so ive been doing terrestrial VHF/UHF for a few years now and finally decided to take the plunge on satellite ops. picked up a used FT-817 and cobbled together a pair of yagis for 2m and 70cm, nothing fancy, hand-rotated for now. been using Gpredict for tracking and honestly that part has been easier than i expected.

my question is about the linear transponders on something like AO-73 or FO-29. i get the basic concept — inverting transponder, USB on uplink becomes LSB on downlink or whatever — but im struggling with the doppler compensation. like do i need to be correcting for both uplinks AND downlinks simultaneously or is just tracking the downlink close enough? ive read conflicting stuff about this. some guys say just chase the downlink beacon and let the uplink drift a little, others say you need to correct both or youll walk all over other stations on the passband.

also is there a way to practice this without actually keying up and potentially causing problems? feel bad just throwing RF at a transponder when i still dont fully know what im doing

the doppler thing trips up pretty much everyone at first. short answer — yes you really want to correct both, but not for the reason most people think. the inverting transponder means your doppler corrections on uplink and downlink actually work in opposite directions, so if you only track one you end up with a larger net error than if you tracked neither. Gpredict can drive a rig via CAT if you set it up right, and it'll handle both simultaneously which takes a lot of the mental load off.

as for practicing without stepping on people, just listen for a while first. a few passes of just receiving lets you get a feel for where signals sit in the passband and how fast things move. FO-29 has been a bit hit or miss lately but AO-73 is usually pretty active when its in sunlight. you'll hear guys making contacts and can get a sense of the pace before you transmit. nobody expects perfection on the first couple passes honestly, just keep your power low and youll be fine.

yeah what he said on the doppler. i had the same confusion when i started. one thing that helped me was just watching the waterfall on the downlink for a few passes before i ever tried transmitting — you can actually see how the passband shifts and get a feel for the rate of change. the correction rate is way faster at AOS and LOS than it is near max elevation, which i kind of knew in theory but seeing it is different.

hand rotating yagis is totally doable btw, lots of people do it. gets tiring on longer passes but for a 10-12 minute pass its fine. i did that for probably a year before i built an az-el mount. just watch your elevation on Gpredict and try to keep up, high elevation passes are way harder to track manually but theyre also shorter horizon to horizon so its kind of a wash