[evlatests] Preliminary Ka-band antenna sensitivity and efficiency results

[Rick Perley]

     Bob Hayward and I performed single-antenna sensitivity and 
efficiency tests on Friday on the prototype Ka band receiver, mounted on 
antenna ea4.    The weather was ideal for these:  clear, light winds, 
air temperature near 30C, DP = 0C.   We chose a frequency near 33 GHz, 
with a RF filter of 100 MHz. 

    We did the usual suite of tests, as has been (and will be) described 
in EVLA memos.  In brief:

    A) Hot (300K), Cold (77K), and sky (at the zenith) total power 
observations.  All with the cal on and off.  These provide the receiver 
temperature, cold-sky system temperature, and cal temperature.  The 
latter can be compared to lab measurements. 

    B) Sky dips -- from 90 degrees to 8.0 degrees in 20 equal steps in 
Sec(Z).  These measure the atmospheric emission, and variation of 
spillover with elevation. 

    C) Efficiency Tests -- on-off observations of sources of known flux 
density, to determine the antenna temperature, and hence the antenna 
efficiency. 

    Results:

    A)  We find the total receiver temperature to be 24.2 K -- about 4K 
above Bob's lab measurement.  He is pondering where the losses might 
be.  The cold-sky zenith system temperature (on a very hot day) was 
58.0K, so the atmosphere/spillover contribution is about 34K.    The cal 
temperature came out to 5.20K, compared to Bob's lab measurement of 5.4K. 

    B  The sky dip showed the system temperature at elevation of 8 
degrees to be 103K.  At an elevation of 30, it was 64 K, at an elevation 
of 20, it was 71.5 K.  This suggests a rather high spillover 
contribution (roughly 26K!) -- a proper analysis will be done later this 
week. 

    C)  After determining the local pointing offsets using X-band, we 
observed 3C84 and 3C273 on/off about six times each.  The antenna 
temperature from 3C84 is 0.7K (error about 0.1K), and from 3C273 we see 
1.63K (error about .05K).  The elevation for both sources was rather low 
-- 29 for 3C273, 26 for 3C84.  Determining the efficiency requires 
knowledge of the flux densities -- these are not known accurately, and 
we will need to make accurate measurements of these against 3C286 at K 
and Q bands.  However, by using the values in the calibrator manual 
(interpolated between the listings, which are for Ku and Q bands), we 
get 40% from 3C84, and 46% from 3C273.  I'm more inclined to believe the 
latter (and not just because of my usual optimism ...) 

    These are very good results indeed.  All indications are that the 
receiver and antenna will perform at a level better than the EVLA 
requirements. 

    If the weather permits (clear, with light winds), we plan on 
returning on Monday and/or Wednesday, to repeat these observations at 
frequencies near 27 and 39 GHz.