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

[Rick Perley]

    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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