# [amsat-bb] Doppler shift question

KC9SGV kc9sgv at gmail.com
Thu Jun 6 02:39:00 UTC 2019

```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

> 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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> _______________________________________________
> 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
```