[evlatests] New Pdif results on modified antennas

[Rick Perley]

    Keith et al. adjusted the output powers of the receivers on ea18, 
ea21, and ea25, so their levels met the specification in the block 
diagram. 

    We ran the Crab-Compression test this morning to see if these 
changes have helped.  The test is an on-off Crab observation at L, S, C, 
X, and Ku bands. 

    The short answer is:  Most PDifs ratios (Crab/OffCrab) for these 
antennas improved.  Some improved a lot. 

    Below is a table, for these antennas only. 

Antenna -band       A              B              C               D     
     comment     
----------------------------------------------------------------------
ea18         L         100           99             96             94   
        much better
ea18         S           96           93             94             92   
        slightly better
ea18         C          93            93            89             91   
        slightly worse
ea18         X          97            95            97             95   
        no change
ea18         U          99            98            97             97   
        better on AC side
ea21         L          98            99             99             98   
        much better
ea21         S           99            97            99             99   
         better
ea21         C          87            93            91              94   
        no change
ea21         X          99           99             100            99   
        better
ea21         U         100          100           100            98     
      no change
ea25         L           98            98            99             100 
        much, much better!
ea25         S           99            97             98            101 
         much better
ea25         C          91            96             94              96 
         much better on BD side
ea25         X        100            99            99              100   
      better
ea25         U          99            97             98              100 
       no change
----------------------------------------------------------------------------------------

    So we're heading in the right direction, but clearly this is not the 
sole problem. 

    Also noted:

    A)  ea18 changed its power at L-band between scans -- quite 
dramatically.  I had carefully arranged to sit on blank sky for well 
more than the 1 minute recommended for 'set and remember' to remember 
the levels.  But the subsequent observations for this antenna had 
dramatically different powers:
    ea18Aand B         went up by a factor of two, between two  scans. 
    ea18C and D        went up by 21% (1 dB), between a different pair 
of scans!

    It may be that the T304 is not to blame here, since A and B behaved 
identically, and differently than C and D. 

    B)  ea12, on all four IFs, had a power jump upwards by a factor of 
about 30, between the last two scans. 

    C) ea23, on RCP, has zero mean PDif values, but good PSum.  It would 
appear the noise diode is not switching.