PA8W's Radio Direction Finding Technology

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

(Revised march 2020: disable auto-reset by adding 220nF, see wiring diagram)

Developed in 2017, this almost ready to use RDF41kit will give you semi professional performance for a modest price tag.
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:

If you are interested in having an RDF41 yourself:
I can help you with a working assembly without housing or with a ready to use RDF41 in aluminum housing.
Just let me know.

The RDF41 DIY radio direction finder


Professional soft switching for highly reduced noise floor.
High sensitivity: Suitable for weak signals.
Accuracy in good conditions, using a UHF doppler array: 2,2 degrees averaged, or 1,5 degrees using a UHF Amplitude array.
Wide frequency range, 27MHz-2GHz depending on antenna array.
Easy calibration over 360 degrees using the potentiometer.
Several antenna array designs available.
New: Dual mode: Normal mode for mobile applications: Burst mode for catching very short transmissions down to 200mSec.
Quality weighing of measurements, using the best measurements to generate a long term average.
Automatic Adaptive Averaging continuously optimizes the amount of averaging in the current situation.
Best measurements are automatically sent over USB to computer in order to plot bearing lines on map. (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 term average.
Digital display of long term 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.
(11V up to 15V 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 term average and signal Quality.

How does the RDF41 know good from bad measurements?

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 audio reaction from 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? We're in a distorted RF field!

The RDF41 compares the pulses of both pairs and calculates a Quality rating 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 term average,
thus resulting in a much more stable bearing indication, especially in difficult multipath conditions.

Soft Antenna Switching:

The above picture also shows one of four antenna control signals (yellow).
Like all professional pseudo doppler designs the RDF41 uses soft commutation, massively reducing the system noise floor.
Simple hard switching RDF's (almost all amateur designs!) perform very poor on weak signals and are very susceptable to strong signals near the hunting frequency!

Normal mode using Automatic Adaptive Averaging:

The long term average obviously is an averaged bearing and therefore lags the current measurements a bit.

Starting March 2018 the RDF41 uses a special algorithm to automatically set the amount of averaging, depending on the circumstances:

Some examples:

 You are driving on an country road, with lots of reflections from trees and bushes:
The Averaging will go up (256 max) to enhance accuracy and bearing stability.
Also the immunity to reflections will be served by this.

Now you are taking a turn.
Normally very high Averaging will make the RDF respond very slowly to that turn.
The RDF41 will recognise your change of heading so it lowers averaging a lot to rapidly adapt to your new heading.

The same happens if the station you are hunting goes off air and another station starts to transmit;
The RDF "sees" this sudden change and reduces the amount of averaging to be able to focus on the new signal more rapidly.

Note that this response to changing circumstances may not be exaggerated, since it would make the RDF respond to strong reflections as well...
Therefore this algorithm is tweaked "on the road" extensively to get the best possible performance in real life conditions.

Burst mode for very short transmissions

Starting August 2018 the RDF41 has an improved second mode specialized for hunting very short transmissions down to 1/5th of a second.
It doesn't  build a long term average: simply because there's no time for that.
Instead, it gathers and displays real time measurements, until the signal starts to weaken.
That means that the last bearing is calculated using the strongest part of the signal burst, not the much weaker part just before the squelch closes.
Depending on modulation content of the tracked signal, the RDF41 manages to get good accuracy (within 5 degrees) on bursts of only 1/5th  of a second.

Doppler plus Amplitude mode!

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 needed.
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 needed.
An Amplitude RDF can be used to find ALL kinds of signals, including sparking electric connections, noise sources, etc.
In this website I published several amplitude antenna arrays.
Go check them out!

An RDF41 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!

Now let's go into the details:

This screenshot shows a reading in a fast right hand curve; the long term 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.

Averaging is automatic, and will in this case be automatically reduced to speed up the long term average indicator.

The following settings are factory set:
Rotation frequency: 500Hz,
Squelch: 1

Starting in the 2021 version, an OVERLOAD warning may appear right above the bearing,
indicating that the input audio level is too high.

The center dot in the pelorus indicates that a measurement was good enough to be accepted,
although Quality is just 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 in the left bottom.
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 can put the RDF 41 in antenna test mode.
In this mode the RDF slowly steps 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.
Below you see a compact plastic enclosure which -after some adaptations- does the job.
I checked for RF leaks through that plastic housing, but the level of RF leaking to your receiver is pretty low, 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 matter.
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 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 in the following picture:

A 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 8/16 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.
So, for a single receiver setup you won't need an extra potentiometer.

Mode switch is optional.
If you leave it out, the RDF will stay in normal operation mode with Automatic Adaptive Averaging.
However, adding the switch will give you Burst mode and Antenna test mode as extras.
The antenna test mode gives you a quick idea of the proper condition of all 4 antennas, simply by listening to the signal and watching the signal strength on the 4 antennas.
The Burst mode enables you to hunt very short signal bursts.
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 of this page.

This way the switch center position is normal RDF mode with Automatic Adaptive Averaging (optimized for mobile purposes),
to the left is Burst mode with Averaging = zero, and to the right is antenna test mode.
The display shows the chosen setting.

The 12V dc input is protected against reverse polarity.

The voltage should be somewhere between 11V and 15V
The current draw will be around 100mA.
So a 200mA 12V dc power supply will do fine. Or the 12V of your car battery.
If you want to use a tiny lightweight battery you could pick a 3-cell Lithium-Polymer pack. A 1200mAH version would run the RDF41 for 10 hours or so.

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 30m.
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 really managed to mix them up you will see erratic behavior of the RDF...

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

The below picture shows how to connect the RDF41.
First remove the Arduino board, and you will see the PCB's copper side as illustrated.

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.

Model RDF41 b:
Later I re-designed the PCB (For better routing, schematic and performance is identical)

The RDF41 resets as soon as a serial connection with a computer (running RDF-Mapper) is made.
This can be disabled by adding a 220nF capacitor according to below diagram.


Model RDF41c:
In the summer of 2020 I re-designed the PCB for a little higher control current (40mA), schematic and performance is identical.



I use the following connector standards when I build a RDF41 in a housing:
For DC power I use a 5,5/2,1 mm bus, center pin is +12V.
Audio input: 6,3mm bus, tip is hot.
Antenna control: 5-pin DIN bus, antennas wired as in below picture.
(antennas are always numbered clockwise, looking down on the array)
The 4-pin encoder bus does not apply for the RDF41.

System testing:

For testing purpuses you may take the antenna driver outputs as testing signals.
Use a 220k series resistor to feed an antenna driver signal into the audio input.
Or in case of an attached speaker in the RDF, use a 1k series resistor.
If you calibrate to 0 degrees on one driver output, the others will show 90, 180 and 270 degrees plus or minus 2 degrees averaged.
If this is the case you know the RDF41 itself works like a charm.
Even if one of the antenna drivers should have a 4 degrees error that would pose no problem:
When hunting a radio signal, the error would be smeered out over the full circle and therefore have a 1 degree impact on the bearing at max.
That's also true for a comparable deviation in one of 4 antennas.

Cheers, PA8W.