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