[evlatests] One more EVLA phase issue

[Rick Perley]

    Antenna 24 is showing a phase ramp, at L-band only.  The two IFs 
behave identically -- the phase declines by about 20 degrees from the 
start of an L-band observation for about 3 minutes, following which it 
is more-or-less stable until the end of the 5-minute scan.  There is no 
such effect at X-band on this antenna.  No other antenna shows ramps of 
anywhere near this magnitude.  (Typically, the 'delay clunk' causes ~5 
degree changes, on a much faster timescale than that reported here).