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Where Does Doppler Occur?



I have been wondering...we all observe Doppler shift when we monitor a
downlink signal from a satellite.  But where precisely does it occur?  I
have thought about this at great length, and I have a theory I wish to run
by some of our "experts" out there.

Let's say a low orbit satellite is transmitting on 443.000 MHz.  If I'm
listening for that signal as the bird just breaks over the horizon, I would
most likely hear that signal around 443.010 MHz--about 10 kc above where
it's actually being transmitted from the satellite.

If I continue to monitor that signal throughout the pass, I will find that
the signal appears to be exactly on frequency when the satellite is right
overhead.

Then as the satellite starts to go away from me, and eventually nears the
horizon again, the signal appears to decrease in frequency by about 10 kc
right before LOS.

Exactly where does that frequency shifting take place?  At first I wondered
if it was occurring on some linear scale over that path between the
satellite and me.  But I have abandoned that theory for what I believe to be
a more plausible one.

I'm convinced that the shifting occurs at the moment the signal strikes my
antenna and is converted from an electromagnetic wave into voltage.  If the
satellite is coming towards me, the waves arrive more frequently, so the
frequency becomes higher.

I tend to think of it in an over-simplified way also.  I picture the
incoming waves being "squashed" closer together when the satellite is coming
towards me, and being "stretched" when it's going away.  (Over-simplified, I
know.)

Am I right about where Doppler occurs--at the receive antenna--or does this
shifting process take place in some other way.

Since I'm not on the AMSAT mailing list directly, please reply directly to
me in addition to the reflector so I can see your response.  Your opinions
and theories are appreciated as I venture into the world of amateur
satellites.

73, James Alderman, KF5WT
Dallas, TX



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