[daip] [Fwd: Cross-hand amplitude problems for BR and FD at 15 GHz]

[Craig Walker]

Matt,

I see my email did not include the forwarded message - not sure why. 
But this is about the high Tsys and high cross polarized amplitude that 
you see in bl178bi at BR and FD.  We see the issue, at least for Tsys, 
in a variety of other projects now that you have called attention to it. 
  But at BR, it seems to have resolved itself for recent projects 
without known intervention on our part.  We don't know about FD because 
it has been down with a broken axel for quite a while (We unexpectedly 
had no usable spare and had to make a new one).

Talking to Jim Ogle, Vivek Dhawan, and Mark Claussen, we are suspicious 
that something, probably water, contaminated the feed but has since 
dried or been removed.  It's hard to be sure.  We will get the integrity 
of the feed windows checked and otherwise look for signs of contamination.

Meanwhile, your high cross hand amplitudes are likely real, not just a 
calibration problem.  I presume that they show up in the uncalibrated 
data too since the calibration process, as you note, has no independent 
scaling for the cross hands.  If you were to try to scale them 
independently from the parallel hand amplitudes, you would likely end up 
very confused.  If the problem is water in the feed, the effect could 
depend on pointing elevation as the water moves around.  Also, the 
amount of water may be variable.  This might make calibration somewhere 
between difficult and impossible.

Please let us know if this continues to be a problem.

Cheers,

Craig




On 12/02/2013 09:59 AM, Eric Greisen wrote:

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     R. Craig Walker            Array Operations Center
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