[amsat-bb] Mode B doppler shift question

John Geiger af5cc2 at gmail.com
Tue Feb 14 15:18:35 UTC 2017

Hi Paul,

Thanks for the reply and your insight. I think you missed the question I
was asking. I understand that people tune the uplink signal on Mode B, and
the downlink signal on Mode J, where the 70cm band is the one being retuned.

I was discussing where you never tune the transmitter, you stay on the same
frequency, and the effect that doppler shift has on the downlink signal.
To me it seems that the downlink signal, all by itself due to doppler
shift, moves in different directions on the two different modes-it seems to
decrease in frequency in Mode J as the satellite approaches you and then
passes you, but seems to increase in frequency in Mode B when approaching
you.  Is that correct?

73 John AF5CC

On Tue, Feb 14, 2017 at 3:50 AM, Paul Stoetzer <n8hm at arrl.net> wrote:

> You are correct in your observation.
> The difference is that by tuning the higher of the two frequencies,
> you are canceling out much more of the Doppler shift since Doppler
> affects the higher frequency more. If you fix your transmit frequency
> on a Mode B satellite, you'll likely drift over other QSOs in
> progress.
> See this article for a more detailed explanation:
> http://www.amsat.org/?p=1489
> 73,
> Paul, N8HM
> On Mon, Feb 13, 2017 at 10:39 PM, John Geiger <af5cc2 at gmail.com> wrote:
> > I wonder if someone can help me figure this out or think through this.  I
> > have Mode J doppler shift worked out-you start high in frequency, and the
> > frequency lowers as the satellite moves towards you, and then away from
> > you.  For example, if I transmit in the exact center of the FO29
> > passband-145.950-the received signal from FO29 starts out above the
> center
> > of the receive passband, around 435.859 or so at the beginning of the
> > pass.  When the satellite is overhead it is at the center of the
> > passband-435.850, and then moves lower in frequency as the satellite
> moves
> > away from me.  This is assuming I keep the uplink frequency at 145.950
> the
> > entire time.
> >
> > Is it my imagination or is the situation reversed on Mode B. It seems
> that
> > if I transmit on the center frequency of the uplink passband, the
> downlink
> > signal is below the center at the beginning of a pass, and then moves up
> > and ends up above the center of the downlink passband at the end of the
> > pass-assuming I keep the transmit frequency the same. Is that correct?
> Why
> > would Mode B behave differently than Mode J?
> >
> > You can see I still have some things to learn about the satellites.
> >
> > 73 John AF5CC
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