[amsat-bb] Mode B doppler shift question
pconver at gmail.com
Tue Feb 14 04:52:47 UTC 2017
It is a useful explanation on link provided by Paul,
Perhaps another approach is try to keep the QSO on the same frequency
correcting both stations transmit and receive frequencies accounting
dynamically for Doppler.
Transceivers with CAT, or two rigs with CAT allow to implement this.
I do this with a vintage FT-736, and QSO is just as a normal HF QSO,
no tuning, hands free during all the pass, providing both stations use
the same approach.
There are very nice programs as SATPC32, Orbitron and others that allow do this.
I use http://lu7abf.com.ar/pass.htm (the .exe part) along with wispDDE Driver.
Using this, I see how transmit and receive Dopplers moves as the
satellite passes, thus even not having CAT Control, one can manually
correct Uplink/Downlink, allowing easily hear own return.
To contact a station heard, Downlink frequency could be changed,
application will show what Uplink to use.
Wishing nice and rewarding contacts thru the birds.
73, LU7ABF, Pedro
On Tue, Feb 14, 2017 at 12:50 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
> 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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> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions expressed
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> Not an AMSAT-NA member? Join now to support the amateur satellite program!
> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
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