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

[Terry Cotter]

The UX converter is not bolted directly to the wall. There are two 
layers of aluminum uni-strut. One layer runs vertical and the other 
horizontal to allow ease of installation. Further, the converter plates 
run hot and need air flow. There is a 4" HVAC air line running directly 
to the converter and there are two fans within the converter to move air 
across the plates.

On 3/14/2017 2:45 PM, Steven Myers wrote:
> by "same cryo" I mean connected to the same compressor, sorry
>
> On Mar 14, 2017, at 2:44 PM, Steven Myers wrote:
>
>> Are these bands in the same cryo? Do these trends correlate with any cryo monitoring (return pressure, stage temperatures, etc.)? We used to see stuff like this in the OVRO 40m due to daily changes in the compressor output over the course of a day.
>>
>> On Mar 14, 2017, at 2:35 PM, Rick Perley wrote:
>>
>>>   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
>>>
>>>
>>>
>>>
>>> <13-L.png><13-S.png><13-C.png><13-X.png><13-U.png><13-K.png><13-A.png><13-Q.png>_______________________________________________
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>>
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