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# Re: Doppler and Light Speed

• Subject: Re: [amsat-bb] Doppler and Light Speed
• From: "Pieter Ibelings" <elpieter@xxxxxxxxxxx>
• Date: Sun, 16 Dec 2001 14:17:44 -0500

```Hi James,

I do not think doppler is created anywhere. A moving receiver toward a
signal source cannot tell the difference between what it sees and a higher
frequency source if it was stationary. The receiver just sees a higher
frequency because it encounters the wavefront at a shorter period. Let's
leave that one there :)

Now on the propagation of radio waves. You commonly hear in school text
books that radio waves start at 20 or so KHz and everything below is sound
waves. In reality, radio waves could be any frequency even 20 KHz. So you
can have a 20 KHz sound wave that you can hear and also have a 20 KHz radio
wave traveling through the air. What makes the differencee between sound and
radio waves is how they are propagating.  Sound travels in a longuitudinal
wave by compressing and rarefying air. The wave travels in the direction of
propagation. Radio waves , on the other hand, will propagate in transverse
waves because the wave moves at right angles to the direction of
propagation. In other words, the radio waves look like water waves because
the motion of the water is up and down, or at right angles to direction of
propagation. An interesting point to make is that sound waves travel faster
in denser mediums and slower in low density ones. Sound waves in the gound
will travel a lot faster than in air and will not travel in a vacuum. Radio
waves are the contrary. They will travel at the speed of light (abbreviated
"c" which is the speed of light in a vacuum). In order to make a radio wave
speed up, you would have to lower the density of the medium by removing all
of the molucules and interactions that might slow the light down. A vacuum
already has nothing in it so that is as fast as you can go. If someone ever
figures out how to make light go faster than the speed of light in a vacuum,
please let me know. Another very inportant point is that for something to
travel at the speed of light, it cannot have any mass.

Back to basic radio propagation. When radio waves propagate through a
medium, they interact electrically and magnetically with the medium. There
is a measure on how much the medium interacts electrically and magnetically.
These are the permittivity and permeability of the medium. Some materials
have more of one than the other. The important thing is that the speed of
light will be around 299,792.458 km/s in a vacuum and it can be derived from
a simple formula c=square root of (mu0/e0) where mu0 = the permeability of
free space and e0 if the permittivity of free space. This means that you can
measure the permeability and permittivity of lets say glass and you can
easily calculate how fast light will travel through it. Free space is
another word for a very good vacuum. Since yiou cannot remove anything from
free space, there is your maximum speed. You can get lower, but not faster.
Some people will argue that things can travel faster, but I am not holding
my breath.

You can speed the RF in a coax by removing things that are slowing it down.
You not necessarily need more energy. A boat with a 100 HP engine will move
faster in water than in tar. A rf signal will travel faster in an air
dielectric coax than in a teflon dielectric coax. To make it go faster than
in an air dielectric coax will involve removing the air. Even after you do
all these things, you wont get any faster than the speed of light in a
vacuum.

Some bonus interesting points:

1. The speed of light seems to be changing with time :). The permeability
and permittivity of free space appear not to be constant with time

2. When you travel faster than the speed of sound, you get a sonic boom. A
bullet can travel faster than the speed of sound.

3. Travelling faster than the speed of light in a transverse mode is pretty
much ruled out. Pretty much nothing can travel any faster in this mode.
Unless something really weird is out there :)

4. Gravity travels at the speed of light and does not seem to be affected by
permeability and permittivity. If it travels at squareroot of (mu0/e0), why
is it not affected by them? By the way, gravity could possibly travel a
little faster than light.

5. When you travel faster than the speed of light, you get a light boom. You
can make these in the lab :)

----- Original Message -----
From: "James Alderman, KF5WT" <kf5wt@verizon.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, December 16, 2001 12:57 PM
Subject: [amsat-bb] 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
>
> 73, James Alderman, KF5WT
> Dallas, TX
>
>
>
>
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