[evlatests] Ku-band Tcal stability over time

[Wes Grammer]

Regarding the variation seen in L-band Tcals before 2019, that was the 
period we were doing solar cal upgrades to L, S, C, X, and Ku band 
receivers. On L-band specifically, all 30 receivers were upgraded with 
solar cal capability, which involved adding an amplifier and adjustable 
attenuators into the existing cal injection path. The Tcal levels were 
re-characterized after each upgrade.

The final receiver of 102 modified (L-011) was done on 6/13/2019, 
completing a 6 year-long campaign. That coincidentally lines up with 
what you observed in the data.

-Wes

On 9/3/2026 1:57 PM, Rick Perley via evlatests wrote:
> 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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