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<font face="monospace">Hello all,<br>
<br>
Tis the season of morning dew on the feed horn windows, and we
have been seeing the wave across calibrated amp vs freq again
commonly over the last two months. I am wondering if any further
effort has been made since this conversation died down last year?
I asked Rick and he said to check in here or talk to Catarina. I
showed Catarina the issue, she looked for a work order and didn't
find any relating to this.<br>
<br>
- Edward<br>
</font><br>
<div class="moz-cite-prefix">On 8/27/21 11:53 AM, Ken Sowinski via
evlatests wrote:<br>
</div>
<blockquote type="cite"
cite="mid:69248b90-5c75-2d1-ecd-5298c3580d9@aoc.nrao.edu">On Fri,
27 Aug 2021, rperley via evlatests wrote:
<br>
<br>
<blockquote type="cite">OK, now that a probable cause is located,
we have to think about how to manage this.
<br>
Other than installing 'window wipers' on the windows (ha! --
think about that for L and S bands ...),
<br>
the only good mitigation is more frequent bandpass calibration.
Probably every couple of hours, if the weather is likely to lead
to dew condensing on the windows. (All feeds from C upwards
have heat lamps, so I would hope the problem is much rarer for
this bands).
<br>
</blockquote>
<br>
I had forgeotten that L and S do not have feed heaters. Is that
<br>
because they are too large to be heated practically? Gimbaled
umbrellas?
<br>
<br>
Ken
<br>
<br>
<br>
<blockquote type="cite">For *most* continuum observations, the
effect of this changing reflection is pretty minor -- (so long
as the imaging is done over spectral widths much larger than the
observed period in frequency space). For continuum observations
requiring channelwidths less than this, self-calibration (if
viable) will fix things up. All other cases will need better
bandpass calibration, if the amplitude of the effect (a few
percent) is important.
<br>
<br>
Rick
<br>
<br>
<br>
On 2021-08-27 12:49, Wes Grammer via evlatests wrote:
<br>
<blockquote type="cite"> This all makes sense. The radome on its
own wouldn't produce much of
<br>
a mismatch/reflection, but with dew it would be much greater.
The RF
<br>
mismatch at the vacuum window is higher because of the
thickness and
<br>
density of the foam window, so a standing wave could be set
up between
<br>
the two interfaces.
<br>
<br>
Since 2013, we've had to replace the original foam windows in
the S-
<br>
and C-band receivers with denser ones because their marginal
strength
<br>
caused them to deform over time. The denser foam had a
slightly higher
<br>
dielectric constant ,which led to a small increase in
mismatch.
<br>
<br>
-Wes
<br>
<br>
On 8/27/2021 10:08 AM, Rick Perley via evlatests wrote:
<br>
<br>
<blockquote type="cite"> So, on a probably drier day, the
effect is nearly invisible.
<br>
Support for the idea that water/dew on the window is a
contributing
<br>
factor ...
<br>
<br>
Rick
<br>
On 8/27/21 9:42 AM, Drew Medlin wrote:
<br>
Here's an observation from a drier time of year.
2021-03-27,
<br>
afternoon observation, operator recorded Sky cover 40%.
Cumuliform
<br>
clouds.
<br>
<br>
<br>
</blockquote>
20A-092.sb39518402.eb39548999.59300.832225300925.ms-J0303+4716-bb12-PHASE-amp_vs_freq-LL_RR.png
<br>
<blockquote type="cite">
<br>
<br>
Drew Medlin
<br>
<a class="moz-txt-link-abbreviated" href="mailto:dmedlin@aoc.nrao.edu">dmedlin@aoc.nrao.edu</a>
<br>
<br>
Scientific Data Analyst - VLA Scientific User Support Group
<br>
National Radio Astronomy Observatory
<br>
<br>
On Aug 27, 2021, at 9:18 AM, rperley
<a class="moz-txt-link-rfc2396E" href="mailto:rperley@nrao.edu"><rperley@nrao.edu></a> wrote:
<br>
<br>
Certainly is reasonable. In the case I analyzed, the start
of the
<br>
run was around sunrise, the end around noon. The period of
the
<br>
oscillations (and the phase) did not appear to change,
however. The
<br>
change in amplitude was quite obvious.
<br>
<br>
I don't know if there was rain on the day I looked at --
but perhaps
<br>
dew on the window would do the trick ...
<br>
<br>
Rick
<br>
<br>
On 2021-08-27 11:11, Ken Sowinski wrote:
<br>
On Fri, 27 Aug 2021, Rick Perley via evlatests wrote:
<br>
Drew, et al.:
<br>
Perfect! Period shown in that C-band plot is about 100
MHz, which
<br>
corresponds pretty closely to the expected value
corresponding to
<br>
the length of the horn. Good evidence for a mismatch
between the
<br>
window and the OMT….
<br>
The change in the sinusoiid should correlate with
temperature.
<br>
Might the reflections have been exacerbated by moisture
from
<br>
all the recent rain?
<br>
</blockquote>
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