so i finally got around to trying AO-73 yesterday after months of just watching the waterfall on SDR and never actually transmitting. i've been doing terrestrial VHF/UHF for a while so i figured how different could it be right. very different apparently.

the doppler correction alone was kind of a nightmare to keep up with manually. i was using gpredict for tracking which worked fine, but trying to actually tune my uplink and downlink at the same time while the frequencies are sliding around... i dont know how people do this without a rig that does it automatically. i've got an FT-818 for uplink on 435 and an SDRplay for downlink monitoring on 145 and keeping both corrected by hand during a 9 minute pass was just chaos.

i did hear myself in the transponder for maybe 30 seconds which was exciting but i was barely able to have any kind of qso. the other station i heard kept drifting out of my passband and i wasnt sure if that was him not correcting or me not correcting or both. ended up logging nothing which is a bit disappointing but whatever, first pass. i'm assuming the doppler on the downlink moves faster than the uplink since it's a different band? like the 145 MHz side shifts less per second than the 435 side? i think i read that somewhere but want to make sure i understand it right before my next attempt tomorrow evening.

yeah you've got the doppler thing right — the frequency shift in Hz scales with the carrier frequency so your 435 uplink is going to move roughly 3x as much as the 145 downlink over the same pass. for a low LEO bird like AO-73 at a decent elevation pass you can see something like +/- 10 kHz on the 435 side and maybe 3-4 kHz on the downlink. so if you're doing manual correction you really gotta stay on top of the uplink more than anything.

the trick most guys use is to keep the downlink tuned to your own signal and just leave it there — let the transponder inversion work for you basically. since it's an inverting transponder if you tune your uplink up your downlink signal goes down, so you kind of develop this feel for it after a few passes. honestly after maybe 5 or 6 attempts something clicks and it stops feeling impossible. the guys running IC-9700 or the old IC-910H with doppler tuning via hamlib have it way easier but people were doing this stuff with split rigs long before computer control was a thing.

i had the exact same experience when i started on the linear birds, felt like trying to pat my head and rub my stomach while someone was slowly moving the floor. what helped me was just doing a few passes where i only listened and tracked my downlink tuning manually in gpredict without even trying to transmit. just getting comfortable with how fast things drift at different elevation angles and then adding the tx part later. low elevation passes are almost worse because the doppler rate of change is higher when the satellite is coming over the horizon weirdly enough, it levels off near max elevation. anyway dont get discouraged, AO-73 has pretty good uplink sensitivity so once you get the doppler thing sorted you wont need to push the 818 hard at all.