1. SKA AA-Low Station Configurations and Trade-off Analysis Nima Razavi-Ghods, Ahmed El-Makadema AAVP 2011, ASTRON, Dwingeloo 12-16 Dec 2011 1

2. Overview Requirements for AA-low configuration design Possible geometries and their limitations Controlling trade-offs Typical configuration design (based on DRM specs) AA-low: single array versus dual-band array Xarray: Code to evaluate AA design parameters Conclusions and Future work 2

3. Configuration Design Space Sensitivity ◦Depends on diameter, number of elements, and their configuration Beam-width (Calibration) ◦We can increase this by either reducing station size or using a tapering function but at the cost of A/T Side lobes (Noise suppression) ◦We can reduce this by tapering and irregular configurations like GRS Filling Factor (one used figure of merit) 3

4. 4 A/T Requirements

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9. 9 Possible Geometries for AA-Low

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11. A eff /T sys for a typical observation 11

12. Pattern vs. Array Size 12

13. Coherent and Incoherent Regimes 13

14. 14 255 m 447 m Random Gaussian Taper (2.5) Chebyshev Taper (100 dB) Chebyshev Taper (70dB) 390 m 345 m Spatial Tapering Controllable Beam-width = K  /D

15. Filling factor  Defined either as the ratio of A eff to A phys or the number of antennas in the array divided by the number of elements required to Nyquist sample the wavefront at each frequency point. Is it as vital as we all think? Beamwidth can be controlled by D and tapering A/T is maintained by N, D, and configuration Side-lobe adds to noise when FF<1 but can be controlled too. 15

16. Example SKA1 Station Distribution Using Single or Dual Array Solution Single Band 70-450MHz Random (dense packed) N = 2440 elements D = 90m Avg. Spacing = 1.43m Element BW = ±35 Trec = 0.1*Tsky + 40 Rad. Efficiency = 93% 16 Dual Band 70-180MHz, 200-450MHz Random N1 = 1540, N2 = 2440 D1 = 80m, D2 = 50m BW1 = BW2 = ±35 Rad. Efficiency = 93% (~63% extra elements) (~50% if lower gain) Aim for A/T of 1000 m 2 /k @ 45  Scan (100 to 450 MHz)

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19. Side-lobe Control The system noise temperature increases by sources out side the main beam. Side-lobe level requirement can be driven from station sensitivity and the maximum noise source flux to be suppressed down to the thermal noise level. The minimum peak side lobe level of an un-tapered station is -17 dB (uniform circular aperture). However, a lower side lobe level can be achieved with a tapered station. 19

20. 20 Xarray Tool: MATLAB GUI sites.google.com/site/xarraytool/

21. Conclusions AA configuration design space should be based on optimising A/T, FOV and mean SLL. Typical increase of A/T can be defined by N and D but some configurations can result in a very rapidly changing A/T. Beam-width is defined by K/D, where K can be changed by use of tapering but should be done cautiously. Mean SLL can be controlled by tapering to achieve better than typical -17dB. Smaller arrays can be beneficial in this regard as well as for larger FOV. Single vs. Dual argument should be thought about carefully with more realistic assumptions. 21

22. Future Configurations work We MUST use OSKAR 2 to test station configurations from an interferometric aspect. Initial first–level station design can be made in Xarray which includes a sky and receiver model. Design Can be further checked and validated with MoM-MBF code developed at UCL, Belgium which work along-side Commercial software packages such as CST and HFSS. Develop optimisation tools which can be analytical (collaboration with UCL). 22

23. Thank You. 23