PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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What's a (pseudo-) Doppler Radio Direction Finder

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.



A Pseudo Doppler Radio Direction Finder does more or less the same thing; it uses a circular arrangement of (often 4) antennas 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.

The antenna array is rotated electronically at around 500 revolutions per second, so a FM receiver connected to this electronically rotated antenna will produce four phase pulses per cycle, which can be heard as a 500Hz audio whining tone in the receivers audio.

A microcontroller based radio direction finder can calculate the Angle Of Arrival comparing the phase jump pulses in the receivers audio.



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.

Advantages:

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

Disadvantages:

A pseudo doppler RDF 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 difficult, wide digital signals and noise are simply impossible.


Accuracy:
Generally, the mean accuracy of
a 4 antenna pseudo doppler will be within +/- 5 degrees in good operating conditions.

73, Wil.


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