Interesting question,

It is a useful explanation on link provided by Paul, http://www.amsat.org/?p=1489

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@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@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 _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. 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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Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb

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@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@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@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 _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb