[mmaimcal] ACA Antenna Specs

[Al Wootten]

Hi Todd

I admit I only quoted the specs which were changed from the ALMA antenna 
specs and recs.  I would guess velocity remains unchanged so I suppose 
the max velocities of all antennas are the same but I will check this.

I do think the FE group needs to comment on the temperatures.  I'm not 
sure they know about this whole can of worms yet.  I forgot to pursue 
this with Tetsuo; I do not know why this spec is changed (there are two 
vents on their antenna vs three on the others but this is all I know).

Thanks,
Al

Todd R. Hunter wrote:
> Hi Al,
> 
> Section 5.2.1 Velocity and Acceleration
> 
> This spec is incomplete.  The only numbers given are for acceleration. 
> We also need to see the max velocity for each antenna type.  For 
> example, an antenna with a lower acceleration but a higher ultimate 
> velocity will reach a new source quicker if it is beyond some threshold 
> distance. If you are only concerned with slews of a few degrees, then 
> the Melco will lose. But if you are also concerned with slews of tens of 
> degrees (say, when switching between track-shared science sources), then 
> we really must consider the max velocity before making conclusions.
> 
> Section 6.3.4 Thermal requirements
> 
> Two comments.  First, probably a more important spec than absolute 
> temperature is the temperature stability (in terms of both rms and max 
> excursion allowed). Some components in the IF/LO chain will have 
> significant phase vs. temperature coefficients, which argue for having 
> the smallest temperature fluctuations.  Second, it is worth noting that 
> teflon undergoes a phase transition around 19C.  If it is used as the 
> insulator in any IF/LO coax cable, then the cabin temperature can 
> potentially influence the electrical length vs. temperature coefficient 
> of this cable. See: 
> http://ieeexplore.ieee.org/iel1/23/4444/00173201.pdf?arnumber=173201
> 
> We might want to get the FELO group to comment on whether there is any 
> specific temperature to avoid when selecting a cabin temperature.
> 
> Todd
> 
> 
> On Thu, 14 Dec 2006, Al Wootten wrote:
> 
>> Folks,
>>
>> Yesterday we discussed the science implications of changes proposed 
>> for the ACA 7m antenna specifications and requirements.  I tried to 
>> summarize these. In the spaces of today's LPR and LLC discussions I 
>> spoke about these with Tetsuo.  On the basis of our chat, I think I 
>> understand the reasons for some of these changes.  In principle, I 
>> think it is not good to have many different versions of antenna 
>> specifications and requirements.  But ALMA is a project of 
>> practicalities, not principles.
>>
>> I added some further words to the discussion document at:
>> http://www.cv.nrao.edu/~awootten/mmaimcal/ACASpecReqsDiffs.txt
>> Please comment.
>>
>> Clear skies,
>> Al
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> 
> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
> Todd R. Hunter  | NRAO Assoc. Scientist / GBT PTCS Instrument Scientist
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