[amsat-bb] Doppler shift question

[k6vug at sbcglobal.net]

 "A perceptive question deserves a proper answer..."
- Umesh, k6vug

    On Thursday, June 6, 2019, 7:34:51 AM PDT, Philip Jenkins via AMSAT-BB <amsat-bb at amsat.org> wrote:  
 
 Thanks for all the replies, both on and off list.

I should have given my satellite background in my first email; it would
have saved some of you quite a bit of typing! :-)

I'm an AMSAT Ambassador, AMSAT-NA Life Member, and my very first satellite
QSO was on AO-13 in 1993 (but then I was inactive on the birds for 20
years) - and did learn a few things I didn't know at Hamvention's AMSAT
Academy. :-)  I have given "Satellite 101" talks to about 10 clubs in the
past 2 years,  have manned info tables at a few hamfests, and may tackle a
couple of hamfest forums solo during the next year.

I had hoped my question about receive versus transmit was specific enough
(since I know how to operate the birds and explain doppler when I give
presentations at clubs), but as I said, it would have been helpful to you
respondents if you had already been familiar with my background. Perhaps I
could have worded it better, and been more succinct. I'm glad that I got so
many answers - and I did get a comprehensible, clear, answer right off the
bat - and hopefully your responses will help those who just "lurk" on the
BB to understand doppler better and how to work the sats more effectively.

So thanks again to all who replied!

Philip N4HF EM85

On Wed, Jun 5, 2019 at 1:41 PM Philip Jenkins <n4hf.philip at gmail.com> 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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