[evlatests] Antenna 8 cross-polarization at 6cm

[Robert Hayward]

Before the Front-End Group gets too exited about whether this receiver's
axial ratio has changed (supposedly it looked good before it left the lab)
someone should check to see if anything has fallen on to the vacuum
window. In the past, when metal shards from the machining of the feed
corrugations have fallen on the window, the D-terms have gotten ugly.
Admittedly, if this was the case, one would have thought the stability
would be lousy too. Simply dropping the receiver to inspect the window
would confirm this theory.

If it's not debris on the window, and the axial ratio hasn't changed since
it was last in the lab, then we might have to start worring about whether
this receiver has polarization ellipses with position angles that look
significantly different than the other C-Band receivers. The lab
measurements, assuming they were done consistently, should help us
quantify this effect as well.

-Bob


>     C-band polarization observations made over the weekend using antenna
> 8 (outfitted with the wideband OMT) show that this antenna has
> anomalously high polarization -- typically 10%, reaching 20% at the band
> edges.  This is much larger than any other wideband C-band antenna.
> Something is amiss.
>     However -- the polarization is very stable, as it is with all other
> antennas.
>
>
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