[evlatests] S-Band Sensitivities

[rperley at nrao.edu]

Bob Hayward and I, assisted by Bob Stupak, spent most of last Friday
determining the S-band sensitivities, using the standard methods.  We
tuned to 9 different frequencies, between 1.928 and 3.976 GHz, using a 200
MHz-wide filter.  A full report will follow, but the essential results are
given in the table below.  Note that in the absence of a calibrated cold
load, we have to assume a cold-sky temperature.  This we have taken to be
15K (3 from the CMB, 1 from the atmosphere, and 11 from ground spillover).
 A variation of 5K either way makes no significant difference to the
determination of system temperature.

The efficiency is determined from on-off observations of Cygnus A.

These are preliminary results.

Freq.   Trec    Tsys   Tcal   Effic.
--------------------------------------
1.928    23      38    1.36    0.52
2.184    21      36    1.49    0.56
2.440    19      34    1.33    0.52
2.696    16      31    1.32    0.51
2.952    15      30    1.36    0.53
3.208    14      29    1.34    0.54
3.464    12      27    1.25    0.55
3.720    12      27    1.19    0.53
3.976    14      29    1.04    0.54
-------------------------------------

These values of Trec are about 5 K higher than those reported by Gordon --
to have our values equal his will require a spillover contribution about
5K higher (i.e. about 15K) -- possible but unlikely, I think.  Without a
calibrated cold load (quite impossible with a 4-foot wide horn!), we
cannot measure the difference.

Sky dips show the expected, high spillover -- starting at about an
elevation of 30 degrees.  At eight degrees elevation, the system
temperature is nearly double the value at the zenith.

RFI is very strong at 2.184 and 2.440 GHz due to satellite radio, plus one
other unidentified source.  There is also good evidence of RFI in the top
two tunings.