[evlatests] Another 128 MHz comb!

[Jim Jackson]

A 128 MHz comb at Q-band is interesting. This is not seen in any 
lower bands (Ku to Ka)?   I wouldn't have thought any of our hardware 
(even the comb generators) would produce much energy at 40-50 GHz. 
Unfortunately, if it does, I also would not expect that most of our 
shielding (modules, racks or correlator room) is particularly 
effective at 40-50 GHz.

Jim




At 02:12 PM 3/15/2011, Rick Perley wrote:
>     Ken ran the full 'spectra sweep' program, covering 1 through 50 GHz
>with 250 kHz channel resolution.
>     Some issues were discovered which prevent full utilization of the
>data -- these are understood and a new test will be done later.
>
>     The data we got are certainly sufficient to uncover some interesting
>new effects:
>
>     Another 128 MHz comb is found -- this one at Q-Band!  Seen are all
>harmonics from 378 through 390 (48384 through 49920 MHz).  These are
>seen on 9 baselines (there were 13 antennas in the array, so 78
>baselines were present).  Seven of these 9 baselines include antenna
>14!  The other antennas involved show no spatial correlation, so it's
>clear that the coupling is not through free space.  This is an internal
>tone, by far the strongest of which is in antenna 14, which is
>coherently seeing weaker tones from the other antennas.  The other
>antennas involved are 7, 10, 8, 12, 24, 26, and 27 (but note that the
>tones are only seen on a few of the baselines).
>
>    A 'grid' of close-spaced tones is seen centered near 26300 MHz.
>
>     A very strong tone is seen at/near 29400 MHz.   Another near 35176
>MHz.  (exact frequencies will require a re-observation -- these current
>set are mislabeled).
>
>     An expected tone at 8192 MHz is seen.
>
>     The usual (and well known) RFI at the low bands (L, S, C) are seen.
>
>     A fuller report will be made when the corrected data are available.
>
>
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