Radio Direction Finding Technology
What is a typical pseudo doppler Radio Direction Finding
1, You need a NBFM receiver for a pseudo doppler Radio Direction
or an AM receiver
for the Amplitude version.
The receiver has to be tuned to the frequency of interest.
2, You need a RDF antenna array, like 4 dipoles or whips in a pseudo
or a switchable directional antenna array for the Amplitude version.
3, You need a RDF processor, like the RDF41, 42, 43, to control the
antenna array and process the audio coming from the attached receiver.
Due to the automatic switching of the antennas, the audio contains the
information necessary for the RDF processor to calculate the Angle of
Arrival (or Bearing) of the received signal.
How do reflections complicate radio direction finding?
Well, there are actually two main groups of reflections:
1, Reflections that add to the direct signal and “pull” the bearing off
Generally due to objects nearby, like trees, buildings up to a few
kilometers away, etc.
They can be recognized by looking at the symmetry indicator, it will
generally show bad symmetry.
2, Reflections without direct signal, since the source may be
from behind the horizon, buildings or mountains.
In these cases, a large bridge, tower, hill or mountain may have direct
sight on the
transmitter and mirror the signal to the receiver.
In this case there’s no way of knowing it is a reflection other than
using common sense and user experience.
The Radio Direction
will indicate a pure signal that seems to come
from that particular bridge or mountain.
Can your Direction Finders find wildlife trackers?
Yes, sort of...
Wildlife trackers generally transmit in very short bursts.
That is not the biggest problem, but generally these trackers are very
and therefore the standard tracking method uses a directive antenna
with gain (yagi, HB9CV, LogPer) in combination with a receiver in SSB
In terms of receiving sensitivity, a pseudo doppler really can't
compete with that classic technique, for the following reasons:
1, It needs omnidirectional antennas with almost no gain,
2, It needs a receiver in NBFM mode, which is less sensitive than
a receiver in SSB mode,
3, Even our soft-switching technique does add some additional noise,
increasing the problem.
And for an Amplitude RDF the following handicaps are true:
1, It needs a receiver in AM mode, which is less sensitive than a
receiver in SSB mode,
2, Even our soft-switching technique does add additional noise,
increasing the problem.
So, for wildlife tracking the old fashioned way is very hard to beat.
However, the strong point of a pseudo doppler- (or automatic Amplitude-) Radio Direction
Once you receive a faint signal, you will have an instant direction
indication with fair accuracy.
And once moving in a car, the doppler will exploit every short moment
in which the signal pops up out of the noise.
Especially at locations where an old fashioned yagi-measurement is out
of the question, like on an elevated highway or a highway bridge, the
automatic RDF proves its strength.
What audio input signal/ level does a Radio Direction Finder
My RDF41/42/43 need an audio signal from the radio that is
attached to the RDF array.
Because the electronic antenna rotation will cause jumps/spikes in the
receiver's audio that contain the angular information for the Radio Direction
The audio can be taken from any line output, headphone output, speaker
output or -for a pseudo doppler setup- a discriminator output.
To deal with all possible audio levels, my Radio Direction
are equipped with an
audio gain trimmer on the interface board.
To set the right level, make sure that the horizontal lines of the
symmetry indicator fill the space between the outer borders
for 70% -90%.
In the following picture, the top situation is showing a bit
too little audio.
The bottom situation shows a bit too much audio, and the middle
position shows a perfectly good audio level.
The elevation indicator (RDF42 and 43) can also be used as
such, when the elevation calibration is default:
Adjust the audio level until the elevation indicator
shows just a few degrees above zero. (that is, on earthbound signals of
A higher audio level will increase the Q-rating a measurement gets, but
the audio level is not very critical for best bearing accuracy.
Starting with 2021 RDF's, an OVERLOAD warning will appear on the screen
when the input audio level is too high.
your Radio Direction Finders find IoT / Wifi/ Cellphone signals?
Nope, wide spectrum or spread spectrum signals are impossible to track
using a normal pseudo doppler or Amplitude variant.
But the amplitude variant can track analog Television signals and
continuous cellphone signals quite well.
Why do you use 4 antennas in your arrays? Why not 5 or even
3 antennas is the minimum to have no ambiguity. Any number higher than
2 is feasible.
4 antennas are convenient for the control logic.
4 antennas are convenient in calculating bearings (X and Y values)
4 antennas offer the possibility of quality weighing the measurements
as I do in the RDF40/41/42.
4 antennas on a car roof can be located in a way that all 4 antennas
are in very similar conditions concerning the mass area they “feel”.
4 antenna designs are mechanically easy, especially an advantage in a
4 antennas perform excellent in a good design, so why go to higher
(I designed and built two different 8-antenna dopplers but they were
not really worth the extra cost and effort)
existing antennas interfere with a RDF array?
Yes, they do have an impact on the measured bearing when they are
within 1/2 wavelength distance from the array.
In a test setup I managed to achieve bearing shifts of + and - 15
degrees by moving a resonant extra antenna close to one of the array
So the impact is not dramatic, but it does exist.
UHF antennas do not interfere in a VHF array, since they are much
smaller than 1/4 wavelength.
2m band antennas however do interfere with a 70cm array, since they are
3x 1/4 wavelength...
There are very professional wideband amplitude radio direction finders
based on small EWE antennas (terminated wire or plate)
These non-resonant antennas have very low gain (down to -20dB) but they
are extremely wideband with a high front to back ratio.
Their (cardioid) radiation patterns and F/B-ratio are easily destroyed
by anything resonant in the vicinity.
Even at 2 wavelengths distance the influence of a resonant antenna can
be quite serious.
Does it matter whether the hunted signal is modulated or not?
An unmodulated carrier is the easiest signal for a Radio Direction Finder to produce a
Transmitter modulation is in fact a negative factor as it masks the
“doppler” tone that is produced by rotating the antenna array.
So if no countermeasures are taken, it will make the reading of the
direction very nervous or even impossible.
The worst are modulation frequencies that are very close to the antenna
In all of my Radio Direction Finder designs I go to great length to minimize the impact of
modulation on the calculated bearing.
First I apply a switched capacitor filter that is only a few Hz (or
even less than a Hz!) wide to get rid of most of the noise and
Additionally, the effects of modulation will decrease the Q-rating I
give to every measurement,
and therefore these polluted measurements have very little impact on
the calculation of the long time Averaged Bearing.
So overall, normal modulation has very limited impact on the
performance of the RDF41/42/43.
But the (modulation dependant!) signal bandwidth is a key parameter
here as well.
In below table the (im-) possibilities are shown.