[evlatests] L-band Sensitivity, cont.

[Rick Perley]

    On April 20, I ran a sensitivity test in L-band covering the entire 
frequency range available to the VLA -- 1275 through 1750 MHz.  There 
were 7 different frequency pairs, using spectral line mode and 12.5 MHz 
BW.  I observed Cygnus A, cold sky, and the known flux density 
calibrator, 3C48.  The goal (then) was to calibrate the Tcals ( from 
measuring the apparent Tsys difference between Cyg A and cold sky, and 
assuming the efficiency as known), and to determine the actual 
sensitivity, from AIPS Weights on 3C48, and noise statistics on cold 
sky.   Note that all of these frequency settings used the same L301 
value:  12928 MHz.
    Other interests have prevented full calibration of these data. 
    Given the interest in frequency-dependent sensitivity, I recovered 
these data, and measured the AIPS weights on 3C48. 

    Only antennas 13 and 14 worked properly in this experiment.  Antenna 
16 gave fringes only on the IF 2 (B/D) side. 

    In the following table, I give the AIPS weights for EVLA antennas 
13, 14, and 16,  There are two values for the lowest 6 frequencies -- as 
we observed 3C48 twice at these frequencies.  A mystery is why the EVLA 
aips weights are so very different between these pairs -- which are 
separated by 90 minutes in time, and 9 degrees in elevation (from 65 to 
74 degrees).   Checking the Tsys values shows no difference in Tsys, so 
the apparent change in sensitivity is a real change in gain 
(efficiency), not due to an increase in Tsys.   

    Remember:  AIPS Weights are proportional to (efficiency/Tsys)^2.   
High values are good.    Typical VLA values are 80 to 100 in the primary 
frequency range of 1350 -- 1500 MHz.   'xx' means no data were 
obtained.  The values at 1565 MHz are dicy -- lots of RFI! 

                                           
13                                                 
14                                                         
16                                  
 Frequency          A           B          C          D      |   
A              B           C           D       |        A           
B             C          D      
-----------------------------------------------------------|----------------------------------------------|----------------------------------------------------
1275                98/76                 96/77               |   
96/63                   93/77                 |      
xx/xx                   xx/xx
1310                            105/79               98/87   
|             104/102                110/97    |                  
102/80                   89/73
1340                97/79                 93/77               | 
105/67                105/87                  |      
xx/xx                   xx/xx
1370                             107/67              104/88 
|              104/107               116/111  |                  
102/81                   87/73
1410                69/56                  57/70              | 
106/72                 111/94                 |     
xx/xx                    xx/xx
1440                             74/51                 72/61  
|                92/99                 108/93    |                  
82/70                     81/75
1470                   34                       26                |    
65                         83                    |      
xx                           xx
1510                                33          42                
|                   83                      78        
|                     64                         69
1565                   26                       23                |    
48                         58                    |      
xx                          xx
1590                                xx                      xx    
|                   68                      68        
|                     50                         66
1620                   25                       20                |    
36                          45                   |      
xx                          xx
1670                                13                       29   
|                   68                       70       
|                     52                         68
1706                   15                         8                |    
38                          45                   |      
xx                          xx
1750                                  3                       13   
|                   57                       55       
|                     43                        52
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------

    Conclusion:

    The general loss of sensitivity with increasing frequency is 
clear.   However, the decline is very different for different antennas, 
with antenna 13 showing (by far) the most extreme variation. 

    If there is time later today, I'll repeat this, with 4 (maybe 5 if 
antenna 24 comes up at L-band today) EVLA antennas.  We need to monitor 
the power levels into the sampler, to see if any of these variations are 
due to power level variations.