[evlatests] Strange differential bandpasses

[Barry Clark]

How about a fan?

On 9/30/2022 12:28 PM, Edward Starr via evlatests wrote:
> 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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