[evlatests] 128 MHz comb at S band

[Rick Perley]

    We long ago found a fairly strong 128 MHz comb, worst in S-band, but 
also visible in L and the lower half of C-band.  Since then, some 
engineering changes have been made to remove this birdie.  ea24 has had 
the 'full' treatment.  I looked at the results from the RFI sweep done 
on Tuesday to see which antennas are good, and bad, and whether ea24 is 
now the best. 

    It's complicated ... 

    For the 3 -- 4 GHz span, antennas 14, 23, 24, 25 and 26 are all 
clean of the 128 MHz comb, while most of the others have a strong 
signal.  Worst offenders are 10, 13, 15, 16, 18, 21 22 27 and 28. 

    However, in the 2 -- 3 GHz apan, the situation is not the same.  
antennas 14, 23, 25 and 26 are all clean, but ea24 does have some 128 
MHz multiple response -- much less than the worst, but still present.  
It's especially visible on baselines with 22 and 27.  Also, some of the 
worst offenders in the 3 -- 4 GHz span are pretty clean in the 2 -- 3 
tuning. 

    As ea24 was not working at L-band in this test, I can't make any 
conclusions. 
    The comb is much weaker at C-band (and poses no real threat to most 
observing).  Some antennas show it much stronger than others:  notable 
are ea10, 14, 16, and 18.  It is weakest on ea4, ea27, and ea28. 

    Note that these subsets are not the same at the various bands!