is as follows:

Merits

Because of less transformer weight the power supply is light and compact. Very wide input voltage range between 90 to 260 V AC. Very good hold up time, typically 25-milli sec. Efficiency is quite high 70 to 80%. Demerits Higher output noise. Higher ripple content. EMI/RFI generation. Higher design complexity.

.

Fig. SMPS

Interior view of an SMPS: belowA: input EMI filtering; A: bridge rectifier;B: input filter capacitors;Between B and C: primary side heat sink;C: transformer;Between C and D: secondary side heat sink;D: output filter coil;

E: output filter capacitors.  

Antenna

Principle

Antenna is usually a metalic device (as a rod or a wire) used for radiating or receiving electromagnetic waves.The radio frequency power developed at the final stage of a transmitter is delivered through cables/feeders, without themselves consuming any power to the transmitting antenna.This travels in the free space in the form of radio waves (electromagnetic waves).The receiving antenna picks up the radio waves and delivers useful signal at the input of a receiver for reception of signals.The transmitting and receiving antennae are reciprocal in the sense, any characteristics of the antenna in general applies equally to both.

Antenna Radiation Resistance

The input impedance ’Zin’ of an antenna is the ratio of voltage to current at its input terminals where the power is fed to the antenna.

Zin = Ra + jXa

Ra = Resistive part of impedance

Xa = Reactive part of impedance

Ra = Rr+Ri

Rr = Radiation resistance of the antenna

Ri = Ohmic loss resistance of the antenna.

It is through the mechanism of radiation resistance, power is transferred from the guided wave at antenna input to the free space wave.

The reactive part of the input impedance is due to the storage of electric magnetic field (capacitive and inductive reactances) in the near field of the antenna.The net reactive impedance of the antenna can be matched with the conjugate impedance of the source driving the antenna.

Radiation Resistance is a fictitious term.It is equivalent of resistance which would dissipate the same amount of power as being radiated by the antenna when fed with the same amount of power.

Radiation Efficiency

The radiation efficiency determines the effective transfer of power from the input to free space, and given by

Radiation Efficiency =

Rr

R i + R r .

Isotropic AntennaIt is an imaginary (non-existent) point (dimensionless) antenna which radiates equally with unity gain in all directions in three dimensional planes.

Power Gain of Antenna

Unlike the isotropic antenna, any practical antenna has physical dimension.The field at any point away from the antenna is the vectorial sum of the individual fields received at that point from a large number of elementary portions of the whole antenna. Depending upon the path length of these individual waves, they may reinforce or cancel at such equidistance points around the practical antenna and thus contribute different levels of field in different directions, but at equal distances around the antenna. What however actually happens is that instead of laying equal field, field is accentuated in certain directions and suppressed in other directions.

Extending this principle, very large power gain can be achieved in any plane by stacking in a particular way, a number of antenna elements in a perpendicular plane.

The power gain in a given direction is the ratio of the power to be fed to the isotropic antenna to actual power of the antenna in question to lay field at a given receiving point in that direction.

Radiation Pattern (Polar Diagram)

Graphical representation of the directional radiation properties of the antenna as a function of space coordinates in three dimensions is called the radiation pattern. Such a representation will be usually very complicated to interpret. It is usual practice to represent the same in two dimensions for both horizontal and vertical planes. The length of vector from the centre or the reference point is proportional to the power gain in that direction.

Half Power (3 dB) Beam Width

The angle between the two directions in which the radiation intensity is one half (3 dB below) the maximum value of the beam.

Bandwidth of Antenna

The range of frequencies within which the performance of the antenna with respect to certain characteristic (such as input impedance, pattern, beam width, polarisation, side lobe level, beam direction, gain)conforms to a specified standard. More commonly in broadcasting the characteristics of importance are gain and input impedance.

Polarisation

The plane containing the electric vector in the electro magnetic wave describes the polarisation of the radiated wave. Ideally maximum signal is coupled if the antennae (both transmitting and receiving) are oriented in the plane of polarisation of the electro -magnetic wave. A vertical radiator radiates/picks up vertically polarised wave, horizontal radiator radiates/picks up horizontally polarised wave.

There are number of well defined polarisations such as horizontal (HP), vertical (VP), slant (+ 45o (SP), circular (left or right) (LCP, RCP), dual (DP), mixed (MP), elliptical (left or right) (LEP/REP) etc.

HP : The electric vector is in horizontal plane. TV broadcasting in India use horizontal polarisation.

VP : The electric vector is in the vertical plane. The self-radiating MW masts of AIR radiate VP waves. Electric supply undertakings use vertical polarisation for their VHF communications.

CP: The electric vector in circular polarisation rotates in a circular motion. They may be considered as the resultant of equal amplitude of vertical and horizontal polarised components combined in phase quadrature (90o).

The polarisation is said to be right or left circular polarised (RCP or LCP) depending on the rotation of electric vector of the propagating wave clockwise or anti clockwise respectively, as seen from the transmitting point or by an observer with his back to the transmitter.

INSAT down link signals are left hand circularly polarised.

Apreture of an Antenna 'A'

This term usually relates only to receiving antenna. Aperture (or effective area) of a receiving antenna is the ratio of power delivered to the load (connected to the Antenna) to the incident power density.

A=Gλ2

4 π

where G is the gain with respect to the isotropic antenna.

Different Antennae Used

1. Yagi-Uda Antenna

A Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element (typically a dipole or folded dipole) and additional parasitic elements (usually a so-called reflector and one or more directors).The name stems from its inventors, as the Yagi-Uda array was invented in 1926 by Shintaro Uda of Japan, with his colleague Hidetsugu Yagi. The reflector element is slightly longer (typically 5% longer) than the driven dipole, whereas the so-called directors are a little bit shorter. This design achieves a very substantial increase in the antenna's directionality and gain compared to a simple dipole.

A Yagi-Uda consisting of a reflector, driven element and a single director as shown here. The driven element is typically a λ/2 dipole or folded dipole and is the only member of the structure that is directly excited (electrically connected to the feedline). All the other elements are considered parasitic. That is, they reradiate power which they receive from the driven element (they also interact with each other).

One way of thinking about the operation of such an antenna is to consider a parasitic element to be a normal dipole element with a gap at its center, the feedpoint. Now instead of attaching the antenna to a load (such as a receiver) we connect it to a short circuit. As is well known in transmission line theory, a short circuit reflects all of the incident power 180 degrees out of phase. The fact that the parasitic element involved isn't exactly resonant but is somewhat shorter (or longer) than λ/2 modifies the phase of the element's current with respect to its excitation from the driven element. The so-called reflector element, being longer than λ/2, has an inductive reactance which means the phase of its current lags the phase of the open-circuit voltage that would be induced by the received field. The director element, on the other hand, being shorter than λ/2 has a capacitive reactance with the voltage phase lagging that of the current. If the parasitic elements were broken in the center and driven with the same voltage applied.

Fig. Illustration of forward gain of a two element Yagi-Uda array using only a driven element (left) and a director (right). The wave from the driven element excites a current in the passive director which reradiates a wave having a particular phase shift. The addition of these waves (bottom) is increased in the forward direction, but leads to cancellation in the reverse direction.

2. Parabolic Dish Antenna

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it is highly directive; it functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, that is they can produce the narrowest beam width angles, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which wavelengths are small enough that conveniently sized dishes can be used.

The operating principle of a parabolic antenna is that a point source of radio waves at the focal point in front of a paraboloidal reflector of conductive material will be reflected into a collimated plane wave beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the focal point.

Fig. In a parabolic antenna, incoming parallel radio waves (Q1 - Q3) are reflected to a point at the dish's focus (F), where they are received by a small feed antenna.

Paraboloidal or dish – The reflector is shaped like a paraboloid truncated in a circular rim. This is the most common type. It radiates a narrow pencil-shaped beam along the axis of the dish

Fig. Main types of parabolic antenna feeds

3. Cassegrain Antenna

In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed radiator is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam.

The primary reflector is a paraboloid, while the shape of the convex secondary reflector is a hyperboloid. The geometrical condition for radiating a collimated, plane wave beam is that the feed antenna is located at the far focus of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid.[1] Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However in offset Cassegrain configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam.

Fig. Different Cassegrain Satellite Communication Antennae

4. Offset Dish Antenna

An off-axis or offset dish antenna is a type of parabolic antenna. It is so called because the antenna feed is offset to the side of the reflector, in contrast to the common front-fed parabolic antenna where the feed is in front of the dish, on its axis. As in a front-fed parabolic dish, the feed is located at the focal point of the reflector, but the reflector is an asymmetric segment of a paraboloid, so the focus is located to the side.

The design is most widely used for small parabolic antennas or "mini-dishes", such as common Ku band home satellite television dishes, where the feed structure is large enough in relation to the dish to block a significant proportion of the signal. Another application is on satellites, particularly the direct broadcast satellites which use parabolic dishes to beam television signals to homes on Earth. Because of the limited transmitter power provided by their solar cells, satellite antennas must function as efficiently as possible. The offset design is also widely used in radar antennas. These must collect as much signal as possible in order to detect faint return signals from faraway targets.

Offset dish antennas are more difficult to design than front-fed antennas, because the dish is an asymmetric segment of a paraboloid, with different curvatures in the two axes.

Fig. Home Satellite Television Dish

Wave Propogation

Propagation of Radio waves takes place by different modes, the mechanism being different in each case. Based on that it can be classified as :

1. Ground (Surface) waves2. Space (Tropospheric) waves3. Sky (Ionospheric) waves

Allocation of frequencies for Broadcasting

Medium Wave (MW) Band

MF - 30 - 3000 kHz

531 kHz to 1602 kHz

Channel spacing - 9 kHz

Short wave (SW) Band

HF - 3 - 30 MHz

VHF

30 –300 MHz

Band 1 40 – 68 MHz TV channel # 4Channel spacing – 7 MHzBand 2 88 – 108 MHz FM Sound BroadcastingChannel spacing – 100 kHz

Band 3 174 – 230 MHz TV CH # 5 – CH #12Channel spacing – 7 MHz

UHF 300 – 3000 MHz

Band 4 470 – 606 MHz CH # 21 – CH # 37Channel spacing – 8 MHzBand 5 606 – 798 MHz CH # 38 – CH # 61Channel spacing – 8 MHz

Ground (Surface) wavesMedium wave (MW) propagates along the surface of the earth.It is vertically polarized to prevent short circuiting of the electric component.

Medium waves induces currents in the ground over which it passes, thus loses some energy by absorption; which is made up to some extent by energy diffracted down ward from the upper portions of the wave front.

Received signal strengthV=

120 πhr ht I

λd

120 = Characteristic impedance of the free space

ht = effective height of the transmitting antenna

hr = effective height of the receiving antenna

I = Antenna current

D = distance from the transmitting antenna

When ground wave and sky wave signals are received, fading occurs in those areas where the signals are of comparable strength and the area is called as fading zone. This fading zone should be kept as far as possible from the transmitter, and the optimum antenna that achieves this objective is of height 0.55, where is the wavelength of the operating frequency.

Space (Tropospheric) WavesThey travel more or less in straight lines. As they depend on line of sight conditions, they are limited in their propagation by the curvature of the earth except in very unusual circumstances.Space wave can be direct or reflected from earth surface. Direct wave will be steady and strong.Line of sight (LOS)

LOS = √2a (√ht+√hr ) m

Where

a = radius of earth = 6370 Kms.=6.37 x 106 m

ht = Transmitting antenna height in metres.

hr = Receiving antenna height in metres.

Radio waves normally goes in curved path due to refraction in troposphere. It can be noted that not only the transmitting antenna height, but also the receiving antenna height is equally important.

Fresnel Zone

Propagation is not by single thread like ray. Certain volume around the ray called “First fresnel zone” is significant for propagation. Therefore, just LOS alone is not sufficient First fresnel zone must be clear.

Environment Effects

Effects of buildings

Built up area has little effect on low frequencies (few MHz). But above 30 MHz obstruction loss and shadow loss becomes important. The attenuation of brick wall may be 2-5 dB at 30 MHz and increase to 10-40 dB at 3000 MHz.

Effects of trees and vegetation

The effect of thick vegetation is to absorb RF and it is particularly more for vertical polarization than horizontal polarization. This is one of the reasons that TV broadcasting mostly uses horizontal polarization.

Clutter losses

The loss due to natural and man made obstruction can only be statistically evaluated and a certain allowance made in the calculations of field strength. Such losses in general are grouped and referred to as “Clutter losses”. This loss is dependent on frequency of operation and the area surrounding the transmitter.

Definitions

Effective radiated power (ERP)

ERP is the product of Intrinsic power of the transmitter and the gain of the transmitting antenna over a dipole. Alternatively it is the sum of these parameters if they are expressed in decibels.

ERP = transmitter power in kW x antenna gain.= Transmitter power in dBm + antenna gain in dB.

Effective isotropic radiated power (EIRP)It is similar to ERP, except that the gain is expressed relative to isotropic antenna.Gain of a dipole = 1.64 times or 2.15 dB

EIRP = ERP (dbw) + 2.15 dBwor = 1.64 x ERP

Field Strength

Maximum signal requirements for satisfactory reception with receiving antenna at 10 m height.Doordarshan has adopted 40 dBV/m

Received field Strength in dBm = 134.8 + 10 log P – 20 log d – F dBv/mP = EIRP in watts d = distance of receiving point in metres.F = Loss experienced in propagation.Interference in VHF/UHF television signals

Co-Channel Interference

If the TV signal exceeds the Interference signal by a voltage ratio of 55 dB or more, no interference will be noted. When the two signals become very nearly same “Venetian blind” interference occurs. This is seen as horizontal black and white bars super imposed on the picture and moving up or down. As interfering signal strength increases the bars become more prominent until at a signal interference ratio of 45 dB or less the interference becomes intolerable. “Offset” method is used for co-channel interference reduction. The “offset” frequency is 2/3 line scan frequency i.e. 15625 x 2/3 = 10416.67 Hz or some odd multiple, there of since the averaging process is then optimum.In case of three stations one station can have a carrier offset above the second and the other below. 1 & 3 carriers would then be offset 15625 (line scan frequency).

Adjacent Channel Interference

Stations occupying adjacent channels present a different problem.Adjacent channel interference may occur as the result of beats between any two of these carriers. The difference of 1.5 MHz (as shown in fig) produces a coarse beat pattern.

Fig. Adjacent Channel Interference

BASIC TRANSMISSION LINESThere are three types of transmission lines used at RF. They are :

(i) Open wire feeder lines

(ii) Co-axial feeder lines

(iii) Wave guides

Characteristics Impedance Of Feeder LinesCharacteristics impedance (Zo) is defined as the input impedance of an infinite line. This is determined wholly by the geometry of its cross section. A transmission line can be represented as having R, L,C.

Zo of a Feeder Line

The inductance, resistance, capacitance and conductance of the line determine the characteristics impedance. G is the conductance of the line.

The characteristic impedance is given by the following basic formula

Zo=√R+ jωLG+ jωC

At higher frequencies R ∧G becomes negligible with respect to reac tan ces of L∧C . There fore

Zo ∝ √LC

TYPES OF FEEDER LINES1. On basis of circuit, they are :

Balanced lines : Where there are equal and opposite potential in both wires. Unbalanced lines : Here one wire is at high potential and the other side is at low

potential.2. Structurally there are two basic forms :

(I) Open wire line (ii) Enclosed line.

Open wire feeder linesZ0=276 log 2 S /d

In MW band, normally the feeder lines used are unbalanced and has following characteristics.

6 wires, 230 Ohms16 wires, 120 Ohms 24 wires, 60 Ohms

In SW, normally the balanced feeder lines are used. The impedances are

300 ohms, 4 wire

600 ohms, 2 wire

3. Basic Applications of feeder line :

To guide energy from transmitter to Antenna. In this mode energy move along the lines in a single travelling wave.

For Storing energy in excess of that dissipated in load, in the form of standing waves.

LOSSES IN THE FEEDER LINES There are four types of losses. They are :

Copper Loss : It is due to the heating of conductor. Earth Loss : It arises due to imperfect earth conductivity. Insulation Loss : It is due to insulation loss and is minor in a well designed

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

lines.

CHOICE OF FEEDER LINE IMPEDANCE When the feeder line impedance is chosen low, feeder current will be more, resulting increase in copper loss and earth loss. When feeder line impedance is high, feeder voltage will be high resulting in the use of higher voltage rating insulators. So the choice depends upon the availability of components and technology in use.

In AIR, following types of feeder lines are used.

230 ohm 6 wire (open wire) lines – for all old 100 kW as well as 10/20 kW. 60 ohm quasi coaxial feeder line - megawatt of Chinsuraha, Rajkot and Nagpur. 120 – ohm quasi coaxial feeder - all 300 kW and all 100/200 kW new version.

120 ohm feeder line is now standardised for modern transmitters.

Measurement Of Characteristic Impedance, Zo

Zo of a feeder line is given by the relation

Zo = √ Zoc . Zsc

Zoc = Open circuit Impedance, measured at input by keeping the feeder line end open

Zsc = Short circuit Impedance, measured at input by keeping the feeder line end short

Generally Zoc & Zsc are either capacitive or inductive depending upon the length of feeder line as multiple of /4.

Zoc & Zsc can be measured with VIM or RF bridge by keeping the line open and shorting high potential wire (inner) with ground wire (outer) at other end.

Another method utilises the fact that when the feeder line is terminated by its characteristic impedance, its input impedance is equal to the characteristic impedance. Input impedance is measured for various termination. The characteristic impedance is equal to that termination for which input impedance is same as the termination itself.

Cables

The coaxial transmission system comprising of cables and connectors are primarily used for efficient transmission of electromagnetic energy. Interconnection of an antenna and RF transmitter or receiver is the most common use of coaxial transmission lines. Their electrical efficiency and uniformity must be maintained over the desired frequency range and in a wide variety of installations and environments. The coaxial cable consists of an outer tubular conductor completely surrounding a concentric inner conductor and separated by a die electric medium. The field components present in the dielectric medium are radial electric field Er and a concentric magnetic field H.

Cross-section of a Co-axial cable There are certain specifications for the cables to help an individual decide which type is suitable for a particular type of application. Each item plays a role in helping the system to perform optimally.

Characteristic Impedance VSWR Capacitance CW power rating Maximum Operating Voltage Attenuation Velocity of propagation Electrical Length Stability Pulse Response Shielding Cut-off frequency Flexibility or cable design and construction Operating temperature range Cable Noise Environmental resistance

Some Other Important Instruments Used

Low-noise block blockconverter

A low-noise block downconverter (or LNB) is the receiving device of a parabolic satellite dish antenna of the type commonly used for satellite TV reception,although this acronym is often incorrectly expanded to the incomplete descriptions, low-noise block or low-noise block converter.

The LNB is a combination of Low-noise amplifer, block downconverter and IF amplifier. It takes the received microwave transmission, amplifies it, downconverts the block of frequencies down to a lower block of intermediate frequencies where the signal can be fed to the indoor satellite TV receiver using relatively cheap cable.

The signal from the dish is picked up by a feedhorn and is fed to a section of waveguide. In this waveguide a metal pin, or probe, protrudes into the waveguide at right angles to the axis and this acts as an aerial, and feeds the signal to a printed circuit board in the LNB.

Fig. (a) LNB , (b) disassembled LNB

Amplification and noise

The signal received by the LNB is extremely weak and it has to be amplified before downconversion. The Low Noise Amplifier section of the LNB amplifies this weak signal while adding the minimum possible amount of noise to the signal.

The low-noise quality of an LNB is expressed as the noise figure (or sometimes noise temperature). This is the ratio of the amount of noise in the output to the amount in the input, in decibels (dB). The ideal LNB would have a noise figure of 0dB.

Every LNB off the production line has a different noise figure because of manufacturing tolerances. The noise figure quoted in the specifications, is important for determining its suitability, is usually representative of neither that particular LNB nor the performance across the whole frequency range, since the noise figure most often quoted is the typical figure averaged over the production batch.

Block downconversion

The purpose of the LNB is to use the superheterodyne principle to take a block (or band) of relatively high frequencies and convert them to similar signals carried at a much lower frequency (called the intermediate frequency or IF). These lower frequencies travel through cables with much less attenuation, so there is much more signal left at the satellite receiver end of the cable. It is also much easier and cheaper to design electronic circuits to operate at these lower frequencies, rather than the very high frequencies of satellite transmission.

The frequency conversion is performed by mixing a fixed frequency produced by a local oscillator inside the LNB with the incoming signal, to generate two signals equal to the sum of their frequencies and the difference. The frequency sum signal is filtered out and the frequency difference signal (the IF) is amplified and sent down the cable to the receiver:

C-Band: IF frequency = local oscillator frequency - received frequency

Ku-Band: IF frequency = received frequency - local oscillator frequency

The local oscillator frequency determines what block of incoming frequencies is downconverted to the frequencies expected by the receiver.

Waveform Monitor

A waveform monitor is a special type of oscilloscope used in television production applications. It is typically used to measure and display the level, or voltage, of a video signal with respect to time.

The level of a video signal usually corresponds to the brightness, or luminance, of the part of the image being drawn onto a regular video screen at the same point in time. A waveform monitor can be used to display the overall brightness of a television picture, or it can zoom in to show one or two individual lines of the video signal. It can also be used to visualize and observe special

signals in the vertical blanking interval of a video signal, as well as the colorburst between each line of video.

Waveform monitors are used for the following purposes:

To assist with the calibration of professional video cameras, and to "line up" multiple-camera setups being used at the same location in order to ensure that the same scene shot under the same conditions will produce the same results.

As a tool to assist in telecine (film-to-tape transfer), color correction, and other video production activities

To monitor video signals to make sure that neither the color gamut, nor the analog transmission limits, are violated.

To diagnose and troubleshoot a television studio, or the equipment located therein. To assist with installation of equipment into a television facility, or with the

commissioning or certification of a facility. In manufacturing test and research and development applications. For setting camera exposure in the case of video and digital cinema cameras.

A waveform monitor is often used in conjunction with a vectorscope. Originally, these were separate devices; however modern waveform monitors include vectorscope functionality as a separate mode. (The combined device is simply called a "waveform monitor")

Fig. Waveform monitor in 2-line mode, showing color bars

Pattern Generator

The composite test pattern consists of a central field of different signal components surrounded by the grid lines and the circle.

Grid Pattern

The grid pattern consists of several crossed lines in the surrounding area of the central pattern. There are 15 horizontal and 19 vertical lines arranged to get perfect square blocks on the screen. These lines are superimposed on a background whose gray value is 20 to 30% of the maximum white level.

Circle

The brightness of the circle corresponds to 100% white level. The sine-squared pulse at the

central horizontal region of the circle has a half amplitude pulse duration of 200 ns. The circle

passes through eight grid line intersection point.

The circle presents an extra sensitive way of checking the geometric distortion.

Colour Bar

This pattern consists of a sequence of eight equidistant colour regions.The sequence of colours

are chosen according to the decreasing values of their luminance levels. The sequence of colours

from left to right is as follows :

White, yellow, cyan, green, magenta, red, blue, black.

The chrominance control in the colour TV set can be adjusted to set the colour saturation by

observing the above colours on the TV screen.

Staircase Signal

The staircase signal follows the colour bar signal in the vertical direction. This area consists of five gray steps with the signal level 0%, 25%, 50%, 75%, 100% and white.

This signal is used to check the reproduction of tonal gradation and the white balance. The contrast and the brightness controls of a TV set are adjusted such that the five gray steps are distinguishable.

White to Black and Black to White RegionWhite to black and black to white region follows the staircase signal in the vertical direction. The transitions have the rise time and the fall time approximately equal to 180 n sec. The white area has 100% white level.

The signal is used to check the transient characteristics of the TV receiver.

Caption SignalThe black area in the signal can be used for inserting a set of characters to identify the transmitting station or the time or the captions indicating the programme interruption. There is an in-built dedicated caption generator facility using Read Only Memories for this purpose.

A maximum of 8 characters is possible for the caption.Therefore, to specify a caption, two separate words of a maximum four characters each are to be indicated

Fig. Pattern Generator

Thruline Power Meter

The Bird thru line power meter is the most popular meter for measuring RF powers in

Doordarshan and AIR. The heart of the thru line meter is the directional coupler transmission

line assembly as shown in figure 1. It is connected in series with the antenna or dummy load.

The plug in directional element can be rotated 180o to measure both the forward and reverse

power levels.

A sample loop and diode elements are contained within each plug in element. The main RF barrel is actually a special coaxial line segment with 50 ohms characteristic

impedance. The thruline sensor works due to the mutual inductance between the sampling loop and the centre conductor of the coaxial element. As it is using a special

point contact diode as a detector, the power measured is the average power. While taking power measurement of TV signal the RF input to the thruline power should

be modulated with sync only and audio drive should be removed. This is because the RF power during normal transmission will vary with video signal and the meter

indication under such case should be taken as an indication of your transmitter working and should not be taken as the real output power. It may also be noted that

audio drive is also present during normal transmission. To get the peak power value in case of TV signals the average block power is to be multiplied by a factor of

1.674.

The output voltage E of the coupler then is proportional to the mutual inductance and the frequency.

The thruline power meter is not a VSWR meter, but VSWR can be determined from the formula or the graphs.

Fig. Thru-line Power meter

Communication Process In Doordarshan

Fig. Block Diagram of Process of Communication

Original Photographs Of the Instruments Used In Communication at Doordarshan Relay Centre Ujjain

Fig. Direction Coupler

Fig. Satellite System Unit

Fig. Audio-Video Switcher

Fig.Exciter

Fig. PDA and Transmitter Hall

Fig.Transmitting Antenna