RFI at GMRT:
Detection, Suppression and Co-existence

Pravin Raybole, S.Sureshkumar GMRT, India
Acknowledgement
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Yashwant Gupta, GMRT Chief Scientist
A. Praveen Kumar
S. Suresh Kumar
B. Ajith Kumar
Suresh Sabhapathy
M. R. Sankararaman
Field Team members
 Amit Sawant, Manoj Chavan, Rahul Durgude, Dhairyashil Dhamdere,
Mukund Deshmukh, Milind Thorat and Suresh Kale
Contents
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Introduction to GMRT
RFI Environment at GMRT
Interference from TV boosters
Interference from Cell Phone towers
Power line interference
RFI detection using the GMRT Software Backend
The GMRT
• GMRT is designed and
built by NCRA – TIFR
Pune. Located 80 km N of
Pune and 160 km E of
Mumbai.
• 30 antennas of 45 meter
diameter covering 50MHz
to 1450 MHz
• 12 dishes in central
compact (1km) array
• Remaining along 3 arms of
Y-array, longest baseline
(25kms)
1 km x 1
km
14
km
RF Spectrum at GMRT : 0 – 500 MHz
Day 12:00 hours
Night 22:00 hours
SATELLITE
FM
ATC
SW
POLICE WIRELESS
TV
RF Spectrum at the GMRT : 500MHz – 1GHz
Day 12:00 hours
Night 22:00 hours
TV
CDMA
GSM
RF Spectrum at the GMRT : 1GHz – 2GHz
Day 12:00 hours
Night 22:00 hours
GSM
GMRT : Operating Frequency Bands
Frequency
Radio Astronomy
Band
System Bandwidth
Source of Interference observed near
Radio Astronomy band.
150 MHz
150 – 153.6 MHz
130 – 230 MHz
Air Traffic control, Fixed Mobile, TV
Transmission,
235 MHz
230 – 235 MHz
213 – 253 MHz
Planned:
250 – 500 MHz
Digital Audio Broadcast ( DAB ),
Aeronautical Radio Navigation, Leakage
from Cable TV signals.
327 MHz
322 – 328.6 MHz
295 – 365 MHz
Fixed Mobile, Aeronautical Radio
Navigation
610 MHz
604 – 616 MHz
570 – 660 MHz
Planned
550 – 900 MHz
TV Transmission
1060 – 1420
MHz
1400 - 1427
885 – 1450 MHz
Fixed Mobile Broadcasting, Satellite
RFI environment at GMRT
Intentional Radiation
Unintentional Radiation
TV and FM
Power Line RFI
Police Wireless Communication
TV Boosters
Air Traffic Control
Industries
Mobile Communication
Electronics Instruments, Computers
Air Force
Networking Devices
Monitoring the RFI environment at the GMRT

Regular monitoring of the RF spectrum.
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Regular field survey to check radiation from new
installations.
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Close contact and a shared knowledge base with
government agencies, local industries, cellular phone
operators, electric companies and individuals.

Regular studies of internal ( i.e. self generated ) RFI, e.g.
from GMRT computers, instruments.
Important RFI sources at the GMRT
Sources
TV Boosters
Nature of RFI
Cellular phones
Broad-band, Narrow
band or very narrow lines
Narrow-band
Power lines
Broad-band
Interference from TV boosters
Frequency Range
VHF and UHF band
Gain
40 – 60dB
No of stages
2 – 3 cascaded stage
• Measurement setup
• Log periodic antenna
• Spectrum analyzer
• 20dB Amplifier
• Generator for backup.
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Locating TV Booster
Did a field survey of TV dipole antennas.
Visited every location for RFI measurement.
RFI from TV Boosters : Broad-band
Frequency 40MHz to 650MHz
RFI from TV Boosters : Broad-band
Frequency 314MHz to 375MHz
RFI from TV Boosters : Narrow-band
Frequency 40MHz to 650MHz
Method adopted
Problem
:
Saturation and oscillation of TV booster amplifiers
Solution
:
New resistor added in series with existing variable resistor
Resistance Values :
900 Ohm to 1.2KOhm
A 25 paise solution
Before Putting Resistor
Variable Pot
After Putting Resistor
Corrected TV booster
Frequency 40MHz to 650MHz
FM
TV SIGNAL
Broadband RFI from TV boosters
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Field survey of 25 nearby villages in and around GMRT with the
help of the local administration.

Inspected nearly 1800 TV boosters over a 3 years period.
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Reduced the gain of the amplifier to prevent saturation and
oscillation, thereby ensuring good signal reception.

Follow-up monitoring studies found no cases of recurring
problem over a 3-year period.
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Recent field surveys have found that 66% of users have switched
to Direct-to-Home services which have Ku–band downlink
frequencies. These do not produce any RFI at GMRT
frequencies. The situation is likely to continue to improve over
the next year.
Interference from Cellular Phone
towers
Co-existing with cellular operators

Cell phone towers use CDMA (824-896 MHz), GSM (935-960
MHz) and (1811-1880 MHz) and they operate with 20 Watt and
40 W transmitters.

After detail discussions, most of the GSM operators within a
radius of 20 km of GMRT, agreed to shift GSM transmissions to
the 1800 MHz band, which does not affect GMRT observations.
A few towers beyond 20 km have also been asked to change to
1800 MHz GSM band.
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CDMA operators operating at 824-896 MHz are still a serious
source of RFI for the GMRT especially with the planned
extensions in frequency coverage. At present, the CDMA
operator have been request to use low power transmitters and to
focus their transmitters away from GMRT antennas. Longerterm solutions are now being examined.
GSM transmission at 950 MHz band shifted to 1800 MHz
GSM Transmission at 950 MHz
GSM transmission at 950 MHz
NARAYANGAON IDEA RTT 25m tr. TOWER(ABOVE RAJGURUNAGAR BANK) GAMA SECTOR 950MHz BAND
TOWARDS HIGHWAY DIRECTION(16-02-2010).
0
-10
-20
-30
-50
-60
945435000.00, -91.28
945900000.00, -90.13
Power(dBm)
-40
-70
-80
-90
-100
9.25E+08
9.30E+08
9.35E+08
9.40E+08
9.45E+08
9.50E+08
Frequency(Hz)
9.55E+08
9.60E+08
9.65E+08
9.70E+08
9.75E+08
Power Line Interference
11KV/415V AC 3 Phase Transformer :
Power line interference measured using Ultrasound detector
Transformer installation
Ultrasound detector
Accurate and efficient in pin- pointing
problems with disc, pin insulators, fuse
links and cut joints at transformer sites.
Survey of transformers along the GMRT West arm
W06
W05 W04
W03
W02
W01
Survey of 3 Phase Transformer installation along
GMRT antennas ( 16th March 2010 )
SR.
ANTENNAS
NO.
TRANSFORMERS
PROBLEMATIC
AVERAGE
GOOD
ATTENDED
1
EAST ARM
5
E-02,E-05
E-03,E-04,E-06
2
WEST ARM
6
W-01,W-03
W-02,W-04,W-05,W-06
3
SOUTH ARM
5
S-01
S-02,S-03,S-04,S-06
TOTAL
16
5
11
SR. NO.
DETAILS OF RADIATION
EAST
WEST
SOUTH
BAD 11KV DISC INSULATOR
1
W-01
2
BAD DO
E-02
W-01
3
BAD TRANSFORMER
E-02,E-05
W-01, W03
4
BAD H.T. BUSHING PIN INSULATOR
W-01
5
BAD L.T. BUSHING PIN INSULATOR
W-01
S-01
Uses measurements made with the GMRT for the
localisation of RFI.
An RFI map with the GMRT Software backend at 150 MHz
A Technique developed by
Ue-Li Pen and collaborators for
Epoch of Reionization work at
the GMRT.
Uses measurement made with
Interferometer for localization
of RFI
Location of RFI sources are marked in blue
Antenna location are marked in red
Hanging stray wire on 440 KV
AC EHT line
Hanging stray wire on 440 KV AC EHT line
Abandoned Telephone line near HT line
Summary
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The GMRT is located in a region of high population
density, and must hence co-exist with multiple sources of
RFI.

The primary sources of RFI at GMRT are from cellular
phone transmissions, power line signals, and TV boosters.

Regular field surveys have been carried out to locate
sources of strong RFI. Direct repairs of individual TVs
were used to solve the problem of malfunctioning TV
boosters.
Summary
cont…
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Close interaction with the Cell phone operators have
resulted in a significant fraction of these transmission
being shifted to 1800MHz, out side the GMRT bands.

Similar interactions with electric companies have been
used to repair malfunctioning power line transformers

RFI environment at GMRT is still quite acceptable for
cutting-edge low frequency radio astronomy. The
frequency range is now being extended to cover 250-500
MHz and 550-900 MHz and there are no obvious showstoppers from the RFI point of view that will limit system
performance.