[evlatests] Temperature-sensitive gains at Ku, K, Ka, and Q bands

[Michiel Brentjens]

The other Q-band holography observation that day was on 3C454.3, and began right after sunset. The PDIF values 
systematically *increased* over the folowing 6 hours, while the antennas cooled down.

- Michiel

On 10/31/2011 05:00 PM, Jim Jackson wrote:
> I just looked - the temperature inside one UX converter cycled over
> only about 5C over a 24 hr period.  I'll have to talk to the guys
> tomorrow about whether that could explain anything like this - I'm
> skeptical that it could.
>
> Jim
>
>
> At 02:04 PM 10/31/2011, Rick Perley wrote:
>>      The elevation-gain test taken early Saturday morning strongly
>> supports Michiel's evidence for temperature-sensitive gains -- and at
>> all the high frequency bands (Ku, K, Ka, and Q).
>>
>>      A strong change in PDif was noted in the tests taken the preceding
>> week -- of a source setting from 82 degrees elevation to elevation 8.
>> All high frequency PDif values *increased*, by typically 10%, during
>> this period.  It was difficult to be sure that the visibilities changed
>> by a similar amount, since the test was of a single source setting
>> during the period.
>>
>>      The data taken Saturday morning were of a source which was rising at
>> midnight, and transited at dawn.  Temperature effects were minimized,
>> while the elevation vs time was reversed (i.e., the source was rising,
>> rather than setting).  The PDif values for this run rise during the run
>> -- but by a much smaller amount than the previous week's test.
>>
>>      We can rule out an elevation dependency of the PDif.  And we can
>> strongly favor a temperature-dependent gain, since the vertex room
>> temperatures should considerably lag the external air temperature.  Both
>> of my runs were taken when the air temperature was declining (the first
>> one strongly so, the second one only slightly), while Michiel's test --
>> which showed the opposite variation in PDif -- was taken with strong
>> heating.
>>
>>      Application of the PDif values gives an elevation-dependence of the
>> gains which appears to be the same for both of my runs -- this
>> strengthens an explanation that the PDif changes are due to real gain
>> variations, and not due to a changing value of the noise diode power.
>>
>>      Rick
>>
>> -------- Original Message --------
>> Subject:        [evlatests] Q-band holography d.d. 2011-10-29
>> Date:   Mon, 31 Oct 2011 12:37:02 -0600
>> From:   Michiel Brentjens<mbrentje at nrao.edu>
>> To:     EVLA Tests<evlatests at nrao.edu>
>>
>>
>>
>> ** [2011-10-31 Mon 12:11] Q-band holography d.d. 2011-10-29
>>
>> Temperature sensitive gains?
>> ----------------------------
>>
>> A 3C273 Q-band holography observation from past Saturday shows a
>> systematic decrease of PDIF over the course of six hours in virtually
>> all antennas. This *decrease* in accompanied by a similar slow,
>> systematic *decrease* in visibility amplitude. Typically, PDIF
>> decreases by 20%. The visibility amplitudes also show a 20% decrease,
>> although they are much noisier than the PDIFs.
>>
>> The source had a fairly constant elevation between 36 and 57
>> degrees. The observation began at 08:00 MDT and ended at 14:00
>> MDT. Are we seeing a temperature dependent Q-band gain here? Notable
>> exception is IF 3C, which shows nicely constant PDIFs, yet also
>> decreases its amplitudes along with all others.
>>
>>
>> Other phenomena
>> ---------------
>>
>> For some reason, 11R lost all signal around 16:50 UTC. After 17:30
>> UTC, the PDIFs of 20L become increasingly more noisy, until they
>> suddenly snap together at a higher value around 19:05 UTC. 12A PDIFs
>> are consistent with noise. 4L PDIFs behave nicely until 17:00, at
>> which point they increase by 50% and begin behaving erratically.
>>
>> - Michiel
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