[evlatests] X and C Band sensitivity and efficiency tests, basic results (and a couple of mysteries ...)

[Rick Perley]

    Bob Hayward and I spent two days at the site last week, doing 
calibration testing on the 'golden antenna', ea24. 

    Because we now have a proper cold load for the C and X band feeds 
(and it's quite large!), we can now properly calibrate these tests, and 
not have to rely on what we think the spillover should be at these bands 
-- this important quantity can now be directly measured. 

    The tests comprised the usual suite:

    1) Hot Load/Cold Load/Sky observations at the vertical to calibrate 
the power scale, determine the switched power calibration, and determine 
the system temperature.  
    2) On/Off observations of Cygnus A, to determine the efficiency.
    3) Sky dips to permit a reasonable separation of the atmospheric 
emission from the spillover. 

    This suite of observations were done at nine frequencies in C-band, 
and at nine frequency in X-band, spanning the full tuning range of 
each.  Weather conditions were ideal -- clear, dry, light winds.   Full 
details will be presented in an upcoming Memo.  A summary follows:

    A) X-Band.  (This is on the new wide-band system). 

    All properties are remarkably uniform across the entire passband!  
We find:
    - The cold-sky system temperature at vertical runs from 24K at 
mid-band to 27 K near band edge.
    - The receiver temperature is 11 K at mid-band to 14 K at band edge. 
    - The zenith opacity (from the tips) is 0.010, which Bryan assures 
me is what is expected. 
    - There is essentially *no* variation in spillover with elevation -- 
a remarkable result!  There is no more than 1K extra noise above that 
expected from atmospheric emission, down to an elevation of 8 degrees. 
    - The efficiency, using the Baars' formula for Cyg A, with a factor 
included for its attenuation by the main beam, is 70%!  This is an 
unbelievable number, much higher than it can possibly be.  We expect 
values near 60%, perhaps as high as 65%.  All data have been carefully 
reviewed for signs of non-linearity -- there is none.  An on/off 
observation of 3C147, utilizing the stationboard generated switched 
power, indicates an efficiency of 60%.   Further evidence that our 
Cygnus A-based efficiency is wrong comes from the SEFD:  The combination 
of 25K system temperature and 70% efficiency predicts an SEFD of 200 
Jy.  What we measure, using noise on blank sky, is 250 Jy. 
    Is there a problem with the Baars value for Cygnus A?  (I shudder at 
the very thought of this ...). 
    A total-power observation of Cyg A and two flux density calibrators 
whose values are based on Mars (3C286 and 3C295) is being made tonight.  
Stay tuned...


    B) C-Band. 

    We have good data from two years (except that we didn't have a 
calibrated cold load), with which to compare our new data. 
    - The receiver temperature in this antenna has risen by 10K in the 
lower half of the band (4 -- 6 GHz) and by 14 to 16 K in the upper half 
of the band (6 -- 8 GHz) since the observations made 2 years ago.  We 
are sure this is the case, because both the calibration diode 
temperatures and the sky dip differential (Tsys at 8 degrees - Tsys at 
90 degrees) are exactly the same as two years ago.  Furthermore, I 
checked the reported system temperature from some recent data, and the 
reported value of 40K is consistent with our measurements. 
    *** Is there water in this feed***? 
    - The variation of system temperature with elevation is very 
different than that seen at X-band.  At C-band, a significant excess, 
above the obvious atmospheric contribution, begins at an elevation of 15 
degrees, and rises for lower elevations.  In addition, in the lower half 
of the band, a super-atmospheric excess, increasing with decreasing 
elevation, begins at much higher elevations (at 30 degrees at the bottom 
edge of the band).  These additional noise powers represent the 
'spillover excess', but are not very large, never more than 5K at 8 
degrees elevation for 3976 MHz, and only 1 -- 2 degrees excess at 
mid-band.  The source is likely far-out sidelobes of the feed through 
which ground radiation is seen at lower elevation. 
    - Antenna efficiency is good -- and very likely too good in the 
upper half of the band.  We are getting efficiencies of about 60% in the 
lower half, and 63 -- 65% in the upper half.  These are quite a bit too 
good, and I wonder, again, about the accepted value for the flux density 
of Cygnus A.