Docstoc

aasshort.ppt - NRAO Socorro_ New Mexico - National Radio

Document Sample
aasshort.ppt - NRAO Socorro_ New Mexico - National Radio Powered By Docstoc
					Optimization of Compact Array Configurations to
      Minimize Side-Lobes for Two Cases:
   The New E-configuration for the EVLA and
       Phased Array for the LWA Station
                      1                   1              1     2
        L. Kogan , F. Owen , J. Ott , A. Cohen
       1
        National Radio Astronomy Observatory Socorro, NM USA
       2 The Johns Hopkins University, Applied Physics Laboratory




   January 13, 2011       217th AAS Meeting-Seattle,WA
                    Configuration figures of merit…
•   Minimum side lobes
•   Gaussian shape of the main beam
•   Minimum gaps in the UV coverage
•   Others

We optimize the array configuration minimizing the maximum positive side
lobe inside of the primary beam!

Kogan 2000, IEEE Transaction on Antennas and Propagation, 48, 7, 1075

The algorithm is coded in AIPS as task CONFI

The achieved small side lobes for the considered arrays promise high image
fidelity for maximum scientific results!!!




       January 13, 2011      217th AAS Meeting-Seattle,WA
                                             The VLA-E configuration




  Left:  Mathematically created mask to prevent appearance of antennas (during optimization) on the prohibited places:
         proximity to tracks, proximity to fixed antennas……
 Center: The designed configuration.11 existed antenna pads: red; 16 new antenna pads: blue.
         Diameters of the circles are in scale with 25m.
 Right: The tracks and connections are included.




               The two dimensional beam is at the left. The two orthogonal cross sections are at the right

          1.          The designed VLA-E configuration has maximum side lobe ~12% for snapshot observation. VLA-D for
                      comparison has ~60%.
          2.          The brightness temperature sensitivity is expected ~10 times better than VLA-D
          3.          The cost of the array is minimized using the 11 existing antenna pads and the existing track at the north–
                      east sector at the central part of VLA. Only one extra track (at the north-west sector) is added.
          4.          Obviously the same 27 antennas are used



January 13, 2011                                 217th AAS Meeting-Seattle,WA
        LWA station: 256 dipoles inside of ~100m diameter circle




                                                                        The final LWA configuration design.
                                                       Left: The configuration itself . The minimum spacing is increased to 5m
Beam pattern at 80MHz. Minimum spacing 2m.
The optimization is done for sin( z)  1
                                                              to compromise with the mutual coupling between the dipoles.
                                                             The optimization is done for sin( z )  2
 The maximum side lobe is 0.6% within the
                                                       Center: The two dimension beam pattern.
optimizing region.
                                                       Right: The two orthogonal cross sections of the beam pattern.
 The left beam pattern is phased towards the
zenith. The right beam pattern is phased
                                                       The side lobes of the designed LWA station configuration are never
towards z 0  60 , az 0  45 . The low right part
                            
                                                              bigger than 1.6% at any point in the sky regardless of phased
of the hemisphere is not covered by the
                                                              direction or operating wavelength.
optimizing region.
                                                       The side lobes can be achieved much lower, if the minimum spacing is
The optimization for sin( z )  2 is required!
                                                              chosen less.




            January 13, 2011                        217th AAS Meeting-Seattle,WA
                   Thanks




January 13, 2011   217th AAS Meeting-Seattle,WA
          LWA station: 256 dipoles inside of ~100m diameter circle

We carried out the optimization inside of the whole hemisphere for the shortest
wavelength of the LWA’s operating frequencies and for zenith direction.
The circle of optimization for other (longer) wavelengths may be only larger.

Therefore if side lobes are optimized inside of the whole hemisphere for the
shortest wavelength of the LWA’s operating frequencies they will be optimized for
any (longer) wavelength!

Phasing the array to another (not zenith) direction, the whole beam pattern will be linear
shifted, if the sine of zenith angle sin(z ) is used for the coordinates instead of the zenith
angle itself (see the following slide).

Therefore if we want to optimize the side lobe inside of the whole hemisphere for
any phasing direction we need to optimize at zenith using the radius of
optimization sin( z)  2 ! That is exactly what is done optimizing LWA station!!!




      January 13, 2011             217th AAS Meeting-Seattle,WA
                                                                          The final LWA configuration design.
Beam pattern at 80MHz. Minimum spacing 2m.              Left : The configuration itself. The minimum spacing is as big as 5m
The optimization is done for sin( z )  1.                                                                     e
                                                                to compromisewith themutual coupling between th dipoles.
The maximum side lobe is 0.6% within                            The optimization is done for sin( z )  2
the optimizing region.                                  Center : The two dimension beam pattern.
The left beam pattern is phased towards the             Right : The two orthogonalcros sections of the beam pattern.
zenith. The right beam pattern is phased
towards z  60 , az  45. The low right part          The side lobes of the designed LWA station configuration are never
of the hemisphere is not covered by the
                                                        bigger than 1.6% at any point in the sky regardless of
optimizing region.
                                                        phased direction or operating wavelength.
The optimization for sin( z )  2 is required!
                                                        The side lobes can be achieved much lower, if the minimum spacing is
                                                        chosen less.




        January 13, 2011                         217th AAS Meeting-Seattle,WA
                              Conclusions
1.       The side lobes of the designed LWA station configuration are never
         grater than 1.6% at any point in the sky regardless of phased
         direction or operating wavelength.
2.       The minimum spacing is as big as 5m to compromise with the mutual
         coupling between the dipoles.
3.       The side lobes can be achieved much lower, if the minimum spacing
         is chosen less.




     January 13, 2011        217th AAS Meeting-Seattle,WA
January 13, 2011   217th AAS Meeting-Seattle,WA
                                                          Conclusions
1.       The designed VLA-E configuration has maximum side lobe ~12% for snapshot observation. VLA-D for comparison has ~60%.
2.       The brightness temperature sensitivity is expected ~10 times better than VLA-D
3.       The cost of the array is minimized using the 11 existing antenna pads and the existing track at the north–east sector at the central part of VLA. Only one extra
         track (at the north-west sector) is added.
4.       Obviously the same 27 antennas are used




     January 13, 2011                                    217th AAS Meeting-Seattle,WA
January 13, 2011   217th AAS Meeting-Seattle,WA
The beam pattern of the optimized E-configuration. The two dimensional
beam is at the left. The two orthogonal cross sections are at the right.




 January 13, 2011        217th AAS Meeting-Seattle,WA
                   UV coverage for different declinations


January 13, 2011             217th AAS Meeting-Seattle,WA
The beam pattern of the optimized E-configuration. The two dimensional
beam is at the left. The two orthogonal cross sections are at the right.




 January 13, 2011        217th AAS Meeting-Seattle,WA
Sensitivity loss due to shadowing for different declinations and hour angles.




  January 13, 2011         217th AAS Meeting-Seattle,WA
                                                             Conclusions

1.       The designed VLA-E configuration has maximum side lobe ~12% for snapshot observation. VLA-D for comparison has ~60%.
2.       The brightness temperature sensitivity is expected ~10 times better than VLA-D
3.       The cost of the array is minimized using the 11 existing antenna pads and the existing track at the north–east sector at the
         central part of VLA. Only one extra track (at the north-west sector) is added.
4.       Obviously the same 27 antennas are used




     January 13, 2011                          217th AAS Meeting-Seattle,WA
                    L W A




January 13, 2011   217th AAS Meeting-Seattle,WA
 Beam pattern at 80MHz. Minimum spacing 2m. The optimization is done for
 sin(z)  1 The maximum side lobe is 0.6% within the optimizing region.
 The left beam pattern is phased towards the zenith. The right beam pattern is
 phased towards z 0  60 , az 0  45 . The low right part of the hemisphere is not
                                   

covered by the optimization area. The optimization for sin( z )  2 can help!


      January 13, 2011          217th AAS Meeting-Seattle,WA
Left:  The designed 110mx100m LWA station configuration.
       The minimum spacing is increased to 5m to compromise with the
       mutual coupling between the dipoles.
       The maximum side lobe is 1.6% within the optimizing region
Center: The two dimensional beam pattern.                         sin( z)  2
Right: The two orthogonal cross sections of the two dimensional beam
        pattern.




    January 13, 2011       217th AAS Meeting-Seattle,WA
                              Conclusions
1.       The side lobes of the designed LWA station configuration are never
         grater than 1.6% at any point in the sky regardless of phased
         direction or operating wavelength.
2.       The minimum spacing is as big as 5m to compromise with the mutual
         coupling between the dipoles.
3.       The side lobes can be achieved much lower, if the minimum spacing
         is chosen less.




     January 13, 2011        217th AAS Meeting-Seattle,WA
                                      Abstract

An optimization algorithm designed by Leonid Kogan (L. Kogan “Optimizing a Large
Array Configuration to Minimize Side Lobes”, IEEE Transactions on Antennas and
Propagation, vol 48, No 7, July 2000, p 1075) to minimize side lobes in the point
spread function has been applied in the design of two new radio-interferometric arrays:
(1) the most compact (E) configuration of EVLA and (2) the phased-array station for
the Long Wavelength Array (LWA). Scientific programs for the EVLA’s E-configuration
includes galactic and local HII, molecular gas, cosmic web, radio continuum, radio
lobes, SZ effect, cosmology and pulsar searches. The LWA will operate at frequencies
10-88 MHz and will study a wide range of scientific programs including clusters of
galaxies, high-redshift radio galaxies, pulsars, SNR’s, extra solar planets, solar and
ionospheric physics. Both arrays need to be compact and to have the smallest side
lobes possible. The EVLA’s E-configuration was designed minimizing the cost by
requiring only one new track and using the 11 existed antenna pads. The achieved
side lobe level for snapshot observation is ~12% within the antenna primary beam for
any operating VLA wavelength. For comparison the VLA-D configuration has
sidelobes ~60%. For the LWA station configuration the side lobes are never grater
than 1.6% at any point in the sky regardless of phased direction or operating
wavelength. Such a small side lobes for both arrays promise very high image fidelity
for maximum scientific results.




    January 13, 2011            217th AAS Meeting-Seattle,WA
Why do we need the most compact E-configuration?
    To get better Surface Brightness Sensitivity.
Tb  FILFACfilling factor
                        2
          D        
 FILFAC   eff
          D        
                    
           ant     
                 
w here Deff          is the diameter of the equivalent circular dish w hich has the given beam w idth
                 0.5
       at the level 0.5 :
      Dant  D N is the diameter of the equivalent circular dish w hich has the geometric area equaled
      to the effectivearea of all array antennas.
      D is diameter of the antenna
       is the antenna efficiency
      N is number of antennas in the array.




January 13, 2011                        217th AAS Meeting-Seattle,WA
January 13, 2011   217th AAS Meeting-Seattle,WA
                 Configuration figures of merit.
•   Minimum side lobes
•   Gaussian shape of the main beam
•   Minimum gaps in the UV coverage
•   Others

Optimizing figures of merit other than “Minimum side lobes” improves the
side lobes implicitly, but the direct minimum side lobe optimization may
produce better result.

We optimize the array configuration minimizing the maximum positive
side lobe inside of the primary beam!

Kogan 2000, IEEE Transaction on Antennas and Propagation, 48, 7, 1075




      January 13, 2011     217th AAS Meeting-Seattle,WA
The final configuration. The minimum spacing is increased to 5m to
compromise with the mutual coupling between the dipoles. The
optimization is done for sin( z)  2



January 13, 2011        217th AAS Meeting-Seattle,WA
Mathematically created mask to prevent appearance of antennas
(during optimization) on the prohibited places: proximity to tracks,
proximity to fixed antennas……


January 13, 2011         217th AAS Meeting-Seattle,WA
One dimensional beam (along RA) of the optimized E-configuration



January 13, 2011       217th AAS Meeting-Seattle,WA
One dimensional beam (along DEC) of the optimized E-configuration



January 13, 2011      217th AAS Meeting-Seattle,WA
January 13, 2011   217th AAS Meeting-Seattle,WA
January 13, 2011   217th AAS Meeting-Seattle,WA
         Beam pattern at 80MHz phased towards z 0  60  , az 0  45 
         Minimum spacing 2m.



January 13, 2011          217th AAS Meeting-Seattle,WA
January 13, 2011   217th AAS Meeting-Seattle,WA

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:0
posted:3/27/2013
language:Unknown
pages:32