[evlatests] An interesting RFI event yesterday

[Rick Perley]

    I ran a short 'stress test' late yesterday afternoon.  The L-band 
data show a remarkable RFI event that I thought might we worthy of a few 
lines of description. 

    On the A/C side only, most of the VLA and EVLA antennas, at L-band 
(frequency/BW = 1465/50 MHz) showed moderate to large drops in coherence 
coefficient at two different (but adjacent) times.    The B/D side 
(1385/50) showed no effects whatever. 

    1) The first occurred between 22:44 and 22:46 IAT (4:45 PM, MDT), 
and clearly was of local origin, located near to antenna 23 (located at 
E36), for which the loss of coherence, and a parallel rise in measured 
Tsys, was by far the largest.  The same effect was seen, with decreasing 
strength as a function of distance from E36.  The effect was completely 
absent only for the five antennas at the end of the north arm (2, 25, 
27, 26, and 7).  The fact that the end antennas on the west arm did see 
the signal, coupled with the rapid rise of signal strength for antennas 
at the end of the east arm, would indicated the source radiation pattern 
is not isotropic. 

    2) About three minutes later, more RFI was visible.  However, this 
'burst' did not appear to have the same clear spatial origin, being seen 
more or less equally on all east arm antennas.  Notably, this one was 
not seen on the same five north arm antennas as the first. 

    My quick test ended while this latter 'burst' was still ongoing. 

    I looked at the data both as raw correlation coefficients, and as 
Tsys-corrected visibilities.  The Tsys mechanism made a valiant -- but 
generally unsuccessful -- attempt to fix things up (not surprisingly).