[evlatests] Meeting to discuss temperature-induced gain variation

[Rick Perley]

    The power from the noise diodes used to monitor the receiver gains 
are known to be temperature sensitive -- this has been demonstrated by 
monitoring their output over a 24-hour cycle with the antennas parked.  
    The diodes are attached to the feed horns, which prove to be good 
heat pipes, coupling the diodes to the outside temperature much more 
effectively than we had thought.  The upshot of this is that the 
corrected visibilities should show a variation closely coupled to the 
air temperature. 
    The 'flux densities' run has shown this to be the case.  At Ku and K 
bands, the post-correction gains are seen to vary, with a clear 24-hour 
signature, by about 2% (in power) about the mean -- a total ~4% 
excursion.  The effect is also visible at X-band, but at a lower 
level.   Some antennas show a larger effect than others. 

    A meeting to discuss the ramifications of this is scheduled for this 
Thursday, 16 February,  at 9AM in the auditorium.  We will show the 
evidence, and discuss how to reduce (or remove) this induced gain 
variation.