[evlatests] Early Ka-band holography (sort of)

[Rick Perley]

    Ken and I attempted some holography with antenna 4, using its 
Ka-band receiver.  To do this, Ken tricked the system into thinking the 
signal from the Ka band receiver was actually from K-band.  We ran this 
at 26.025 GHz. 
    The motivation for this was a wild idea of mine, that the apparently 
10K excess spillover might be due to a significant alignment error of 
the feed, such that the centroid of the illumination is in fact located 
partway to the edge, thus increasing the edge illumination sufficiently 
to receive a significant extra contribution from the ground.  The error 
would have to be in elevation, such that the effective centroid of 
illumination is on the down side of the antenna, so that when we tip 
downwards, the spillover stays on the ground.  If the offset were to the 
'up' side, the excess ground noise would quickly disappear as we tip 
downwards -- this is not seen.  If the offset were to the side, 
something in between would result. 

    The expected consequences of this offset would be higher sidelobes, 
and a phase gradient across the primary beam. 

    The holography raster was done with azimuth changing fastest, with 
phase calibration every two scans -- once every 6 minutes.

    The holography worked well -- in amplitude.  Antenna 4 fringed 
nicely at 26 GHz, with equal correlation coefficient to that obtained 
from the other antennas.  The horizontal beam cuts show nice symmetry, 
with the expected sidelobe size.  The phase gradients in each horizontal 
cut are flat.  All is well.

    The anticipated action is in the vertical direction -- and here we 
have two problems:

    1) It is abundantly clear that the subreflector rotation algorithm, 
used to offset the 2nd order focus shift, is not turned on.  There is a 
strong coma lobe from the focus drift, which is dominating any other 
effect giving rise to higher sidelobes in the vertical direction.  This 
problem can only be repaired by proper subreflector rotation. 
    2) To reconstruct the phase from the holography data in the 
vertical, we have to select the appropriate points in the sequential 
horizontal rasters.  The phase stability was poor (afternoon, big 
clouds, etc.), and there is some evidence that antenna 4's phase is 
changing from scan to scan, more than seems reasonable from atmospheric 
instability.  (However, 4 is at the end of the arm, so perhaps this is 
not surprising).  This problem can be avoided by performing vertical 
cuts, which we shall try tomorrow. 

    Conclusion:  All looks well in the horizontal cuts, but no judgement 
of illumination, or beam symmetry can yet be made in the vertical.