[amsat-bb] S band or any backup?

G0MRF@aol.com G0MRF at aol.com
Sun Sep 10 03:03:31 PDT 2006


On the subject of a parallel S band TX.   That may or may not be  simple 
depending on what else is available inside Eagle. But in any event  it should not 
require switches.
About two years ago when there was a similar period of outrage on the BB,  
the hot topic was the wisdom of flying an untried or tested SDX  transponder 
without a switchable conventional 'linear' backup.  I beleieve  it was Bob who 
announced "There will be no conventional linear transponder in  Eagle" I was 
very pleased to see Rick, W2GPS, leap quickly into the debate and  reassure 
everyone that the complicated DSP + logic in the SDX would be backed up  by 
conventional linear electronics. 'just in case'
In a recent mail the UV transponder has been referred to as having  
unbelievably high audio quality.  An apparent and very well justified  comment on the 
quality of the SDX.
What I can't recall seeing, is any reference to any fall back  position of a 
linear system as a back up.
It's worth remembering that 3 - 5 years time will be the peak of the  sunspot 
cycle. High solar activity, or an unfortunate error with the orbital  change 
burn as AO-10, could result in the satellite receiving a great deal of  
It would be good to know that the linear backup plans have not been quietly  
filed in the waste paper basket.  
If it hasn't, then a parallel transmitter for S or any other band is very  
simple to implement. Just split the IF, wire the other transmitter in parallel.  
5W to an S band patch or whatever, and control it by turning the DC power 
line  on or off.  S band amplifiers no longer need to run in class A with 30%  
efficiencies, so an up to date class AB design would not be a large burden on  
the power budget.
I suppose my quiet concern is not really the lack of S band, but; If the  SDX 
or 2m TX fails, what is left in the current design to provide a service to  
the 90+% who will not have, or are not interested in C band?
Good engineering is important, but planing for the unexpected is  vital.
Incidentally. It occurs to me that a disadvantage of having one major  UV 
transponder operating 24/7 is that while that concept removes the need for  
complicated scheduling it will occupy 100kHz or so of the 2m band permanently  
which may not be totally desirable in a limited 200kHz  bandwidth. Presumably with 
Eagle 2 and 3 under design / construction  at the time of launch they will 
have differnt bands. - Maybe 2390 - 2400  ish

Enjoying the debate
David   G0MRF

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