"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@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@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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