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.
A Pseudo Doppler Radio Direction Finder does exactly the same thing; it uses a circular arrangement of antenna elements which are activated one after the other in a very fast sequence, simulating one antenna element being swung around in a circle at very high speed.
This means that the received carrier of the tracked radio signal will be moved up in frequency as long as the antenna element swings towards the transmitter, and shortly after that the frequency will apparently be lowered below the true frequency due to the antenna element moving away again.
The antenna array is rotated electronically at around 500 revolutions per second, so a FM receiver connected to this electronically rotated antenna will produce a 500Hz audio tone.
The zero crossings of that tone (plus or minus some phase shift etc. etc. ) will mark the points where the doppler shift is zero.
These points are easy to detect electronically, and after the necessary compensation for phase shift mentioned above, one of these points is used as a trigger to light the LED at the correct point in the circle.
(Microcontroller based radio direction finders however don’t look for zero crossings,
instead they calculate the Angle Of Arrival directly comparing the signals of the different antennas.)
The simplified Block Diagram of a conventional pseudo doppler RDF:
Top-left we have the 4-element vertical dipole array, meant for non-mobile applications.
(for mobile applications there’s a similar arrangement of 4 magnet mount whip antennas)
The antenna array is “rotated” by the antenna multiplexer, which activates only one element at the time.
From the antenna array, a coax runs to our FM-receiver. The audio output of the receiver is fed into the RDF processor.
We should realize that particularly a fixed (base) RDF accuracy will suffer from signal reflections from objects in its vicinity,
which may introduce bearing deviations much larger that the intrinsic accuracy of this RDF.
A mobile RDF has the same problem, but since it changes location constantly, the changing deviations can be averaged into a much smaller bearing error.
Especially a microcontroller based mobile RDF can use smart algorithms to reduce the effects of multipath reception.
A pseudo doppler RDF is relatively simple and uses a standard amateur grade narrow band FM receiver.
A pseudo doppler RDF needs a more or less constant signal WITHIN ITS BANDWITH to perform properly.
So, NBFM, AM, SSB and narrow band digital modes are ok.
WBFM (radio broadcast) is difficult, wide digital signals and noise are simply impossible.
Generally, the mean accuracy of a 4 antenna pseudo doppler will be within +/- 5 degrees in good operating conditions.