[evlatests] Ku-band Tcal stability over time

[Rick Perley]

The procedure to generate 'sky-calibrated' Tcals for our noise diodes allows investigation of long-term stability.
The results for Ku-band are especially illuminating.

As described earlier, we use the known flux density of 3C286 to generate corrected values for the noise diode temperature.

For Ku-band, there are 14 antennas whose receivers have not changed since 2019.  I show below the derived values of Tcal at 13.936 GHz.

Ant  Rcvr#  2019Jan31   2019Oct24    2021Sep09   2023Oct03   2025Jan31   2025Mar09
                               RCP   LCP     RCP   LCP     RCP   LCP      RCP   LCP      RCP   LCP      RCP   LCP
--------------------------------------------------------------------------------------------------------------------------------------------------------
01    16    1.56   1.85     1.65   1.86     1.52   1.74     1.53   1.73    -    -       1.51   1.80
03    13    0.70   0.78     0.69   0.81     0.68   0.75     0.69   0.80     0.67   0.79     0.69   0.79
04    17    1.93   1.88     1.99   1.94     1.89   1.84     2.00   1.87     2.00   1.88     2.00   1.85
05    14    2.23   2.35     2.32   2.41   —-        ---       2.37   2.48     2.42   2.44   --         --
07    05    1.79   1.82     1.78   1.78     1.86   1.81         --       --         1.41   1.25     1.86   1.82
09    19    1.79   1.77        —       --         1.69   1.73     1.70   1.76     1.81   1.82     1.71   1.74
11    27    1.30   1.26     1.30   1.30     1.31   1.45     1.33   1.41     1.34   1.39     1.34   1.44
15    28    1.24   1.30     1.27   1.36     1.40   1.46     1.45   1.57     1.38   1.47     1.33   1.41
18    12    1.89   1.87     1.99   2.00     2.25   2.29     2.34   2.39     2.32   2.34     2.28   2.31
19    02    1.90   1.90     3.58   3.18     1.50   1.42     1.58   1.50     1.52   1.47     1.56   1.47   Values in 2019 questionable
22    07    3.00   2.99     2.91   3.04     3.06   2.82     3.20   2.97     3.19   2.98     2.99   2.89
23    24    1.70   1.71     1.68   1.74     1.70   1.68     1.74   1.67     1.75   1.67     1.73   1.66
26    25    1.24   1.15     1.19   1.20     1.17   1.17     1.20   1.16     1.25   1.16     1.19   1.18
28    06       —-      ---       1.47   1.35     1.51   1.43     1.55   1.43     1.51   1.44     1.51   1.45
----------------------------------------------------------------------------------------------------------------------

Except for ea19 (whose very different values in 2019 have no ready explanation), the rms stability over time, for a given antenna/receiver/polarization is between 2 and 4%.  Curiously, for the four antennas with the same receivers prior to 2019 (dating back to 2014) show Tcals about 10% lower than the numbers shown above.  Other bands, particularly L-band, show considerable variations in their derived Tcals for dates before 2019.   After this date, all bands that I've investigated so far (L through Ku) show similarly to the numbers above, for nearly all antennas.

The few percent variation from year to year for a given antenna is easily accounted for by temperature variations of the switched diode, pointing variations, and sky opacity differing from the simple model used in the gain calibration.


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