[evlatests] Strange R-L phase symmetries
Rob Long
rlong at nrao.edu
Fri Mar 25 17:21:55 EDT 2022
Looking at this thread, it seems the cabling we would be considering
would have to be inside the LO and/or utility racks. These cables are
fairly well secured and shouldn't be moving around. If it were one or
two I might suspect loose connections, but.....
Another thought might be temperature in the racks, especially depending
on the angle. Some of the patterns almost look like jumps. Possibly the
Teflon knee or heat slosh in the modules? AC/BD comparison would be
interesting.
Rob
On 3/25/2022 2:57 PM, Frank Schinzel via evlatests wrote:
> Just to add to Paul's second point, a similar behavior is also seen at
> X-band, as noted in EVLA memo #205.
>
> On Mar 25, 2022 14:14, Paul Demorest via evlatests
> <evlatests at listmgr.nrao.edu> wrote:
>
> Yes, I was also going to suggest maybe it's a cable being
> stretched/flexed, for reasons similar to what Barry mentioned. In
> addition to the AC/BD comparison, a few more ideas:
>
> 1. Look at the noise diode R-L phase vs elevation to see if the
> effect
> can be seen there. If so I think that pretty definitively rules out
> optics-based causes. Also, this is a single-antenna measurement so
> removes reference antenna confusion. And it does not require
> astronomical sources so can be done any time.
>
> 2. Repeat the test at X and Ku bands. L/S/C all share a very similar
> signal path so the results at some higher bands might be a
> informative
> comparison.
>
> -Paul
>
> On 2022-03-25 13:43, Barry Clark via evlatests wrote:
> > 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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> >> evlatests at listmgr.nrao.edu
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> >
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