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 - visit the "Getting Chips" web page  for more information.
"How do I get programmed chips?"

If you are interested in the chips mentioned here, send me an email!  At the present time, the following programmed chips are available:
  • Doppler I - This can be a (nearly) drop-in replacement for the original VE2EMM that can provide improved performance - and even a lot of additional features.  Read about it here.
  • Doppler II - This chip can simply drop in or, with a bit of modification, be used to significantly enhance the performance of the original Doppler II.  Read about it here.  It is functionally very similar to the enhanced firmware for the Doppler III.
  • Doppler III - Similar to the Doppler II, this hardware was further enhanced with the addition of an LED compass rose and more stable filtering.  The enhanced firmware offers improved performance and functionality over the original.  Read about it here.
    • Clock generator for the Doppler III - This chip provides a precise clock signal for the Doppler III's input filtering.  You may get the chip from me, or you can program it yourself with the .HEX file found on this page.  This is not needed for the Doppler II unless you were to modify it to use the same audio filtering as the Doppler III.
  • Compass Rose - Originally designed as an upgrade for the Doppler III, it can function as a stand-alone or remote unit or even be interfaced with the enhanced code of the Doppler II unit.  Read about it here.
  • Comb filter - This will remove the switching tone from the audio to make long-term listening more tolerable!  This is designed for use with the enhanced Doppler I, II and III firmware, but a version is available for use with Jacques' original Doppler II and Doppler III firmware.  If you have a need to remove a particular frequency from audio (e.g. your own DF unit, to remove the Morse ID from a repeater link, etc.) then let me know!  Read about it here.
  • "Mini" Comb filter - Similar to the above, but in a much smaller form factor (only 8 pins instead of 18) and fewer bells and whistles - but it still works very well for DF units!  Read about it here.
  • PIC TDOA - This is a simple "TDOA" (Time-Direction of Arrival) DF unit that will work with nearly any narrowband FM receiver with no modification.  This unit indicates whether the signal source is left, right or straight ahead (or behind) using tone frequency, tone loudness, LED-based indicators or even a plain old analog meter - or all of those at once!  This unit can also be used as a short-range audible field strength meter when you are in fairly close proximity to the transmitter.  Read about it here.

Important notes for those interested in the source and object code:

  • For reasons that I'm sure that you'll understand, I almost never give out any source code unless I know the person with whom I'm dealing.  I will occasionally give out the .HEX file if I'm working with someone that I trust who already has a previous version of code and we have been working together to make improvements and/or bug fixes.
  • For some reason I tend to get a lot of requests at certain times of the year (maybe near the end of school semesters?) apparently wanting code for a class project.  Interestingly, these requests have never been part of a dialog of "How would I go about doing this in my design?"  While  I am not going to just hand over code, I'm willing to engage in back-and-forth correspondence to help them with their design and give them some practical hints and ideas as to how accomplish the task - but this had better be started well before the end of the semester!

If you are interested in any of these, please visit the "Getting Chips" web page and contact me via email.

Stand-alone ARDF units:  Are they still useful?

I would be remiss if I failed to mention that a number of computer (PC, tablet, etc.) based ARDF solutions exist - some of them quite full-featured.  In general these operate in the following manner:

Advantages of a PC/tablet-based system over a "stand-alone" ARDF system such as that described on this page:
Disadvantages of a PC/tablet-based system over a "stand-alone" ARDF system:

I would recommend that you explore all options to determine what is right for your application.  I don't have links available because they change frequently and I have not used most of the available options:  I'd suggest doing the appropriate research to find these devices and programs.

Important notes:


Warning about using inexpensive Chinese radios with ARDF gear:

It is strongly recommended that you NOT use a radio that has DSP audio processing - and this includes ALL Baofeng and many other inexpensive Chinese radios.  The reason for this is that these radios use inexpensive "all-in-one" transceiver chips that have dedicated DSP cores - but, depending on their operating mode and state, the amount of audio delay can vary wildely even during the reception of a single transmission.

What this means is that if you calibrate the radio with a strong, clean signal, a feature within these radios designed to make weak, noisy signals more intelligible can automatically switch in and out, upsetting the calibration of the ARDF unit - no matter which type - and producing useless/confusing bearings.  This feature cannot be switched off in most - if any - of these radios!

In other words:  Do not attempt to go "DFing" with a cheap, Chinese radio - an older "analog" radio is much preferred!

There is yet another problem with these radios:  When you get very close to another transmitter - or even the transmitter that you are seeking - they can overload very badly:  They are essentially unusable with a tool such as an "Offet Attenuator". (See more information about an Offset Attenuator below, or via this link.)

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 work along the same principles of the so-called "Roanoake" DF units - 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 from this site.


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


The Montreal Doppler II

The Montreal Doppler II - This is a microprocessor-controlled "Doppler" type of RDF unit roughly based on the "Roanoke" design, but using some DSP techniques to process the audio signal.
The "Doppler II" 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 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 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:
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:
Can you "DF" digital voice signals?

The question has been asked:  With new, digital voice systems such as D-Star, Yaesu's System Fusion 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.  (The 'EMM units describe here have no problems at all with D-Star!)

For DFing a narrowband digital signal one could use "normal" (non-D-Star) analog receiver 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 avoid data corruption due to the "switching tone") 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 are also expected to work just as well as they all function using the same, basic techniques.

DFing one station amongst several in a TDMA environment:

There is one notable exception to the above with regard to "narrowband" digital voice operation and that has to do with those modes that involve TDM (Time-Domain Multiplexing) - that is, the "sharing" of several transmitters on the same frequency.  In this mode independent users' transmitters will take turns, briefly transmitting during their assigned "slot", allowing the re-use of the same frequency for several users at the same "time."  Clearly, this can cause confusion of "traditional" direction-finding techniques - particularly those of the "Doppler" type as there is no obvious way to separate the two.

Typically, the radios operating in these modes require that they be configured to transmit some sort of identification which means as long as the user has appropriately configured the radio - and as long as you have access to one - you should be able to identify the user.  Barring that, if you have a radio and are able to differentiate the transmissions of the "station of interest" from those of other stations using the frequency it may be possible to get general bearings of the station through the use of a directional antenna such as a Yagi and attenuator.  Finally, it will be highly likely that not all users on the frequency will always be transmitting at the same time, leaving transmission of the "station of interest" by itself, making it easier to employ more conventional means of location!

The one exception to this rule thusfar is if the digital voice system in question is being used by more than one operator in a TDMA (Time Division Multiple Access) scheme such as is available with DMR:  If there are two stations transmitting on one frequency in a time-interleaved manner, directional readings can be confused.  In the case of a "Doppler" type system that is inherently omnidirectional, if both signals are audible the likely result will be nonsensical readings.  If one is using a Yagi it may be possible to isolate the two, distinct transmitters, but this will depend on the relative strength of the two signals with respect to each other, the separation of their bearings, the prevalance of reflections/multipath and a few other related factors - not to mention needing to be able to discern one signal from the other to determine which one is actually being sought!

If the signal source is very close and strong then the use of attenuators, directional antennas, etc. can help isolate it from other, weaker TDMA signal on the same frequency, allowing both types of systems to be used.


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:

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 20170515

    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: