PA8W Amateur Radio

Wil, PA8W,  E-mail:                  

The PA8W RDF41 kit, pre-assembled and tested:

This almost ready to use kit will give you semi professional performance for a modest price.
You just have to buid a housing around it plus the antenna array of your choice.

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 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.
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 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,
Averaging: 128,
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 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.

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.
Note that a metal housing is preferrable, and if you want to send bearings to a computer you should  keep the on-board USB connector very 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 careless 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 360 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.

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.
The antenna mode however gives you a quick idea of the proper condition of all 4 antennas.

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.

73, Wil.