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The receivers installed in ea03 and ea13 are U-013 and U-021,
respectively. The last noise temperature measurements on these
receivers in the lab (in 2018 and 2014) didn't show anything
abnormal at the frequencies of these resonances. But that doesn't
rule out a change in the interim.<br>
<br>
I suspect a strong cross-pol peak like this could be caused by
undesired mode conversion in the waveguide components upstream of
the LNAs. The reason is likely mechanical: something shifted,
loosened, or broke with temperature cycling in the interval since
they were installed. Possible culprits are the OMT (a gap in the
mated halves, loose sidearm pins), or cracked or misaligned mating
flanges, particularly at the thermal gap. We've seen this kind of
thing happen on EVLA receivers before.<br>
<br>
The initial measurement of axial ratio for both these receivers
looked good as well: it was the usual W-shaped curve, with a max of
1 dB at 12 GHz and below 0.5 dB almost everywhere else. <br>
<br>
I would recommend Rob Long flag these for a check the next
opportunity we have to swap them out. The problem should be evident
in an AR test even at ambient temperature, if it's due to loose or
misaligned waveguide flanges.<br>
<br>
Thanks for sharing the data, Rick!<br>
<br>
-Wes<br>
<br>
<div class="moz-cite-prefix">On 5/30/2020 4:24 PM, rperley--- via
evlatests wrote:<br>
</div>
<blockquote type="cite"
cite="mid:34c3f63cc281be5bfb7b03ab7ea10b05.squirrel@webmail.aoc.nrao.edu">
<pre class="moz-quote-pre" wrap="">The Ku-band polarizers on the JVLA are in general really outstanding. For
almost all antennas, the cross-polarization is below 4% over almost the
entire 12 -- 18 GHz band.
But two antennas, ea03 and ea13, clearly have something wrong with their
polarizers. Both show cross-polarization of over 50% (!!!) at the high
frequency end. Both are fine below 16 GHz.
I attach a plot to show the effect. Yellow is ea03, blue is ea13. The
plot spans 16 -- 18 GHz. At 16 GHz, the polarization is normal -- about 2
-- 3%. But with increasing frequency, both show resonance-like structure
in the cross-polarization.
The top panel shows the leakage from LCP into RCP, the bottom panel the
leakage from RCP to LCP. Each panel shows the phase (narrow plot on top)
and, below this, the voltage amplitude (in tens of percent, so 100 = 10%.
Rick</pre>
<br>
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</pre>
</blockquote>
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