[evlatests] Zero PDifs -- only one left!

[Rick Perley]

    Review of all the switched data from 'Flux Redux' shows that amongst 
all the antennas, bands, and polarizations, there remains only one (1) 
channel with zero PDif (meaning, the detected synchronous power is 
zero-mean noise):

    ea25, in LCP only.  (RCP is fine).  This indicates the diode is 
working, but that its output isn't getting to the LCP side. 

    There are zero PDifs on three other antennas (ea20 at L-band, ea01 
at C-band, and ea08 at S-band), but for all three cases, it appears 
there is no receiver installed (or working).