[evlatests] L-Band Sensitivity, Tsys and Efficiency

[Rick Perley]

    I used an hour of 'Dynamic Time' yesterday afternoon to run some 
more tests on the peculiar frequency dependence of L-band sensitivity. 

    Antennas 14, 16, and 18 were available (13 was 'under repair', and 
24 does not have its LSC converter installed yet). 
I observed the standard calibrator 3C147 in the  usual spectral line 
mode (12.5 MHz bandwidth, 16 channels per correlation, mode '4') at 3 
pairs of frequencies:  1275/1375, 1440/1510, and 1670/1706 MHz 
(AC/BD).    I also observed a piece of blank sky, 5 degrees away from 
3C147, to permit calculation of the true system noise. 

    Data quality from the EVLA antennas was simply outstanding.  No 
flagging needed in any IF.  No drops or spins seen.  Only one case of 
misbehavior to report :  Ant 14, LCP, (IFs C and D) was very weak at the 
last frequency pair (1670/1706 MHz).  No cause is known.   The 
subsequent report does not include these weak data. 

    The data were calibrated with the correct flux density of 3C147 for 
each frequency.  Bandpasses were calculated and applied.  AIPS weights 
were then calculated for the central channel.  To check that these 
weights can be safely used as a proxy for real sensitivity, I generated 
noise histograms for both VLA-VLA baselines, and for EVLA-EVLA 
baselines.  In all cases, these noise histograms (which are beautiful 
gaussians) confirm the validity of AIPS weights.  (It's almost like 
magic -- but I think this actually means the Tsys system is working well 
for EVLA antennas). 

    I again remind readers that AIPS weights are proportional to 
(antenna efficiency/Tsys)^2.  A high number is good.   The actual values 
given below are not important -- the ranking, mean, and spread are. 

    Results:

    I used the convenient AIPS program ANBPL to calculate the 
antenna-based AIPS weights, applying all calibrations.  I then made 
'sensitivity histrograms', of all VLA and EVLA antennas.  Some very 
interesting results came from this:

    At 1275 MHz:  VLA antennas span a wide range in weights:  from 8 to 
20, with a median of about 14.  EVLA antennas 14 and 18 are at the very 
top -- weights of 21 to 23.  Antenna 16 is a little worse -- weight = 
18, which is still much better than the VLA median. 

    At 1375 MHz:  VLA antennas span a range of 11 to 23, with median of 
17 -- the array is notably more sensitive at this frequency.  The EVLA 
antennas however slip considerably in their ranking:  16 is in the 
bottom third, 14 is barely in the upper half, but 18 is still near the 
best. 

    At 1440 MHz, the VLA median weight is about 15 (down a bit from 
1375), and the rankings of EVLA antennas continue to slip:  16 is at the 
bottom (weight 11), 14 below the median (weight 14), 18 is now the 5th 
best antenna. 

    At 1510 MHz, the VLA median is now at 13, antenna 16 remains at the 
bottom, 14 and 18 are both in the middle of the distribution. 

    At 1670 MHz, the VLA median continues to drop -- now at 11, and all 
the EVLA antennas are below the median.  However (and this is different 
than in past measures) -- the EVLA antennas are not below the bottom of 
the VLA distribution. 

    Finally, at 1706 MHz, the VLA median is down to 10 (in other words, 
the array sensitivity is now 75% of its best value near 1375 MHz), and 
the EVLA antennas are all in the bottom half, but not worse than the 
poorest VLA antennas. 

    Summarizing the above:  The trend noted many times before of the 
EVLA antennas have very good performance at low frequencies, with 
decreasing sensitivity, (both absolutely, and relative to VLA antennas) 
is confirmed.  What is different in this test is that at no frequency 
were any of the EVLA antennas found to be worse than the worst VLA 
antennas.    I can't explain why the relative sensitivities of the EVLA 
antennas are better than we have seen in the past at these higher 
frequencies. 

    I also observed the absolute flux density calibrator, 3C274 (aka 
Virgo A).  This source provides enough flux to raise the system 
temperature by an amount (typically 20K) far greater than the noise in 
these measurements and variability in ground pickup and atmospheric 
emission, so an attempt can be made at determining antenna efficiency.  
    In the following, I analyzed only the RCP channel 'A' (1275, 1440, 
and 1670 MHz).  There are no reasons to believe the results will be 
different in other channels. 

    The expected antenna temperature (that is, the change in system 
temperature when going onto Virgo A) is 17.8, 16.0, and 14.1 kelvin at 
1275, 1440, and 1670 MHz) assuming an efficiency of 0.43 (which was 
measured by Bob Hayward and me two years ago on antenna 13), and the 
published Baars et al. flux (232, 209 and 184 Jy, at 1275, 1440, and 
1670 MHz, respectively).    Virgo A has a large halo, contributing 
significant flux on a scale comparable to the antenna beam --  I have 
ignored this effect in what follows.  I have also ignored the 
contribution of 'cold sky' -- this should be small for  Virgo, which 
sits far from the galactic plane.
   
    The observed change in Tsys must be corrected for the error in Tcal 
-- the VLA system knows only the value of Tcal at the default frequency 
of 1465/1385 MHz.  The Tcal values have been carefully measured by the 
front-end group, and I've made these necessary adjustments for each of 
the frequencies, for each antenna.  (I trust that the final EVLA system 
will do all this for me!) 

    The resulting rise in Tsys (= Tant) can be interpreted in two 
distinct ways:

    a) Assume the efficiency is known, and use Tant to calculate Tsys 
(cold sky), and Tcal. 
    b) Assume the Tcals are correct, and use Tant to calculate the true 
efficiency. 

    I give both answers in the following.

    At 1275 MHz:  We have at this frequency an 'embarassment of 
riches'.  The system is better than we expect. 
      
        a) Assuming the efficiency really is 0.43 (as measured on 
antenna 13), we find:

     Antenna 14A:  Tsys = 24 (rather than the system's claim of 33).
     Antenna 16A:  Tsys = 25 (rather than 31)
     Antenna 18A:  Tsys = 25 (rather than 32). 

    Applying a beam dilution factor will lower these system temperatures. 

          All of these Tsys values are better than expected. 

       b) Assuming the Tant values are correct (meaning the Tcals are 
correct, and ignoring beam dilution), we find efficiencies of 0.58, 
0.53, and 0.53 for 14, 16, and 18, respectively.  These are much better 
than the measured 0.43 -- but I note that this measurement was made on 
antenna 13.   Applying a beam dilution correction will (by the way) make 
these efficiencies even higher!
    I'll note here that all these Tsys values are 'back-end', and for 
reasons not clear, they are higher than the corresponding front-end 
values (for VLA antennas -- there is no 'front-end' Tsys on EVLA 
antennas yet).  The effect is not believed to reflect a true rise in 
Tsys -- it is likely an error in the VLA's system.  The scale of the 
effect is I think 5 to 10%, which would decrease the efficiencies 
derived above by the same factor.  Not enough, however, to lower them to 
0.43. 

    At 1440 MHz: 

    a) The system temperatures are 24, 26, and 22 K, for an assumed 
efficiency of 0.43.  The Tsys values given by the system are 24, 33, and 
19, for antennas 14, 16, and 18, respectively. 
    b) The efficiencies are 0.43, 0.54, and 0.37 for the three antennas, 
if we assume the Tsys values are correct.  These values are disturbingly 
different, but they reflect the variation in Tsys. 

    At 1670 MHz:

     a) Tsys values are 24, 30, and 26, for antennas 14, 16, and 18, 
rather than the listed values of 24, 30, and 26 K (after correction for 
Tcal error). 
    b) Efficiencies are 0.46, 0.49, and 0.43, assuming the Tsys values 
are correct. 

    The three antennas are in decent agreement, and all are performing 
at about the expected level. 

    So:  From these measurements, it seems the real mystery is why the 
EVLA antennas are so good at low frequencies (where the efficiency is 
expected to be decreasing).  The next question is why the EVLA antennas' 
performance is not improving at the higher frequencies.