[amsat-bb] AO-91: *No* uplink frequency changes needed for Doppler shift
matthew at mrstevens.net
Fri Nov 24 08:03:05 UTC 2017
Have you tried operating this way during a pass?
It doesn’t work on AO85 really. If you’re more than 5khz off on the uplink your audio tends to sound terrible (for example, using my IC-821H). AO-91 doesn’t seem to be any different in my initial tests over the past couple of nights. You don’t have to be exactly on frequency, especially if you are running power, but it is much easier to get in and your audio sounds much better if you do adjust reasonably close for doppler during the pass.
Maybe somebody else from engineering or ops can speak to this after the frequency sweeps they did over the last few days. Mine are just informal user tests based on my experiences making a few hundred QSOs on AO-85, and now about 20 on AO-91.
Congrats on the great receiver sensitivity by the way! The new bird is performing extremely well. I was able to get in AOS to LOS this evening, testing at 50mW with a handheld 7 element Arrow II and TH-D72 antenna.
- Matthew NJ4Y
Sent from my iPhone
> On Nov 23, 2017, at 20:56, John Klingelhoeffer via AMSAT-BB <amsat-bb at amsat.org> wrote:
> I am puzzled why I am seeing announcements going out from apparently several sources indicating user UHF uplinks need to be changed to adjust for Doppler shift near to their own AOS and LOS. I'm seeing this posted from the AMSAT-UK website as well as other places.
> The FOX1-B UHF receiver has automatic frequency control (AFC) that will slew the uplink users signal to the middle of the IF filter and the discriminator if s/he is anywhere within about +/- 10 kHz of the nominal uplink frequency. From what I remember, the worst case UHF Doppler shift was about +/- 8.5 kHz.
> There should be *NO* need at all to 'predistort' the uplink transmission frequency to correct for uplink Doppler shift - that was the whole purpose of going through the complexity of adding AFC to the receiver I designed. The AFC was certainly working fine during testing. Just use the posted nominal uplink center frequency - that's all.
> This, by the way, is true for 1-A as well. Only one of 1-Cliff or 1-D has the AFC knowingly disabled. I guess we will see how that 'experiment' works when they get out of the barn.
> AMSAT FOX RF Design Team
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