LOFAR Antenna Systems

Dion Kant, Wim van Cappellen

AAVP 2010

8 – 10 December 2010, Cambridge, UK.

DK, WvC, 2010/12/10AAVP 2010

Outline

• Requirements and design considerations

• Low Band Antenna

• High Band Antenna

• Summary

DK, WvC, 2010/12/10AAVP 2010

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

DK, WvC, 2010/12/10AAVP 2010

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

DK, WvC, 2010/12/10AAVP 2010

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

DK, WvC, 2010/12/10AAVP 2010

High Band Antenna

High Band Antenna

• 768 x 2 dipoles per station

• Sparse rectangular grid

• Analog beamformer per tile (4x4 elements)

DK, WvC, 2010/12/10AAVP 2010

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 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

DK, WvC, 2010/12/10AAVP 2010

Low Band Antenna

• Frequency range: 15 – 80 MHz ( 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

DK, WvC, 2010/12/10AAVP 2010

LBA noise performance

• Single element simulation and measurement

DK, WvC, 2010/12/10AAVP 2010

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).

Tsky

Estimated Tsys

Simulated Tsys

DK, WvC, 2010/12/10AAVP 2010

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

DK, WvC, 2010/12/10AAVP 2010

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

DK, WvC, 2010/12/10AAVP 2010

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

DK, WvC, 2010/12/10AAVP 2010

High Band Antenna

• Fat dipole antenna elements in 4x4 tiles

• Rectangular array

• Element spacing (1.25 m) = /2 at 120 MHz

• True time delays integrated in elements

• Beamformer is ‘simple’ combiner

• ‘Matched’ LNA

• Per tile digitization

DK, WvC, 2010/12/10AAVP 2010

HBA Tsys for various scan angles

• Simulated Tsys

DK, WvC, 2010/12/10AAVP 2010

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.

DK, WvC, 2010/12/10AAVP 2010

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 =

DK, WvC, 2010/12/10AAVP 2010

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