Phased Array Feed Design - Australia Telescope National ... · PDF file• Why phased array...
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Phased Array Feed Design
Stuart Hay23 October 2009
CSIRO. Connected Array CBFM
Outline
Why phased array feeds (PAFs) for radioastronomy? General features and issues of PAF approach Connected-array PAF approach in ASKAP
CSIRO. Connected Array CBFM
Why PAFs?
High sensitivity Wide field of view (FoV) (instantaneous and contiguous)
Survey speed (apAphy/Tsys)2 x FoV Possibilities for discovery (eg transient phenomena)
Large frequency range (0.7-1.8GHz)
CSIRO. Connected Array CBFM
PAFs in general
Concentrator (optics)
Low-noise amplifier (LNA) +Digital conversion +Polyphase filter
Beams (weighted sums of inputs)
Beamformer (digital)
CSIRO. Connected Array CBFM
PAFs vs existing multibeam feeds
PAF gives complete high-sensitivity sampling of FoV
Beamformer (digital)
max/2
4 2 0 2 40
0.5
1
1.5
2
2.5
Angle (deg)
Sen
sitiv
ity (
m2
/ K)
Beams (colors) and combined correlations (black)
4 2 0 2 40
0.5
1
1.5
2
2.5
Angle (deg)
Sen
sitiv
ity (
m2
/ K)
Beams (colors) and combined correlations (black)
CSIRO. Connected Array CBFM
PAFs vs existing multibeam feeds
PAF has significant mutual coupling effects However beam Tsys amplifier Tmin with optimum (active) noise
matching of array/amplifiers and optimum beamforming
0 50 100 1500
0.5
1
1.5
2
2.5
Number of beamformer inputs
Sen
sitiv
ity (
m2
/ K)
0 50 100 1500
50
100
150
200
250
Number of beamformer inputs
Noi
se te
mpe
ratu
re (
K)
Beam Tsys
Amplifier Tmin
CSIRO. Connected Array CBFM
PAF optics - focal vs image plane
#elements determined by magnification All elements must be beamformed for
good FoV limited by subreflector size
focal plane
imageplane
fD
#elements (f/D)2 and area of FoV Compromise between f/D and Can beamform subsets of elements Can expand FoV by adding more
elements (upgrade path)
CSIRO. Connected Array CBFM
PAF optics - beam stability
Beam stability with respect to the astronomical sources is required for high dynamic range image formation
Possibly solutions in clear-aperture (offset-fed) optics and electronic beam rotation through beamformer weights
ASKAP will use a 3-axis antenna design
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PAF optcis - beam stability
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PAF/LNA issues
Uncooled LNAs with Tmin in 20-40K range over 0.5-2GHz have been developed
Tsys/ap < 70K realistic target that would make PAFs competitive Cooling PAFs is not straightforward
Many coupled elements distributed over large area However should be further considered
Differential LNAs and LNAs with >50 noise-match impedance are desired for some PAF designs
Reduce balun loss Minimize LNA Tmin LNA modelling and measurements require further work
CSIRO. Connected Array CBFM
Connected-array approach to PAF
Broaden PAF investigation Previously focussed on Vivaldi (ASTRON and DRAO)
Planar connected arrays Alternative viewpoint emphasizing mutual coupling Possible advantages of planar and low-profile structure
Cost Loss Integration Noise coupling? Cooling? Other?
Prototype connected-patch FPA
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Connected-array approach
Current continuity enhances bandwidth (element spacing
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Approach to the design
Develop modelling capability Electromagnetic modelling of array Electronic modelling of LNA
Numerical and experimental investigations of prototype
Resonances and matching to LNA Radiation pattern in chamber Parkes 12m testbed
Optimize larger design for ASKAP
150 100 50 0 50 100 15035
30
25
20
15
10
5
0
(deg)
Pow
er (
dB)
CBFM pol. 1CBFM pol. 2GEMS pol. 1GEMS pol. 2CBFM pol. 1 (no diel.)CBFM pol. 2 (no diel.)
y (m)
x (m
)
0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06
0.06
0.04
0.02
0
0.02
0.04
0.06
50
40
30
20
10
0
10
20
30
y (m)
x (m
)
0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06
0.06
0.04
0.02
0
0.02
0.04
0.06
CSIRO. Connected Array CBFM
Connected-array matching
Zdiff~377 LNA optimum noise-match impedance in mutually coupled array environment
Lower Zdiff modifications to array will lessen sensitivity to parasitics
0.5 1 1.5 2200
100
0
100
200
300
400
500
600
700
Frequency (GHz)
Impe
danc
e (o
hm)
real Zopt
imag Zopt
real Zin
imag Zin
ZSE ZSE
Vi+ Vi-
Patch Patch
A (Vi+-Vi-)
Groundplane
ie ZC=ZSE / 2ZD=ZSE x 2
Contact UsPhone: 1300 363 400 or +61 3 9545 2176
Email: enquiries@csiro.au Web: www.csiro.au
Thank you
CSIRO ICT CentreStuart Hay
Phone: +61 2 9372 4288Email: Stuart.Hay@csiro.auWeb: www.csiro.au/group