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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. Key LOFAR Antenna requirements. Frequency band: 15 – 240 MHz Excluding FM-band 80 – 110 MHz

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LOFAR Antenna Systems


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lofar antenna systems

LOFAR Antenna Systems

Dion Kant, Wim van Cappellen

AAVP 2010

8 – 10 December 2010, Cambridge, UK.

outline
Outline
  • Requirements and design considerations
  • Low Band Antenna
  • High Band Antenna
  • Summary
key lofar antenna requirements
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
lofar 2 antennas
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
slide5

Low Band Antenna

  • 96 Low Band Antennas per station
  • Station diameter: 45 – 85 m (LBA)
  • Sparse pseudo-random configuration
high band antenna
High Band Antenna

High Band Antenna

  • 768 x 2 dipoles per station
  • Sparse rectangular grid
  • Analog beamformer per tile (4x4 elements)
lba and hba in a nutshell
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
low band antenna
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
lba noise performance
LBA noise performance
  • Single element simulation and measurement
lba station system temperature
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

to receiver filter (not

included in simulation).

Simulated Tsys

Tsky

Estimated Tsys

lba station simulation
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

Tsys

Aeff/Tsys

lba temperature dependency
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
lba environmental tests
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
high band antenna14
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
hba station estimated tsys
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.

hba station rotation
HBA Station rotation
  • HBA tiles have a different orientation in every station
  • The product beam suppresses grating lobes
  • Individual dipoles are rotated back for calibration purpose

x

=

summary
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