[evlatests] ea28 L-band Polarizer -- All Better!

[Rob Long]

Thanks Rick,

We're looking closely at the L-band in the lab.

Rob

On 3/17/2022 2:58 PM, Rick Perley wrote:
> Attached is the new plot (amplitudes only) for ea28 at Lband, from 
> observations taken earlier this morning.
>
> It's now completely normal.  Note that the scale runs from 0 to 10% 
> (the former plot tan from 0 to 60%).
>
> I have no idea how or why it's better -- but it is ...
>
> Rick
>
> On 3/17/22 11:49, Rob Long wrote:
>> Hi Rick,
>>
>> The OMT on the L-band receiver from ea28 was definitely built 
>> backwards. Wes, Dan and I discussed this and we are a bit confused as 
>> to how this could have gone undetected for so long (or your current 
>> test results). Lets definitely check ea28 again and verify that 
>> everything looks normal now.
>>
>> Anyway, fyi.
>> Rob
>>
>> On 3/17/2022 10:02 AM, Rick Perley wrote:
>>>  Rob:
>>>
>>> This is hard to understand -- the polarizations are right 'in the 
>>> end' -- so there must have been a 'counter-swap' elsewhere. If one 
>>> antenna has reversed polarization, and the others do not, the result 
>>> is very easy to see in the data. This is not seen.
>>>
>>> And this certainly won't explain why the cross-polarization is so 
>>> bad at the low frequency end, but ok at the high frequency end...
>>>
>>> Rick
>>>
>>> On 3/17/22 09:41, Rob Long wrote:
>>>> Hi Rick,
>>>>
>>>> We put this L-band on the bench and discovered that the 
>>>> polarizations are actually swapped. We aren't sure how this 
>>>> receiver was missed, but it is very clearly cross-pol'd. We are 
>>>> going to verify the OMT, hybrid and retest the receiver, but it's 
>>>> also strange that you only saw the problem below 1.3 GHz.
>>>>
>>>> It would be interesting to verify that everything is good in ea28 now.
>>>>
>>>> Rob
>>>>
>>>> On 3/15/2022 8:45 AM, Rick Perley wrote:
>>>>> Rob, et al.:
>>>>>
>>>>> Attached is a plot of the cross-polarization.  The narrower plots 
>>>>> are the phases, the larger plots are the amplitudes, with y-axis 
>>>>> in 'milli-fraction'. RCP above, LCP below.   So, '600' means 60%. 
>>>>> You can see the cross polarization reaches 70% at 1.02 GHz.
>>>>>
>>>>> The 'spiky' polarizations are due to RFI -- I made no attempt to 
>>>>> clean this up.
>>>>>
>>>>> No other antennas look like this.
>>>>>
>>>>> Rick
>>>>>
>>>>> On 3/15/22 08:37, Rob Long wrote:
>>>>>> Thanks Rick! We thought it was worth a look as a first pass. We 
>>>>>> will be swapping the L-band receiver on maintenance day.
>>>>>>
>>>>>> Rob
>>>>>>
>>>>>> On 3/15/2022 8:32 AM, Rick Perley via evlatests wrote:
>>>>>>> Removing the RFI monitoring equipment made no difference to the 
>>>>>>> L-band polarization problem on ea28. Observations last evening 
>>>>>>> show the same signature -- cross-polarization over 60% at 1.0 
>>>>>>> GHz, declining to normal values (~5%) around 1400 MHz, and 
>>>>>>> staying near that level up to 2 GHz.
>>>>>>>
>>>>>>> Rick
>>>>>>>
>>>>>>> On 3/14/22 16:00, Dan Dillon via evlatests wrote:
>>>>>>>> We have made a change and removed the RFI monitoring equipment 
>>>>>>>> from the L-band to help isolate the problem.  Can we repeat the 
>>>>>>>> testing to see if this made any difference before we just 
>>>>>>>> replace the receiver?
>>>>>>>>
>>>>>>>> On 3/13/2022 9:07 AM, Wes Grammer wrote:
>>>>>>>>> That definitely sounds like a receiver issue, either with the 
>>>>>>>>> OMT or (more likely) the quad hybrid coupler that immediately 
>>>>>>>>> follows it. This L-band should come back to the lab for 
>>>>>>>>> evaluation and repair.
>>>>>>>>>
>>>>>>>>> On 3/12/2022 12:40 PM, Frank Schinzel via evlatests wrote:
>>>>>>>>>> I second this observation. I have been seeing this behavior 
>>>>>>>>>> of ea28 at L-band in 5 independent observations since 
>>>>>>>>>> February 26.
>>>>>>>>>>
>>>>>>>>>> On 3/12/2022 12:19 PM, Rick Perley via evlatests wrote:
>>>>>>>>>>> Something is seriously amiss with the L-band polarizer in ea28.
>>>>>>>>>>>
>>>>>>>>>>> Normal cross-polarizations are typically 5%, and never more 
>>>>>>>>>>> than 10%.  For ea28, these values are seen above 1300 MHz, 
>>>>>>>>>>> but below this, the cross-polarization rises quickly to more 
>>>>>>>>>>> than 50% at 1050 MHz.
>>>>>>>>>>>
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