PA8W's Radio Direction Finding Technology

Frequently Asked Questions:

What is a typical pseudo doppler Radio Direction Finding setup?

1, You need a NBFM receiver for a pseudo doppler 
Radio Direction Finder, 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 doppler setup,
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 a bit:
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 originating 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.
Radio Direction Finder 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 low power,
and therefore the standard tracking method uses a directive antenna with gain (yagi, HB9CV, LogPer) in combination with a receiver in SSB mode.

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 Finder is speed:
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 need?

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 Finder to work with.
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 Finders 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.

Radio direction finding FAQ  

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 course)
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.

Can 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 more?

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 dipole array.
4 antennas perform excellent in a good design, so why go to higher antenna count.
(I designed and built two different 8-antenna dopplers but they were not really worth the extra cost and effort)

Can 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 antennas.
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 rocksteady bearing.
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 rotating frequency.
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 modulation.
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.

Cheers, PA8W.