MINI PROJECTON

HIGH POWER TRANSMISSION (HPT)

BY

A.KAVITHA

J.SWATHI

CH.MAHALAKSHMI

CH.N.V.MADHAVI

K.MOHAN SUDHAKAR

ABSTRACT

The main function of High Power Transmitting station is that, it downlinks the signals from the satellite with the help of a parabolic dish antenna and boosts the signal in such a manner that it is suitable for a good TV reception & transmission. The parabolic dish antenna is placed in such a manner that its elevation and azimuth is calculated according to the looking angle of the satellite. Here two signals are to be transmitted i.e. audio signal & video signal. Modulation is required for these signal where AM is used for video signal & FM is used for audio signal.

The amplification of the signals is done by power amplifiers. The audio signal is boosted to 1 kw & video to 10kw and sent to the duplexer. These signals are fed to the turnstile antenna with the help of coaxial cables which inturn transmitted to various places around 120 km.

PARABOLIC DISH ANTENNA

The PDA which is used in HPT station is under receiving mode. It downlink the signals from the satellite at a particular frequency. A beam of parallel rays which are incident on a parabolic surface are focused at a point called focal point.

The parabolic reflector can be use to concentrate the radiation from an antenna located at the focus & to produce a sharply defined beam of radiation. The design of parabolic reflector involves the principal of equality of path length.

The PDA is placed according to the looking angle of the satellite INSAT 2E, INSAT 3A, INSAT 4B.Satellite are used for DD1 & DD2 transmission.

2. TVRO SYSTEM

Presently Doordarshan is uplinking its national metro and regional services to INSAT-2A (74°C) and INSAT-2B (93.5°E) and INSAT 2E (83°C) Downlink frequency bands being used are C-band (3.7GHz - 4.2GHz) and Ex-C Band (4.5GHz – 4.8 GHz).

Parabolic Dish Antenna (PDA) is widely used to receive the satellite signal. Gain of a PDA is directly proportional to the square of the frequency being received and its size. After reflection from PDA, signal strength is maximum at the focus of PDA, so feed is placed at its focus with the help of a mounting plate. For the reception of S-band signal we use helical feed while for C-band signal, we use feed-horn.

3. SOLID STATE EXCITER

The solid state exciter consists of 3 units.IF unitDG corrector unitUp converter unit IF unit modulates the video and audio signals and output the VIF and AIF signals. Required ±T! line offset is also introduced in this unit which is selectable.

All the PCB’s in the IF unit are fully connectorised to enable quick replacement whenever required. The IF unit is self contained, required only audio, video and 230V AC supply. The IF unit consists of the following PCB’s Visual modulator. Video processor. IF synthesizer. ICPM corrector. Rectifier and Regulator board. Display board.

Video Modulator

The video modulation generates the vision modulated IF signal at the IF of 38.9MHz. The IF signal is taken from the IF synthesizer PCB and the video signal from the video processor PCB.

The video signal is clamped to its pedestal level to stabilize the block level of the modulated RF signal and also to remove the hum present in the video waveform.

The synchronous is first separated from the signal passed through pulse shaping circuit and then a clamping pulse is generated which clamps the video signal at its back porch.

The clamped video signal and the IF signal is amplified in a two stages tuned amplifiers are fed to a using modulator to get the modulated IF signal.

Video Processor

The video processor board processes the input video signal before feeding it to the video modulator. It consists of a peak white limiter circuit, gap delay equalizer and receiver pre-corrector. The peak white limiter video signal in excess of 1V to avoid RF carrier cutoff due to over modulation. The delay equilizer corrects the difference in delay between frequency in the video band as they pass through the transmitter. The receiver pre-corrector gives extra correction for delay occurring in the domestic receiver.

IF Synthesizer

The IF synthesizer PCB consists of two separate synthesizers. One synthesizer generates the vision IF carrier at 38.9MHz with a facility to offset if by ±T! line frequency. The other synthesizer operates at5.5MHz working as the aural modulator. The incoming audio is amplified and 50µs pre-emphasis is

ICPM corrector:

This board pre correct the ICPM of vision carrier occuring in the power amplifier. Depending on the level of the incoming video, the 38.9 MHZ. If signal is phase modulated in the opposit direction to that accuring in the PA’s. This pre-corrected IF is then fed to the visual modulator board for IF Modulation.

Rectifier & Regulator Board:

The rectifier & regulator board generates clean DC power supply for all the PCB’s in the IC synthesizer unit. The AC

voltages taken from Transformer are rectified and passed through the fixed voltage regulators. The transformer and the rectifier & regulator board are designed to work even with -15% to +10%

variations in AC supply voltage of 230V .

DG corrector unit:

The DG corrector unit houses the DG corrector PCB. It requires a power supply of +29V which is fed from a power supply unit located in the Transistor rack.

DG corrector PCB:

It acts as a drop in corrector board. The PCB doesn’t provide any loss (or) gan to the vision IF signal. The DG corrections also varies with the average picture level (APL) of the input video as the amount of DG introduced by the solid state PA’s which varies with the APL.

The DG corrector PCB consists of AGL amplifier stage, APL detector stage, a luminance corrector stage, a DG corrector stage and a response corrector stage. The vision modulated IF signals at 38.9 MHZ from IR is fed to the DG unit at levels of 0dBm. The output of DG corrector after DG pre correction is fed to the up convertor unit at 0dBm.

Up Converter Unit

The up converter units translate the modulated vision IF and aural IF signals to the respective channel frequency suitable for transmission: Power supply to the unit is inbuilt allowing ease of bench testing. The PCB’s contain on board regulation to generate required voltages. The upconvertor unit contains the following PCB’s. AGC and unconverter vision PCB. AGS and upconverter aural PCB. Low synthesizer PCB. Display board.

TRANSMITTER CONTROL SYSTEM

CONTROL AND SWITCH BOARD EL CONTROL TRANSMITTER CONTROL TRPA MONITOR TRANSMITTER MONITOR TRANMITTER INTERFACE

Power supply

AIR CIRCUIT BREAKER(ACB):

The output of the transformer is fed to ACB. In ACB, the air acts as a quenching media. ACB output is spitted to two transmitters via bus bar. Each bus bar is provided with condenser bank, so as to have better power factor which results in less energy consumption.

CHANGE OVER SWITCH( C/O switch):

When the power supplied by the transformed is off., then the change over switch is operated such that it is connected to the generator. Here two generator are used, one for DD1 transmission and the other for DD2 transmission.The generator are operated manually or automatically.

TRANSFORMER

The transformer used is a step down transformer, ie 11kv is stepped down to 433v. The transformer is of 3. operated at 50hz & 250kva. The primary winding is in delta form where as the secondary winding is in star form. When supply is given to the primary side which produce the flux & links with the secondary winding by mutual induction principal according to faraday’s law of electromagnetic induction.

SUB DISTRIBUTION PANEL (SDP):

The change over switch is connected to the SDP through bus bar chamber. The sub distribution panel divides the entire supply into various units. The SDP distributes the power for DD1 transmission, essential supply and non essential supply.

ESSENTIAL AND NON ESSENTIAL SUPPLY:

Some amount of power, light and water pumps aer essential for the TV transmission. some part of power supply is used for operating these devices. Where as non essential power supply is used for room lighting, power points, street lights and flood lights.,etc.

EARTHING SYSTEM

MEASUREMENT OF EARTH RESISTANCE:

The determination of resistance between the earthling electrode & the surrounding ground is of atmost important. The resistance measurement is of potential fall method. The resistance area of an earth electrode is the area of soil around the electrode within which a voltage gradient measurable with commercial instrument exists. The earth resistance R=V/I. V= voltage drop between E & B. I= current through the earth path. The use of AC source is necessary to eliminate electrolysis effect. The test can be performed with current at power frequency from a double wound transformer by means of a voltmeter and an ammeter or by means of an earth tester.

CONCLUSION

High power transmission is very much required for a good TV reception. The HPT station does the work of collecting the signal from the satellite, modulates it, and then re-transmited it to various local areas around 120km.

The high power transmission station may not transmit the signal to the shadow region. Therefore there are few sub-stations called low power transmission station which receives the signal from HPT stations and re-transmits to the near by areas.

Hence a HPT station or a LPT station is nothing but a T.V station or a broadcasting station which broadcasts various T.V programs.

BIBILOGRAPHY

•ANTENNAS AND WAVE PROPAGATION •PARABOLIC DISH ANTENNA•TURNSTILE ANTENNA by.K.D.PRASAD

•SATELLITE COMMUNICATIONS•SATELLITE LOOK ANGLE, AZIMUTH, ELEVATION•DOWNLINK FREQUENCY CALCULATION by.TIMOTHY PRATT

•MONOCHROME AND COLOUR T.V•TVRO SYSTEM•SOLID STATE EXCITER by.GULATI

•INTRODUCTION TO ELECTRICAL ENGINEERING•POWER SUPPLIES•TRANSFORMER by.M.S.NAIDU &KAMAKSHAIAH

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