[amsat-bb] Doppler shift question

[KC9SGV]

More train Doppler effect.
These are sound waves, but the RF FM principle is the same.
Not sure why they don't use AM in space, like in aviation...I.e. NO Doppler effect.

https://youtu.be/fWX38qjECyE

KC9SGV

Sent from my iPad

> On Jun 5, 2019, at 9:39 PM, KC9SGV <kc9sgv at gmail.com> wrote:
> 
> A band on a train explains Doppler effect the best.
> On board the train, the sounds are normal, I.e. the frequency is constant.
> 
> https://youtu.be/WgMxtT_jYf0
> 
> KC9SGV
> 
> Sent from my iPad
> 
>> On Jun 5, 2019, at 12:54 PM, Paul Stoetzer via AMSAT-BB <amsat-bb at amsat.org> wrote:
>> 
>> 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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>> _______________________________________________
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