[evlatests] Polarization Changes

[rperley]

Rotating the receivers by 90 degrees is (for me) the easiest way to 
directly measure the 'absolute' polarizations of the antennas. The idea 
is to rotate one antenna's receivers by 90 degrees.  Subsequent 
observation of a polarized source, combined with an observation of the 
source in the unrotated state, permits easy determination of the 
polarization.  This is different than the standard polarization 
measurement, which provides relative polarizations (to the antenna 
chosen as reference).

This exercise has been done three times -- in March 2011 (with the 
results written up in EVLA Memos 131 and 151), Feb 2019, and January 
2020.  For the 2011 observations, the rotation was done at L, S, C and X 
bands, although most of the antennas were not yet converted to EVLA 
standards.  For 2019, the rotation was done at L, S and C bands 
(unfortunately, the X band receiver could not be rotated in ea10).  In 
2020, the L, S, C, X and Ku receivers were rotated in ea09, although the 
rotation at X-band was only by 60 degrees, due to physical limitations 
in that antenna).

I have now reduced all these data, with one goal being to compare the 
solutions over the 9-year time span since the first observation.  (The 
recent X-band observation reduction is not yet complete -- some code 
changes are needed in the 'TRUEP' program to account for the 60 degree 
rotation).

Here I note some interesting changes in the polarization.

1) 2011 to 2020:  There is virtually no similarity in the polarizations 
of the EVLA antennas at L, S and C bands between these times.  However, 
many changes have taken place since 2011, notably all the C-band 
polarizers have been replaced.  (Also, only 5 L-band antennas had been 
converted to wide-band, and 9 S-band antennas had been updated to the 
new designs by March 2011).  Likely, numerous other changes have also 
occurred over that 9-year span, so any hope for close similarly over 
this span is likely wishful thinking ...

2) 2019 to 2020:  I have comparison data for 24 antennas -- ea07, 08, 24 
and 25 have observations  on only one date, so no comparison can be 
made.

At S and C bands, the polarization are virtually identical -- typically 
changing by small fractions of 1% over this time.  This is good news, 
for if we ever get to the point of implementing automatic polarization 
calibration, we should be able to employ tabled values of the antenna 
polarizations.  Two C-band antennas did not replicate well:  ea11 and 
ea16.  For the former the changes were not large (~ 1%), but for the 
latter, the polarization was completely different.  Was that receiver 
replaced, or some other major change to it made between Feb 2019 and Jan 
2020?

At L-band, the situation is notably different:  For about half of the 
antennas, the polarizations are very similar (similar to the S and C 
band situation).  But for some others, dramatic changes have taken 
place.  Those with the most dramatic changes are:  ea02, 03, 06, 09, 11, 
19, and 28.  What has happened to these antennas' receivers between Feb 
2019 and Jan 2020?

A memo is being prepared with all the results from these observations.