RDF41

This microcontroller based Radio Direction Finder is the result of 2,5 years of testing and experimenting with my RDF40 development model.
In this period, I found ways to remarkably improve the stability and reliability of bearing estimates produced by the RDF.
Although the working principle of the RDF41 does not really differ from the simpler discrete LED-pelorus versions I designed,
the available computing power of a simple microcontroller can enhance performance substantially.

By weighing the quality of measurements this RDF manages to calculate a long time average bearing even in very poor conditions.
This can only be done “on the move”, since multipath effects -which normally corrupt the measurements- rapidly change randomly during the ride.
Therefore these changes can be considered and treated as noise.
The RDF can still dig out a good bearing estimate simply because it can distinguish good from bad measurements.

It displays 4 real time measurements every half a second, checks their credibility, and displays a long time average using the best measurements.
The necessary algorithms are tested and tweaked over and over again over the last years.

So, now all posibilities are sorted out I decided to develop an almost ready to use kit for those radio amateurs -and other operators- that show interest in this concept.

Explanation.jpg

Features:

  • Professional soft switching for highly reduced noise floor.
  • High sensitivity: Suitable for weak signals.
  • Accuracy better than +-5 degrees in good conditions.
  • Wide frequency range, 25MHz-1GHz depending on antenna array.
  • Easy calibration over 360 degrees using the potentiometer.
  • Several antenna array designs available.
  • Quality weighing of measurements, using the best measurements to generate a long time average.
  • Best measurements are automatically sent over USB to computer in order to plot bearing lines on map. (free mapping program available)
  • Clear 128×64 pixel display, perfectly readable even in bright daylight.
    On screen quality indicator, gives instant insight of multipath distortions.
  • Bearing pelorus showing the four last measurements plus long time average.
  • Digital display of long time average and quality factor.
  • Automatic display freeze below squelch point.
  • Antenna testing mode available.
  • Pre-assembled, programmed and tested kit.
  • Runs on 12V power supply or car battery, consuming less that 70mA.
    (8V up to 13V dc Minus pole connected to mass)
  • Reverse polarity protected.

The RDF41 does 500 cycles per second over 4 antennas, so it collects 2000 measurements per second in four sample buffers of a digital filter.
Here, initial averaging is done and modulation and noise are suppressed.
The 4 buffers are then sampled by the microcontroller, 4 times every half second.
So twice every second the 4 latest samples are displayed, including the newly calculated long time average and signal Quality.

How does the RDF41 know good from bad measurements?

The following oscilloscope screendump shows a few signals in the RDF41, where the blue signal is the output of the connected FM receiver.
In a pseudo doppler, the jump to the next antenna results in a phase jump in the receiver and therefore a pulse in the receivers audio.

The audio pulses 1,2,3 and 4 belong to antennas 1,2,3 and 4 obviously.
Consider the 4 antennas as two pairs of opposing antennas, say a 1 – 3  pair and a 2 – 4 pair.

In theory, a clean RF field will produce a similar reaction in both antennas of a pair but with opposite polarity.
If that is not the case the RF wavefront must be distorted by multipath (Or the receiver is not tuned to the signal properly)
So, in a clean RF field: If we add up audio pulse 1 and 3, the outcome should be close to zero.
The same is true for pulse 2 and 4.
If not? Then we have a distorted RF field.

The RDF41 compares the pulses of both pairs and calculates a Quality figure depending on pulse amplitude and symmetry in both pairs.
This “Q” figure is a very reliable indicator for the accuracy of a bearing measurement.
The RDF41 uses this Q to decide how much this measurement is allowed to have an impact on the long time average,
thus resulting in a much more stable bearing indication, especially in difficult multipath conditions.
pseudo doppler waveforms.gif

The RDF41 compares the pulses of both pairs and calculates a Quality figure depending on pulse amplitude and symmetry in both pairs.
This “Q” figure is a very reliable indicator for the accuracy of a bearing measurement.
The RDF41 uses this Q to decide how much this measurement is allowed to have an impact on the long time average,
thus resulting in a much more stable bearing indication, especially in difficult multipath conditions.


The kit will be delivered as in below picture, including calibration potentiometer but without connectors and switch, so you can pick your own types.
On the bottom of this page you can find a simple wiring diagram.
And this website shows how to build a simple but high grade antenna array.
So any radio amateur can do the job and attend the next foxhunt with semi professional equipment!

Sandwich.jpg


TurnScreen
TurnScreen
  • This screenshot shows a reading in a fast curve; the long time average arrow lags the 4 current measurements.
  • The length of the current measurement arrows show their Quality.
    Overall Quality is very good:  Q=8. This is also clear due to the nice symmetry showed by the symmetry indicator.
  • Averaged bearing is 127 degrees,
  • Battery voltage is 13.1V,
  • Calibration is set to 352.The following settings are factory set:
  • Rotation frequency: 500Hz
  • Averaging: 128
  • Squelch: 1

MultipathScreen
MultipathScreen
  • The center dot in the pelorus indicates that a measurement was good enough to be accepted, although Quality is only 4.
  • The reason for this poor Quality is multipath reception.
  • Reflections add up to the direct signal and distorts the received wavefront.
    This is clearly illustrated by the symmetry indicator.
    Both horizontal lines should be vertically aligned on the vertical line, which is clearly not the case.
  •  A sure sign that this bearing of 245 degrees may be off by a fair amount.
  •  If the Quality drops below 1, the center dot in the pelorus will disappear and the reading will freeze until a good signal is received again.

TestScreen
TestScreen
  • The mode switch puts the RDF 41 in antenna test mode.
    In this mode the RDF steps slowly through all 4 antennas, enabling the user to check if the performance of all four is similar.
  • In most situations there will be substantial difference in signal strength due to multipath reception.
    A defect antenna though will clearly drop out compared to the rest.

 

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