[evlatests] Noisy C-band antennas?

[Rick Perley]

This problem must be highly time-variable.  I have eight wide-band, 
3-bit, C-band observations on hand, taken between 22 June and last week 
-- and *none* of them show any problem in the calibration gains, or any 
hint of any problem.

We really need to find the source of this problem...

Rick

On 8/29/23 16:23, Bryan Butler wrote:
>
> Middle of E and W arms.  See attached.  Drew tells me they see it 
> "quite a lot" - not sure if it's always the same antennas.
>
>     -Bryan
>
>
> Rick Perley via evlatests wrote on 8/29/23 16:14:
>> Where are these eight antennas located? In previous tests, there 
>> seems to be little spatial correlation for the affected antennas -- 
>> in Pedro's note (below), W36 and N28 are far, far apart.  Hard to see 
>> how a single source can affect both so badly.
>>
>> Also note that I claimed on June 29 that the effect was completely 
>> missing.
>>
>> So -- do we really understand what is going on?  Perhaps a more 
>> careful review of data taken at C-band may be needed.
>>
>> Rick
>>
>> On 8/29/23 15:52, Bryan Butler via evlatests wrote:
>>>
>>> Yesterday, I got a pipeline reduction report from the DAs on a 
>>> science observation at C-band, observed on July 27.
>>>
>>> The situation seems really grim - upon digging further (well, asking 
>>> for more information from James), this effect is seen strongly on 8 
>>> antennas (1, 4, 5, 9, 19, 25, 26, 27).  Roughly 1/3rd of them.
>>>
>>> Shouldn't we be telling our C-band observers to observe in 8-bit, 
>>> instead of 3-bit, to avoid (or at least lessen the effect of) this? 
>>> If it really is compression, it's unclear (to me at least) that this 
>>> is recoverable at all, and certainly not without a much larger 
>>> uncertainty on the flux density scale.  If I had known it was this 
>>> bad, I certainly would have observed in 8-bit, and taken the sqrt(2) 
>>> loss rather than the 1/3 (potential) loss.
>>>
>>>     -Bryan
>>>
>>>
>>> Pedro P.B. Beaklini via evlatests wrote on 7/11/23 16:29:
>>>> Rick, and all,
>>>>
>>>> I did a test last Thursday on this. The thing is that the noise 
>>>> depends on the antenna location. The compression is due to an RFI 
>>>> source in the direction of stations W36 and N28 and often can be 
>>>> seen in N32, N36, and W32 stations. For some reason that we are 
>>>> still investigating, ea17 is more affected than others when it is 
>>>> on the RFI path. But the problem can be seen in the other antennas 
>>>> in the RFI path.
>>>>
>>>> On the current configuration, there is no antenna on W36 and N28. 
>>>> EA17 is far away now, at N64, and no compression was seen in my 
>>>> test on ea17, in agreement with your data. However, on the antenna 
>>>> closest to W36, ea05 at W40, I could see a C-band compression on 
>>>> 3-bit. Please, can you take a look at ea05 on your data? I also saw 
>>>> a new noise source leading to compression on the east arm ( peak on 
>>>> EA04 at E48). That is new to me and needs further investigation, 
>>>> but the effect on data is similar.
>>>>
>>>> If someone wants to take a look at pdif varying in time on my tests 
>>>> and past data, I put some shorts movies at. Each frame is 5s, time 
>>>> on top in minutes
>>>>
>>>> /lustre/aoc/sciops/pbeaklin/tsys/C-band-issue/2023-06-06/pdif/
>>>>
>>>> As a last comment, I was not the first one to notice the issue. The 
>>>> DAs, SRDP team, and the support telescopes group have been aware of 
>>>> that for a while. I only decided to take a deeper look at it :-)
>>>>
>>>> Cheers
>>>> Pedro
>>>>
>>>> On 6/29/23 14:55, Rick Perley via evlatests wrote:
>>>>> Some months ago, Pedro noted that certain antennas were especially 
>>>>> noisy (low SNR) for some antennas and spectral windows, when using 
>>>>> 3-bit samplers. From my own data, I confirmed this.
>>>>>
>>>>> The recent data referred to in my last circular allows a check on 
>>>>> this situation -- the data were taken with 3-bit sampler.
>>>>>
>>>>> The 'noisy' effect is completely gone!  All antennas, in all 
>>>>> spectral windows, have exactly the expected sensitivity and gains.
>>>>>
>>>>> Pedro plans to take some data, to confirm.
>>>>>
>>>>> Rick
>>>>>
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