[amsat-bb] Observations on AO-91 AFC

Scott ka9p at aol.com
Sun Nov 26 03:52:06 UTC 2017

Matt’s observations beg the question of what the design basis of the AFC system might be- single signal or a number of signals of varying amplitude and frequency.

In the end it may not matter much, but if understanding better how it works leads to more enlightened use of the bird than a few of the (obviously there’s still learning going on) passes this weekend, it could be very helpful.

Thanks for any info or guidance you engineering guys may have, and again, congrats on a super job.

73 Scott ka9p

Make something good happen!

> On Nov 25, 2017, at 8:09 PM, Matthew Stevens <matthew at mrstevens.net> wrote:
> I’ve done a bit of study on the Fox series AFC. I never really knew
> much about it up until this point, but with all the discussion on the
> amsat-bb about AO-91’s AFC function, and the alleged lack of uplink
> Doppler tuning needed – I figured I should educate myself a bit more.
> I also wanted to test it out some. My observations (which I’ve
> outlined in another BB post
> http://www.amsat.org/pipermail/amsat-bb/2017-November/065569.html),
> making hundreds of QSOs on AO-85 using various antennas, radios, and
> power settings have been that there is definite need to tune the
> uplink frequency for Doppler shift. My initial experiences working
> passes on AO-91 from a few minutes after commissioning, using an HT
> and whip, then an HT and Elk, then and HT and arrow, did nothing to
> change my mind about tuning for the AO-91 uplink.
> However, even though I already made a bunch of QSOs on AO91, I didn’t
> really have enough data to make any kind of real conclusion about the
> AFC on AO-91. So, I figured I’d try some experiments and post them
> here!
> Today was the first pass I had worked on AO-91 with anything other
> than a 0.05-5w Kenwood D72 HT for a radio. I broke out my Icom 821H
> for a 90 degree lunchtime pass, turned up the power to the full ~35w,
> and used my handheld arrow II antenna. Having already heard the
> absolute zoo that has been daytime AO-91 passes, I was curious first
> to hear how it went using more power (as far as being able to get in
> over the dead carriers and multiple hetrodyning uplinks), and second,
> how the AFC functioned with a lot of high-power users.
> What I found: The AFC *does* seem to work! However, there are some
> usage caveats that I have to note. When there is a single signal
> coming into the bird, it seems to compensate fine for being off
> frequency (within reason). I didn’t try too wide of a tuning range,
> since compared to some stations I heard today I didn’t have enough
> power to be way off (more on that in a minute). But I did try +/-
> about 5khz, and it seemed to compensate just fine if I was the only
> signal coming in.
> That’s the rub however. There was virtually no time today where I was
> the only signal coming into the bird! And when there was a bunch of
> different signals, basically, the strongest uplink was the one that
> determined the AFC tuning. You can hear that happening in the
> recording which I’ll link to below. There was more than one instance
> where I began to transmit over a carrier or other signal that I was
> hearing (being careful not to transmit over any actual audible callers
> or QSOs in progress). You can hear my audio go from poor to crystal
> clear as the AFC (apparently) adjusted to my carrier – and I did not
> adjust my transmit frequency at the time. I assume that my signal was
> stronger than the others, and the AFC “picked” mine.
> You can also hear a signal from XE3ARV, who had by far the strongest
> signal I heard during the pass. Because his uplink signal was so much
> stronger than anyone or anything else, it sounded like the AFC
> adjusted to whatever his uplink frequency was. If I, or anyone else
> was transmitting at the time, you can hear it IMMEDIATELY go from the
> other clear (or scratchy) signal (depending on what state the AFC
> tuning was in at the time), to a crystal clear downlink from XE3ARV.
> Another observation of note was the downlink signal from 5K0T. At the
> beginning of the pass you can hear a very clear signal from their
> obviously strong uplink. As the pass progressed, the audio quality on
> their downlink deteriorated a lot. The only reason I can see for this
> is that they may have not been adjusting for Doppler during the pass,
> and were possibly 10khz or more from the correct uplink frequency
> (perhaps transmitting at 435.240 the entire pass?). I can’t confirm
> this as I wasn’t there to see what was happening on their end, but
> that is one possible explanation to why their signal sounded like it
> did at the end of the pass. This hypothesis also is supported by
> having heard another station on a much quieter, early morning pass the
> other day who I know was transmitting 30khz off frequency. His audio
> sounded very similar to 5K0T at the time.
> Yet another observation is that when there are a lot of users on the
> pass, it seems to be beneficial to adjust for Doppler. There were
> several times during today’s pass where I did tune my radio to the
> correct, Doppler adjusted uplink frequency. If there was a high level
> of QRM, tuning for Doppler seemed to help my signal be received by the
> satellite, as opposed to tuning +/- 5khz from the Doppler adjusted
> value.
> Here is my recording:
> https://drive.google.com/open?id=1c8XYCoxFCKCyR4dtfwJCyM9tjfo1mLKd
> TL;DR observations:
> 1. AFC does work within some limitations.
> 2. With multiple strong signals into the bird, the strongest “wins” as
> far as determining the AFC adjustment. This makes other signals tuned
> to other frequencies sound “staticky”
> 3. Transmitting very far off frequency makes your downlink audio sound bad.
> 4. Adjusting for Doppler seems to help your signal capture the sats
> receiver better than other users who are operating at a similar power
> level to you.
> Conclusion so far: In practice, AFC is good for correcting minor
> Doppler tuning errors. However, unless there is only one user
> transmitting into the sat at a point in time, it does not eliminate
> the need to tune your uplink for Doppler.
> I would be curious to hear other observations and interpretations from
> AO-91 passes. I think it’s helpful to support observations with
> recordings, so others can listen and draw their own conclusions about
> your interpretation of what has occurred.
> Also, these are just my initial impressions from operating passes…and
> these are subject to change over time :-)
> 73,
> Matthew nj4y
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