<!DOCTYPE html><html><head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
Hi Rick,<br>
<br>
Have the cross-pol issues noted below been seen in the past (and not
necessarily on the same antennas)? It would be helpful to know
whether the magnitude of cross-pol has changed over time, as well as
the number of problematic receivers.<br>
<br>
In the EVLA Ku-band receiver chain, the most likely culprits are the
waveguide phase shifter and OMT. As these are purely passive
components, mechanical failures or misalignment are likely behind
these issues. Some comments on this, below:<br>
<ul>
<li>Manufacturing yield on the phase shifters wasn't that good,
from what I gather. However, these were all individually
characterized on the bench before installation into a receiver,
weeding out any bad units. In addition, the axial ratio of both
polarizations was also measured on the fully assembled receiver,
though I think it may have been done on a limited number of
frequencies across the band. Nevertheless, I think it's pretty
unlikely these receivers had these issues from the beginning. <br>
<br>
</li>
<li>A weak spot in the phase shifter are the waveguide flanges,
which are separately-machined parts brazed onto the electroform.
We've seen some of these develop cracks over a long period after
a lot of temperature cycling; these could cause reflections and
possibly resonances. Fortunately these are fairly evident on
inspection, and can be repaired.<br>
<br>
</li>
<li>The waveguide OMT's Achilles Heel is the septum, a thin piece
of metal clamped in between the two halves of the OMT housing.
Provided there are no gaps in the split waveguide channels, and
the septum alignment is maintained, everything works well.
However if the screws in the housing loosen over time and
temperature cycling, a gap can then develop. This could create
multiple problems: resonant spikes in the passband, added loss,
and degraded cross-pol. <br>
<br>
</li>
<li>Another potential issue is the alignment of the waveguide
flanges between the polarizer components. The most troublesome
ones are the square waveguide interfaces, which are at three
separate locations (at each end of the phase shifter, and at the
OMT input). These are screwed together, but we've had past
instances where they've loosened enough over time to shift
slightly, creating resonant spikes and possibly degraded
cross-pol as well.</li>
</ul>
<br>
-Wes<br>
<br>
<br>
<div class="moz-cite-prefix">On 7/8/2026 4:44 PM, Rick Perley via
evlatests wrote:<br>
</div>
<blockquote type="cite" cite="mid:MN6PR14MB71474D45180E6F3C0CF2E22CD7FF2@MN6PR14MB7147.namprd14.prod.outlook.com">
<style type="text/css" style="display:none;">P {margin-top:0;margin-bottom:0;}</style>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Observations made over the past two months show some antennas
have problems with the Ku-band polarizers.</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Most of these are at the high frequency end — between 16 and 18
GHz. </div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Bad antennas of note:</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
ea03: cross-polarization of 10 to 20% between 16.7 and 17.6
GHz.</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
ea07: 10 to 40% between 16.2 and 18 GHz. Also shows a
resonance-like bump to 40% at 14.4 GHz. </div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
ea14; 10 to 50% above 16.6 GHz. </div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
ea15; 10 to 25% above 17.2 GHz.</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
ea02 and ea05 show a single high spike in cross-pol (to 40%)
around 17.8 GHz. </div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
All others are generally less than 5% over the entire band. </div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, "Aptos_EmbeddedFont", "Aptos_MSFontService", Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
rick</div>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
<pre wrap="" class="moz-quote-pre">_______________________________________________
evlatests mailing list
<a class="moz-txt-link-abbreviated" href="mailto:evlatests@listmgr.nrao.edu">evlatests@listmgr.nrao.edu</a>
<a class="moz-txt-link-freetext" href="https://listmgr.nrao.edu/mailman/listinfo/evlatests">https://listmgr.nrao.edu/mailman/listinfo/evlatests</a>
</pre>
</blockquote>
<br>
</body>
</html>