[evlatests] ea23 returns, slowly

[Robert Hayward]

Gordon,

I thought I did exactly that this morning when I showed you the "pseudo"
axial ratio plots that I had measured on the ellipticity test stand on
Friday which compared the good polarizer from the Arecibo receiver and the
poor polarizer from the 4th VLBA receiver. I then started mixing and
matching components from the two to find out what was needed to make the
VLBA polarizer better.

The polarizer from the 4th VLBA receiver (i.e., the T-gap + OMT +
phase-trimmed cables + hybrid) was always poor unless I swapped in the OMT
(and only the OMT) from the Arecibo polarizer. With this OMT, it looked
equally fine no matter which hybrid I used. This tells me that it is the
OMT out of the VLBA receiver that is different from the Arecibo OMT (and
thus "not created equal").

On top of that, I have tested all of the hybrids remaining from our 46
units - 3 of them in the 2nd VLBA receiver, 20 of them in the 3rd VLBA
receiver and the last 21 in the Arecibo receiver. I have found no
anomalous hybrids.

I also played with introducing gaps in the RF tree to see if I could make
the good axial ratio in the Arecibo receiver worse. This includes forcing
unrealistically large gaps between the Top Plate and the T-Gap assembly,
and between the T-Gap and the OMT, plus making the gap in the T-Gap
assembly too big. None of this changed the crossover points seen on the
axial ratio.

Since we've not had any spare OMTs to play with, the first real chance
I've had to do a proper good vs. bad set of mix & match measurements was
when I reluctantly tore the Arecibo receiver apart to do this test.

I honestly don't know what exactly is different with the OMT out of the
4th receiver, I only know that it is different than the OMT that was in
the 1st VLBA receiver and the one from the Arecibo receiver. The OMTs in
the 2nd, 3rd and 4th VLBA receivers are unfortunately less good in a
circular polarizer.

I'm hoping you can tell us exactly what is different and hopefully what we
need to do to make them all perform identically good.

-Bob


> Bob,
>
> Please substantiate claim (4) with data.
>
> Regards,
>
> Dr. Gordon M. Coutts, PhD
> Associate Scientist, Research Engineer
> National Radio Astronomy Observatory
>
>
> On Sep 6, 2011, at 4:32 PM, Bob Hayward <rhayward at nrao.edu> wrote:
>
>> Chuck Kutz wrote:
>>> The Ku receiver is waiting on Hayward for test.
>>>
>>
>> And Hayward has been...
>>
>> 1) testing & debugging the first L-Band Solar Receiver, slated to be
>> installed on Ant 24 in less than 2 weeks.
>>
>> 2) testing & debugging the latest upgraded VLBA C-Band receiver, slated
>> to be shipped to Brewster on Sept 19th.
>>
>> 3) testing & debugging the new improved EVLA-style C-Band receiver,
>> which has a new thermal gap assembly from the VLBA installed in it (that
>>  will knock about 5K off the T(rx) at the low-end) and which needs to
>> be ready by Sept 19th when we have a couple of engineers visiting from
>> Arecibo who want to look at the receiver (and hopefully buy one).
>>
>> 4) sorting out annoying axial ratio problems in the C-Band polarizers
>> (it seems all OMTs may not be created equal).
>>
>> 5) preparing for Thursday's "Dog & Pony" show.
>>
>> 6) and writing stupid memos like this.
>>
>> I'll try to fit the Ku-Band S/N 15 receiver testing in when I can find
>> the time over the next few days - unless someone wants any or all of the
>> above dates to slip.
>>
>> -Bob
>>
>>
>>> -----Original Message-----
>>> From: evlatests-bounces at nrao.edu [mailto:evlatests-bounces at nrao.edu] On
>>> Behalf Of Ken Sowinski
>>> Sent: Tuesday, September 06, 2011 3:18 PM
>>> To: Ken Sowinski
>>> Cc: evlatests at aoc.nrao.edu
>>> Subject: Re: [evlatests] ea23 returns, slowly
>>>
>>> On Tue, 6 Sep 2011, Ken Sowinski wrote:
>>>
>>>> X and L band receivers are not yet installed
>>>> S band works
>>>> C band works, but the polarizations are backwards
>>>
>>> C band cables have been reversed; pointing will be done tonight.
>>>
>>>> Ku band rotation setting is unknown
>>>
>>> A rotation setting has been provided, but there seems to
>>> be no receiver.  the downconverters show no power at Ku band.
>>>
>>>> K band works, but is very weak, probably because of poor pointing
>>>> Ka and Q bands not examined
>>>>
>>>> If C band cables are swapped today we can do pointing tonight.
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