[evlatests] Ka-band efficiency

[Rick Perley]

    I've reviewed all the total-power data taken last month.  
Efficiencies are determined by measuring the additional power 
contributed from a source of known flux density.   Roughly speaking, for 
a 25-meter antenna, about 1K of additional noise power is seen for every 
10 to 15 Jy flux density.   For the setup we had, measuring the system 
temperature to an accuracy of better than 0.1K is difficult. 

    For the observations of last month, the only available source, given 
the time and date, was 3C273 -- about 25 Jy.  The preferred sources for 
this band were not available:  Venus was (exactly!) behind the Sun, and 
Mars was too far away. 

    Thanks to Craig, who provided me with the VLBA's estimations of the 
flux density of 3C273, I have derived the following -- preliminary -- 
estimates of the efficiency for the Ka band system on antenna 4:

Freq.       Tant        tau0         Flux      Elev    Eff.     Comments
---------------------------------------------------------------------------------------------------------
26232       2.3       .0267        25.9        28      .50
28024       2.26     .0249        25.9        35      .49
33144      1.80      .0291        24.9        49      .41
33160      1.56      .0326        24.2        29      .36      May have 
missed the source
37000      1.74      .0332        24.0        40      .41
39600       ~2        .0424       23.4        42       .48     Very 
unstable data.  Disregard
------------------------------------------------------------------------------------------------------------

    We expect an efficiency between 40 and 50 percent, based on data 
taken at K and Q bands.  The current data indicate we are close to 
expected values. 

    When Venus reaches maximum elongation (near the end of this year), 
we will likely repeat these observations in order to obtain a more 
accurate estimate.