[evlatests] 128 MHz combs

[Rick Perley]

    Prior spectral sweeps have shown narrow comb emission at various 
bands.  I've gone through the current run, and find all are still with 
us.  But, in all cases, only a subset of antennas is involved.  A 
summary follows:

    1) L-band.  This is seen at 1408, 1676, and 1920 MHz (and probably 
others -- RFI makes them hard to see).  Only about 30 baselines show any 
emission.  Those with (by far) the strongest lines are: 9 x 25, 11 x 16, 
16 x 25, 16 x 26, 25 x 26.  Virtually all affected baselines include one 
of the antennas noted above. 

    2) S-band.  The comb is seen throughout S-band.  Characteristics are 
similar to L-band, but the affected antennas are different!  The lines 
are strongest on 1 x 3, 3 x 10, 3 x 16, 3 x 21, 3 x 25 (especially!), 3 
x 28, 10 x 25.  Antennas 3, 10, 25, and 28 are involved in virtually all 
affected baselines. 

    3) Q-band.  The comb reappears for the highest frequencies only -- 
above 46 GHz.  Again, only about 20 baselines are affected, but the 
antennas involved are different!  Now, antennas 2, 10, 21, and 22 are 
involved in virtually all affected baselines. 

    Note:  None of these emissions is very strong -- typically 1 dB 
above the noise.  This is no emergency! 

    The one line which *is* important is the 'super-spike' -- at 29440 
MHz.  ( = 230th harmonic of 128 MHz).  This guy is a whopper -- a good 
35 dB above the noise.  It would be good to get rid of this ...