[evlatests] 4band, interference and ea14

[Frazer Owen]

     Details of 60 MHz interference:

1) My test was looking at an 8minute average.

2) The test that shows the strongest cross-correlation at 60MHz is 
looking at the NCP (North Celestial Pole).

3) The antennas are ea12 at N9 , ea14 at E8, ea19 at W1

4) Except for the NCP, the signals when averaged over 8 minutes decrease 
in amplitude relative to 2 second averaging.

5) The strength of the correlated 60MHz signal changes with different 
pointing positions (Cas A, 3C48, NCP). Especially the NCP strength is 
very different from the other two.

6) ea14 at E8 has no internal 60 MHz interference based on the total 
power and is near the end of the east arm. However, ea14-ea19 appears to 
have stronger 60MHz interference than ea12-ea19. These signal have been 
through the requantizer step but it seems clear that whatever signal 
ea14 is seeing that correlates with ea19 is not coming from ea14 itself.

---Frazer


On 06/23/2014 09:51 AM, Rick Perley wrote:
> In the (distant) past, a similar effect was commonly noted at 4-band.  
> The accepted explanation was either that:
>
>     1) An exceptionally bad antenna is radiating to the others. This 
> would be most prominent in D configuration.
>     or
>     2) The supposedly incoherent LOs are sufficiently close in 
> frequency that, for periods of seconds to tens of seconds, there is 
> insufficient phase slip between them to destroy the cross-correlation.
>
>     I recall that #2 was in fact the theory I came to embrace, as it 
> was usually noted that the baselines with strong combs were 
> independent of antenna location.
>
>     You could test this by looking at the phase of the line 
> cross-correlation.  If from a single source (i.e., theory #1), the 
> phase should slip with the fringe rate.  But if from different LOs 
> with slightly variable phases, the phase relation will be 
> 'different'.  (I'm not prepared to define 'different', other than it 
> won't be strictly the same as the fringe phase).
>
>     Rick
>
>
> Interestingly the 5MHz comb frequencies correlate on baselines to 14 
> (as they do on 12-19). Since the 5MHz clock is supposed to be 
> incoherent from antenna to antenna, there must be a general source of 
> the comb frequencies, especially 60MHz, which is being broadcast 
> around the site. Could this be one very bad antenna ? ---Frazer 
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