[evlatests] Strange R-L phase symmetries

[Rick Perley]

     This is a long circular -- apologies to all, but the subject is a 
bit complex ...

     Many will remember a meeting called by Frank a few years ago where 
the subject was the very peculiar phase differences seen between the RCP 
and LCP phases when observing a source passing by the zenith.  The 
general conclusion was that 'we have no idea of what is going on'.

     In preparation for an upcoming trip, I am reviewing my extensive 
observations, taken over the past decade or more, from projects with the 
goal of measuring, and implementing the 'absolute' D-terms.  (In other 
words, dispensing with the usual method of measuring the antenna 
polarizations with respect to an assumed standard (usually zero)).

     One observation, taken in January 2019, is especially well suited 
to this task.  I observed four sources, through transit, for five hours, 
at three bands -- L, S, and C.

     The four sources were:

     3C286   dec = 30.5

     OQ208  dec = 28.5

     3C287    dec = 25.2

     3C273    dec = 2.0

     Note that OQ208 is completely unpolarized, while the others have 
varying degrees of polarization.  All sources transit south of the zenith.

     The data are of exceptionally good quality.  The array was in the C 
configuration.

     The attached plots show the R-L phases, using ea10 as the reference 
antenna.  Note that these are *not* the RL or LR correlation phases -- 
they are the differences between the antenna phase solutions using the 
RR and LL data, using ea10 as the reference.  This means the R-L 
dependence of ea10 is impressed on all the other antennas.  We are 
looking at differentials.

     The plots show two antennas -- ea01 and ea12, which represent the 
two different symmetries seen in the data.  The x-axis is HA -- plots 
against time and parallactic angle jumble the results -- the 
dependencies seen are purely a function of HA.

     Colors:  3C286 is red, Light green is OQ208, blue is 3C287, dark 
green is 3C273.

     ea01 is of the even symmetry type.  Antennas 1 3 5 6 8 15 and 22 
have this symmetry.

     ea12 is of the odd symmetry type.  All other antennas show this, 
with the same sign -- positive difference before transit, negative 
difference after, with the possible exception of ea18. (For this 
antenna, the amplitude of the effect is very small, so the signature is 
hard to discern).  Three antennas were out of the array at the time:  7, 
24 and 28.

     Key points:

     1) The phase signatures are *identical* for each band.  Same width, 
same height, same values, same symmetry.

     2) The magnitude of the effect is sharply dependent on how close 
the zenith the source transits.  For 3C273, the effect is almost 
completely absent.

     3) The effect is independent of source polarization.  OQ 208 has 
less than 0.1% polarization, and shows the same symmetry signature as 
the strongly polarized sources 3C286 and 3C287.

     4) The location of the antennas is not related to the signature -- 
the 'even' antennas were located all over the array: W6, W18, E14, N6, 
N1, E12, and W12.

     One conclusion is clear:  The effect has nothing to do with the 
beam squint.  And it is very hard to see how differences in the antenna 
pole direction can do this -- the required tilt magnitudes are just 
unreasonable.  And in any event, the parallactic angle is not a function 
of polarization -- it's an antenna quantity.

     I have shown these data to two of our serious pundits (Barry and 
Steve), hoping for some insight.  None was forthcoming.  We are 
completely stumped.  It seems clear that the signatures are geometric in 
origin -- but how does this translate into such a clear signature in the 
phase *difference* between polarizations?

     Any and all suggestions will be taken seriously!

     Rick
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