[evlatests] Internal Birdie at 1408.25 MHz

[Rick Perley]

    We are seeing in recent tests the presence of a strong birdie, 
centered at 1408.25 MHz.    The characteristics of this RFI strongly 
indicate is is *NOT* of external origin:

    1) It is seen only on baselines associated with antennas 1, 2, 3, 9, 
18, and 25.
    2) It is absent on all baselines associated with antennas 19, 23, 
24, and 28.  
    3) The signal is constant (where seen), or absent entirely, 
throughout the hour long observation. 
    4) It is strongest on the five antennas associated with antenna 9 (9 
x1, 9x2, 9x3, 9x18, and 9x 25), but completely absent on (9x19, 9x23, 
9x24 and 9x28). 
    5) There are no spatial groupings amongst the two subsets -- antenna 
23 (clean) is immediately adjacent to 3 (dirty).  Antenna 24 (clean) is 
immediately adjacent to 1 (very dirty).  Antenna 19 is mid-way between 
24 and 25.  Antenna 28 is midway between 18 and 25.  Baseline 9 x 18 
(where the signal is the strongest of all) is one of the longest in the 
array.   
    6) The phase of the interfering signal rotates at a rate compatible 
with fringe tracking -- indicating the interfering signal has steady 
phase. 
    7) It is perfectly centered on channel 73 (1408.25 MHz) -- as there 
is absolutely no sign of any Gibbs' ringing.  This would be a remarkable 
coincidence if an external signal is the origin.    The sub-band center 
frequency is 1436.25 MHz, and sub-band width is 128.0 MHz. 

    Any ideas?