[evlatests] Polarization Changes

[Rob Long]

Hi Rick,

My first thought about L-band is changes made with the solar upgrades. 
I'll have to do some digging, but I'll let you know what I find.

I'm sure that we have had numerous receiver swaps over the year, but 
again I'll have to do some digging and let you know.

Rob

On 5/11/2020 10:28 AM, rperley via evlatests wrote:
> 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.
>
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/evlatests