[Xastir] RDF Units?

[James Ewen]

> I haven't messed with direction-finding for quite some time.  I
> built portable quads and the double-ducky direction finder way back
> when, but always had my eye on the doppler systems ever since I saw
> the first one in QST back in, what, '78 or so?  The DoppleScant.

It works quite well, but some of the newer designs are supposed to work 
better. The DoppleScAnt needs a fairly healthy signal to overcome the 
antenna switching noise. You still need a beam for the weak ones, but when 
you get closer, the DoppleScAnt makes it like shooting fish in a barrel. You 
also really get an appreciation for multipath. You can almost see the 
signals bouncing off buildings as you drive.

I haven't modified mine from the original 78 schematics. There are upgrades 
that are supposed to make the switching better, and also a serial interface 
to tie into the computer.

> If someone can point me to a theory of operation for decoding the
> audio signal to come up with a bearing, that would be of interest as
> well.  If it's a relatively simple sort of operation (and it must be
> since people are doing it with a PIC), then it might be fun to do
> that myself someday.  Any open-source RDF Doppler projects out there
> I can glean stuff from?

The theory of operation is simple. Doppler shift occurs when an object is 
moving towards or away from the observer. The shift is higher in frequency 
when moving towards the object, and lower in frequency when moving away from 
the object.

The movement is simulated by scanning through an array of antennas. This 
creates an increase and decrease in tone as decoded by an FM radio. The 
DoppleScAnt uses an array of eight antennas, and when you view the waveform 
on an oscilloscope, you can see a pseudo-sine wave made up of eight steps. 
If you smooth that sine wave out, and watch for the zero crossing as the 
signal is decreasing in tone, and you have found the direction towards the 
signal source.

Ramsey describes it in their PDF. 
http://www.ramseyelectronics.com/downloads/manuals/DDF1.pdf

Tom Bruhns had one last summer he was trying to get rid of...
http://www.allaudio.org/detail-9494905.html

Neat toys... everyone should have one... hang on a minute! That means I 
wouldn't have an unfair advantage anymore!

James