|PA8W Amateur Radio|
The PA8W RDF41 kit, pre-assembled and tested:
|This microcontroller based Radio Direction Finder is the result
of 2,5 years of testing and experimenting with my RDF40
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 massively.
By weighing the quality of measurements this RDF manages to calculate a long term 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 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.
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, 30MHz-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 128x64 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. (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.
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.
This RDF41 is capable of using the doppler working principle as well the amplitude principle, depending on the attached antenna array.
For the doppler principle, an external FM receiver is necessary.
A -pseudo- doppler RDF needs some kind of carrier, so it can track FM, AM, FSK signals very well.
Generally, a 430MHz doppler antenna array will work properly from 350MHz up to 500MHz.
If need be, a UHF array can be used down to 140MHz with reduced sensitivity and accuracy.
For the amplitude working principle, an external AM receiver is necessary.
It can be used to find ALL kinds of signals, including sparking electric connections, noise sources, etc.
In this website I published a design of a UHF amplitude antenna array.
It will work well from 400MHz up to 440MHz.
kit will be delivered as in below picture, including calibration potentiometer but
without connectors and switch, so you can pick your own types.
On 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!
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,
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.
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.