[evlatests] X-Band on Ant 18

[Michael Rupen]

Hi Bob,

   the polarization purity was indeed horrible, at least on Aug. 21.  The
usual pol'n leakage is a few per cent; on that date ant. 18's X-band leakage
was between 44 and 67%.  For contrast, on Aug. 5 ant. 18's pol'n leakage
at X band was 4.5-5.3% .  Still the worst in the array, but not so 
stunning.  Good call...

                 Michael

On Wed, 30 Aug 2006, Robert Hayward wrote:

> There have been numerous reports of poor sensitivity on the X-Band
> receiver on Antenna 18. To that end, the receiver was dropped from the
> feed this morning and the following was found:
>
> 1) A curly shard of metal about a 1/4" in diameter was sitting smack dab
> in the center of the vacuum window. This was undoubtedly from one of the
> machined corrugations in the feed which took 20 years to work itself loose.
>
> 2) The shard was sitting in a bead of water, which took up about about
> 1/2 the diameter of the circular waveguide aperture and was about 1/16"
> or more deep at its center.
>
> So we had a double whammy this time. In the past, when we found a metal
> shard on an X-Band window, the polarization purity had gone to pot. I
> don't suppose anybody noticed anything peculiar this time, did they?
>
> The Antenna Mechanics have been made aware of the moisture problem. At
> this point we don't know whether it is due to a leak in the feed/radome
> or whether it is from condensation inside the feed.
>
>
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