so ive been trying to work the linear transponders on AO-73 for about three weeks now and im just not getting it. i can hear signals on the downlink sometimes, pretty faint but theyre there, but whenever i try to transmit i either cant hear myself come back through or the signal just disappears. running a 7el yagi on 145 and a 5el on 435, both homebrew, connected to an IC-9700.

the tracking software is gpredict and i think its correct, im using the latest TLEs from celestrak. passes overhead here are maybe 8-10 minutes when its a good high elevation pass. the problem is i dont really know if im actually hitting the bird or not because the transponder is linear and im used to FM birds where you just hear the squelch break.

someone told me i need to do the doppler correction manually on the 9700 but i thought the 9700 handled that through gpredict? maybe im misunderstanding how the whole thing works. any help would be appreciated, been lurking this forum for a while and figured id finally ask

the doppler thing is probably your main issue. on a linear transponder the uplink and downlink are on different bands so the doppler shift is different for each one and you have to correct both independently. gpredict can drive the 9700 via CAT if you set it up right, there's a radio control section in the settings where you point it at the CI-V port. once thats done gpredict will chase the frequency on both VFOs as the pass progresses.

but honestly even before that, try just listening first. tune around the downlink passband during a good pass, like 60 degrees or higher elevation if you can catch one, and just see if you can pull out any SSB signals. AO-73 isnt always active depending on its power budget too, worth checking amsat's status pages before you go out because sometimes the bird is in economy mode and the transponder is off or limited.

also your homebrew yagis — are they circularly polarized? linear pol works but you'll have deep fades as the satellite tumbles, which AO-73 does. not a dealbreaker but something to be aware of.

yeah what he said about the transponder being off sometimes, i wasted like two weeks before i figured that out lol. check the amsat-uk page they usually have status updates.

one other thing, when you do hear yourself come back through the transponder your signal will be inverted — so if youre transmitting USB on the uplink you'll hear yourself as LSB on the downlink. some people get confused by that. the 9700 in satellite mode should handle it automatically if you set the mode right but just something to double check