[evlatests] Two Remarkable Characteristics

[Rick Perley]

    I've been reducing some 'ECSO' data, taken in July and August.  
There were three runs, each of about 4 hours long, at C-band.  No 
frequency or band retunings.  We observed in 'extended OSRO' mode, 16 
adjacent subbands, extending from 4.2 through 6.25 GHz. 
    In general, the data were outstanding!  Calibration and imaging are 
completed, and we reached the thermal noise. 
   
    But in the course of the calibration, two curious characteristics 
were found, one good, and on bad. 

    1) Good 

    We are all familiar with the 'subband 0' rolloff, of still 
mysterious origin (although I understand there's a good chance this 
originates in the station board?) 
    In determining the bandpass, I reviewed the bandpass solutions.  The 
'rolloff' is completely absent in two antennas -- ea19 and ea21, on IF 
'B' only!  Subband '0', on those two antennas on that IF is beautifully 
sharp and flat, except for the sharp drop due to the anti-aliasing filter. 
    A number of other antennas show a dramatically smaller 'rolloff' on 
various IFs -- but none are flat like ea19 and ea21 on IF 'B'.  This 
bandpass was identical on all three runs. 
    Keith has been informed, and reviewed the T304 data for those 
antennas -- nothing unusual.  It's gotta have something to do with the 
station board.  (Can we make all the others like these two?)

    2) Bad

    The antenna stabilities were all very good ('good' defined as +/- 5% 
or so) in all subbands *except* one -- subband 16 showed huge (up to 
factors of 2) variations in fringe amplitude on timescales of minutes, 
in all four IFs, **** on some antennas only****.  The bad antennas were 
the same in all three runs (which were distributed over more than a 
month in time):  2, 7, 11, 14, 15, 17, 18, 19.  Some others were less 
variable, but still well outside the norm:  27 and 28.  All other 
antennas were as 'good as gold'. 
    There is no spatial relations of the listed antennas.  They behaved 
correctly in all other subbands. 
    Now, subband 16 does contain moderate RFI -- in the lower 64 MHz:  
6100 to 6165 MHz.  But it's hard to see how this could cause such 
instability in some antennas, but not in others...