[evlatests] Strange R-L phase symmetries

[Barry Clark]

In previous looks at this phenomenon we have been looking for clever and 
sophisticated problems.  These data look more like stupid problems.  The 
fact that all bands look the same suggests that the problem does not 
arise in the optics or front ends.  The main stupid thing in the rest of 
the system is the software.  I am inclined to discount it on the grounds 
that in the relevant phase calculations the software does not know what 
polarization it is dealing with.  But of course one never knows about 
software.  The other remaining stupid items are pieces of wire.  In 
particular, it might be the pieces of wire connecting the L101 or L102 
synthesizers to their respective mixers.  These ought to be well behaved 
at this level (couple of ps), but pieces of wire can be almost as 
treacherous as software.  Something to look at is whether IFs BD behave 
the same as AC - different L102s.

On 3/25/22 11:53, Rick Perley via evlatests wrote:
>      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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