[evlatests] Antenna-based RFI at 1400 and 1408 MHz

[Rick Perley]

    Inspection of WIDAR spectra reveal some RFI at 1400 and 1408 MHz, 
which clearly originates within our antennas. 

    I reproduce below what I reported back in July.  The situation has 
now changed a bit:

    The 'guilty' antennas, listed below (1, 2, 3, 9, 18 and 25) are now 
completely clean.  No RFI is seen on any baseline connected to these.   
Antenna 24 also seems good. 

    The RFI is now found on most (if not all) of the other WIDAR 
connected antennas:  4, 5, 8, 15, 27 and 28.  Perhaps we didn't detect 
this earlier on these antennas due to fringe-rate attenuation.  
(D-configuration is very useful to find such problems). 

    Note that we see RFI at both 1400.0 and 1408.0 MHz on these 
antennas.  Some show both, some show one, or the other. 
(The frequency offset of 0.25 MHz between the July report and this one 
is almost certainly due to the header issue which has been resolved.  
It's 1408.0). 



-------- Original Message --------
Subject: 	[evlatests] Internal Birdie at 1408.25 MHz (fwd)
Date: 	Fri, 6 Nov 2009 10:00:06 -0700 (MST)
From: 	Ken Sowinski <ksowinsk at aoc.nrao.edu>
To: 	Rick Perley <rperley at aoc.nrao.edu>



---------- Forwarded message ----------
Date: Mon, 13 Jul 2009 15:17:58 -0600
From: Rick Perley <rperley at nrao.edu>
To: evlatests at aoc.nrao.edu
Subject: [evlatests] Internal Birdie at 1408.25 MHz

    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?



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