[evlatests] Ku-band Tcal stability over time
Claire Chandler
cchandle at nrao.edu
Fri Sep 4 12:23:19 EDT 2026
Hi Brian,
My test last week covered all Ku-band, I'm working on analyzing those data and will send the results to this list when available.
Best,
Claire
________________________________
From: evlatests <evlatests-bounces at listmgr.nrao.edu> on behalf of Brian Svoboda via evlatests <evlatests at listmgr.nrao.edu>
Sent: Friday, September 4, 2026 9:04 AM
To: evlatests at listmgr.nrao.edu <evlatests at listmgr.nrao.edu>
Subject: Re: [evlatests] Ku-band Tcal stability over time
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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