so ive been trying to work AO-73 for a few weeks now and i keep having this problem where my downlink just slowly walks off frequency during a pass. im using a FT-818 for uplink on 70cm and an SDRplay for the downlink on 2m, running gpredict for tracking with a rotor controller on a pair of crossed yagis. the doppler correction is set up through gpredict talking to the radio via hamlib and its... mostly working i guess.

the thing is ill be in a qso and everything sounds fine for maybe 30-40 seconds then the other station just starts drifting up and i have to chase it manually. at first i thought it was my doppler correction not being aggressive enough but ive watched the waterfall during passes where im not transmitting at all and the beacon seems stable so im starting to wonder if its actually my uplink thats off and im the one moving around on the transponder passband and dont realize it.

anyone dealt with this before with AO-73 specifically or linear transponders in general. the inverting transponder thing still kinda messes with my head when i think about it too hard

yeah the inverted transponder thing trips everyone up at first. remember that on an inverting linear transponder if you drift UP on your uplink, your downlink moves DOWN. so if you're hearing yourself drifting up on the waterfall it almost certainly means your 70cm uplink is actually drifting down in frequency. classic thermal drift on the FT-818 if it hasn't been on long enough to warm up. i always leave mine on for at least 15-20 min before a pass and it helps a lot.

the other thing worth checking is whether gpredict has the right TLEs loaded. if they're more than a few days old the tracking can be off enough that your doppler correction is fighting you instead of helping. i grab fresh elements from celestrak every day or two when im doing satellite stuff regularly. AO-73 has been pretty well behaved in my experience once you get the doppler sorted, it's not the bird thats the problem usually

im probably not the most helpful here since i only did a handful of AO-73 passes before switching over to working FO-29 mostly, but one thing i noticed with my 818 is the TCXO option makes a noticeable difference for satellite work. dont know if you have that installed or not. without it mine wandered enough to be annoying on the linear birds even after warmup