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Re: Doppler tuning



Quoting Steve Meuse <smeuse@mara.org>:

> ANTHONY JAPHA expunged (tjjapha@earthlink.net):
> 
> > I've been watching the discussion of tuning on the current SSB up/FM
> down
> > mode, though I haven't had a chance to try it.  Why wouldn't you
> simply set
> > your uplink exactly as you do in FM, and leave it there?  Then tune
> the
> > downlink.  
> 
> This is what I've always done, seems to be the easiest, Tune the higher
> of the two freqs after you find yourself. 
> 
> -Steve
> N1JFU

I'll attempt to answer this question in greater depth, but I'm happy to be
corrected.

First, we need to distinguish between the frequency shift characteristics
of FM and SSB/CW. If you tune off frequency from a FM signal, the result is
poorer reception, but not a change in the tone of the signal. Because with
CW and SSB we are beating against the received signal, tuning off will
result in a different tone. It only takes a kHz or so to end up with one
which is unintelligible. 

So with FM satellites, we tune for doppler shift in order to hear a clearer
signal, not one which is the proper tone. However, with SSB satellites,
both on uplink and downlink, we need to get the tone right. This is
particularly true for AO-51 SSB/FM, because there is only one SSB channel,
as it were. You can't adjust for your uplink by tuning your downlink, nor
can anyone else.

If you set your uplink to AO-51 on a fixed frequency, the doppler shift
will, over time during the pass, make the space-born receiver hear your
voice shift in frequency, and it will retransmit it further and further
from a natural-sounding voice, just as if some gremlin were turning the
knob on your HF rig while you were listening to someone. (This isn't an
issue with the FM receiver on AO-51 for the reasons mentioned above.) We
have to compensate for that shift, either with software, or by hand, as I
heard Mark, VO1ONE do quite successfully yesterday evening.

73, Bruce
VE9QRP
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