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GMRT Intruder Alarm System is installed in all array antennas and it is working fine. It consists of a Passive Infra-Red (PIR) detector, Control Panel and an electric siren. PIR detector senses the variation of the human body temperature while in motions. Hence it is known as Motion Detector. Control Panel process the true signals from the PIR detector and activate the power circuit to drive the electric siren. At the same time, Control Panel also provides the connectivity to the MCM port and give the information to the Control Room at the CEB regarding the intrusion inside the antenna shells.

Features :

• The Intruder Alarm System detects any unauthorised person in the Antenna shell and gives a warning at the Control room.
• The Intruder Alarm System is installed in all Arm antennas. PIR status is transmitted to Control Room through the telemetry system.
• Passive Infra-Red [PIR] Motion Detector is used in the Intruder Alarm System.
• The unit detects the variation of human body temperature within a range of 15m and a sensing angle of 180 deg. when it detects the intrusion, it drives the Electric Siren located in the antenna site.
• A key lock type switch is provided for switching off the Siren by authorised persons entering the Antenna Shell.

GMRT Smoke & Fire Alarm System is installed in all 30 antennas and it is working fine in all antenna. It is consists of Ionisation Smoke Detectors, a Control Panel and an electric siren. This system is monitoring smoke and fire for three zones, one is above the ABR rack, the second is above the Servo rack and third is above the Electrical Distribution Panel located just outside the antenna shells. Any one of this detector sense smoke and fire produces the signal and hence it breaks the electrical loop connected to Control Panel thereby it activates the power circuit to drive the electric siren. At the same time, it gives information to the Control Room at the CEB.

Features :

• The Smoke/Fire Alarm system monitors any smoke in the Antenna shell and transmits warning signal to the Control Room if smoke is detected.
• The System generates a warning Siren at the Antenna site as well as in the Control Room and the Antenna name is flashed on the computer screen.
• System Sensor make Americium-241 based ionization Smoke Detector-1412B is used as the smoke sensor.
• Three Smoke detectors are installed in each Antenna site, one above the Servo System racks, one above the Receiver System racks and one above the Electrical Distribution box.

Correlator is most critical and sensitive area of GMRT. It contains High Speed Digital Networks for data acquisitions and built up with special designed chips whose operating temperature are very critical. The normal temperature maintain in the Correlator Room is about 16 degree centigrade. In case of Air Conditioning failure Correlator Over Temperature Alarm System detects the rise in temperature and triggers to the Power Switching circuit for activating the electric siren.

Metallic Shielded Case provides the best RFI shielding for GMRT antenna shells, Labs and Correlator Room with temperature controlled environment for smooth operations of various sub-systems sitting inside. But many people forget to close its door after opening it causing serious problems to RFI shielding and overloading to Air Conditioning system. Electronics Door Closer can detect such mistake. It is consists of a sensor- Magnetic Door Contact and Control circuitry to drive the Sounder. In case the door is opened for more than the defined time, sounder will be getting activated and produce sound. Prototype unit is tested in the Correlator Room and it is working fine.

It is basically on-line video monitoring system. The Charge Coupled Device (CCD) senses the image information and converted to Video signals. This video output could directly connect to Television or LAN system for monitoring live video information in remote locations. GMRT Closed Circuit Television Network consists of five zone video monitoring system but presently only three zones are getting monitored. CCD cameras are fixed in the critical areas like Correlator, Receiver Room and Control Room and getting monitored in a Television available with Security. All the CCD camera outputs are connected to the Video Multiplexer/ Switcher for amplifications and controlling the switching time of each zone.

GMRT antennas spread over an area of 50 square kilometers. Locally generated RFI are very often appearing in such a vast area that could be seriously affecting to the GMRT Observations. Hence to minimize RFI it is needed a metallic shielded chamber right at the antenna base while processing the Radio signals collected from the sky. GMRT RF Shielded Case is a completely metallic enclosure consists of a door for entry and many holes for cables connected to various sub-system including electrical distribution panels and Air Conditioning systems. To have the better door contact with the metallic case, Beryllium copper is fitted on door contact surface to provide better RF shielding. Also, the holes made above the metallic case for cables are fitted with Feed-through inside the metallic hat to provide the additional RF shielding.

The following jobs are completed prior to RF shielding measurements.

1.Beryllium-copper is fixed in all the antenna shells except very few like C03, C04 and C12. Shri S. Rajamohan took the responsibility for fixing the Beryllium copper in all the antennas, 2.For Front-end and Servo system cables Feed-through is fixed on the top of the shielded case covered by a metallic hat. This job is completed only in C09 and W06.

Hence C09 and W01 are presently taken as prototyping antenna for shielding measurement.

RF SHIELDING MEASUREMENT PROCEDURE

GMRT RF shielding measurements are conducted in the frequency bands of 125, 250, 500 and 1000 MHz. We used two similar kind monopole antennas in each frequency band for transmitting and receiving RF signals. One antenna is placed inside the Shielded Case and connected to the Spectrum Analyzer and another antenna is connected to Signals Generator and placed outside the Shielded Case. The distances between Rx and Tx is about 3 meters along the door side of the Shielded Case. The two measurements are taken when Shielded Case door is opened and closed. The difference between the two measurements gives the shielding effect of the case.

Antenna type : Monopole (for transmitting and receiving)
Instruments used:
1. HP Spectrum Analyzer, Model- L1500A
2. Agilent Synthesized Signals Generator, Model- 8648D

Initially, we have tested C09 and W01 as prototyping antenna for shielding measurement. The average shielding effect is about 40 dBm. Also, we have measured the shielding for rest of the antennas where only beryllium copper is fixed (but without Feed-through and metallic hat for servo and front-end systems). The results show very poor shielding compare to C09 and W01.