[evlatests] EVLA L-band Sensitivity

[Rick Perley]

    Many reports have shown that the EVLA antennas have a poorer 
sensitivity at high elevations than the VLA antennas.  To get a better 
quantitative look at this, I examined some recent data taken at a range 
of elevations, from 20 through 67 degrees, at a frequency of 1425 MHz. 

    There is a striking difference in the *range* of sensitivities for 
VLA and for EVLA antennas.  The VLA antennas are strikingly uniform -- 
the range of Tsys/efficiency amongst the antennas is no more than about 
20%.  For the EVLA antennas, the range is much higher -- currently about 
a factor of four (due to antennas 17 and 21 having either anomalously 
high Tsys, or some other malfunction). 

    This observation encouraged me to use only the best antennas for 
each of the arrays, using the argument that these are the 'poster 
children' for the array capabilities, and that those currently much 
poorer (on the EVLA) are due to maintenance or level tweaking issues 
that we will address shortly. 

    I thus looked at the sensitivities of the best three antennas on the 
VLA (2, 8, and 27), and the best three on the EVLA (1, 18, and 24), at 
1425 MHz, at four elevations:  20, 32, 38, and 67 degrees.  Below is the 
*ratio* of Tsys/Efficiency (or, SEFD, if you prefer) for these antennas 
at the elevations listed.  A ratio greater than one means the EVLA is 
*more* sensitive (i.e., has a lower SEFD). 

    Elevation      Ratio
------------------------------
        20              1.21  
        32              1.06
        38              1.04
        67              0.92
-----------------------------

    I would have obtained the same results had I used the 'best 6', or 
even the 'best 9' antennas in each array. 

    This demonstrates that the EVLA will in general outperform the VLA 
at elevations lower than about 40 degrees. 

    There is some variation with frequency in these ratios -- the 
subject of some future studies, perhaps.