[evlatests] Internal RFI emissions from ea24

[Rick Perley]

    Changes were made to antenna 24 which were intended to remove most 
internal RFI. 
    Observations were made Friday to check the results.  The correlator 
produced spectra in L, S, and C bands, with 128 kHz resolution.  I have 
produced ampscalar-averaged spectra of antenna 24 with all other 
antennas (except 1, which was at the end of the W arm, and 23, which has 
no receiver), and compared these to full-array averaged spectra made 
last March. 

    Results:

    Little has changed at S-band.  Antenna 24 is producing (or picking 
up) every harmonic of 128 MHz, exactly as was present last March.  The 
only RFI birdie which was present in March and which is now absent is at 
2400 MHz.  (Note that this one is not a multiple of 128 MHz (but is the 
75th harmonic of 32 MHz)). 

    I'll check C-band tomorrow. 

    At L-band, the previously known multiples of 512 MHz, seen at 1408 
and 1920 MHz, are weakly present on ea24, most obviously in LCP.  
Baselines which show the presence of this birdie are 24 with 8, 12, 13, 
15, and 28.  These are distributed in various places in the array -- 
there is no evidence that these birdies are from a single source. 
    Antenna 13 is a very strong emitter (or producer) of these two 
birdies.  There is no other antenna anywhere close to what ea13 is doing 
here.