PA8W's Radio Direction Finding Technology

Building the conventional V2,3 doppler radio direction finder

By the end of the year 2011 I started the development of the PA8W Doppler Radio Direction Finder.
Wherever possible I used available knowledge found on the internet.
I studied, filtered, adapted and enhanced existing designs until my dopplers finally performed the way I wanted them to.
As a result, the PA8W Dopplers have a few features that I missed in existing designs.
For example, is uses soft switching of the antennas, giving a massive improvement compared to the generally used hard switching.
It has an automatic gain control to make it highly tolerant to input level variations.
It also features an antenna test mode, in which all antennas are selected in a slow sequence.
And last but not least, I optimized the necessary antenna array using modelling software.
So, despite its simplicity, the "PA8W dopplers" really have a few advantages over existing amateur designs,
resulting in surprisingly good accuracy even on weak signals.
As soon as a FM signal is strong enough just to be readable, the RDF is providing a pretty stable and accurate bearing.
The RDF also produces steady bearings on signals with strong modulation.
The below V2.3 version is the latest version of the conventional LED-pelorus Radio Direction Finders.

But first let's get back to the beginning:
Initial Design Goals:

Simple design, easy to reproduce, using very common components, mainly based on the buffered C-Mos 4000B logic series.
Single sided PCB's only.
No software necessary, no PIC's. Lots of people still don't like handling bits and bytes...
It should however have an output available to feed data into a computer, because there's very nice software freely available for presentation in computerized maps.
Very good performance, avoiding the pitfalls of earlier designs.
Autonomous operation from 12VDC, to support use in cars or all kinds of simple temporary setups.
It should work with practically all NBFM receivers from say 120MHz up to 450MHz.
It should come with optimized antenna designs for base-operation and car-operation.

Features of Version 2,3:

32 LED pelorus display (compass rose) "wobbled" for better than 5,6 degrees dynamic resolution.
Pelorus freeze after a signal has disappeared.
Drives an array of 4 antennas.
Wide band capability: 25MHz to 500MHz using appropriate antenna arrays.
120MHz to 170MHz coverage using a 145MHz array.
Very good signal conditioning using a high grade digital filter with extremely high, variable Q.
Automatic gain control.
On board loudspeaker, since a lot of small receivers ony offer a headphones output, switching off the internal speaker when used.
Full calibration range, to compensate for all possible antenna configurations and receiver properties.
Antenna check mode, selects the four antennas in a slow sequence, and the display shows the selected antenna.
Soft control with extensive overlap instead of hard switching of the antennas,
since hard switching produces a lot of noise and unwanted mixing products.
Hardware accuracy: better than 5 degrees.

How It Works:

First, let's get to understand what Doppler shift is.
Everybody knows of the pitch change of a fast moving object, like a passing car.
The motor or horn sound of a car at constant speed seems to be pitched up as long as it approaches, and it pitches down as soon as it has passed.
This is called Doppler shift.

The same doppler effect would occur if the car would be static and you would be running by at very high speed.
Or imagine you would take a microphone and swing it around at its cord.
As long as it swings towards the car, the microphone will pick up a higher pitched tone of that car.
In the opposite part of the swing, it will move away from the car, pitching down the sound of the car.
There's only two points in the circle, where the pitch is correct, one point going from pitched too low to too high (the closest point), and the other point will be found going from pitched too high to too low.

A Doppler Radio Direction Finder does exactly the same thing; it uses a circular arrangement of antennas which are activated one after the other in a very fast sequence, simulating one antenna being swung around in a circle at very high speed.
This means that the received carrier of the tracked radio signal will be moved up in frequency as long as the antenna swings towards the transmitter,
and shortly after that the frequency will apparently be lowered below the true frequency due to the antenna moving away again.
The antenna array is rotated electronically at around 500 revolutions per second,
so a FM receiver connected to this electronically rotated antenna will produce a 500Hz audio tone.
The zero crossings of that tone (plus or minus some phase shift etc. etc. ) will mark the points where the doppler shift is zero.
These points are easy to detect electronically, and after the necessary compensation for phase shift mentioned above,

one of these points is used as a trigger to "freeze" the LED display at the correct point in the circle.
You may imagine that the cirular LED pelorus display copies the circular movement of the antenna array and only the LED that is exactly at the trigger point is allowed to light.


The PA8W Version 2.3 Doppler has the following controls:

Threshold, to adjust to the "freezing" level of the pelorus, given a specific output level of your receiver.
Speaker on/off.
Calibration, to force the RDF to point into the right direction.
Phase switch, to enhance calibration with 180 degrees shift.
Filter-Q, to set the signal conditioner for fast or slow response.
Antenna check switch, selects the four antennas in a slow sequence, and the display shows the selected antenna.
(In this mode the 500Hz tone is absent so you can listen to the modulation of the tracked transmitter, and check for equal performance of all 4 antennas.)

The simplified Block Diagram:

Top-left we have the 4-antenna vertical dipole array, meant for non-mobile applications.
(for mobile applications there's a similar arrangement of 4 magnet mount whip antennas)
The antenna array is "rotated" by the antenna multiplexer, which activates only one antenna at the time.
From the antenna array, a coax runs to our FM-receiver. The audio output of the receiver is fed into a series of audio filters.
Next, a zero crossing detector marks the point of no phase-shift, and, after a calibration pulse shift section, this triggers a data latch.
This data latch then copies the current state of the address lines, and presents it to the LED driver, which lights the corresponding LED in the pelorus display.
Of course, the entire process is timed by the central clock + address counter.


One could think of a LED pelorus display with 64 or even more LED's to get better display resolution.
Knowing that a 4 antenna array is capable of offering a maximum accuracy of around 5 degrees, it would make sense to use 64 LED's giving the pelorus a resolution of 5,6 degrees.
However, this would make the RDF much more complex.
So I designed the 32 LED pelorus with wobbled response, filling the gaps between 2 subsequent LEDs, giving a better than 5 degrees resolution.
 And if you really want more, a hookup to a computer will give all the features you could want, even a direct plot of the transmitters direction on an electronic road map.

Keep in mind that the simple 4 antenna array does not exactly simulate a smooth rotating antenna.
It will produce 4 phase jumps every cycle, or even only two jumps per cycle, worst case, when two antennas are at equal distance from the transmitter.
Obviously this is pretty hard for the RDF to analyse with some degree of accuracy.
A 8 antenna array already does a better job, and professional systems often use 16 or even more antennas.

As for practical reasons we will have to stick to 4 or 8 antennas max (in my Version3 doppler),
our RDF will need to have an incredibly good signal conditioner, and that's where the digital filter and additional low-pass filters step in:
They integrate the incoming audio, they don't allow for fast signal jumps to pass, but they average everything into a sinus-like waveform.
To achieve this the applied digital filter has an extremely high Q. It has a bandwidth of less than 1 Hertz.
The extremely narrow bandwidth will force about any signal shape into a sinusoidal shape, and it
also effectively cuts off modulation on the tracked signal, which would otherwise blur the display severely.
As far as I know this is the only type of filter this narrow, that is not depending on very high component accuracy.
Its center frequency is determined by the clock frequency only.
In this design, the same clock is used to rotate the antenna, and if the clock would deviate, the filter would track that deviation automatically.
No crystal controlled precision necessary. This is why this type of filter is the eureka part of the design!

This signal smoothening process actually fills-in the gaps between the jumps from one antenna to the next, giving the RDF a highly improved accuracy.
And in fact the softened activation of the antennas plus the considerable overlap on every transition from one antenna to the next also add to that process.
The soft switching has an even bigger advantage: It reduces the level of spurious in the radio band so your reception of the weaker signals will improve by a large amount.
Developing the PA8W Doppler RDF, I experimented quite a bit with a variable timing and overlap for the antennas to find the best spot, so I assume we have achieved pretty much the maximum performance one can squeeze out of this simple antenna configuration.

However, we should realize that particularly a fixed (base) RDF accuracy will suffer from objects in its vicinity,
which may introduce bearing deviations much larger that the intrinsic accuracy of this RDF.
A mobile RDF has the same problem, but since it changes location constantly, the changing deviations will be averaged into a much smaller bearing error.

The following screenshots will illustrate the excellent signal conditioning in the heart of the RDF:

First of all the red wave shows an audio signal right out of the receiver.
It is a bit noisy because it was a rather weak carrier.
But since it is a clean carrier without modulation, this signal is easy to process by the RDF.
A modulated signal would be total chaos to the eye, and still the RDF is able to pick out what's important.
This is achieved mainly by the digital filter, of which the output is shown in yellow.
It resists to all rapid changes in the signal, so what remains is information that is constant over some period of time, like the doppler information.

After the digital filter two low pass filter stages are responsible for filtering out everything above the doppler frequency.
So the output is only the base frequency, an nice 500Hz sinusoid shown in blue, very well suited for a precise phase measurement.
Note there's a considerable phase shift between the yellow and the blue signal, due to the low pass filters.
This is of no meaning as long as it is a constant factor, since the calibration will compensate for this automatically.

Accuracy measurement:

I performed accuracy measurements with the base array on a rotatable glassfiber mast.

The, hardware mean accuracy turned out to be around 5 degrees,
which is in accordance with what has been stated by professionals about 4-antenna doppler RDF's.

The following downloads are still available here:

Main diagram
PCB drawing (will help you to assemble the PCB)
PCB artwork (for those who want to home-brew the single sided PCB)
Operating the V2,3 Radio Direction Finder

A few considerations before you build the V2,3 doppler.

Car setup:
First of all, if you are going to use the doppler in a mobile setup, you might leave out the 12V voltage regulator and instead put in a 1N4004 diode plus a 10 ohm resistor in series.
This would protect the unit against reversed polarity, and the normal battery voltage of a car (13,8V) is perfectly suited for the doppler.
The voltage regulator is not necessary as long as you don't feed in voltages higher than 15V.
A 12V or 15V regulated  (!)  mains power supply would be the right choice for mains applications of the modified unit.

I found that at a certain calibration point, the doppler showed some degree of instability.
It turned out that the wiring to the calibration potentiometer picked up transients from its surroundings, resulting in sporadic mistriggering.
I replaced the wire by a piece of shielded (pickup-) wire, and connected the shield to the PCB's mass and the other side to the potentiometers casing.
This cured the issue completely.

The front:
First drill the front, using the drill template; best is to tape it firmly to the front and mark all hole positions using a steel nail and a small hammer.
Just make small pits marking the spots where the LED's should be positioned as well as all controls.
Then drill the front, and gently clean all holes using a chamfer drill bit.
Now stick on the vinyl front sticker.

After putting the sticker on the front just cut off the excess with a sharp knife.
Pierce all holes from the front side, and remove any residue.
If you planned to have an integrated speaker this is the moment to glue it in place, since as soon as the pelorus is in place, you're too late!
That's also true for the speaker cable!

Building the pelorus:

Put in all wire jumpers first, and solder them except for the LED's side.
Then the LED's have to be put in.
Start with LED#1 (RED) at the marked position and follow up clockwise with 3x green, 1x yellow, 3x green.
Note that the long leg of each LED is pointing to the centre of the pelorus.
Just lower the LED's down to about 20mm bottom spacing (if you want to include the speaker in the perosus centre as I did)  from the PCB, and ONLY SOLDER THE INNER LEG!
Work around the pelorus until all LED's are in place.
Now carefully stick the LED's into the drilled pelorus holes. Make sure the 0-degrees LED is a red one!
Since the LED's are soldered with the inner leg only, you can carefully bend each LED into position to fit its hole.
Small deviations in the drilled pattern will be not be a problem.
Once all LED's have protruded the front for a few millimeters, you can solder the outer legs of all LED's.
Then the flatcables can be soldered in position, simply solder them to the copper side of the pelorus PCB.
Apply some contact glue to bond all LED's to the front plate.

The main board:
The below pictures will help you assemble the main board.
With these pictures and the schematic diagram, you should be able to reproduce the RDF.
(You're a HAM remember?...)
Note that NOT all IC's are oriented in the same direction, and the photo below is not entirely correct.
So take the schematic diagram and the PCB drawing for component values.

If you compare the PCB with the schematic diagram you will notice a similar layout; all IC's are located more or less at the same spot as in the diagram.
So that will make it much easier to understand the layout when troubleshooting or experimenting.
The controls are all wired, so anyone can use his own front layout and even potentiometer and switch size are not critical this way.
Some electronically active HAMs will even be able to build it almost entirely out of the junkbox.

After assembling the board it should be checked for failures.
Leave out the IC's until you've measured a nice 12V supply voltage after the voltage stabilizer.
Then you can put in the IC's and do some simple checks.
Any antenna control output should show a frequency around 500Hz +/- 10%. (not in array test mode!)
If you feed a 100mV tone generator signal into the audio input then you must be able to find a frequency around 500Hz that will result in the LED's racing around the pelorus. (try this with minimum Q and with Threshold at max)
Passing over the central clock frequency will suddenly make the LED's race into the opposite direction.
With no input signal, the LED pelorus should freeze, with the correct Threshold adjustment.

The antenna driver signals should look like this:
(no array attached!)

Note that when not loaded by the array, array drivers 2, 3, and 4 may show a funny type of overshoot at max positive output.
Driver 1 doesn't, because it has a small load formed by the network for the reference output.
As soon as the array is attached these "funnies" will be gone, so they are not important at all.

 Now connect the array and calibrate the RDF to a known station.
Then switch to a second known station and see if the new bearing makes sense.
Or, rotate the array and see if the pelorus is nicely following the rotation into the opposite direction!
If you turn the array clockwise, the pelorus should turn counterclockwise!
If it is running the wrong way you may have made a wiring mistake in the control lines.
In that case, swap two opposite lines, for example of the north and south antenna.
This will make it turn in the right direction.

Looking from the top down the antennas should be selected clockwise:

/     \
4         2
\     /

On this website you will find additional information about antenna arrays for your pa8w doppler radio direction finder.

Cheers, PA8W.