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

[Rick Perley]

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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