so i finally got my az/el rotor setup working well enough to track AO-73 and ive been making contacts on the linear transponder but im still confused about something. when im operating SSB through it, do i adjust the uplink or downlink or both? ive been reading conflicting stuff and my passes are short enough that i dont have time to think it through mid-pass.

my current setup is an ft-818 on the uplink at 435 MHz and an sdr running gqrx on the downlink around 145 MHz. im using gpredict to track the bird and it'll do doppler correction on the sdr side but i havent figured out if i should also be manually tuning the 818 or just leaving it fixed. i can hear myself in the passband when the bird is close to AOS but further into the pass things start sounding weird, like my signal drifts. not sure if thats me, the bird, or just my setup being garbage.

anyone been through this learning curve? feels like there should be a simpler explanation somewhere but everything i find online is either super basic or assumes you already know what youre doing

yeah the doppler thing on linears trips everyone up at first. short version: for SSB through a linear transponder you generally want to tune your downlink to keep your received signal stable, and let the uplink drift a little OR use gpredict to drive doppler correction on both radios if your setup supports it. the key thing to understand is the transponder is inverting — so if you tune up on your uplink your downlink goes down. that catches people off guard when they try to manually chase their signal.

gpredict can actually control two radios at once over hamlib if you set it up right, one for each band. that way it handles the math automatically and you can focus on actually operating. with just the sdr on the downlink being corrected and the 818 fixed youre only doing half the job, which explains the drift you hear later in the pass when the doppler delta is bigger. AO-73 is a fairly low bird so your doppler shift on 70cm uplink can be several kHz across a pass, definitely noticeable on SSB.

im in basically the same situation lol, been trying to get AO-73 contacts for a couple months. one thing that helped me was just listening for a while before trying to transmit, like really getting a feel for where signals are sitting in the passband and how theyre moving. also i found that if i tune my downlink receiver to a beacon or a signal i know and keep that stable, then my uplink is probably in roughly the right place too. its a bit by ear but it works when you cant automate everything.

the inverting thing the other guy mentioned is real and it messed me up a lot at first. i kept tuning the wrong direction and chasing my own signal further off frequency.