[evlatests] Bad Polarizers on ea03 and 13 at Ku-band

[Wes Grammer]

We've definitely had issues in the past with the cryo isolators on this 
band. But that shouldn't affect the polarization purity, just the noise 
temperature and gain (to a minor extent). And it would be confined to 
one channel.

On 5/31/2020 1:29 PM, Denis Urbain via evlatests wrote:
> Could this be a problem with a bad cryo isolator, we had isolators 
> brake due to thermal cycles.
>
> Denis
>
> On 5/30/2020 10:09 PM, Wes Grammer via evlatests wrote:
>> The receivers installed in ea03 and ea13 are U-013 and U-021, 
>> respectively. The last noise temperature measurements on these 
>> receivers in the lab (in 2018 and 2014) didn't show anything abnormal 
>> at the frequencies of these resonances. But that doesn't rule out a 
>> change in the interim.
>>
>> I suspect a strong cross-pol peak like this could be caused by 
>> undesired mode conversion in the waveguide components upstream of the 
>> LNAs. The reason is likely mechanical: something shifted, loosened, 
>> or broke with temperature cycling in the interval since they were 
>> installed. Possible culprits are the OMT (a gap in the mated halves, 
>> loose sidearm pins), or cracked or misaligned mating flanges, 
>> particularly at the thermal gap. We've seen this kind of thing happen 
>> on EVLA receivers before.
>>
>> The initial measurement of axial ratio for both these receivers 
>> looked good as well: it was the usual W-shaped curve, with a max of 1 
>> dB at 12 GHz and below 0.5 dB almost everywhere else.
>>
>> I would recommend Rob Long flag these for a check the next 
>> opportunity we have to swap them out. The problem should be evident 
>> in an AR test even at ambient temperature, if it's due to loose or 
>> misaligned waveguide flanges.
>>
>> Thanks for sharing the data, Rick!
>>
>> -Wes
>>
>> On 5/30/2020 4:24 PM, rperley--- via evlatests wrote:
>>> The Ku-band polarizers on the JVLA are in general really 
>>> outstanding.  For
>>> almost all antennas, the cross-polarization is below 4% over almost the
>>> entire 12 -- 18 GHz band.
>>>
>>> But two antennas, ea03 and ea13, clearly have something wrong with 
>>> their
>>> polarizers.  Both show cross-polarization of over 50% (!!!) at the high
>>> frequency end.  Both are fine below 16 GHz.
>>>
>>> I attach a plot to show the effect.  Yellow is ea03, blue is ea13.  The
>>> plot spans 16 -- 18 GHz.  At 16 GHz, the polarization is normal -- 
>>> about 2
>>> -- 3%.  But with increasing frequency, both show resonance-like 
>>> structure
>>> in the cross-polarization.
>>>
>>> The top panel shows the leakage from LCP into RCP, the bottom panel the
>>> leakage from RCP to LCP.  Each panel shows the phase (narrow plot on 
>>> top)
>>> and, below this, the voltage amplitude (in tens of percent, so 100 = 
>>> 10%.
>>>
>>> Rick
>>>
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>>
>>
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