[evlatests] P-Band Beam 'Pedestal'

[Rick Perley]

     I've previous circulated reports on the elongated 'pedestal' which 
underlies the central beam at P-band.
     In short, the central (~ 2 degree) primary beam sits atop an 
elliptical pedestal, whose major axis is oriented orthogonally to the 
(linear) polarization.  Hence, the vertically polarized beam has a  
horizontal pedestal, the horizontally polarized beam a vertical 
pedestal.  The extent of the pedestal is approximately 6 degrees from 
the beam center.

     Two hypotheses for the origin of the pedestal have been put 
forwards -- (1) the quadrupod support legs, and (2) The asymmetric 
subreflector -- which is normally oriented such that its asymmetic axis 
is aligned with the vertical legs.

     To discriminate between these, we (Michiel and I) generated a 
script which does four beam cuts:  horizontal, vertical, +45, and -45 
degrees.  Following these, the same four cuts, but with the subreflector 
rotated by 45 degrees.   The pedestal would be expected to rotate with 
the subreflector, if the latter is responsible for the former.

     Result:

     The position of the subreflector made no difference to the beam 
profile in any of the four cuts.    This supports the hypothesis that 
the quadrupod legs are the origin of the asymmetric profile.

     We would next like to do a full holography in order to generate the 
electric field illumination on the antenna surface.