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

James Sharp rescue at sunhelp.org
Mon Jun 11 15:19:34 CDT 2001

If you put the receiver you're connecting the DF unit to into FM mode, the
doppler shift induced by the antenna rotation imparts an audible tone in
the recieved audio.  If you churn that through a set of filter and if you
know the speed of the antenna rotation, you can easily compute the doppler
shift and thus the direction of the signal.

All of this is done in discrete components on the board.

There's a really good book about DF techniques and theories.  It's called
"Transmitter Hunting and Radio Direction Simplified".  It gives good
descriptions about how the two-antenna phase-differential and the doppler
systems work.  You should be able to get it at any ham store or through
the ARRL.

> I was just wondering if it did actually use the Doppler
> effect or was just measuring phase of the incoming wave
> at two antennas like the two antenna design that I have.
>  Not really knowing how they are supposed to work (as
> opposed to how they claim to work) and from the
> description given, it just did not look like Doppler to
> me, just measuring phase difference between the antennas.
>  Obviously there has to be some heavy calculations to
> determine the sum and difference frequencies of the
> received signal due to the rotation of the virtual
> antenna to determine the direction of the source.
> Something that an IPX might not be able to do (to get
> this back on track).

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