[amsat-bb] Doppler shift question

Philip Jenkins n4hf.philip at gmail.com
Wed Jun 5 17:41:49 UTC 2019

This came up at AMSAT Academy at Hamvention, and I still can't wrap my head
around it (something simple I'm not getting, I'm sure). I know the
xmit/receive frequencies aren't shifted, stay the same at the satellite.

SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I
lower my receive frequency (and continue lowering it as the bird approaches
LOS). So far so good.

AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I
go up in my transmit frequency.

Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when
receiving a satellite, but *raise* the 435 Mhz frequency when transmitting
to a satelllite?

So, my question boils down to - why should transmit doppler shift go in the
opposite direction from receive on the same band? In both cases, the
satellites are approaching me (from AOS).

Basically, why the difference when I'm transmitting  and when I'm


Philip N4HF

More information about the AMSAT-BB mailing list