Phased Array Feed Design

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					Phased Array Feed Design



  Stuart Hay
  23 October 2009
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)




           <λmin/2




                                    Low-noise amplifier (LNA)
          Beamformer (digital)      +Digital conversion
                                    +Polyphase filter


  Beams (weighted sums of inputs)



CSIRO. Connected Array CBFM
                             PAFs vs existing multibeam feeds
                             Beams (colors) and combined correlations (black)                                Beams (colors) and combined correlations (black)


                       2.5                                                                             2.5


                        2
Sensitivity (m2 / K)




                                                                                                        2




                                                                                Sensitivity (m2 / K)
                       1.5                                                                             1.5

                        1                                                                               1

                       0.5
                                                                                                       0.5

                        0
                             −4        −2           0          2          4                             0
                                               Angle (deg)                                                   −4        −2           0          2          4
                                                                                                                               Angle (deg)
                                  <λmin/2
                                                                                                                  >λmax/2




                                  Beamformer (digital)


                                        • PAF gives complete high-sensitivity sampling of FoV
                             CSIRO. Connected Array CBFM
                             PAFs vs existing multibeam feeds

                                                                                                     250
                                                                                                                                   Beam T
                       2.5                                                                                                                sys
                                                                                                                                   Amplifier T
                                                                                                     200                                        min




                                                                             Noise temperature (K)
                        2
Sensitivity (m2 / K)




                                                                                                     150
                       1.5


                        1                                                                            100


                       0.5                                                                            50


                        0
                         0                  50            100          150                             0
                                         Number of beamformer inputs                                    0      50            100             150
                                                                                                            Number of beamformer inputs




                             • PAF has significant mutual coupling effects
                                   • However beam Tsys→ amplifier Tmin with optimum (active) noise
                                     matching of array/amplifiers and optimum beamforming

                             CSIRO. Connected Array CBFM
         PAF optics - focal vs image plane


                                       focal
                                       plane

     D             f




                                                  image
                                                  plane
•   #elements α (f/D)2 and area of FoV         • #elements determined by magnification
•   Compromise between f/D and η
                                               • All elements must be beamformed for
•   Can beamform subsets of elements             good η
•   Can expand FoV by adding more
    elements (upgrade path)                    • FoV limited by subreflector size


         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?




CSIRO. Connected Array CBFM
                                   Prototype connected-patch FPA
    Connected-array approach
                                                 Patches     Transmission lines

                                                                       Currents




                                                  Digital beamformer


                                  Ground plane

•   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
    CSIRO. Connected Array CBFM
      Approach to the design

• Develop modelling capability
                                                                         0


                                                                       −5
   • Electromagnetic modelling of array
                                                                       −10

   • Electronic modelling of LNA




                                                          Power (dB)
                                                                       −15

• Numerical and experimental                                           −20

  investigations of prototype                                          −25
                                                                                                 CBFM pol. 1
                                                                                                 CBFM pol. 2

   • Resonances and matching to LNA
                                                                                                 GEMS pol. 1
                                                                       −30                       GEMS pol. 2
                                                                                                 CBFM pol. 1 (no diel.)
   • Radiation pattern in chamber                                      −35
                                                                                −150      −100
                                                                                                 CBFM pol. 2 (no diel.)
                                                                                                      −50         0                  50       100        150

   • Parkes 12m testbed                                                                                        θ (deg)



• Optimize larger design for ASKAP                                                                                                    30
                                                  −0.06
                                                                                                                             −0.06

                                                                                                                                      20
                                                  −0.04
                                                                                                                             −0.04
                                                                                                                                      10
                                                  −0.02
                                                                                                                             −0.02
                                                                                                                                      0
                                          x (m)




                                                                                                                     x (m)
                                                     0
                                                                                                                                0     −10

                                                   0.02                                                                               −20
                                                                                                                              0.02

                                                                                                                                      −30
                                                   0.04
                                                                                                                              0.04

                                                                                                                                      −40
                                                   0.06
                                                                                                                              0.06
                                                                                                                                      −50
                                                     −0.2               −0.18   −0.16   −0.14 −0.12   −0.1   −0.08       −0.06
                                                                                                                             −0.2     −0.18   −0.16   −0.14 −0.12   −0.1   −0.08   −0.06
                                                                                             y (m)                                                         y (m)

      CSIRO. Connected Array CBFM
                           Connected-array matching


                                                                                Patch                 Patch
                   700
                                                           real Z
                                                                 opt
                   600

                   500
                                                           imag Z
                                                                    opt                               Groundplane
                                                           real Zin
                                                           −imag Z                        Vi+         Vi-
Impedance (ohm)




                   400                                                 in

                   300
                                                                                          ZSE   ZSE      ie ZC=ZSE / 2
                   200
                                                                                                            ZD=ZSE x 2
                   100

                     0
                                                                                A (Vi+-Vi-)
                  −100

                  −200
                     0.5           1                 1.5                    2
                                       Frequency (GHz)




                           • Zdiff~377Ω LNA optimum noise-match impedance in mutually coupled
                             array environment
                           • Lower Zdiff modifications to array will lessen sensitivity to parasitics

                           CSIRO. Connected Array CBFM
CSIRO ICT Centre
Stuart Hay



Phone: +61 2 9372 4288
Email: Stuart.Hay@csiro.au
Web: www.csiro.au/group




Thank you
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