[amsat-bb] Doppler shift question

[John Brier]

Philip you might have a typo here:

You say, for SO-50, "from AOS I lower my receive frequency (and continue
lowering it as the bird approaches LOS." You do lower the frequency through
the pass, but you actually start by raising the frequency, then you lower.
So with AO-91 you start by lowering the frequency and then raise it through
the pass.

73, John Brier KG4AKV



On Wed, Jun 5, 2019, 13:56 Paul Stoetzer via AMSAT-BB <amsat-bb at amsat.org>
wrote:

> When you're on the ground listening to SO-50 as it approaches you, the
> RF waves are compressed, and thus the frequency you receive at is
> higher than the frequency the satellite is transmitting at.
>
> When you're transmitting to AO-91 as it approaches you, you need to
> compensate for the fact that your RF waves are going to be compressed
> and appear at the satellite at a higher frequency than you are
> transmitting at. So you have to transmit lower than the nominal
> frequency.
>
> 73,
>
> Paul, N8HM
>
> On Wed, Jun 5, 2019 at 1:45 PM Philip Jenkins via AMSAT-BB
> <amsat-bb at amsat.org> wrote:
> >
> > This came up at AMSAT Academy at Hamvention, and I still can't wrap my
> head
> > around it (something simple I'm not getting, I'm sure). I know the
> > xmit/receive frequencies aren't shifted, stay the same at the satellite.
> >
> > SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I
> > lower my receive frequency (and continue lowering it as the bird
> approaches
> > LOS). So far so good.
> >
> > AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from
> AOS, I
> > go up in my transmit frequency.
> >
> > Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when
> > receiving a satellite, but *raise* the 435 Mhz frequency when
> transmitting
> > to a satelllite?
> >
> > So, my question boils down to - why should transmit doppler shift go in
> the
> > opposite direction from receive on the same band? In both cases, the
> > satellites are approaching me (from AOS).
> >
> > Basically, why the difference when I'm transmitting  and when I'm
> > receiving?
> >
> > 73
> >
> > Philip N4HF
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