KA7OEI's Amateur Radio Direction Finding (ARDF) Page


This page contains various links related to Amateur Radio Direction Finding (ARDF) - also known as "Foxhunting" or "DFing"

Important Note about firmware availability:
The "Alternate firmware" for the VE2EMM Doppler I, II and 3 units is still available - see the links below.

Important notes:


Pages related to VE2EMM's "Montreal Doppler" units:

The late Jacques Brodeur, VE2EMM, designed several excellent microprocessor-based "Doppler" type DF units described at his web site (see the links below.) These units works along the same principles that the so-called "Roanoake" DF units do - but the addition of a microprocessor (to do the generation of switching signals and processing of the received audio) adds additional flexibility.  If there are any questions, please use the contact information on the respective page(s.)


Links to the legacy VE2EMM Web pages relating to the Montreal Doppler units


Please note that circuit boards are still available from FAR circuits (see the links below) and the "Alternate" firmware for the Montreal Doppler 1, 2, and 3 units is still available from me.

Note that the chips that may be available from FAR circuits would NOT be the programmed with the "Alternate" firmware!


The Montreal Doppler I
The "Doppler I" (a.k.a. "VE2EMM 32 LED  Doppler Direction Finder") has been obsoleted by Jacques, but the boards are still available from FAR circuits and alternate firmware is still available.
Why "Alternate" Firmware?

Occasionally I get asked how the "Alternate" firmware for the Montreal Dopplers came to be.

I purchased my original Montreal Doppler II from Jacques himself some years ago.  Unfortunately, something went wrong with the programming of the chips and they didn't work:  Since I already had a programmer and "C" compiler, I wrote a simple test program and verified that the hardware was working correctly.

Upon emailing Jacques he kindly sent me a .HEX file of the object code and upon programming it into the chip, everything worked fine.  This started a series of correspondences about some minor bug-fixes and changes - some of which he kindly did.  Then one day, after not having heard from him for several months, he sent to me his original source code in an email and said that it was a gift:  Surprised, I replied and thanked him.

With the code in hand I gradually tweaked the code more to my liking, adding new features and some additional processing and display options that Jacques, in our correspondence, indicated that he would have like to have done, but didn't get around to it.  When the "Doppler III" came out I bought another set of boards from him and with the similarities of hardware, quickly adapted my re-worked "Doppler II" code to work on the new platform.

Ultimately, only a small portion of the original Doppler II code remained -  Some of the low-level signal processing and the general layout of the Doppler II menu structure - which I personally preferred over the Doppler III's menu system:  Occasionally, upon making major changes and producing stable software I would email the source code to Jacques.

And so it is today:  I've since even ported a pin-compatible version of the original "Doppler I" hardware and added about all of the features that I and a few others can think of and the code is very stable - so not too much has been added to it recently - but if someone who uses it has a great idea, I'm all ears!

Please note that the original "Doppler I" (32 LED Doppler direction finder) web page is no longer online, but:

The Montreal Doppler II
The "Doppler II" has been obsoleted by Jacques, but the boards are still available from FAR circuits and alternate firmware is still available.

Please note that the original Montreal Doppler II web page is no longer online, but:

The Montreal Doppler 3

Note:  The following pages are not official pages of VE2EMM and did not necessarily involve his direct contributions.  For questions about the contents of these pages, please use the contact information contained on them.

Other pages related to the Montreal Doppler units - construction and circuit board availability:
Can you "DF" digital voice signals?

The question has been asked:  With new, digital voice systems such as D-Star and APCO-25, can those signals still be located?

To answer this, let's do a quick review:  There are two general types of DF (Direction Finding) techniques:
  • Signal strength.  These use the strength of the signal - often using a directional antenna such as a yagi - to determine the direction from which the signal is arriving.
  • Phase-detection techniques.  These include systems such as two-antenna "TDOA" or the multi-antenna "Doppler" systems that determine something about the direction of the incoming signal.
The "signal strength" system doesn't really care what mode or bandwidth is being used:  As long as the signal can be detected in some way, one can determine its direction.  This could be done using a field-strength meter, a receiver specially designed for direction-finding, or even the S-meter of an existing receiver.

For the phase-detection systems, it turns out that many of the systems designed for analog reception will work with narrowband digital systems, such as D-Star.  Note, however, that the "bandwidth-limited noise" nature of the D-Star  (and similar) signals may reduce accuracy and sensitivity of some units somewhat, depending on their design and filtering.

For DFing a narrowband digital signal, one would continue to use the same analog receiver as before in conjunction with the signal meter or direction-finding unit.  Of course, if one wanted to "hear" the audio being transmitted on the digital signal being tracked, you would need to use a digital receiver (such as a  D-Star-capable receiver) on a separate antenna to do so.

Note that DFing of wideband digital signals (those that are much wider than an standard FM signal, such as the 128kbit D-Star found on 23cm) will likely require "Signal strength" techniques as the phase information imposed by antenna switching (for example) is likely to be badly diluted.

The "alternate firmware" for the Doppler II and III has been tested with D-Star signals and found to work very well.  The "Doppler I alternate firmware" and the original Montreal Doppler firmware versions have not been tested, but is also expected to work just as well as they all function using the same, basic techniques.

KA7OEI's pages pertaining to the Montreal Doppler 1, 2 and 3 DF units:

The following pages are maintained by KA7OEI:  Questions about these pages should be directed using the contact information at the bottom of this page.

"Alternate" firmware for the Montreal Doppler units:
 Other related pages:

Interfacing Doppler DF units with computers and GPS receivers for moving-map displays:

With relatively little effort, it is possible to use a computer (a modest laptop - 200 MHz Pentium-class or faster) along with a GPS receiver equipped with NMEA output, and the Montreal Doppler II or III units to produce a mobile, street-level moving-map display that can display converging bearings on received signals.  There are at least two low-priced software packages that can do this:

Other pages at this site having to do with Doppler RDF:
Using a "Doppler" RDF on SSB signals using an FM receiver

I you were to have asked me several years ago I would have thought that determining the bearing of SSB signals without a real "carrier" to phase modulate could not be done with a "Doppler" type DF unit, but this isn't entirely true.

Some time ago, I happened to tune the FM receiver that I use for Doppler DF at home over to the frequency of the local 2 meter SSB net.  While I was listening to the net (on an SSB receiver) I happened to be watching the compass rose display on the DF unit as it bounced around, responding to the garbled SSB signals that its FM receiver was detecting and noticed that it did, in fact, indicate the bearings of the stations that were talking.

To be sure, the displayed bearings weren't as stable as those that might be detected from an FM signal, but there was most definitely a correlation!  When I listened to the distorted audio from the FM receiver, I could definitely hear traces of the DF unit's antenna switching tone imposed on the audio and this was apparently enough for the DF unit to process.

As the net progressed, I tweaked the DF unit's settings to see if I could improve the quality of the bearings and observed that the following settings seemed to yield the best results using my "Alternate" firmware for the Doppler II/III:
  • The "Q" control for the "Roanoake" filter should set to minimum.  Unless it was added afterwards, the Doppler II does not have such a filter and no adjustment is required.  Note the modification on this page to add a "Q" control to the Doppler III.
  • The "Averaging" control was set to 64 - the maximum setting for the current firmware.
  • The "Average Clear" control was set to 1/4 second.
  • The squelch on the FM receiver was set very tight so that it would close very quickly after the SSB signal dropped below the receiver's threshold - this being done to minimize the "contamination" of the bearing with "squelch noise" where the SSB signal's power went to zero between words and at the end of a transmission.
  • The "Gate" (an audio level based threshold) on the alternate firmware is utilized to detect when the audio from the FM receiver ceases, this being done to assure that data is gathered only when an SSB signal is actually being detected.

It is the nature of SSB that RF power is only present when there is voice energy so, unlike FM, the signal goes away between words and when the operator stops talking.  During these periods of "silence" there is no information on which a bearing can be determined so the trick is to feed audio to the DF unit only when there is actual SSB energy, hence the need for the "fast" squelch and the "Gate" feature.

By using the combination of the audio gate feature of the alternate firmware as well as the "average clear" (that is, the past "history" is deleted after a defined period of no detected audio) and the fast squelch one can help assure that the average being built up during the transmission contains mostly information that resulted from receiving the SSB signal - and not the "squelch noise" between syllables and at the end of the transmission.

Because the alternate firmware's average is a "sliding" average, it will dynamically adjust its size from the beginning of the transmission (where this is no past data on which to base an average) until it contains the maximum number of data points (64 for the newest firmware) and with the audio gating working, and it will only collect data during those portions where SSB "audio" was being detected.  With the settings noted above, 1/4 second after the last audio the averaging history is then cleared in preparation for the next transmission.

As was mentioned, the accuracy and stability of bearings obtained this way aren't nearly as good as those one might get from an FM signal from the same transmitter but they correlated very nicely!  As you might guess, using the DF unit this way is more "visual" than normal as best results can be obtained by watching the compass rose while listening to the received signal on an SSB receiver and in so-doing one's visual skills can be used to determine the direction(s) from which the signal(s) seem to be emanating!

It is worth mentioning that on the SSB net, among the stations that check there is a mix of vertical and horizontal polarization with the majority of them using the same vertical antennas that they use of FM.  For a net, using a vertical omni antenna makes sense as the is no need to constantly move the antenna back and forth to listen to stations that are coming from different directions.  In observing the quality of the bearings I noted that better results were obtained from those using vertical polarity, but strong signals from horizontally-polarized stations also yielded reasonable results.

Having said all of the above, the best way to determine the bearing to an SSB station is the old tried-and-true method using a directional antenna and observing the signals strength as one can adjust a hand-held antenna quickly determine the bearing and accommodate the different polarities that people might use, but the methods described above may be useful if you looking for a signal while in a vehicle.

These pages discusses other devices used with the Montreal Doppler units, but these could be applied to direction-finding units as well.

  • An Add-on Pelorus for the Doppler II (or III) - This page describes an outboard compass-rose display unit that is driven by "Agrelo" formatted data output from the serial port.  This display has been "obsoleted" by the use of the "Alternate Firmware" for the Doppler II, Doppler III, and the compass rose display.
  • An add-on Pelorus using an OLED Display - This project was done mostly to evaluate the use of Organic LED (OLED) displays that are becoming available.  This unit operates from the serial port (using "Agrelo" format bearing/quality data) and provides a graphical representation of the bearing, history, and quality of the signal being received.
  • Audio comb filter for switched-antenna RDFs - When a switched-antenna RDF system operates, an audio tone (used to determine the bearing) results.  This page describes a PIC-based DSP comb filter that can remove this tone (if desired) to make the on-signal modulation more intelligible.  Note:  Even with the comb filter enabled, one can still discern the presence of multipath by the "sound" of the audio and multipath-generated sidebands that get through the filter.
  • DF Antenna Arrays - This page describes some typical antenna arrays (for mobile and home use) that may be used with RDF systems.
  • A "Pre-processor" for Doppler-type DF units - This unit may help to detect and prevent distorted/bad bearings from being received by the DF unit.


    • Miscellaneous other pages having to do with Amateur Radio Direction finding (ARDF)

    These pages contain information on how ARDF gear works as well as the building of other pieces of equipment.

    Note:  The reader should be aware that the authors of the links on this page do not necessarily endorse any of projects, circuits, designs or vendors mentioned above.  The level and satisfaction of performance as well as efficacy and safety of any of the above circuits is largely based on the skill and experience of the operator.  Your mileage may vary.

    Do you have any questions on this or other DF-related topics?  Go here.

    Go to the KA7OEI main page


    This page updated on 20120316

    Note:  This page (and other pages on this site) are not "official" pages of VE2EMM.  These pages are simply set up to aid those who have built or might build the described equipment.

    Since 12/2010: