[amsat-bb] Doppler shift question

[Paul Stoetzer]

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
frequency.

73,

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
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