[evlatests] ea28 L-band Polarizer

[Rick Perley]

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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