[amsat-bb] Doppler shift question

[Greg D]

And to add to this, when the satellite passes by you, what was a
compression of the downlink as it approached turns into an expansion as
it passes away.  So the downward tuning of the received signal
continues.  It started above "actual" at the start of the pass, was
dead-on right as the satellite got to its closest point, and then
continued below actual as it trails off to the setting horizon.

And everything is swapped for your uplink.  You start out below center
at the start of the pass, even with it in the middle, and above towards
the end of the pass.

FM satellites (such as SO-50) benefit from the "FM Capture Effect",
where a strong enough FM signal can be a little off frequency and still
get received correctly.  This can let you not have to mess with the 2
meter band (whether up or down link), and just focus on the 70cm part. 
Tuning both is best, if you can, as it gets you a bit more margin.

Good luck on the birds,

Greg  KO6TH


Dave Taylor via AMSAT-BB wrote:
> Look at it from the satellite’s point of view.  The satellite is receiving your Doppler-shifted signal.  If you continue transmitting on a fixed frequency, the satellite sees the frequency dropping.  To keep the frequency seen by the satellite steady, you need to offset the drop by increasing your transmit frequency at the same rate.
>
> Dave, W8AAS
>
>> On Jun 5, 2019, at 1:41 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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