[evlatests] C-Band RFI

[Rick Perley]

    Bob and I conducted the final C-band efficiency/system performance 
tests on antenna 24 last week.  While conducting these, we noted some 
interesting (and strong) RFI.  Using Walter's bandpass tool, we captured 
some spectra for subsequent review.  Below is a summary. 

    1) A weak *internal* birdie appears at about 3605 MHz (outside the 
C-band passband).  The strength is about 4 dB above the noise in a 2 MHz 
channelwideth.  This is of no consequence to observing, but may indicate 
some internal issue. 
    2) A narrow, and strong, birdie near 6580 MHz is seen -- only at low 
elevations towards the west.  The width is no more than 2 MHz, and the 
strength is up to 15 dB over the noise.  The birdie is undetectable 
above 20 degrees elevation, and gets stronger as elevation decreases. 

    3) An extremely wide and strong 'birdie' is seen between 5990 and 
6020 MHz.  This has already been detected by Emmanuel in his 6cm 
sensitivity tests.  The signal is seen in all our observations, and 
appears to be at least as strong when at the vertical as when pointed to 
the horizon.  It is likely time variable.  Its strength is 15 to 20 dB 
above the noise over its 30 MHz width -- thus contributing about as much 
power as is in the entire 1 GHz band on cold sky! 

    I think it is important that we learn the origin of the last two 
signals -- particularly the last one, which if omnipresent, will prevent 
future observations within that frequency window.