so ive been licensed for about two years mostly doing HF and some local 2m FM stuff but i finally decided to try working through the linear transponder birds, specifically AO-73 and sometimes FO-29 when it cooperates. got myself a decent yagi setup, two separate radios so i can do full duplex, and i have gpredict running on the laptop pulling fresh keplerian elements every few days.

my problem is the doppler correction. i understand the concept — uplink goes up in frequency as the bird approaches and then drops off, same deal on the downlink but in reverse since its a linear inverting transponder. i have gpredict set to control the radios via hamlib and it does move the frequencies around during a pass but honestly it feels like im always slightly off. i can hear signals on the downlink that kind of sound like SSB but they're just a little mushy or like slightly off pitch. im not sure if its my radio tuning resolution, the doppler tables, or if im just not understanding how the passband works on these transponders.

also not sure if i should be listening for my own downlink or just trying to work someone. feels like trying to pat your head and rub your stomach at the same time honestly

the mushy audio thing is almost certainly doppler thats just a hair off, and it could also be that the other station isnt correcting properly either — on a linear bird both ends have to be doing their doppler homework or it sounds like garbage. one thing that helped me a lot was to not fully trust the automated hamlib control at first and instead tune manually on the downlink to find a clear signal and then just ride it by ear. gpredict is good but the timing of updates through hamlib can lag a tiny bit depending on your polling interval.

on the inverting transponder thing — remember that if you tune UP on the uplink your signal moves DOWN in the passband on the downlink. so when youre listening for yourself, put out a carrier and tune around until you hear it come back, it takes a few passes to get a feel for it. honestly once it clicks it becomes pretty natural. FO-29 is a bit easier to start with imo because the passband behavior feels more predictable to me than AO-73 sometimes, especially with the AGC doing weird things when theres a strong station in the passband.

yeah the head-patting thing is real lol, i remember my first few passes i was so focused on trying to find my own downlink signal that i completely missed a CQ from someone and then the bird was gone. what i do now is just pick a spot in the passband a little above center on the downlink and call CQ once the bird is like 5-10 degrees elevation on the way up, before the doppler shift gets too wild. and dont worry too much about perfect doppler at first, close enough usually gets the job done on SSB, were not talking about narrow digital modes here