[evlatests] Temporal Antenna Gain Changes

[Rick Perley]

The 24-hour flux density run allows us to check system performance in 
many ways.  One of these is checking antenna gain stability over time.

The data for this epoch was taken on a day with a large temporal 
temperature change -- from about +10C at dawn to +30 in the afternoon.  
As is known from past observations, this induces a large gain change in 
the antenna signals.  The effect varies dramatically with band.  The 
following table, made from examination of the switched power, shows the 
effect.  Shown is the maximum gain range observed over the 24 hour 
period.  Nearly all antennas show the same pattern and ratio.

Band    Gain ratio (max/min)

L            <1

S              3

C             10

X              8

Ku             20

K               20

Ka             25

Q              20

-----------------------------------------------------

Note that the high frequency bands are much more affected.  The working 
theory is that the gain changes are due to the temperature sensitivity 
of the post-amps, which are mounted directly onto the feed horns, which 
are acting as good thermal conductors to the outside temperature.

Attached are two plots showing the effect on the gains at Ku-band.  The 
first ('TempWind.png') shows the wind and outside temperature during the 
24-hour run.  The second ('TempGain.png') shows the Ku-band gain over 
the same period.  (These are *voltage* gains, and a low value means a 
higher signal output).  Note the lag between the outside temperature 
minimum and maximum gain (minimum gain voltage) -- in keeping with the 
idea that it's the post-amp temperature change .  The scatter in the 
voltage gain plots are due to the range in elevations over which the 
data were taken (10 to 85 degrees) -- this is an increasingly important 
effect at the higher frequencies.  The four antennas shown were selected 
on the basis of their flat gain curves.  (Other antennas, like ea15, 
ea18, and ea19 are *much* worse).

The switched power mechanism has been shown (from past runs) to be 
highly effective in removing these temperature-induced gain variations.  
I have not attempted to use this for these data yet, as the system was 
not operating for one of the antennas, and deriving gain curves was one 
of the major goals of this experiment.

*R-L Phase Changes*

VLA polarimetric calibration *requires* at least one antenna whose R-L 
phase in stable.  A change in the R-L phase of the phase reference 
antenna is equivalent to rotating the source polarization over time.  
Finding the most stable antenna is often quite a chore.

For the K-band data, I've looked at the R-L stability.  Using ea28 as 
the phase reference, the following two plots show four antennas showing  
small changes (clearly looking like a temperature induced instability) 
(GoodAnts.png), and four antenna with dramatic changes in the R-L phase 
over time (BadAnts.png). Using any of these antennas for the initial 
phase reference antenna will dramatically degrade the polarization 
calibration accuracy (and the subsequent polarimetric imaging).

No obvious origin for the instability in these four antennas has yet 
been identified.

Rick


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