[evlatests] Temporal Antenna Gain Changes

[Jim Jackson]

With regard to the 4 higher frequency receivers being worse and similar  - these all go through the T303 UX converter which is also mounted against an exterior wall.

There are two temp sensors in that unit, RCP_TEMP and LCP_TEMP,  though I'm not remembering exactly where they are in the box.

Cheers,
Jim

-----Original Message-----
From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of Rick Perley via evlatests
Sent: Tuesday, September 21, 2021 11:37 AM
To: evlatests at aoc.nrao.edu
Subject: [evlatests] Temporal Antenna Gain Changes

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