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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.....<br>
<br>
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.<br>
<br>
Rob<br>
<br>
<div class="moz-cite-prefix">On 3/25/2022 2:57 PM, Frank Schinzel
via evlatests wrote:<br>
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<div>Just to add to Paul's second point, a similar behavior is
also seen at X-band, as noted in EVLA memo #205.<br>
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<div class="elided-text">On Mar 25, 2022 14:14, Paul
Demorest via evlatests <a class="moz-txt-link-rfc2396E" href="mailto:evlatests@listmgr.nrao.edu"><evlatests@listmgr.nrao.edu></a>
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<p dir="ltr">Yes, I was also going to suggest maybe it's
a cable being <br>
stretched/flexed, for reasons similar to what Barry
mentioned. In <br>
addition to the AC/BD comparison, a few more ideas:
<br>
<br>
1. Look at the noise diode R-L phase vs elevation to
see if the effect <br>
can be seen there. If so I think that pretty
definitively rules out <br>
optics-based causes. Also, this is a single-antenna
measurement so <br>
removes reference antenna confusion. And it does not
require <br>
astronomical sources so can be done any time.
<br>
<br>
2. Repeat the test at X and Ku bands. L/S/C all share
a very similar <br>
signal path so the results at some higher bands might
be a informative <br>
comparison.
<br>
<br>
-Paul
<br>
<br>
On 2022-03-25 13:43, Barry Clark via evlatests wrote:
<br>
> In previous looks at this phenomenon we have been
looking for clever
<br>
> and sophisticated problems. These data look more
like stupid
<br>
> problems. The fact that all bands look the same
suggests that the
<br>
> problem does not arise in the optics or front
ends. The main stupid
<br>
> thing in the rest of the system is the software.
I am inclined to
<br>
> discount it on the grounds that in the relevant
phase calculations the
<br>
> software does not know what polarization it is
dealing with. But of
<br>
> course one never knows about software. The other
remaining stupid
<br>
> items are pieces of wire. In particular, it
might be the pieces of
<br>
> wire connecting the L101 or L102 synthesizers to
their respective
<br>
> mixers. These ought to be well behaved at this
level (couple of ps),
<br>
> but pieces of wire can be almost as treacherous
as software.
<br>
> Something to look at is whether IFs BD behave the
same as AC -
<br>
> different L102s.
<br>
> <br>
> On 3/25/22 11:53, Rick Perley via evlatests
wrote:
<br>
>> This is a long circular -- apologies to
all, but the subject is a <br>
>> bit complex ...
<br>
>> <br>
>> Many will remember a meeting called by
Frank a few years ago <br>
>> where the subject was the very peculiar phase
differences seen between <br>
>> the RCP and LCP phases when observing a
source passing by the zenith. <br>
>> The general conclusion was that 'we have no
idea of what is going on'.
<br>
>> <br>
>> In preparation for an upcoming trip, I
am reviewing my extensive <br>
>> observations, taken over the past decade or
more, from projects with <br>
>> the goal of measuring, and implementing the
'absolute' D-terms. (In <br>
>> other words, dispensing with the usual method
of measuring the antenna <br>
>> polarizations with respect to an assumed
standard (usually zero)).
<br>
>> <br>
>> One observation, taken in January 2019,
is especially well suited <br>
>> to this task. I observed four sources,
through transit, for five <br>
>> hours, at three bands -- L, S, and C.
<br>
>> <br>
>> The four sources were:
<br>
>> <br>
>> 3C286 dec = 30.5
<br>
>> <br>
>> OQ208 dec = 28.5
<br>
>> <br>
>> 3C287 dec = 25.2
<br>
>> <br>
>> 3C273 dec = 2.0
<br>
>> <br>
>> Note that OQ208 is completely
unpolarized, while the others have <br>
>> varying degrees of polarization. All sources
transit south of the <br>
>> zenith.
<br>
>> <br>
>> The data are of exceptionally good
quality. The array was in the <br>
>> C configuration.
<br>
>> <br>
>> The attached plots show the R-L phases,
using ea10 as the <br>
>> reference antenna. Note that these are *not*
the RL or LR correlation <br>
>> phases -- they are the differences between
the antenna phase solutions <br>
>> using the RR and LL data, using ea10 as the
reference. This means the <br>
>> R-L dependence of ea10 is impressed on all
the other antennas. We are <br>
>> looking at differentials.
<br>
>> <br>
>> The plots show two antennas -- ea01 and
ea12, which represent the <br>
>> two different symmetries seen in the data.
The x-axis is HA -- plots <br>
>> against time and parallactic angle jumble the
results -- the <br>
>> dependencies seen are purely a function of
HA.
<br>
>> <br>
>> Colors: 3C286 is red, Light green is
OQ208, blue is 3C287, dark <br>
>> green is 3C273.
<br>
>> <br>
>> ea01 is of the even symmetry type.
Antennas 1 3 5 6 8 15 and 22 <br>
>> have this symmetry.
<br>
>> <br>
>> ea12 is of the odd symmetry type. All
other antennas show this, <br>
>> with the same sign -- positive difference
before transit, negative <br>
>> difference after, with the possible exception
of ea18. (For this <br>
>> antenna, the amplitude of the effect is very
small, so the signature <br>
>> is hard to discern). Three antennas were out
of the array at the <br>
>> time: 7, 24 and 28.
<br>
>> <br>
>> Key points:
<br>
>> <br>
>> 1) The phase signatures are *identical*
for each band. Same <br>
>> width, same height, same values, same
symmetry.
<br>
>> <br>
>> 2) The magnitude of the effect is
sharply dependent on how close <br>
>> the zenith the source transits. For 3C273,
the effect is almost <br>
>> completely absent.
<br>
>> <br>
>> 3) The effect is independent of source
polarization. OQ 208 has <br>
>> less than 0.1% polarization, and shows the
same symmetry signature as <br>
>> the strongly polarized sources 3C286 and
3C287.
<br>
>> <br>
>> 4) The location of the antennas is not
related to the signature <br>
>> -- the 'even' antennas were located all over
the array: W6, W18, E14, <br>
>> N6, N1, E12, and W12.
<br>
>> <br>
>> One conclusion is clear: The effect has
nothing to do with the <br>
>> beam squint. And it is very hard to see how
differences in the <br>
>> antenna pole direction can do this -- the
required tilt magnitudes are <br>
>> just unreasonable. And in any event, the
parallactic angle is not a <br>
>> function of polarization -- it's an antenna
quantity.
<br>
>> <br>
>> I have shown these data to two of our
serious pundits (Barry and <br>
>> Steve), hoping for some insight. None was
forthcoming. We are <br>
>> completely stumped. It seems clear that the
signatures are geometric <br>
>> in origin -- but how does this translate into
such a clear signature <br>
>> in the phase *difference* between
polarizations?
<br>
>> <br>
>> Any and all suggestions will be taken
seriously!
<br>
>> <br>
>> Rick
<br>
>> <br>
>>
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