Re: [amsat-bb] Thoughts on future "FM birds" (was: proposal for maximizing fm le o contacts)

[Mike Gilchrist <mike@xxxxxxxxxxxxxx>]

> > At 09:18 AM 8/17/00 -0400, Bob Bruninga wrote:
> > >My design thoughts for a new analog SSB satellite for newcomers is:
> > >   10 meters UP with users using 25 watt Radio Shack $139 10m rigs
> > >    2 meters down for minimum doppler and greatest link budget
>
>Mike Gilchrist responded:
> > Many newcomers to the satellite ranks do not have 10 meter 
> privileges.  The
> > RS birds use a 2 meter up/10 meter down transponder.  Most of your other
> > comments apply to this mode (A) also.

At 01:19 PM 8/17/00 -0400, Bob Bruninga retorted:
>Yes, but still, the doppler tuning of mode A transmitter is 5 times
>greater than doing the same on a 10m uplink.  Yes, the combined up/down
>total is the same, but with 10m up, the smaller shift is on the uplink,
>so everyone in the passband retains their relative position about 5 times
>better and the whole group then moves greater relative to any particular
>receiver.  Thus a 5 times reduction in the "sliding QSO" problem...

The Doppler shift on a 10 meter downlink is negligible.  The Doppler shift 
on a 2 meter downlink however is around 3 kHz on a LEO satellite.  Since 
convention has the operator tuning the higher frequency of the transponder 
pair, the operator would be tuning the 2 meter uplink on mode A, or in your 
example, the 2 meter downlink; same difference, no net "savings," or 
reduction in any sliding QSO problem.

While a 3 kHz Doppler shift is trivial on the FM birds (including the 
digital versions,) it bears correction on a SSB signal.

73, Mike



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