[evlatests] T304 attenuator changes

[rperley at nrao.edu]

I reported earlier on the amplitude changes seen in the C-band data, on
most antennas.  All of these changes occurred between scans, and most
corresponded to a 1 dB change in power, strongly suggesting that the
origin is the T304 attenuator chanaging.

I later reported on apparent unstable C-band bandpass responses -- unlike
at X-band where perfect stability is seen.  Closer inspection of these
apparently unstable bandpasses shows that they changed shape between
scans, and were always stable within any one 5-minute scan.  Indeed, most
of the bandpass solutions look as good as the X-band ones -- it's a subset
of the scans which show non-standard shapes.

Ken suggested that I look for a correlation between the two phenomena
reported above -- and it is certainly there.  Most (but not all) of the
scans showing a change in bandpass shape are the ones with a change in
power level, likely corresponding to a change in T304 attenuator.  For
each power (attenuator) setting, there is a different bandpass shape.  If
we take one of these as the 'right one', the other differs by a few
percent, and has a distinctly sinusoidal shape with a long period -- over
100 MHz.  Not all antennas show this -- of the ten, perhaps one or two are
much less affected.