[amsat-bb] Mode B doppler shift question

Paul Stoetzer n8hm at arrl.net
Tue Feb 14 03:50:01 UTC 2017

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

See this article for a more detailed explanation: http://www.amsat.org/?p=1489


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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