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!

T sky Simulated T sys Estimated T sys • Deviation at 80 MHz due 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

A eff T sys A eff /T sys

AAVP 2010 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 SO 2 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