[evlatests] C-band Test Results

[Rick Perley]

    Bob Hayward and I spent yesterday on antenna 24 measuring the C-band 
efficiency and system temperature.  This antenna is a fully compliant 
EVLA antenna, with the new horn, new receivers, and final OMT. 
    On-Off observations of Cygnus A were used to determine the 
efficiency.   A resolution correction is needed -- this varies from 2% 
at the lowest frequency to 9% at the highest. 
    Dips from 90 to 8 degrees were made to measure atmosphere and 
spillover variations.
    A hot load and cold sky observation were used to calibrate the 
system gain.  Because a proper liquid nitrogen cold load is not feasible 
for this band (the horn is too big!), we have to assume a value for the 
spillover when observing cold sky.  These results assumed a total of 15K 
from all outside sources.  A 5 degree variation in this has little 
effect (about 2%) on the  derived values of Tsys, but a large effect on 
the derived value of Tr.  The assumed value of 15K gives Tr values close 
to those measured in the lab -- support for our spillover value being 
about right. 

    The results are given in the attached table.  These are preliminary 
results, but I don't expect them to change much after more careful 
calibration.  The results are in good agreement with those from analysis 
of corrrelator coefficients:  Tsys/effic is about 50K for frequencies 
below 6 GHz, and close to 40K for frequencies above 6 GHz.  (Multiply by 
5.62 to get to SEFD). 




   
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