so ive been chasing LEO satellites for a few months now mostly just FM birds like SO-50 and PO-101, got pretty comfortable with that whole thing. decided to jump into the linear transponder world with AO-73 last week and man its a whole different beast.

my main issue is that i keep losing my own signal during the pass. i know about the doppler shift obviously and ive got gpredict running on my laptop feeding corrected frequencies to my IC-9700 but somewhere in the middle of the pass i just... lose myself. i can hear other stations fine, the transponder is clearly working, but i cant find my own downlink anymore. im transmitting SSB on the uplink and listening on the downlink passband and i just drift off apparently.

is this a case where i need to be manually tweaking the RX frequency even with doppler correction running? i thought gpredict handled all that but maybe im missing something in the rig control setup. or is this more of an antenna issue, im running a pair of yagis on an az-el mount that i built last year. any ideas would be appreciated, this has been frustrating me for a few passes now.

yeah the linear transponder doppler thing trips up pretty much everyone at first. so the deal with inverted linear transponders like AO-73 is that your uplink and downlink doppler corrections actually work in opposite directions through the passband -- if you tune up on the uplink your signal appears lower on the downlink, so manually chasing your own signal kind of fights itself if you dont have that relationship set correctly in gpredict.

check your gpredict rig config, theres a setting for inverting the downlink tracking that you need to enable for inverting transponders. once thats set right gpredict should keep your signal roughly in the same spot in the passband throughout the pass. you'll still need to do small manual tweaks especially near AOS and LOS when doppler rate is highest, but it shouldnt be drifting way off on you. also make sure your CAT control polling rate isnt too slow, ive seen people set it at like 5 seconds and then wonder why their frequency is always a bit behind.

this was me about six months ago lol. the inverting transponder thing is confusing and i dont think gpredict makes it super obvious in the UI. one thing that helped me was just doing a few passes where i wasnt even trying to make contacts, just found a clear spot in the passband, sent a carrier, and watched where it went while gpredict was correcting. gave me a feel for how much manual intervention i actually needed vs what the software was handling. AO-73 is a good bird to learn on though, passband is decently wide and the transponder seems pretty robust.