Antennas and Front

Report
LOFAR Antenna Systems
Dion Kant, Wim van Cappellen
AAVP 2010
8 – 10 December 2010, Cambridge, UK.
Outline
• Requirements and design considerations
• Low Band Antenna
• High Band Antenna
• Summary
AAVP 2010
DK, WvC, 2010/12/10
Key LOFAR Antenna requirements
• Frequency band: 15 – 240 MHz
– Excluding FM-band 80 – 110 MHz
• Sky noise dominated
• Large collecting area
• Large beamwidth (120 deg)
• Height < 2.0 m
• Cost effective
AAVP 2010
DK, WvC, 2010/12/10
LOFAR: 2 antennas
• Two antennas:
– Low Band Antenna (LBA) 15 – 80 MHz
– High Band Antenna (HBA) 110 – 240 MHz
• Because:
– Completely different sky noise temperatures in low and high bands
– One antenna could not meet performance requirements
over whole band
– Antenna configuration can be different for LBA and HBA:
• If one antenna was used, the array would be too dense at 15 MHz
or too sparse at 250 MHz.
– RFI (FM band) in the middle of LOFAR band
AAVP 2010
DK, WvC, 2010/12/10
Low Band Antenna
•
•
•
96 Low Band Antennas per station
Station diameter: 45 – 85 m (LBA)
Sparse pseudo-random configuration
AAVP 2010
DK, WvC, 2010/12/10
High Band Antenna
High Band Antenna
•
768 x 2 dipoles per station
•
Sparse rectangular grid
•
Analog beamformer per
tile (4x4 elements)
AAVP 2010
DK, WvC, 2010/12/10
LBA and HBA in a nutshell
• LOFAR Low Band: 15 – 80 MHz:
– Tsky varies from 125,000 to 1,750 K
– Array must be very sparse at lowest frequency for high Ae
– Per-element digitization
– Randomized configuration to smear out grating lobes
– Electrically small elements (height <0.1 l at lowest freq)
• LOFAR High Band: 120 – 240 MHz
– Tsky varies from 600 K to 110 K
– Large number of antennas needed for high collecting area
– RF beamforming within tile (16 elements), digitization per tile
– Regular configuration to reduce costs and ease calibration
– Grating lobes suppressed by station rotation
AAVP 2010
DK, WvC, 2010/12/10
Low Band Antenna
• Frequency range: 15 – 80 MHz (l from 20 m to 3.75 m)
• Active balun:
– Senses open terminal voltage of antenna
– Small wrt wavelength to meet environmental requirements
– Inefficient radiator at low frequencies, but acceptable due to
very high sky noise
– But: No RFI filtering possible ahead of active circuits
• Low frequency performance set by Tsys
• High frequency performance set by pattern degradation
AAVP 2010
DK, WvC, 2010/12/10
LBA noise performance
• Single element simulation and measurement
AAVP 2010
DK, WvC, 2010/12/10
LBA station system temperature
• Estimated Tsys from measured Ae/Tsys of LOFAR station towards
zenith and simulated Ae
• Simulated Tsys (=Tant + Tsky from previous slide)
• Excellent agreement!
• Deviation at 80 MHz due
Tsky
Simulated Tsys
Estimated Tsys
to receiver filter (not
included in simulation).
AAVP 2010
DK, WvC, 2010/12/10
LBA Station Simulation
• Station simulation (96 elements)
– Using combined EM and circuit simulation
(using in-house CAESAR code)
– Results shown at 60 MHz
• Used to optimize station configuration
– dense vs sparse
– Regular vs irregular
Aeff
AAVP 2010
Tsys
Aeff/Tsys
DK, WvC, 2010/12/10
LBA Temperature dependency
•
Gain temperature dependency
– Measured from -30 to 80 °C
– Max. change rate: 0.005 dB / °C
•
(0.1% / °C)
Phase temperature dependency
– Max. change rate: 0.06 deg / °C
AAVP 2010
DK, WvC, 2010/12/10
LBA Environmental tests
• Lifetime > 15 years
• Suppliers indicate their materials will be OK, but cannot
guarantee.
• The LBA has been extensively tested:
– Ozone, salt-spray and SO2
– Solar radiation (1000 hr)
– Tent peg pulling test
– Dipole wire pulling test
– Liquid penetration test of molded LNA
AAVP 2010
DK, WvC, 2010/12/10
High Band Antenna
• Fat dipole antenna elements in 4x4 tiles
• Rectangular array
• Element spacing (1.25 m) = l/2 at 120 MHz
• True time delays integrated in elements
• Beamformer is ‘simple’ combiner
• ‘Matched’ LNA
• Per tile digitization
AAVP 2010
DK, WvC, 2010/12/10
HBA Tsys for various scan angles
• Simulated Tsys
AAVP 2010
DK, WvC, 2010/12/10
HBA station estimated Tsys
• Tsys estimated from measured Ae/Tsys and simulated Ae.
• At this scan angle (7° from zenith) the system is
sky noise dominated below 150 MHz.
AAVP 2010
DK, WvC, 2010/12/10
HBA Station rotation
x
=
• HBA tiles have a different orientation in every station
• The product beam suppresses grating lobes
• Individual dipoles are rotated back for calibration purpose
AAVP 2010
DK, WvC, 2010/12/10
Summary
• The LOFAR antenna design is highly sky noise dominated:
– High sky noise enables electrically small LBA in a very sparse
configuration
– Lower sky noise in high band forced ‘matched’ LNA’s
• But there were many more aspects impacting the design:
– RFI
– Required array configuration
– Performance requirements
– Reliabilioty, lifetime
– Maintainability
– Costs
• LOFAR successfully demonstrated a split-band design
AAVP 2010
DK, WvC, 2010/12/10

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