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For some time, the new VLA 22 GHz (K-band) receivers have been
contaminated by water during wet weather, with resulting system
temperatures 50-200 K greater than normal. This can affect data for
periods from several days to several weeks depending on the length
of time it takes for the receiver feed and window to completely
dry.
Several attempts were made to seal the feed horns from moisture
contamination without removing them from the antennas. This
approach didn't yield acceptable results and it was decided that
each new K-band receiver had to be removed, retrofitted, retested
and then reinstalled. The feed upgrade included sealing of the feed
horns with O-rings and RTV sealant, and adding insulating foam
below the dewar window to prevent condensation. A passive desiccant
system, which had been installed previously, along with a dry
nitrogen purge of the horn will help to prevent new moisture
problems in the feed horns. More stringent use of the feed heaters
has also been implemented to minimize the effect of condensation on
the outer goretex cover.
The retrofits are now completed and appear to have been
successful as demonstrated in the figure which shows a comparison
of the system temperature T0 due to all sources except
the atmosphere, for antenna 19 during year 2001 and 2002.
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National Radio
