Safety Measures in Super Power and High Power Transmitters in All India Radio R.K.Singh and V.K Gupta

Present Status of Transmitters in AIR All India Radio is operating a large Number of Transmitters ranging from 1kw to 1000kW In addition large no of FM transmitters are also operating All these Transmitters radiate through MW, SW Antennas and high FM Towers .

Present Status of Transmitters in AIR Working on these Transmitters, Antennas and very high Towers calls for great care and safety measures to avoid accidents And safety of very costly equipments and operators .

Major components/systems involved in Broadcasting 1.RF Chain 2.AF Chain 3.Power Supply System 4. Metering System 5.Protection System 6.Cooling System 7.Feeder Line 8.MW/SW Antenna System 9.Control System .

Frequency Range of Broadcasting MW Range: 540kHz to 1605 kHz SW Range: 3MHz to 30MHz FM Range: 87.5MHz to 108MHz

Salient features of RF Chain RF chain develops Carrier(RF) of rated Frequency Very costly transmitting valves are used It requires special cooling system like Condensed Vapor cooling. Special protection system is provided to these very costly transmitting valves. For example in 300 kW MW BBC transmitter the valve used is CQK 350 costing about 25 LACS

Salient features of AF Chain This chain provides desired amplification to audio signal (content) before it is superimposed on Carrier. This process is known as ‘Modulation’ In the final stage of this chain costly transmitting Valves are used These valves are also provided special protection. For example in 300 kW MW BBC transmitter the valve used is CQK 50.

Power Supply System This system provides DC power supply to various stages of RF chain and AF chain. In final stage required may be 15 KV DC or more. In HPTs and SPTs Thyristor controlled DC power supply is used. This has special protection system like costly bank of protection diodes. This Thyristor controlled power supply must be protected against any reflected power or transients.

Metering system of Broadcast Transmitter Constant observation of voltage, current, or any other parameters of various stages of the transmitter, transmission line and mast/tower is very essential to monitor the health of the broadcast transmission system.

Metering system of Broadcast Transmitter Essential voltage, current and power, temperature , cooling water temperature/pressure are picked up from various points and displayed on the panel in front of the operator. These readings are recorded manually or automatic at regular interval.

Metering system of Broadcast Transmitter One such important measurement is VSWR. This reading gives the idea about how much transmitter power is being radiated and how much power is reflected back to transmitter. This reflected power is very dangerous and may damage costly equipments. Readings are recorded manually or automatic at regular interval.

Protection System of Broadcast Transmitter Very fast and effective protection system is Safe guard operators and costly equipments. A knob in red color is provided in front panel of the transmitter. In case of any major problem like fire, severe Sparking etc. this knob is pressed to switch off the Transmitter completely. These protection devices should be very fast.

Protection System of Broadcast Transmitter In old generation transmitters electromagnetic relays were used. These relays were not fast acting. Now modern generation transmitters uses digital relays. These solid state device relays are very fast acting.

Cooling System of Broadcast Transmitter High power transmitting tubes needs very effective cooling to protect it from damage. Forced air cooling and water cooling were used In old generation transmitters. Modern transmitters use condensed vapour Cooling which is very efficient in removing heat from the valve and protecting it from the damage.

MW/SW Antenna System Broadcast Transmitter Very high self radiating mast are used for MW Transmitters. These masts are supported by guy Wires and insulated from the ground by mast Base insulator. Verticality of the mast regular maintenance of guys Is essential to avoid any serious accident. SW aerial are called curtain, dipole. These are supported in the air with the help of towers. Regular maintenance of Aerial field is essential.

Control Circuit System of Broadcast Transmitter Control circuit of any transmitter is most important It is called Brain of the transmitter. It controls every operation of the transmitter like switching on/off in defined sequence Operators working on the transmitter must be very familiar with the Control circuit. Control system is important part of the safety measures of broadcast transmitters

Special precautions before switching on the of Broadcast Transmitter Operating staff work in shift in AIR. There is over lapping period when persons of two shifts are on duty. It is mandatory for engineer on duty to blow hooter for some specified time before applying HT voltage to transmitter. Some time technician or engineer may be working or doing some maintenance work inside the transmitter or in the aerial field Hooter is to alert everyone.

Special precautions before starting maintenance work in the Broadcast Transmitter or in the Aerial field Proper entry should be made in the log book. Before touching any component in the cubicle or In ATU it must be touched by the Earth Rod. Cubicles and components should be examined for any sparking spot Components should be touched with due care for any overheating. Maintenance schedule must be strictly followed.

Special Safety features in Broadcast Transmitters It is interesting to note that in any high power Broadcast transmitter there is strong RF field present in the cubicles of the transmitter, inside The transmitter building and in the aerial field. This RF field is capable of producing voltage of the order of 50to100 Volts or even more. Digital control circuit operate on 5volts. This digital circuit gives command to operate various stages of the high power transmitter

How Digital circuit is protected from strong RF field in the transmitter? Digital control circuit is part of the transmitter and located in one of the cubicle of the transmitter. RF field in the cubicle can easily generate more than 5 Volts in Digital circuit force it to give wrong command to transmitter. This may create malfunctioning of the transmitter and may cause serious damage to equipment and threat to operators.

What is Filter Wall? How it protects Digital circuit from strong RF field in the transmitter? Digital control circuit is located in a cubicle which has filter walls all around. This filter wall has large number of holes and each hole is fitted with RF bypass capacitor. All the wires entering into the digital control circuit or going out from the control circuit has to pass through these holes fitted with RF filter condenser. This removes any RF voltage associated with wire. Hence Digital circuit can not malfunction.

Digital Broadcasting in India Introduction of DRM transmitters in AIR About 40 DRM transmitters have been installed At various stations of All India Radio. These transmitters can operate in both Analog and Digital mode. These Digital transmitters with modern technology have excellent protection devices and Safety measures.

We welcome your Questions THANKS We welcome your Questions R.K.Singh and V.K Gupta