so ive been licensed for about 3 years and mostly just do HF but i picked up an FT-818 a while back thinking id eventually get into portable ops and someone at the club mentioned satellite work so i started going down that rabbit hole last month

anyway i understand the basic concept of doppler shift and i know you have to correct for it in real time during a pass but what i dont fully get is the practical side of it with a linear transponder. like on FM birds you just have one frequency to chase but on something like AO-73 or FO-29 youre dealing with both your uplink and downlink shifting at different rates? and you have to tune them somewhat independently to keep your downlink signal in the right spot while also not drifting off the transponder passband on the uplink?

i tried a pass last weekend with gpredict doing the doppler correction automatically through CAT to the radio and i could hear signals on the downlink but i couldnt find my own signal at all. either i was putting out too little power or i was completely off frequency or both. using a dual band yagi, probably about 15 watts on 70cm uplink. just not sure what im doing wrong or where to even start troubleshooting this

the two-VFO doppler thing trips up basically everyone at first. the way to think about it is the transponder inverts the passband, so as the satellite comes toward you your uplink freq needs to go up while your downlink also goes up but they move at different rates because 70cm and 2m have different wavelengths. gpredict handles this but you have to make sure youve got the right transponder data loaded for whichever bird youre working, and the uplink/downlink offsets have to match the actual passband edges, not just some generic entry.

on the power thing, 15w into a decent yagi should be way more than enough for FO-29 or AO-73, like people work those with a handheld and a handheld yagi. so id suspect youre just off frequency or the CAT connection isnt updating fast enough. what rig control mode are you using in gpredict? some of the older radio driver implementations have lag issues and you end up chasing the doppler instead of leading it. try tuning manually for one pass just to get a feel for how much the freq actually moves and whether you can find yourself in the passband, then go back to automated correction once you know the basic setup is working.

yeah i had the exact same problem when i started, couldnt hear myself for like the first 4 or 5 passes and it was incredibly frustrating. one thing that helped me a lot was just listening first without transmitting for a whole pass or two, get familiar with where signals are sitting on the downlink and how they drift, then when you do transmit you have a mental picture of where things should be. also make sure youre not in the middle of the passband when you start, a lot of people aim for the edges to avoid being on top of someone. FO-29 can be pretty busy on a good pass