[evlatests] ea28 L-band Polarizer

[Rob Long]

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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