[hpsdr] Direction Finder using Dual-Mercury Diversity Operation

[Curt, WE7U]

On Thu, 22 Sep 2011, Joe Martin K5SO wrote:

> I don't know anything about "doppler scanning" so I couldn't say.  Maybe someone else knows.
>
> The problem with frequency diversity and related "frequency agile" approaches is that the source needs to be transmitting on more than one frequency, right?

When I referred to frequency agile I meant tuning the fixed-point receiving stations all to the same band/frequency in order to do DF'ing on an unknown transmitter.  This could be used for repeater-jammers, airplane transponders, FRS/GMRS/CB/Ham transmitters, whatever.  At the time of trying to do DF'ing, the DF'ing receivers would be fixed-frequency.

Doppler scanning arrays use multiple antennas, usually four, all attached to one receiver.  You switch ON one antenna at a time (or short out the other three) repetitively in a circle, causing a phase shift in an FM receiver which can then be filtered and displayed to determine the direction of the transmitting station in relation to the DF'ing array.  If you sync the switching of the antennas with the recovered signal from the switching of antennas, you can determine the direction.  I first read about this in the DoppleScant article in '78 or so, QST or 73 magazine I think.  It used four antennas and had 16 LED's on the display to determine the direction.  It could lock on and hold a direction with a TX time of 250mS.  The double-ducky direction finder was a simpler version with two antennas and a zero-center analog meter for the readout:  I built one of those and had fun with it.

I believe this method only works for FM receivers, as the switching of the antennas creates phase changes which introduce an audio tone in an FM receiver.  Whether this can be done with a single Mercury and switching of four antennas is the question, or whether it would work for different types of transmissions are the questions.

-- 
Curt, WE7U.        http://www.eskimo.com/~archer
The world DOES revolve around me:  I picked the coordinate system!"

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