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

[Denis Urbain]

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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