[amsat-bb] Mode B doppler shift question

[Paul Stoetzer]

John,

Let's take a look at the examples.

If you fix your transmit frequency on an overhead pass of FO-29 at
145.950 MHz, your signal at the satellite at the beginning of the pass
will appear to be 145.953 MHz. The transponder will retransmit your
signal at 435.847 MHz, which will appear to you around 435.857 MHz. At
the middle of the pass, your signal at the satellite will be at
145.950 MHz so the transponder will retransmit your signal at 435.850
MHz, and you'll receive it at 435.850 MHz.

Say you fix your transmit frequency on an overhead pass of XW-2F at
435.340 MHz. Your signal at the satellite at the beginning of the pass
will be around 435.350 MHz. The transponder will retransmit your
signal at 145.980 MHz, which will appear to you at 145.983 MHz. At the
middle of the pass, your signal at the satellite will be 435.340 MHz,
the transponder will retransmit your signal at 145.990 MHz, and you'll
receive it at 145.990 MHz.

The reason for the difference is the different magnitude of the
Doppler shift between the two bands (435 MHz having about triple the
Doppler shift of 145 MHz), so, yes, a fixed transmit station will
drift down on a Mode J satellite and up on a Mode B satellite. If you
fix your transmit on Mode J, your signal will drift about 7 kHz on the
transponder during an overhead pass, however on Mode B, your signal
will drift about 20 kHz on the transponder.

73,

Paul, N8HM



On Tue, Feb 14, 2017 at 10:18 AM, John Geiger <af5cc2 at gmail.com> wrote:
> 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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>