[amsat-bb] AO-91: *No* uplink frequency changes needed for Doppler shift
n8hm at arrl.net
Fri Nov 24 14:57:21 UTC 2017
I worked a station last night who didn’t check the frequencies before the
pass. He was transmitting at 435.220 MHz when I worked him - 30 kHz low. He
sounded terrible, but it worked.
I’m still not sure it’s a great idea to advise that stations stay at center
frequency, I suppose time and experimentation will tell.
On Fri, Nov 24, 2017 at 03:04 Matthew Stevens <matthew at mrstevens.net> wrote:
> 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.
> > 73
> > John...
> > WB4LNM
> > AMSAT FOX RF Design Team
> > _______________________________________________
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