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This all makes sense. The radome on its own wouldn't produce much of
a mismatch/reflection, but with dew it would be much greater. The RF
mismatch at the vacuum window is higher because of the thickness and
density of the foam window, so a standing wave could be set up
between the two interfaces.<br>
<br>
Since 2013, we've had to replace the original foam windows in the S-
and C-band receivers with denser ones because their marginal
strength caused them to deform over time. The denser foam had a
slightly higher dielectric constant ,which led to a small increase
in mismatch. <br>
<br>
-Wes<br>
<br>
<div class="moz-cite-prefix">On 8/27/2021 10:08 AM, Rick Perley via
evlatests wrote:<br>
</div>
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cite="mid:be9b27f2-ca96-7fe0-d273-fc4c960c2d82@nrao.edu">
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<p>So, on a probably drier day, the effect is nearly invisible.
Support for the idea that water/dew on the window is a
contributing factor ...</p>
<p>Rick<br>
</p>
<div class="moz-cite-prefix">On 8/27/21 9:42 AM, Drew Medlin
wrote:<br>
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Here's an observation from a drier time of year. 2021-03-27,
afternoon observation, operator recorded Sky cover 40%.
Cumuliform clouds.
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<div class="">20A-092.sb39518402.eb39548999.59300.832225300925.ms-J0303+4716-bb12-PHASE-amp_vs_freq-LL_RR.png</div>
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<div class=""><img apple-inline="yes"
id="558F8D95-5CB0-464C-A12A-876BD21ABEF4"
src="cid:part1.0D43032E.5FFA1C71@nrao.edu" class=""></div>
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Scientific User Support Group</div>
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<blockquote type="cite" class="">
<div class="">On Aug 27, 2021, at 9:18 AM, rperley <<a
href="mailto:rperley@nrao.edu" class=""
moz-do-not-send="true">rperley@nrao.edu</a>> wrote:</div>
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<div class="">
<div class="">Certainly is reasonable. In the case I
analyzed, the start of the run was around sunrise, the
end around noon. The period of the oscillations (and
the phase) did not appear to change, however. The
change in amplitude was quite obvious.<br class="">
<br class="">
I don't know if there was rain on the day I looked at
-- but perhaps dew on the window would do the trick
...<br class="">
<br class="">
Rick<br class="">
<br class="">
On 2021-08-27 11:11, Ken Sowinski wrote:<br class="">
<blockquote type="cite" class="">On Fri, 27 Aug 2021,
Rick Perley via evlatests wrote:<br class="">
<blockquote type="cite" class="">Drew, et al.:<br
class="">
Perfect! Period shown in that C-band plot is
about 100 MHz, which corresponds pretty closely to
the expected value corresponding to the length of
the horn. Good evidence for a mismatch between
the window and the OMT….<br class="">
</blockquote>
The change in the sinusoiid should correlate with
temperature.<br class="">
Might the reflections have been exacerbated by
moisture from<br class="">
all the recent rain?<br class="">
</blockquote>
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