[rescue] Radio DF equipment (was Ham radio license question)

James Sharp rescue at sunhelp.org
Wed Jun 13 01:37:13 CDT 2001

> OK, so how does it determine direction?  The only reason
> I ask besides being brain dead at times is that I've
> seen so much crap in ARRL books and ham magazines about
> certain areas that I'm a bit skeptical when I hear
> claims that devices work "this way (tm)".

Okay...Imagine you're being swung around on a rope at a high speed and
there's the noise maker in front of you.  You know two things at all
times...the apparent frequency of the sound you're hearing and your
angular position in the circle you're being swung around in.

Picture this from a top down perspective.   Put the noise source at the 12
o'clock position and you're being swung clockwise, starting at the 12 o
clock position as well.  As you progress from 12 to 6, you will be
proceeding away from the noise source and the apparent frequency will be
lower than the actual frequency due to doppler shift.  At the 3 oclock
position, doppler shift will reach maxium because your velocity vector is
parallel to a line drawn from the noise source to the center of the
circle...in effect, you're headed directly away from the source.

As you progress from 3 to 6, the doppler shift will begin to decrease as
relative motion away from the source decreases.  At 6'oclock, your
velocity vector is perpendicular to the line from the noise source to the
center, therefore you have no relative motion toward or away from the
source and thus have no doppler shift.

>From 6 back to 12 is basically the same, except you experience a positive
doppler shift because you're approaching the source.  At 12 oclock, you
once again have no doppler shift...and the cycle begins again.

So now you have two points and can draw a line on which the noise source
lies.  I forget exactly how the ramsey kit determines which doppler null
is the one that points toward the target...I'd have to go dig up the books
from the basement...but I remember it having something to do with audio

Now...when you induce a doppler shift on a signal and attempt to decode it
through an FM detector, the doppler shift induces a tone in the audio.
that tone is indicative of the doppler shift amount.  If you know that and
your angular position in the described circle, you can plot your nulls on
the circle and then draw your line.

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