[mmaimcal] ACA Antenna Specs

[Todd R. Hunter]

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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