[evlatests] X-Band on Ant 18

[Robert Hayward]

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.