[evlatests] More L-band birdies ...

[Rick Perley]

    The last 'L-band birdie report' came from careful perusal of the 
1713 -- 1905 MHz portion of the spectrum -- region almost completely 
free of RFI. 

    This report comes from careful perusal of the 1329 -- 1521 MHz 
portion -- almost as clean of RFI as the higher chunk. 
  
    First -- the obvious external RFI:

    1) There are two clear radar signals at 1330 MHz and 1337 MHz.  The 
first has a period of 6 seconds, the second has a period of 12 seconds.  
Each pulse is narrower than my time resolution (1 second).  These are 
easy to clip out. 
   
    2) The GPS L3 signal at 1381 MHz.  This is intermittent, detectable 
approximately every few tens of minutes, and occasionally strong enough 
to affect the other channels within the subband.   When it is 'on', it 
can last from a few seconds to a few minutes. 

    Then we have the mysterious 'birdies' -- all of those below are 
unresolved in frequency (< 1 MHz wide):

    1)  1408 MHz.  This is seen on a small fraction of the baselines -- 
but all of them involve ea06 and ea28.  It is stronger in RCP than in 
LCP.   The two antennas centrally involved are widely separated -- this 
birdie is very likely internal in origin. 

    2) 1375 MHz.  This one is seen on all the baselines involving 
ea05,9,11, 13, and 24, on both polarizations.  As all these antennas are 
adjacent, on pads spanning W2 to W12, the origin of this signal *must* 
be on-site, probably in the technical services area. 

    3) 1438 MHz.  This odd one was seen only on ea05 x ea09 (and was 
weak).  The pads are W8 and W10.  Control building origin, perhaps?

    4) 1350 MHz.  This one is seen on most antennas to ea13.  The birdie 
was strongest on 13 x 15, 13 x 22 and (especially!) 13 x 24.  The 
detected birdie gets weaker as the baseline gets longer.  It is far 
stronger in RCP than in LCP.  These characteristics are consistent with 
a single source of emission, from ea13, which is radiatively coupled to 
the adjacent antennas. 

    5) 1400 MHz.  This is seen only on ea13 x ea24 -- adjacent antennas 
(W2 to W4).  It is almost certainly the same situation as the 1350 MHz 
birdie, but much weaker.