so ive been messing around with ISS APRS and some of the FM birds for a while now and figured id finally try one of the linear transponder sats, specifically AO-73 since its supposed to be pretty active. got a yagi setup outside, nothing fancy, just a handheld yagi for 2m and a second one for 70cm and im manually pointing them which is already a pain but whatever.

the doppler thing is killing me though. i understand the concept, frequency shifts as the bird comes toward you and then goes away, fine. but in practice when im tuned to where i think the downlink should be i either hear nothing or i hear like a sliver of a signal that disappears immediately. im using gpredict on the laptop to track it and it shows me the doppler corrected frequencies but i dont fully understand if i should be adjusting my TX or my RX or both as it passes. my radio is a FT-818 paired with a cheap SDR for receive, thinking maybe that combo is making it more complicated than it needs to be.

anyone whos done this before, what did your first few passes actually look like in terms of what you were doing with the dial? like step by step almost. i know thats asking a lot but im really struggling to picture it.

yeah doppler on the linear birds is genuinely confusing at first, dont feel bad. the way i think about it is this — on a linear transponder the whole passband is inverting, so if the satellite is coming toward you the uplink needs to go UP in frequency and your downlink is also coming DOWN. gpredict should be handling the math for you if you have your radio connected via CAT control, but if youre doing it manually youre basically chasing two moving targets at once which is rough.

for your first few passes honestly just lock the TX frequency and dont touch it, just try to find yourself on the downlink and see if you can hear your own signal. once you can do that the rest starts to make sense because you can hear what the doppler is doing in real time. the FT-818 with SDR setup is a bit awkward but people make it work. bigger issue is probably that AO-73 isnt always on, it goes into eclipse mode sometimes and shuts the transponder down to save batteries so check amsat status page before you go outside with the yagis.

i went through exactly this like two years ago. spent three passes hearing absolutely nothing and then on the fourth one i finally caught my own signal coming back and it was one of the coolest moments ive had in ham radio honestly. the key thing nobody told me early on is that the linear transponder passband is only like 40khz wide on AO-73 so if youre even slightly off youre just gone. and with manual doppler tracking that window moves fast especially near AOS and LOS when the rate of change is highest.

also are you accounting for your own local oscillator drift on the SDR? cheap SDRs can wander a bit especially when they first warm up and that adds another layer of confusion on top of the doppler. might be worth letting it run for 10-15 minutes before the pass so it stabilizes. once you get one clean contact through a linear bird youll be hooked, fair warning.