[evlatests] ea13 birdie @1400 MHz, ad nauseam

[Emmanuel Momjian]

On 04/28/2015 07:28 PM, Rick Perley wrote:
> Fabulous result!
>
> But what are those 'block' offsets seen at the low frequency end?


The 'block' offsets are streams of RFI that show up intermittently. In 
this particular case they were seen for less than 2.4 hours during a 6 
hour session on April 26, 2015 (see attached, which shows the data of a 
single baseline ea01-ea27 (N12-N08) in RR spanning 6 hours of observing).

Each block spans 11 channels (~172 kHz) centered at ~1397.7 MHz and 
1398.1 MHz. To the left of the 'blocks' there are also weaker and 
narrower streams of interference.

The 'block' offsets are stronger in RR, and can be seen on all the 
baselines among the antennas on the north arm and the near-array-center 
antennas on the east and west arms (note that the array was in 
B-configuration).

The absence of the signal on baselines to the antennas on the east and 
west arms that are far from the center of the array seems due to fringe 
winding.

Emmanuel







>
>
>> On Apr 28, 2015, at 6:12 PM, Emmanuel Momjian <emomjian at aoc.nrao.edu> wrote:
>>
>> Over the last weekend we got high spectral resolution L-band observations that allowed me to check the "birdie situation" on ea13.
>>
>> The results are clearly showing that this antenna no longer has the 1400 MHz birdie and the combs associated with it: the 468.75 kHz comb and the 25 MHz comb. Note that for the latter I checked four of its components (at 1300, 1325, 1400, and 1425 MHz) to establish that they are no longer present.
>>
>> Attached are two example spectra showing the situation before and after the work that Dan, Rob and Ricardo did on ea13 about a week ago.
>> The file ea13_1400_before.png shows the 1400 MHz birdie and its 468.75 kHz comb as seen in data taken on April 4, 2015.
>> The file ea13_1400_after.png shows their disappearance in the new data taken on April 26, 2015.
>>
>> Emmanuel
>>
>>
>>> On 04/22/2015 01:10 PM, Dan Mertely wrote:
>>> Rob Selina, Ricardo Contreras, & I went out to ea13 for an
>>> RFI hunting mission and had a lot of success.
>>>
>>> One of the main problems was that the LAN switch and media
>>> converters up in the vertex room were plugged into an un-
>>> switched AC outlet.  They should have been plugged into a
>>> switched outlet (LAN devices of all kinds are very noisy!)
>>> Rob re-routed the cables and plugged the wall-wart power
>>> supplies into the proper switched AC outlets.
>>>
>>>>>> I ask that everyone working on antennas be aware of the
>>> fact that the LAN switches and media converters are very, very
>>> RF noisy.  Please make sure that the Vertex Room switches turn
>>> off when the room lights are off, and please also make sure
>>> that the LAN switch and media converter down in the Ped Room
>>> is unplugged when not in use.  This is important. <<<
>>>
>>> In addition, the LOIF and Utility rack RF gasketing "fingers"
>>> were greatly damaged due, in part, to a couple of the doors
>>> being misaligned.  We fixed the gaskets & re-aligned the doors
>>> by adding spacing washers on the hinges.
>>>
>>> Now ea13 may be one of the quieter antennas out there in L-band.
>>> Please make another check at your convenience.  TNX.
>>> -Mert
>>>
>>>
>>>> On 4/21/2015 10:42 AM, Rob Selina wrote:
>>>> HI Mert,
>>>>
>>>> Will you be out at the site tomorrow? I was hoping we might have some time to talk about EA13's 1400MHz birdie and perhaps go do a sweep at the antenna. Assuming you've looked into this one before, is there any pertinent info in /evla/techdocs/rfi ? I did a quick search but didn't see anything.
>>>>
>>>> We can talk about it tomorrow (hopefully) so I won't write a book here, but the long and short of it is that Emmanuel, Rick and others have suggested that we focus tracking down some of our self-generated RFI, starting with this birdie on EA13. I wanted to see if this was something we  could perhaps work on together.
>>>>
>>>> Cheers,
>>>>
>>>> Rob
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: Emmanuel Momjian [mailto:emomjian at aoc.nrao.edu]
>>>> Sent: Wednesday, April 15, 2015 8:59 PM
>>>> To: Rob Selina
>>>> Cc: Bryan Butler; Rick Perley
>>>> Subject: ea13 birdie @1400 MHz
>>>>
>>>> Hi Rob,
>>>>
>>>> Attached is a spectrum that shows the birdie of antenna ea13 at 1400 MHz. What I don't recall seeing before are the little spikes near the main feature; there are two on the left and two on the right. These weaker ones are also only seen on baselines to ea13. It looks like the birdie has a whole family now!
>>>>
>>>> The separation between these features is exactly 30 channels, which is
>>>> 468.75 kHz.
>>>>
>>>> cc-ing Rick and Bryan to check if they recall seeing the weaker features before.
>>>>
>>>> These data were taken on April 4, 2015.
>>>>
>>>> I'll look into the 1408 MHz birdie later.
>>>>
>>>> Cheers,
>>>> Emmanuel
>>>>
>>> On 1/20/2013 11:51 AM, Rick Perley wrote:
>>>> The latest 'spectra sweep' shows clear evidence that ea13 is
>>>> seriously polluting our spectrum at L-band.  In trolling through the
>>>> spectra provided from the data, some baselines common to ea13 show
>>>> strong RFI at multiples of 25 MHz, extending from ~1200 MHz through
>>>> ~1500 MHz.  There are only a few baselines on which this comb is easily
>>>> visible -- these are 13 with 9, 14, 24, 25, and 26.  All of these are
>>>> antennas adjacent to 13!
>>>>       It's quite clear from this that ea13 is probably the sole source of
>>>> the interference.
>>>>
>>>>
>> <ea13_1400_before.png>
>> <ea13_1400_after.png>

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