[evlatests] Strange differential bandpasses

[Edward Starr]

Hello all,

Tis the season of morning dew on the feed horn windows, and we have been 
seeing the wave across calibrated amp vs freq again commonly over the 
last two months. I am wondering if any further effort has been made 
since this conversation died down last year? I asked Rick and he said to 
check in here or talk to Catarina. I showed Catarina the issue, she 
looked for a work order and didn't find any relating to this.

- Edward

On 8/27/21 11:53 AM, Ken Sowinski via evlatests wrote:
> On Fri, 27 Aug 2021, rperley via evlatests wrote:
>
>> OK, now that a probable cause is located, we have to think about how 
>> to manage this.
>> Other than installing 'window wipers' on the windows (ha! -- think 
>> about that for L and S bands ...),
>> the only good mitigation is more frequent bandpass calibration. 
>> Probably every couple of hours, if the weather is likely to lead to 
>> dew condensing on the windows.  (All feeds from C upwards have heat 
>> lamps, so I would hope the problem is much rarer for this bands).
>
> I had forgeotten that L and S do not have feed heaters.  Is that
> because they are too large to be heated practically?  Gimbaled umbrellas?
>
> Ken
>
>
>> For *most* continuum observations, the effect of this changing 
>> reflection is pretty minor -- (so long as the imaging is done over 
>> spectral widths much larger than the observed period in frequency 
>> space).  For continuum observations requiring channelwidths less than 
>> this, self-calibration (if viable) will fix things up.  All other 
>> cases will need better bandpass calibration, if the amplitude of the 
>> effect (a few percent) is important.
>>
>> Rick
>>
>>
>> On 2021-08-27 12:49, Wes Grammer via evlatests wrote:
>>>  This all makes sense. The radome on its own wouldn't produce much of
>>>  a mismatch/reflection, but with dew it would be much greater. The RF
>>>  mismatch at the vacuum window is higher because of the thickness and
>>>  density of the foam window, so a standing wave could be set up between
>>>  the two interfaces.
>>>
>>>  Since 2013, we've had to replace the original foam windows in the S-
>>>  and C-band receivers with denser ones because their marginal strength
>>>  caused them to deform over time. The denser foam had a slightly higher
>>>  dielectric constant ,which led to a small increase in mismatch.
>>>
>>>  -Wes
>>>
>>>  On 8/27/2021 10:08 AM, Rick Perley via evlatests wrote:
>>>
>>>>  So, on a probably drier day, the effect is nearly invisible.
>>>>  Support for the idea that water/dew on the window is a contributing
>>>>  factor ...
>>>>
>>>>  Rick
>>>>  On 8/27/21 9:42 AM, Drew Medlin wrote:
>>>>  Here's an observation from a drier time of year. 2021-03-27,
>>>>  afternoon observation, operator recorded Sky cover 40%. Cumuliform
>>>>  clouds.
>>>>
>>>>
>>>  20A-092.sb39518402.eb39548999.59300.832225300925.ms-J0303+4716-bb12-PHASE-amp_vs_freq-LL_RR.png 
>>>
>>>>
>>>>
>>>>  Drew Medlin
>>>> dmedlin at aoc.nrao.edu
>>>>
>>>>  Scientific Data Analyst - VLA Scientific User Support Group
>>>>  National Radio Astronomy Observatory
>>>>
>>>>  On Aug 27, 2021, at 9:18 AM, rperley <rperley at nrao.edu> wrote:
>>>>
>>>>  Certainly is reasonable.  In the case I analyzed, the start of the
>>>>  run was around sunrise, the end around noon.  The period of the
>>>>  oscillations (and the phase) did not appear to change, however.  The
>>>>  change in amplitude was quite obvious.
>>>>
>>>>  I don't know if there was rain on the day I looked at -- but perhaps
>>>>  dew on the window would do the trick ...
>>>>
>>>>  Rick
>>>>
>>>>  On 2021-08-27 11:11, Ken Sowinski wrote:
>>>>  On Fri, 27 Aug 2021, Rick Perley via evlatests wrote:
>>>>  Drew, et al.:
>>>>  Perfect!  Period shown in that C-band plot is about 100 MHz, which
>>>>  corresponds pretty closely to the expected value corresponding to
>>>>  the length of the horn.  Good evidence for a mismatch between the
>>>>  window and the OMT….
>>>>  The change in the sinusoiid should correlate with temperature.
>>>>  Might the reflections have been exacerbated by moisture from
>>>>  all the recent rain?
>>>
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