[evlatests] 4band, interference and ea14

[Rick Perley]

     Mike Revnell just showed me VLITE spectra from ea14.  There's a 
super-strong resonant feature ('birdie') right around 60 MHz.  Looks 
like external RFI.
     We note the API is nearby ...  any chance it's from there?


On 06/23/2014 11:33 AM, Frazer Owen wrote:
> 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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