[evlatests] Sensitivity, and the L301s and L302s

[Rick Perley]

    Previous observations have suggested that the settings of the L301 
and/or L302 might be influencing the EVLA sensitivity.  With with 
discovery of strong birdies floating about within the T304, we might 
expect that the setting of the L302 may be a factor. 

    I drew up an experiment to test this idea.  We observed at two 
frequencies within C-band -- 4885 and 4629 MHz, separated by 256 MHz, so 
that each can be tuned with different combinations of the same L301 and 
L302 settings.  For example, 4885 MHz (with 12.5 Mhz BW) can be tuned 
to   with L301 and L302 settings of 14976/12977, and 14720/12721 MHz.  
Similarly, 4629 MHz is obtained with 14976/13233 or 14720/12977 settings 
of the L301/L302. 

    I optimistically expected a clear resolution of our variable, 
frequency/IF dependent sensitivity.  I was disappointed.    In fact, no 
significant variation in sensitivity was found by *any* of the above 
settings, in any combination of the two IF pairs.  To within a few 
percent, the sensitivities are the same at both frequencies, and at both 
IFs.  The only clear effect is that antenna 16 is less sensitive than 13 
or 14 (antenna 18 is out at C-band) -- but we already knew that. 

    As always, antennas 13 and 14 are the best in the array. 

    My next tests will likely try repeat the C-band results I reported a 
couple of weeks ago, and to try a variation of the variable L301/L302 
test at L-band, where the difference between 1384 and 1484 MHz has been 
very marked, and very repeatable with the standard settings.