Transcript SKA AAVP Antenna Array developments at University of Cambridge

SKA AA-Low Station Configurations and
Trade-off Analysis
Nima Razavi-Ghods, Ahmed El-Makadema
AAVP 2011, ASTRON, Dwingeloo 12-16 Dec 2011
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Requirements for AA-low configuration design
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Possible geometries and their limitations
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Controlling trade-offs
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Typical configuration design (based on DRM specs)
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AA-low: single array versus dual-band array
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Xarray: Code to evaluate AA design parameters
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Conclusions and Future work
Overview
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Sensitivity
◦ Depends on diameter, number of elements, and
their configuration
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Beam-width (Calibration)
◦ We can increase this by either reducing station
size or using a tapering function but at the cost
of A/T
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Side lobes (Noise suppression)
◦ We can reduce this by tapering and irregular
configurations like GRS
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Filling Factor (one used figure of merit)
Configuration Design Space
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A/T Requirements
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A/T Requirements
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A/T Requirements
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A/T Requirements
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Possible Geometries for AA-Low
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Possible Geometries for AA-Low
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Aeff/Tsys for a typical observation
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Pattern vs. Array Size
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Coherent and Incoherent Regimes
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345 m
255 m
255 m
Random
Random
390 m
Gaussian Taper (2.5)
447 m
Controllable
Beam-width
= K/D
Spatial
Tapering
Chebyshev Taper (70dB)
Chebyshev Taper (100 dB)
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Defined either as the ratio of Aeff to Aphys or
the number of antennas in the array divided
by the number of elements required to
Nyquist sample the wavefront at each
frequency point.
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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.
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Filling factor 
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Single Band
70-450MHz
 Random (dense packed)
 N = 2440 elements
 D = 90m
 Avg. Spacing = 1.43m
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Element BW = ±35
Trec = 0.1*Tsky + 40
Rad. Efficiency = 93%
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 m2/k @ 45 Scan
(100 to 450 MHz)
Example SKA1 Station Distribution
Using Single or Dual Array Solution
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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.
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Side-lobe Control
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Xarray Tool: MATLAB GUI
sites.google.com/site/xarraytool/
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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.
Conclusions
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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).
Future Configurations work
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Thank You.
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