ok so ive been doing SO-50 passes for a few months now with a handheld and a cheap arrow antenna and that part i mostly have figured out. but i keep reading about linear transponder birds like AO-73 and FO-29 and i guess im confused about the fundamental difference in how you actually operate them vs a straight FM repeater like SO-50.

with SO-50 i just dial in the uplink and downlink, correct for doppler manually as best i can and key up. simple enough. but from what i understand with a linear transponder youre putting in SSB and the whole passband can have multiple QSOs happening at once? and you have to track your own downlink and adjust? i tried a FO-29 pass last week with my FT-817 and a yagi i threw together and i could hear what sounded like two guys talking but i couldnt figure out what i was supposed to be transmitting on to even try to get in there. doppler was all over the place and i kept losing them.

is there a good way to get started with this or should i just stick to FM birds until i get better tracking software sorted out. im running gpredict right now on a linux box which works ok for predicting passes but i havent figured out how to get it to drive the radio for doppler correction automatically yet.

yeah the jump from FM birds to linear transponders is a bit of a head scratcher at first. the key thing to understand is that on a linear transponder the sat is basically just frequency inverting a chunk of spectrum, so everyone in the passband gets their own slice. you transmit SSB on the uplink and listen on the downlink but because of the inversion if you're using LSB up you hear USB down, or vice versa depending on the bird. AO-73 is USB on the downlink if i remember right.

the doppler thing is what really gets people. the trick is you dont chase the doppler on your transmit, you chase it on your receive. find your own signal on the downlink first before you even try to work anyone, that way you know you're actually getting into the bird. with gpredict you can set up hamlib to control the 817 and it'll handle doppler on both VFOs automatically which is huge. took me a while to get that working but once it clicks its a game changer. look up the gpredict hamlib rig control docs, theres a decent walkthrough somewhere on amsat's site.

FO-29 has been a bit spotty lately in terms of when they have it in linear mode so AO-73 might be more reliable to practice on. good luck, it's worth the effort once it starts coming together.

honestly i was in the same spot maybe a year ago. what finally made it click for me was just spending a couple passes just listening and not transmitting at all. like really listening and trying to follow individual signals through the doppler shift, you start to get a feel for how fast things are moving frequency wise especially near AOS and LOS vs mid pass when its more stable.

also dont sleep on the AMSAT pass schedule stuff, some of those linear birds have scheduled periods where they switch modes and if you hit it when its in CW beacon only mode youll just be confused. i wasted like three passes on FO-29 before someone told me that lol