Radio Direction Finding Technology
a (pseudo-) Doppler Radio Direction Finder?
First, let's get to understand what Doppler shift is.
Everybody knows of the pitch change of a fast moving object, like a
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
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
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
These 4 pulses per cycle will sound as a whining tone in the receivers
A microcontroller based radio direction finder can calculate the Angle
Of Arrival by comparing the phase jump pulses in the receivers audio to
the antenna commutation.
A pseudo doppler Radio Direction Finder is relatively simple and uses a
grade narrow band FM receiver.
A pseudo doppler Radio Direction
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
Generally, the mean accuracy of a 4 antenna pseudo doppler Radio Direction
within +/- 5 degrees in good operating conditions.
We should realize that particularly a fixed (base) Radio Direction
Finder accuracy will
suffer from signal reflections from objects in its vicinity,
which may introduce bearing deviations much larger that the intrinsic
errors of this Radio Direction
A mobile Radio Direction
has the same problem, but since it changes location
constantly, the changing deviations can be averaged into a much smaller
Especially a microcontroller based mobile Radio Direction
can use smart algorithms
to reduce the effects of multipath reception.