[evlatests] Elevation Gain Curves

[Rick Perley]

    I have determined the gain curves for most of the VLA antennas for 
all bands where there are receivers.  The gain coefficients have been 
sent to Joe and Bryan, for distribution to the appropriate places.  I 
can send a 'personal copy' to anybody who wants these. 

    A few comments on these coefficients:

    1) They were determined from observations of a single sources while 
it rose from 8 degrees through 87 degrees elevation.  The data were 
taken from midnight to just before dawn on a calm and clear night -- by 
doing this we minimized the residual temperature sensitivity. 
    2) The data utilized the PDif monitor values to remove (generally, 
quite effectively) any amplifier or band switch gain variations.  A 
small temperature variation of the noise diode potentially remains -- 
we'll not know the effect of this until more tests are done.  However, 
for the high frequency bands, I expect this residual variation to be 
small compared to the variation in actual antenna gain. 
    3) Antenna 19 was out of the array, and antenna 20 was stowed by 
operations after only 1 hour -- hence there are no coefficients 
available for these. 
    4) For nearly all antennas, the elevation dependence for the gain is 
very low (2% or less) at L, S, C, and X bands. 
    5) The gain variations for the high frequency bands is generally 
parabolic, with the maximum gain near 50 degrees -- as we intend it to 
be.  But there are some interesting variations, with a few antennas -- 
notably 11, 15, 19, and 26, showing much larger than normal gain 
dependency at the high frequency bands.  This is almost certainly due to 
incorrect subreflector coefficients for the 'subreflector rotation 
trick' -- we will try correct these errors with measurements taken later 
this winter. 

    To illustrate the range of behavior, I attach two plots -- for ea16 
and ea18.  The former shows a 'normal' antenna gain, the latter one of 
the more extreme. 


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