[evlatests] Zero mean PDif

[Rick Perley]

    From today's all-band test, I have checked for 'zero PDif' -- where 
the PDif shows only noise with zero mean -- indicating no detectable 
switched power. 

    L-Band:  Antennas 9 and 17, both polarizations.  Antenna 9 gave good 
fringes, while 17 did not (but was not flagged). 

    S-Band:  as noted earlier, antenna 12. 

    All high frequency bands (Q, Ka, and K) give no fringes, and zero 
PDif for antenna 13 in RCP only.  LCP looks fine.

    All other antennas which gave good fringes also gave good PDif values.