so ive been trying to work AO-73 for a few weeks now and i finally got a decent pass last night where i could actually hear myself in the downlink which was pretty exciting. running an FT-818 with a couple of arrow antennas, nothing fancy, just handheld pointing by eye which honestly works better than i expected once you get the feel for it.

anyway the part thats tripping me up is the doppler compensation. i understand conceptually that you have to tune as the satellite moves but in practice its like im always chasing it and never quite locked in. i was using Gpredict on my laptop to get the timing and elevation but i wasnt doing any of the rig control stuff, just manually reading the predicted frequency offset and trying to adjust on the fly.

the other thing i noticed is that when i transmit and try to find my own signal in the passband it keeps drifting around and im not sure if thats me not compensating fast enough or if theres something weird going on with the transponder itself. someone on another thread mentioned inverting sidebands but i havent fully wrapped my head around why that matters yet. any tips from people who have actually done this a bunch would be helpful, im sure im just missing something obvious

the inverting sideband thing is actually key and once you get it it kind of clicks. on a linear inverting transponder like AO-73 if you tune your uplink VFO up, your signal moves DOWN in the downlink passband. so doppler correction ends up being kind of counterintuitive because you're compensating in opposite directions on the two VFOs. Gpredict can handle the rig control for both VFOs if you set it up right and that takes a lot of the manual juggling away, its worth spending an hour getting that configured trust me.

as for chasing your own signal, what helped me early on was to just find a clear spot in the passband first before transmitting, listen for your own tone, then barely whisper on the uplink so youre not blasting through the whole transponder. the AGC on these birds will pull everyone down if one station is too loud and people get annoyed fast. once you can hear yourself consistently the doppler starts to make more sense because you can actually feel the relationship between your tuning and what comes back down.

yeah i went through the exact same thing a few months ago. honestly the manual doppler correction by hand during a fast overhead pass is pretty rough, LEO passes are short and the rate of change is steepest near AOS and LOS which is also when youre fumbling with the antenna angle anyway. i gave up trying to do it all manually and just got the rig control working in Gpredict, made a huge difference. the arrow antenna pointing by eye thing is legit though, works fine up to maybe 10 degrees elevation or so and then you kinda lose it but by then the pass is usually almost done anyway