Doppler shift:

First, let's get to understand what Doppler shift is.
Everybody knows of the pitch change of a fast moving object, like a passing car.
The motor or horn sound of a car at constant speed seems to be pitched up as long as it approaches, and it pitches down as soon as it has passed.
This is called Doppler shift.

The same doppler effect would occur if the car would be static and you would be running by at high speed.
Or imagine you would take a microphone and swing it around at its cord.
As long as it swings towards the car, the microphone will pick up a higher pitched tone of that car.
In the opposite part of the swing, it will move away from the car, pitching down the sound of the car.



If a receiving antenna is physically rotated, we get the same situation as in our microphone example:
The transmitters frequency seems to go up and down in frequency due to the doppler effect,
which is sufficient to calculate a bearing estimate of the recived transmitter.
This would be a true doppler direction finder.

But if the antenna array is virtually "rotated" as in a 4-antenna pseudo-doppler direction finder, 
the connected FM receiver will "see" four phase jumps per cycle, resulting in 4 pulses in its audio output.
These 4 pulses per cycle will sound as a whining tone in the receivers audio.

A microcontroller based radio direction finder can calculate the Angle Of Arrivall using these phase jump pulses in the receivers audio.



Advantages:

A pseudo doppler Radio Direction Finder is relatively simple and uses a standard amateur grade narrow band FM receiver.

Disadvantages:

A pseudo doppler Radio Direction Finder needs a more or less constant signal WITHIN ITS BANDWIDTH to perform properly.
So, NBFM, AM, SSB and RTTY (narrow band digital modes) are ok.
WBFM (radio broadcast) is more difficult and requires a WBFM receiver.
Wide digital signals and noise are simply impossible to locate using this principle.

Accuracy:
Generally, the mean accuracy of a 4 antenna pseudo doppler Radio Direction Finder will be within +/- 5 degrees in good operating conditions.
We should realize that the accuracy will be reduced by signal reflections from objects in its vicinity,
which may introduce bearing deviations much larger that the intrinsic errors of this Radio Direction Finder.
That is also the case when you are hunting with a mobile (car) RDF, but since it changes location constantly, the changing deviations can be averaged into a much smaller bearing error.
Especially a microcontroller based mobile Radio Direction Finder can use smart algorithms to reduce the effects of multipath reception.

The following accuracy measurements are made in fairly open terrain (our local model airplane field) using the UHF mobile array.




Cheers, PA8W.