PA8W Amateur Radio

Wil, PA8W,  E-mail:                  

The PA8W RDF41 pseudo doppler radio direction finder kit,
pre-assembled and pre-programmed:

This almost ready to use kit will give you semi professional performance for a modest price.
You just have to build a housing around it plus the antenna array of your choice.
The RDF41 can work as pseudo doppler as well as amplitude direction finder, depending on the connected antenna array.

This is not the toy quality you find in hard switching amateur designs!

Look down this page for features and specifications.

A test drive using an Amplitude Array can be seen here:

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.


Professional soft switching for highly reduced noise floor.
High sensitivity: Suitable for weak signals.
Averaged accuracy in good conditions, using a UHF doppler array: 2,2 degrees. 
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 than 80mA.
(8V up to 14V 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.

The above picture also shows one antenna control signal.
Like all professional pseudo doppler designs the RDF41 uses soft commutation, massively reducing the receiver's noise floor.

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, as long as the signal fits within the receivers passband.
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 390MHz up to 470MHz.
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!

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 now factory set:
Rotation frequency: 500Hz,
Averaging: 256, can be set to 32 by grounding pin A, see wiring diagram.
Squelch: 1

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 distort 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 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.
Note that the numbering of the antennas is not absolute: The antenna called number 1 may physically be number 2 or 3 or 4, it depends on the moment you switch to antenna test mode.

The numbers are only there for you to recognize that one and the same antenna drops out in performance.
Which antenna that physically is can only be determined by measurement.

Housing considerations:

A nice enclosure for the RDF is Conrad # 523232, measuring 103x56x168mm, offering plenty of room for all parts.
But of course many types of housing will do. I made one out of one sided PCB sheet.
Below you see a compact plastic enclosure which -after some adaptions- does the job.
I checked for RF leaks through that plastic housing, but the level of RF leaking to your receiver is pretty low, so nothing to worry about.
If you want to send bearings to a computer you should  keep the on-board USB connector close to the right hand side of the housing,
so the USB is accessable through an opening in the side panel. I simply drilled a 20mm hole in the side for that purpose.

The calibration potentiometer is an essential control; it will enable you to make the RDF point into the right direction regardless of the orientation of your array.
So, antenna 1 may be the one left, front, right, or rear, it doesn't care.
The potentiometer has a range of 365 degrees so you will always be able to set things right.
The calibration potentiometer can be mounted inside if you are going to use only one type of array. 
In that case one single calibration session would do.
Or you can mount it recessed, so you can correct it from the outside using a screwdriver.
And the last option is to mount the potentiometer the classic way, with or without knob.
Putting a knob on will make calibration a bit more comfortable, but at the same time the chance of accidentally moving the knob will increase, corrupting your calibration.
I solved this by putting the knob in a cup, glued to the enclosure, as you can see on the following picture:

The volume potentiometer and the loudspeaker are optional additions, depending on the type of receiver you are going to use.
The speaker in those cases where the radio's speaker is turned off as soon as you plug in the audio cable. (earphone socket)
A small 16 ohm speaker will do in most cases.
A volume control can set a proper input volume for your RDF when you swap between different receivers.
The RDF interface board has a small blue trimmer that allows you to reduce input sensitivity.
(At the component side, visible after removing the display board)
So, for a single receiver setup you won't need an extra potentiometer.

The antenna test mode switch is optional.
If you leave it out, the RDF will stay in normal operation mode with Averaging = 256.
The antenna mode however gives you a quick idea of the proper condition of all 4 antennas.
I used a 3- position (on-off-on)switch: Connect the center pin to ground, one pin to the Mode-switch contact on the PCB,
and the other pin soldered to the PCB contact with the "A" label. See the wiring diagram at the bottom.

This way the switch center position is normal RDF mode with Averaging = 256,
to the left is RDF mode with Averaging = 32 (faster response of the long time average),
and to the right is antenna test mode.
The display shows the chosen settings.

The 12V dc input is protected against reverse polarity.
You may use a 12V car battery or a 12V dc regulated mains adapter. 
Don't go beyond 14V dc to avoid damage to the RDF41.
Internally the RDF runs on 12V and 5V.
(Don't use the on-board dc socket of the Arduino!) 

The antenna control outputs speak for themselves.
You need a cable and connector system of at least 5 conductors: 4 antenna control signals plus ground.
General current willl be around 10mA so thin signal cable (cat-5) will do fine for cable runs up to 25m.
4 wires for the antennas, and the rest of the wires for ground.
The antennas themselves have to turn/run clockwise looking down on them.
If you discover that you hooked them up the other way around you just have to swap antenna 1 and 3  (or 2 and 4) to get it right.

If you really manage to mix them up you will see eratic behavior of the RDF...

Display contrast may be adjusted by a tiny trimmer at the back of the display.
Be very careful adjusting this tiny part...


The above picture shows how to connect the RDF41.
First remove the Arduino board, and you will see the PCB's copper side as illustrated.
If you connect the pin labeled "A" to ground, the Long Time Averaging will be 4x faster than normal.
See the above text for a nice option with a 3- position switch.

The PCB on the left is a standard combiner for a doppler array, but with the possibility to add a MMIC preamp.
Of course you can build the array without preamp as well.

73, Wil.