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{ "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/VRKTM7EDUTR4NNI4FLSNWH5VAHCZDHTB/", "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/", "message_id": "CAN8zKtSgaa42kYdThngDwqdgL+ASrNeT8Kq=ndc_bWcTEJyGcg@mail.gmail.com", "message_id_hash": "VRKTM7EDUTR4NNI4FLSNWH5VAHCZDHTB", "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/VRKTM7EDUTR4NNI4FLSNWH5VAHCZDHTB/", "sender": { "address": "matthew (a) mrstevens.net", "mailman_id": "9211e31bd97d4743ace6c47863af3e1b", "emails": "https://mailman.amsat.org/hyperkitty/api/sender/9211e31bd97d4743ace6c47863af3e1b/emails/" }, "sender_name": "Matthew Stevens", "subject": "[amsat-bb] Observations on AO-91 AFC", "date": "2017-11-26T02:09:52Z", "parent": null, "children": [ "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/VYPM5IFSQ24PXA4ORGUM3IPKRZYXJBUJ/" ], "votes": { "likes": 0, "dislikes": 0, "status": "neutral" }, "content": "I’ve done a bit of study on the Fox series AFC. I never really knew\nmuch about it up until this point, but with all the discussion on the\namsat-bb about AO-91’s AFC function, and the alleged lack of uplink\nDoppler tuning needed – I figured I should educate myself a bit more.\n\nI also wanted to test it out some. My observations (which I’ve\noutlined in another BB post\nhttp://www.amsat.org/pipermail/amsat-bb/2017-November/065569.html),\nmaking hundreds of QSOs on AO-85 using various antennas, radios, and\npower settings have been that there is definite need to tune the\nuplink frequency for Doppler shift. My initial experiences working\npasses on AO-91 from a few minutes after commissioning, using an HT\nand whip, then an HT and Elk, then and HT and arrow, did nothing to\nchange my mind about tuning for the AO-91 uplink.\n\nHowever, even though I already made a bunch of QSOs on AO91, I didn’t\nreally have enough data to make any kind of real conclusion about the\nAFC on AO-91. So, I figured I’d try some experiments and post them\nhere!\n\n>>\n\nToday was the first pass I had worked on AO-91 with anything other\nthan a 0.05-5w Kenwood D72 HT for a radio. I broke out my Icom 821H\nfor a 90 degree lunchtime pass, turned up the power to the full ~35w,\nand used my handheld arrow II antenna. Having already heard the\nabsolute zoo that has been daytime AO-91 passes, I was curious first\nto hear how it went using more power (as far as being able to get in\nover the dead carriers and multiple hetrodyning uplinks), and second,\nhow the AFC functioned with a lot of high-power users.\n\nWhat I found: The AFC *does* seem to work! However, there are some\nusage caveats that I have to note. When there is a single signal\ncoming into the bird, it seems to compensate fine for being off\nfrequency (within reason). I didn’t try too wide of a tuning range,\nsince compared to some stations I heard today I didn’t have enough\npower to be way off (more on that in a minute). But I did try +/-\nabout 5khz, and it seemed to compensate just fine if I was the only\nsignal coming in.\n\nThat’s the rub however. There was virtually no time today where I was\nthe only signal coming into the bird! And when there was a bunch of\ndifferent signals, basically, the strongest uplink was the one that\ndetermined the AFC tuning. You can hear that happening in the\nrecording which I’ll link to below. There was more than one instance\nwhere I began to transmit over a carrier or other signal that I was\nhearing (being careful not to transmit over any actual audible callers\nor QSOs in progress). You can hear my audio go from poor to crystal\nclear as the AFC (apparently) adjusted to my carrier – and I did not\nadjust my transmit frequency at the time. I assume that my signal was\nstronger than the others, and the AFC “picked” mine.\n\nYou can also hear a signal from XE3ARV, who had by far the strongest\nsignal I heard during the pass. Because his uplink signal was so much\nstronger than anyone or anything else, it sounded like the AFC\nadjusted to whatever his uplink frequency was. If I, or anyone else\nwas transmitting at the time, you can hear it IMMEDIATELY go from the\nother clear (or scratchy) signal (depending on what state the AFC\ntuning was in at the time), to a crystal clear downlink from XE3ARV.\n\nAnother observation of note was the downlink signal from 5K0T. At the\nbeginning of the pass you can hear a very clear signal from their\nobviously strong uplink. As the pass progressed, the audio quality on\ntheir downlink deteriorated a lot. The only reason I can see for this\nis that they may have not been adjusting for Doppler during the pass,\nand were possibly 10khz or more from the correct uplink frequency\n(perhaps transmitting at 435.240 the entire pass?). I can’t confirm\nthis as I wasn’t there to see what was happening on their end, but\nthat is one possible explanation to why their signal sounded like it\ndid at the end of the pass. This hypothesis also is supported by\nhaving heard another station on a much quieter, early morning pass the\nother day who I know was transmitting 30khz off frequency. His audio\nsounded very similar to 5K0T at the time.\n\nYet another observation is that when there are a lot of users on the\npass, it seems to be beneficial to adjust for Doppler. There were\nseveral times during today’s pass where I did tune my radio to the\ncorrect, Doppler adjusted uplink frequency. If there was a high level\nof QRM, tuning for Doppler seemed to help my signal be received by the\nsatellite, as opposed to tuning +/- 5khz from the Doppler adjusted\nvalue.\n\nHere is my recording:\nhttps://drive.google.com/open?id=1c8XYCoxFCKCyR4dtfwJCyM9tjfo1mLKd\n\nTL;DR observations:\n\n1. AFC does work within some limitations.\n\n2. With multiple strong signals into the bird, the strongest “wins” as\nfar as determining the AFC adjustment. This makes other signals tuned\nto other frequencies sound “staticky”\n\n3. Transmitting very far off frequency makes your downlink audio sound bad.\n\n4. Adjusting for Doppler seems to help your signal capture the sats\nreceiver better than other users who are operating at a similar power\nlevel to you.\n\nConclusion so far: In practice, AFC is good for correcting minor\nDoppler tuning errors. However, unless there is only one user\ntransmitting into the sat at a point in time, it does not eliminate\nthe need to tune your uplink for Doppler.\n\nI would be curious to hear other observations and interpretations from\nAO-91 passes. I think it’s helpful to support observations with\nrecordings, so others can listen and draw their own conclusions about\nyour interpretation of what has occurred.\n\nAlso, these are just my initial impressions from operating passes…and\nthese are subject to change over time :-)\n\n\n\n73,\n\nMatthew nj4y\n", "attachments": [] }