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 metallic case, Beryllium coper are 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.
jobs are completed prior to RF shielding measurements. 1.Beryllium-coper
are fixed in all the antenna shells except very few like C03, C04 and
C12. Shri S. Rajamohan took the responsibility for fixing the Beryllium
coper in all the antennas, 2.For Front-end
and Servo system cables Feed-through are 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 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 other 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.
Difference between the two measurements gives the shielding effect of
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
coper are 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.