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Doppler and Light Speed



Greetings Fellow Satellite Enthusiasts,

Thanks to all of you who responded to my question about Doppler shift.  I
must say I learned a lot more than I expected.  We truly have a vast
reservoir of knowledge out there in the amateur satellite community.

The general answer to my inquiry seems to be this:  Doppler is not really a
condition that "exists", like voltage drop across a resistor, or AC phase
shift across a capacitor.  Those conditions would be present whether I were
there measuring them or not.

Doppler is more of a phenomena that is "observed."  Doppler is an observed
phenomena similar to the strobing effect we observe in some movies where a
car's wheels may appear to be turning backwards.  Of course, the wheels
aren't really turning backwards.  They just appear to be because of the
frame rate of the motion picture film.  (We might see this same type of
observed phenomena when a fan appears to be turning backwards, or very slow,
under a fluorescent light.)

In a like manner, Doppler shift from a satellite may be observed by one guy
on the ground, while at the same time a guy flying along in space with the
satellite would observe no Doppler at all.

As for my question as to where Doppler occurs, I'm convinced that it
"happens" at the point where it is observed.  If I'm standing along side a
train track, the Doppler "occurs" when those compressed sound waves strike
my ear.

If I'm monitoring a satellite beacon, Doppler for me "occurs" when the RF
strikes my antenna and is converted to voltage which my receiver picks up.
In other words, if we must say that Doppler occurs somewhere, it happens
wherever you receive the signal.

Now I have a question about the speed of radio waves.  We read in the books
that radio waves travel at the speed of light in free space.  We also read
that in the atmosphere, they travel a bit slower than light speed.  If
atmosphere can slow a signal down, can lack of atmosphere speed it up?

If a satellite is beaming a signal to me, the RF travels at light speed
until it enters the atmosphere, then it slows down.  If I beam a signal to
the satellite, my RF travels slower than light speed through the atmosphere.
What happens when the signal breaks into free space?  Does it speed back up
to light speed?  If so, where does it get the energy needed to increase its
speed?

I'm convinced that RF must have the ability to increase in speed.  RF
travels at about 60% of light speed in a transmission line (assuming the
velocity factor or the coax is .6), and at nearly full speed when it reaches
the antenna and is sprayed out into the air.

Here's the question:  Am I right that RF does increase in speed?  If so,
where does the energy come from which accelerates the waves?

Since I'm not subscribed directly to the reflector, please also reply
directly to me so I can conveniently see your response.  Thanks for your
technical advise.

73, James Alderman, KF5WT
Dallas, TX




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