[evlatests] Switched Power Troubles at S-band

[Rick Perley]

    After all those rosy description of PDif working at C through Q 
bands, I give a dose of reality for Sband. 

    Things are not so simple here. 

    1) Bad PDif values. 
         - ea6 in LCP has rapidly varying (timescales of seconds) PDif 
values which are not seen in the visibilities. 
         - ea16A also varies in PDif, but not in the visibilities. 
         - ea07 in LCP has very low PDif amplitudes (1/10 of normal), 
and 7C in particular has very high noise.
         - ea12C also has high noise.
         - ea09A is dominated by noise. 
         - ea27C and D are very noisy. 
      *** note that none of these gives a zero PDif -- all have 
reasonable values, on average.  But there is no evidence that these 
values relate to system gain! 

    2) Changes in PDif not related to gain.
         -  For a number of antenna-IFs, the PDif varies over time in a 
way that makes it look likely it is responding to elevation or 
temperature.  In particular, ea15, on all IFs, shows a 15% rise in PDif 
over a couple of hours while the antenna was rising from 8 through 30 
degrees elevation.  This change is *not* seen in the visibilities, so 
must represent a change in something else, such as the noise diode 
power.  (The effect is by far the largest in this antenna). 
          - ea28, in LCP, also shows this effect, but at the 10% levels. 
          - A significant number of antennas show the effect at lower 
levels. 

    3) Troubles in Tsys or Tant.
          - The temporal variability described in Section (1) above 
shows up in the estimates of system temperature -- these variations are 
thus all (or mostly) in the values of PDif, and not in PSum. 
          - The claimed Tsys for ea07 in LCP exceeds 200K.  This might 
be right. 
          - Due to the variability, or other effects, the expected ~1K 
increase in Tsys due to observing 3C84 is completely invisible in ea07 C 
and D, ea09A, ea12C, and ea27 C and D. 

    These are significant issues, which need some investigation.