[evlatests] Demise of the 'Heat-Pipe' Theory?

[Rick Perley]

     We long ago noted the very large diurnal gain changes seen with the 
EVLA receivers -- particularly at the four high frequency bands.  
Typical power gain changes of 15% are noted, and in some cases greater 
than 20%.  These are clearly driven by external temperature -- and are 
expected if the actual temperature of the post-amplifiers change by 
significant -- but plausible -- values.   It was also noted that the 
low-frequency bands have similar, but considerably smaller (~5%), 
diurnal variations.

     To explain the effect, Bob Hayward posited the 'heat-pipe' 
mechanism:  That the horns, being exposed to the cold night air, were 
strongly thermally coupling the external temperature changes to the 
post-amplifiers -- which are mounted with good thermal coupling to the 
horns, and which are known to have a strong temperature coefficient.

     My recent analysis of data taken on 10/11 February casts some 
significant doubts (at least in my mind) on this theory.  I've attached 
eight plots, showing the change in PDif (hence, the change in system 
gain) at the eight cassegrain bands over an 8-hour period, starting at 
3AM in the morning, and finishing at about 11AM, for ea13.  This is not 
a 'specially chosen' antenna (except that it is one of only a few for 
which all switched power data are really clean at all bands).

     Some points stand out:

     1) The curves are all essentially identical.  Most importantly, the 
all maximize at the same time (7AM, local time). There is *no lag* 
between the time of maximum gain and the time of minimum temperature at 
any band.

     2) The four high frequency bands have exactly (to 1%) the same gain 
change -- 15%.

     3) The four low frequency bands also have the same gain change -- 
but at 5%.

     If the 'heat-pipe' explanation were the dominant cause, I would 
expect the S and (especially) L band gain maxima to be significantly 
later than that seen at the high frequencies.  This is not the case.  
Further, we might expect the X-band and Ku-band gain curves to be at 
least fairly close in amplitude -- since the horns are not very 
different in size.  This is also not the case.

     Furthermore, the gain changes seen at Q and Ka bands are exactly 
the same as at Ku and K bands.  Yet the horns for Q band (and I think 
also for Ka band) do *not* extend outside -- they are underneath weather 
windows.  Hence, externally-driven temperature changes should be much 
less at these two highest frequency bands than they are for K and Ku 
bands -- who horns do indeed extend well out into the night air.   But 
in fact, they are all the same.

     So how is the external temperature changes so strongly correlated 
with the gain changes?  And why are the four high frequency bands so 
similar?  (And the four low frequency bands also similar, but different 
than the high?) One thing connects all four high frequency bands -- the 
U/X converter.  Is it possible that this unit -- which is bolted to the 
side of the receiver cabin -- is the largest contributor to the observed 
diurnal effect?

     Rick




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