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Phased Array Feed Design Stuart Hay 23 October 2009

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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 (ηapAphy/Tsys)2 x ΩFoV

• 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)

<λmin/2

CSIRO. Connected Array CBFM

PAFs vs existing multibeam feeds

• PAF gives complete high-sensitivity sampling of FoV

Beamformer (digital)

<λmin/2 >λmax/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 η• FoV limited by subreflector size

focal plane

imageplane

fD

• #elements α (f/D)2 and area of FoV• Compromise between f/D and η• 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

CSIRO. Connected Array CBFM

PAF optcis - beam stability

CSIRO. Connected Array CBFM

PAF/LNA issues

• Uncooled LNAs with Tmin in 20-40K range over 0.5-2GHz have been developed

• Tsys/ηap < 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 >50Ω 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

CSIRO. Connected Array CBFM

Connected-array approach

• Current continuity enhances bandwidth (element spacing <λ/2)• Target 0.7-1.8GHz frequency range• 9x10x2 elements for ~30 sq deg FoV• D=12m front-fed reflector with f/D=0.5

Patches Transmission lines

Ground plane

Digital beamformer

Currents

CSIRO. Connected Array CBFM

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 50 100 150−35

−30

−25

−20

−15

−10

−5

0

θ (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~377Ω 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 2−200

−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: [email protected] Web: www.csiro.au

Thank you

CSIRO ICT CentreStuart Hay

Phone: +61 2 9372 4288Email: [email protected]: www.csiro.au/group