so ive been doing FM sats for a while now, worked SO-50 and a bunch of passes on the ISS during special events and felt pretty comfortable with that whole thing. but ive been wanting to try one of the linear transponder birds for ages and finally had a clear enough pass yesterday to give AO-73 a real shot.

got my yaesu FT-818 set up on the uplink and a separate VX-8DR just for monitoring the downlink while i twisted the dial, which by the way is way harder than it sounds because you're basically fighting the doppler the whole pass and trying not to trample on other guys at the same time. i could hear signals pretty clearly on the downlink passband but my own signal i only caught once or twice and im not even sure it was me.

the tracking software i was using was gpredict and it worked fine for az/el and the doppler prediction, but i noticed the frequency offsets it was calculating seemed slightly off compared to what i was actually hearing. like i had to shift maybe a couple hundred hz more than it wanted me to. could be my reference oscillator being slightly off i guess, or maybe the satellite itself has drifted a bit? anyone else notice this with AO-73 specifically or is this just normal and you kind of calibrate by ear as you go

yeah the frequency offset thing is pretty common and its almost never perfectly matched to what the software says. gpredict uses the published transponder center freq but those can shift a bit over time and your rig's TCXO or whatever reference you have is probably slightly off too, even a few ppm adds up when you're dealing with doppler corrections at 145 MHz. i just use a known beacon on the downlink to calibrate before i start seriously working the pass, get your radio zeroed on that and then you have a reference point for the rest.

the dual radio thing is the right approach but it takes some getting used to. honestly the hardest part for me early on was remembering to tune the uplink opposite to the downlink doppler direction when you're in inverting mode — which AO-73's linear transponder is. if you tune up on the downlink to find your signal you actually have to tune down on the uplink, catches a lot of people off guard the first few times

im kind of in the same boat as you, been FM sat for maybe a year and just started poking at linear transponders. the inverting transponder thing the previous guy mentioned took me forever to wrap my head around. i drew a little diagram to remind myself which way to turn the knob lol. also what antenna setup are you running? im using a pair of arrow yagis and hand tracking and i feel like im losing a ton of signal just from not being perfectly pointed during a low elevation pass