[evlatests] Strange differential bandpasses

[Dave Parker]

Yes but all the antennas have the 4 band elements mounted.

Dave

On 8/26/2021 8:56 PM, Walter Brisken via evlatests wrote:
> 
> VLITE antennas are 28, 2, 12, 3, 4, 6, 9, 24, 7, 11, 19, 14, 20, 15, 17, 
> 26, 27, 21, 8 .  Given that ea05 is not amongst these, connection to 
> VLITE seems unlikely.
> 
> -W
> 
> -------------------------
> Walter Brisken
> NRAO
> Deputy Assistant Director for VLBA Development
> (505)-234-5912 (cell)
> (575)-835-7133 (office; not useful during COVID times)
> 
> On Thu, 26 Aug 2021, Steven Myers via evlatests wrote:
> 
>> Are these VLITE antennas by any chance? I can?t find a list of those.
>>
>>> On Aug 26, 2021, at 3:41 PM, Rick Perley via evlatests 
>>> <evlatests at listmgr.nrao.edu> wrote:
>>>
>>> One of our DAs, Edward Starr, has shown me some strange bandpass 
>>> effects, whereby properly calibrated data, after application of 
>>> calibration, shown undulating sinusoids with frequency.  There appear 
>>> to be (at least) four separate observations affected by this, all 
>>> taken in July or August.
>>>
>>> Only one of these is accessible to AIPS at the present time.  I 
>>> loaded this one (21A-033, S-band, observed 23 July 2021) into AIPS to 
>>> investigate.
>>>
>>> What I found is most peculiar:  The bandpasses of nearly all antennas 
>>> systematically change over the course of the observation (13:30 to 
>>> 17:35 IAT).  The effect is very easily seen if one uses the initial 
>>> scan of 3C48 to calibrate the bandpasses, then generate 
>>> 'differential' bandpass solutions on the subsequent data (following 
>>> normal calibration).  By the end of the run, the observation of 3C84 
>>> shows bandpass ripples up to 2.5% in amplitude, and 2 degrees in phase.
>>>
>>> The character of the change in bandpass is exactly as Edward 
>>> originally described -- a nearly perfect sinusoid, which steadily 
>>> grows in amplitude over time.  The period is very close to 43 MHz, 
>>> which corresponds to 3.5 meters in free space, if due to a standing 
>>> wave.  The characteristics shown in the plot make it virtually 
>>> certain that this is a beat phenomenon due to a reflected signal.
>>>
>>> I have attached an example -- the most spectacular antenna is ea05.  
>>> This is the differential bandpass, from 3C84, observed at the end of 
>>> the run, using the bandpass from 3C48, taken at the beginning.
>>>
>>> All antennas look like this, but none have amplitudes as large as 
>>> this one.  The 'beat' pattern is the same in all antennas on which 
>>> the effect can be discerned.  Other antennas which have the largest 
>>> effect are ea08, 12, 20, and 26.  There is no spatial relationship of 
>>> these -- they are evenly spread about the array. This is not an 
>>> elevation effect, as the growth of the pattern in the phase 
>>> calibrator (J0204+1514) neatly fits the final observation of 3C84.  
>>> The last observation of J0204+1514 was taken at an elevation of 32, 
>>> the subsequent observation of 3C84 was taken at elevation 54.  The 
>>> original calibration, on 3C48, was at elevation 79.
>>>
>>> The two polarizations give identical results -- both in amplitude and 
>>> phase, including the frequency location of the peaks and troughs.
>>>
>>> I've seen sinusoids before, but nothing quite like this case.
>>>
>>> Ideas?
>>>
>>>
>>>
>>> <S-ea05.png>_______________________________________________
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>>
>>
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