so ive been wanting to get into satellite ops for a while now and finally had a decent pass of AO-73 yesterday where i actually heard some signals, which was exciting. but im really struggling with the doppler thing and i think i might be doing it wrong.

my setup right now is an FT-817 and i borrowed a dual band yagi from a club buddy to get started. im using Gpredict for tracking which seems to work fine, it shows the doppler corrected frequencies and stuff. the issue is when i try to actually tune around on the downlink i can hear voices fading in and out but they sound like chipmunks or sometimes really low, and i cant tell if thats doppler or if im just tuned wrong to begin with.

also someone told me i need to tune the uplink inversely from the downlink on a linear transponder but i dont fully get what that means mechanically. like do i just set up two vfos and move them in opposite directions manually or does gpredict handle all of this. feels like theres something fundamental im missing here about how passband transponders work vs FM.

yeah the inverse tuning thing trips up basically everyone at first. so on a linear transponder the passband is inverted, meaning if you tune your uplink higher in frequency your downlink signal comes out lower. so if you hear yourself drifting up on the downlink you actually need to tune your TX down, not up. its backwards from what feels natural.

gpredict can handle doppler correction automatically if you have it connected to your rig via CAT, but it corrects both VFOs independently based on the bird's velocity so you dont have to think about it much. the chipmunk thing you're describing is almost certainly doppler thats not being corrected, especially near AOS and LOS where the rate of change is fastest. mid pass it slows down considerably.

one thing that helped me a lot starting out was just to listen only for the first few passes and not even bother transmitting. get a feel for how signals drift and where the busy part of the passband is before you jump on. AO-73 can get pretty crowded on a good pass and if you're not correcting doppler you'll just be drifting through other peoples QSOs anyway.

the 817 is actually a decent starter rig for this, ive used one on sats before. one gotcha with it is the CAT control can be a little flaky with gpredict depending on which cable/driver you're using, so double check that its actually responding to frequency commands and not just pretending to. i had an issue once where gpredict thought it was connected but nothing was actually changing on the radio.

also on AO-73 specifically the linear transponder is LSB on the downlink so make sure you're not in USB or you'll hear everything all garbled. took me an embarrassingly long time to figure that out the first time lol.