so ive been messing around with satellite work for a few months now, mostly just receiving APRS stuff off the ISS and doing some FM passes on SO-50 which went fine. figured it was time to try a linear transponder bird and picked AO-73 since it seems like the go-to suggestion for beginners on the linear stuff.

problem is i cant seem to get my doppler correction right. im using gpredict to track and i have it linked to my ft-818 via hamlib but every time i think im on a signal i either lose it or i hear like a echo of myself drifting all over the place. i know you have to tune opposite directions on the uplink vs downlink but i feel like im chasing my own tail here. the pass was pretty decent elevation too, went up to like 52 degrees, so it wasnt a geometry issue i dont think.

also not sure if my cross yagis are phased right. built them myself from a M2 design i found somewhere, 2m and 70cm, running them into an arrow style phasing harness i cobbled together. preamp on the 70cm side. anyone dealt with this kind of thing when starting out with linear birds, any tips on getting the doppler workflow sorted before the pass starts rather than scrambling during it

yeah the doppler thing on linear transponders trips everybody up at first. the key thing people dont realize is that gpredict's rig control will move both VFOs if you set it up right, but you have to configure the satellite entry correctly — make sure you have the uplink and downlink frequencies set as the inverting transponder type, not just two separate frequencies. if gpredict thinks its a non-inverting transponder it'll move your uplink the wrong direction and you'll spend the whole pass chasing nothing.

also the echo you're hearing is almost certainly your own signal coming back down, which actually means you ARE in the passband, you're just not tuned to where another station is or you're hearing your downlink slightly off. on AO-73 the transponder is pretty narrow so even a few hundred hz puts you outside where you want to be. what i do is set everything up about 10 minutes before AOS, get gpredict locked and syncing, and just do a quick beacon check on the downlink first before i even try to work anyone. the beacon is at the lower edge of the passband and gives you a reference point. once you can hear that clearly you know your downlink is working and you can start worrying about the uplink.

the phased harness thing could also be an issue. if the polarity is off you'll lose like 3dB or more depending on the pass geometry. how are you checking that?

i had the same frustration on my first few linear passes, honestly its kind of a rite of passage lol. one thing that helped me was switching to manually correcting doppler during the pass instead of relying fully on the hamlib link — not because automated is bad but because it forced me to really understand what was happening. you tune the downlink to keep what you're hearing centered, and the uplink chases accordingly. once you feel that in your fingers a few passes in it starts to click.

AO-73 can be a little quiet honestly depending on how many ops are on. have you tried FO-29 or even TEVEL-2 when its operational, sometimes a busier bird helps because you have more signals to orient yourself with. also worth mentioning — whats your uplink power? the 818 is only 5w and with some antenna setups that can be on the edge for getting through cleanly on the linear birds, especially at lower elevations.