[evlatests] C-Band Sensitivity Issues

[Charles Kutz]

 >It is probable that the existing Tcal values are incorrect, leading
 >to incorrect Tsys.

If someone can dump the C-Band Tcal values out of the data base for me, 
I will cheerfully investigate and resolve this issue.


Chuck

Rick Perley wrote:
>     The data shown by Emmanuel yesterday morning indicated that many or 
> most EVLA antennas are performing far short of requirements at C-band, 
> and there is some evidence that the more recent antennas are less 
> sensitivity that the older ones.
>     I took some data yesterday afternoon at L, C and X bands to 
> investigate this issue a bit.  The primary goals were to see if the 
> spread in sensitivity could be correlated with system temperature, and 
> if any correlations could be seen in performance with band. 
> 
>     A)  X-Band.  This band is useful as a diagnostic, as the receivers 
> are unchanged between EVLA and VLA, and there is minimal electronics. 
>     The EVLA sensitivity requirement is for the antenna SEFD to be 300 
> Jy (or, Tsys/effic. = 53K). 
>     The data from yesterday show that eleven antennas (9 EVLA, 2 VLA) 
> antennas are performing near, or better than, this level.  Recent EVLA 
> antennas numbered 1, 2, and 4 are in this group.  Eight antennas (5 VLA, 
> 3 EVLA) are high by about 10% (not bad) -- recent EVLA antenna #1 is in 
> this group.  Five antennas are high by 20 to 30% (3 VLA, 2 EVLA). 
>     Two antennas are particularly poor:
>     17 is high by a factor of about two on all four IFs.   The Tsys is 
> claimed to be 130K, so this is the likely cause.
>     25 is high by a factor of nearly three.  Tsys is claimed to be 25K 
> -- so the receiver is likely o.k. 
>    
>     Overall -- EVLA performance is better than VLA, and although the 
> mean sensitivity is a bit worse than spec (320 Jy, cf. requirement of 
> 300 Jy), there's no reason for alarm. 
> 
>     B) C-Band.  Here the EVLA requirement for SEFD is 245 Jy (this is a 
> tough spec!), equivalent to Tsys/effic = 44K. 
> 
>     As noted yesterday, *none* of the EVLA antennas meet this spec -- 
> not even close.  The best four antennas are all 'old' EVLA antennas (13, 
> 14, 16 and 18) -- their mean SEFD is 310 Jy.    The average SEFD over 
> all the EVLA antennas (excluding two which are clearly discrepant) is 
> about 380 Jy.  Pretty poor. 
>     The mean SEFD amongst VLA antennas is definitely worse than that of 
> the EVLA -- about 440 Jy, with a remarkable spread, from about 350 Jy 
> (for 27, 20, and 22) to 540 Jy (6, 8, 10, and 15).  There is good 
> evidence that most of the variation amongst the VLA antennas is due to 
> system temperature, as there is a good correlation between the claimed 
> Tsys and the SEFD values. 
> 
>     I have searched for a similar correlation between SEFD and Tsys for 
> EVLA antennas -- and although I believe there is one, it's crystal clear 
> that the recorded values of Tsys are only approximately correct:
>     a) The four best EVLA antennas all have  believable Tsys values -- 
> about 30K. 
>     b) All other EVLA antennas have recorded Tsys values which are far 
> too high -- typically 80 to 100 K.  But these are not believable numbers 
> -- (the SEFDs are not THAT bad), and there is little correlation between 
> the Tsys values and the SEFDs. 
> 
>     It is probable that the existing Tcal values are incorrect, leading 
> to incorrect Tsys.    It is difficult to diagnose problems in 
> sensitivity without believable Tsys values, so I think it important that 
> we get modern, correct values of the Tcals into the EVLA parameters 
> database. 
> 
>     Three antennas are particularly poor, and could use quick attention:
> 
>     12:  SEFD = 700,  with a (believable) Tsys = 75.  (
>      19:  SEFD = 900 (four times too high!), with a claimed Tsys of 190 
> -- this could be close to correct.
>     24:  SEFD = 600, with a claimed Tsys of 50 (better than most!). 
> 
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