[evlatests] Curious P-band RFI

[Rick Perley]

    Adding to my last note:

    The test used only two spectral windows (to reduce the data load), 

    SP1:  312 -- 328 MHz
    SP2:  328 -- 344 MHz

    Very curious RFI was found at:

    1) 324 MHz in 'LCP' ( = vertical dipole).  This was absolutely 
constant in both amplitude and phase, and nearly completely absent in 
the opposite polarization.  Also seen on all antennas. I suspect 
something internal, due to the constant nature. 

    2) 333 MHz in 'RCP' (= horizontal dipole).   This one rotates in 
phase, and is I think due to the Abq. airport ILS signal.  (We saw this 
back in the 'old days').  Nearly invisible in vertical polarization.  I 
would be good for Dan to confirm the origin, and the polarization ... 

    In every one of the tests I've done so far, I have seen short 
'amplitude dropouts' -- lasting a few seconds, probably on all spectral 
windows, in both polarizations, and on all antennas.  The frequency of 
occurence is a few minutes to a few tens of minutes.  In this current 
short (15 minute) test, there were four occurrences.  I ran SPFLG on 
these data, and the effect is easy to see.  What is notable is that in 
the two spectral windows, a single channel in each of the two IFs had a 
huge (hundreds or thousands of Jy) spike.  The frequencies of these two 
'spikes' are 323 and 330 MHz.   It seems that these impulses are 
saturating the receivers sufficiently to reduce the gain.  I can't tell 
from these data if the real origin lies outside the two small frequency 
windows (that is, the 'spikes' aren't the primary cause). 

    Does anybody have a speculation on the origin of these 'dips'?