so ive been trying to work AO-73 for a few weeks now and i keep hearing myself on the downlink but either im way off frequency or something is just not clicking with the full duplex thing. running a yaesu ft-817 for uplink on 145 and monitoring downlink on a cheap SDR dongle through gpredict for tracking. the doppler correction on gpredict seems to be working okay i think but when i try to actually make a contact the whole thing falls apart — like im hearing myself fine when the bird first comes up but then by the time its at like 30 degrees elevation my signal seems to wander all over the place and i lose myself on the downlink

i know the linear transponder is inverting so USB up means LSB down or whatever but i feel like im missing something fundamental here. do i need to be doppler correcting both VFOs independently or is there a smarter way to set this up with gpredict controlling the radio directly via hamlib. the passes here are only like 8-10 minutes and im spending half of it just chasing my own signal around

any help appreciated, been doing this for about 6 months overall but satellites are completely new to me

yeah the inverting transponder thing trips everyone up at first. so what you want to do is set up two separate doppler correction profiles in gpredict — one for the uplink and one for the downlink, and because the transponder inverts, as the satellite comes toward you the downlink frequency goes UP and your uplink needs to go DOWN to compensate. if you're chasing yourself around that's almost certainly a doppler correction issue on the uplink side.

the hamlib integration with gpredict works pretty well once you get it configured but honestly for AO-73 a lot of guys just manually tune and once you get the feel for it you can keep yourself in the passband without too much trouble. the trick is to find yourself on the downlink first and then just leave the downlink alone and tune the uplink to keep yourself stationary. sounds backwards but it works. also make sure youre actually inverting — if youre going up SSB on both ends nothing is going to work right on a linear bird

the 817 and an sdr dongle is kind of a rough combo for this because your eyes are in two different places and its hard to react fast enough. i used that setup for a while and switched to using the 817 for downlink monitoring too just so both VFOs are in one place. also gpredict has a radio control window where you can assign uplink and downlink separately and tell it the transponder is inverting — thats the key setting most people miss when they first set it up. once you enable that it should be correcting both sides with the right sign. good luck, AO-73 is a fun one when it all clicks