[amsat-bb] Doppler shift question

Paul Stoetzer n8hm at arrl.net
Wed Jun 5 17:54:13 UTC 2019

When you're on the ground listening to SO-50 as it approaches you, the
RF waves are compressed, and thus the frequency you receive at is
higher than the frequency the satellite is transmitting at.

When you're transmitting to AO-91 as it approaches you, you need to
compensate for the fact that your RF waves are going to be compressed
and appear at the satellite at a higher frequency than you are
transmitting at. So you have to transmit lower than the nominal


Paul, N8HM

On Wed, Jun 5, 2019 at 1:45 PM Philip Jenkins via AMSAT-BB
<amsat-bb at amsat.org> wrote:
> 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
> receiving?
> 73
> Philip N4HF
> _______________________________________________
> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions expressed
> are solely those of the author, and do not reflect the official views of AMSAT-NA.
> Not an AMSAT-NA member? Join now to support the amateur satellite program!
> Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb

More information about the AMSAT-BB mailing list