[evlatests] Ku-band Tcal stability over time

Fri Sep 4 11:04:29 EDT 2026

Hi Rick,

I know it's laborious, but would you be able to check antenna ea01 in Ku 
Band at 15.0 or 17.0 GHz in LCP instead of 13.9 GHz?

Ku S/N 016 on ea01 is an interesting case because the lab-measured and 
derived curves are quite different (see the first page of the PDF I 
circulated in a separate thread), and it's been on a long time since Dec 
2016. It just so happens that at 13.9 GHz the two curves cross and the 
numbers are almost the same around ~1.6 K. But at 12, 15, and 17 GHz 
they are close to a factor of two different. I'm very curious if you see 
the same overall values:

ea01 LCP (Ku Rcvr# 16)

12 GHz: 0.8 K
15 GHz: 2.8 K
17 GHz: 0.5 K

If not, then that means that something must be going wrong with the 
tipping scan method systematically for this receiver and antenna for 
some reason. Otherwise there's good agreement just eyeballing the values 
in your table against the charts.

Clear skies,
Brian

On 9/3/26 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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-- 
Brian E. Svoboda, PhD
Associate Scientist
National Radio Astronomy Observatory (NRAO)

Office: DSOC 312
Office Phone: +1 (575) 835-7246
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