[evlatests] Antenna 4 -- Gone Wild!

[Rick Perley]

    The 'stress test' has revealed some very peculiar behavior with VLA 
antenna 4.  It makes little sense to me -- I describe what I see, in the 
hope that others can see through the fog ...

    a) P-Band:  There are no fringes, and all data are flagged.

    b) L-Band:  Short dropouts are seen on all four IFs.  The duration 
of any drop is a single .416 second integration.  The amplitude of the 
drop is typically a factor of two.  There is no phase effect.  The 
frequency of the drops is about once every 10 seconds -- but there is no 
regularity.  All four IFs drop at the same time.  These are the same 
characteristics that I have noted before. 
          In addition to these, there are 'extra' phenomena on IFs A and 
C only -- short (few record) amplitude *rises* of a couple seconds 
duration.  Similar effects may be in B and D, but are less clear.  These 
perturbations may be correlated with very odd Tsys behavior. 
          None of these bad data were flagged. 

    c) C-Band.  Everything looks quite normal at C-band -- except that 
IF 'A' shows a regular 'beat' in amplitude, dropping by about 3% over ~5 
seconds, then abruptly returning to the normal level.  The 'beat' period 
is about  20 seconds.  There is no corresponding effect in Tsys, so this 
originates in the correlation coefficient.  No data were flagged. 

    d) X-Band:  IFs B and D are entirely normal.  But in IF 'A', we 
again see the 'beat', but the sense of it is reversed -- the antenna has 
about 10 seconds in the low state, then about 5 second in a higher, 
normal, state.  The amplitude of the beat is much higher -- about a 
factor of 2.  In IF 'C', the gains are low by a factor of 10, and there 
are intermittent short (few seconds) pulses of normal amplitude.  Tsys 
values of IF 'A' are fixed at 203K, and are large and erratic in IF 
'C'.    No data were flagged. 

    e) U, K, Q bands.  All is normal at these bands.