Amateur Radio Direction Finding (ARDF) Page
contains various links related to Amateur Radio
Direction Finding (ARDF) - also known as "Foxhunting" or
Important Note about firmware availability:
firmware" for the VE2EMM Doppler I, II and 3 units is
available - visit the "Getting Chips" web
page for more information.
"How do I get programmed
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
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
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
- 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.
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
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
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
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:
Some sort of interface connects to the computer to drive the
antenna array. This often takes the form of a device being driven
by pulses from the audio output
of the device.
The audio from the receiver
connected to the antennas being rotated is applied to the sound card
input along with some sort of timing reference pulse from the antenna
driver hardware to allow precise synchronization, making it insensitive
to the varying buffering found in practically any computer with a large
Advantages of a PC/tablet-based system over a "stand-alone" ARDF system such as that described on this page:
Somewhat less hardware is needed: The same type of
antenna/switching system is required along with some hardware used to
drive these antennas from the PC's audio output.
Many people already own a PC or other device that is suitable:
It need not be particularly "powerful" which means that
older/cheaper units may be suitable.
Updates are done simply by installing a new version of the software on the computer.
Disadvantages of a PC/tablet-based system over a "stand-alone" ARDF system:
The device will require more power than a typical stand-alone unit. (This unit will take only about 100-200mA at most at 12 volts.)
Any such device will take, perhaps, a minute or two to boot up while a stand-alone unit will be online in only few seconds.
In most jusdictions there are specific laws prohibiting the use
of of any sort of computer by the driver. In many jurisdictions
having a computer/video display that is visible to the driver - or
even in a front seat - is not allowed, requiring the use of another
person any time the device is used. A simple, stand-alone
unit might not run afoul
of such laws. It's worth noting that having a second person to
interpret the ARDF readings and act as a navigator is always a good
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.
It is with regret that I note the passing of Jacques,
VE2EMM, in 2009. Jacques' pages are
still online at the time of this writing and
email to his address may be being
answered by a friend of his.
Firmware for the VE2EMM Doppler I, II and 3
units is still available from this
web site - see
the links below for more information.
Unless stated otherwise, the
modifications and code noted on the pages linked
below were originally built upon Jacques' fine
work. In the cases below, the new/added code
is entirely original, enhancing the capabilities
and functionality of the original firmware.
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
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!
page, in both English and French: Jacques
had interests other than just RDF.This page has
been maintained in-memorium of VE2EMM.
The Montreal Doppler I
The Montreal Doppler I - This is
a microprocessor-controlled "Doppler" type of RDF unit roughly
based on the "Roanoke" design.
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:
Please be aware that Jacques may or may not be able to
provide support for this project and your
understanding on this matter is appreciated. Feel
free to ask me (via the link at the bottom of this page) if
you have any questions.
The "Alternate Firmware" for the Doppler II is still
available - go here
for more information on pre-programmed chips
The Montreal Doppler 3
Montreal Doppler III (English page.)This unit is a newer Doppler unit, similar to the Doppler II
but with the addition of a 36 LED compass rose and better audio
Note: The V3.2 boards
are available from FAR circuits and
alternate firmware is
still available. (As of 2/2008, a set of boards was
available from FAR Circuits for $19 - $45 with processors with
the original VE2EMM code - but check the price and
availability of code before ordering.)
Here are the archived board-layout files that were on
the Doppler 3V2 web page in "RAR" format. These
files are referenced in the above .PDF. These files
are for the "sPlan" software available in demo versions
Software in Germany - namely "sPlan
Viewer Ver. 6" for the schematic and "View
Layout Ver. 5" for the circuit board.
The "Alternate Firmware" for the Doppler III is still
available - go here
for more information on pre-programmed chips.
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
site: Various pictures of the Doppler III
(mostly in French.) This site has many pictures of
the Doppler III unit in various stages of construction including
additional details and hints pertaining to their assembly and
use. Even though it is largely in French, pictures are
still worth a thousand words - no matter what language you may
HERE for a link to the English section of the website
that details some of the construction of their
pertaining to the Montreal Doppler 1, 2 and 3 DF units:
following pages are maintained by KA7OEI:
Questions about these pages should be directed using
the contact information at the bottom of this page.
firmware for the Montreal Doppler units:
firmware for the "Montreal Doppler I" DF unit
- This firmware adds some of the features of the Doppler II/III
to Jacques' original PIC-based 32 LED Doppler DF
unit such as a serial port, multi-color display, and DSP-based
filtering. Note that the Doppler I alternate firmware
does not support direct multiplexing of a GPS receiver
through the DF unit as is done by the Doppler II and III
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
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:
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
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.
- If you didn't read the documentation too carefully,
some of the info here might remind you - and maybe cover a bit
more ground. This description applies to VE2EMM's original
- An illuminated display is very handy when trying to
use this at night...
additional switched-capacitor filter for the Doppler II
- One of the shortcomings of the original Doppler II (but
corrected in the Doppler III) is the lack of a very
narrow switched-capacitor filter. This page describes such
an add-on unit. Note that the "averaging" feature in
the Alternate Firmware
does a simulation of this switched-capacitor filter, providing
an equivalent of some of the filtering action provided by it.
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:
APRS (Automatic Packet Reporting
System) - This software, originally
conceived by Bob Bruninga (WB4APR) was originally intended to
provide a means of interfacing GPS receivers, packet radio, and
computers to display the location(s) of stations on a map.
This software has provisions to be interfaced with RDF gear and
using a variety of methods (including signal strength
measurement and bearings supplied by external equipment) it can
determine the bearing/location of a remote transmitter.
While I am aware of these capabilities, I have yet to talk to
anyone else who has any personal experience with APRS and RDF
gear. The APRS software is shareware (that is, a donation
is expected if you are going to use it more than just "trying it
out...") Over the years I have seen conflicting
information as to whether the fully-functional ARDF interface is
included with the base APRS package, or an extra-cost
option. I am currently researching the use of
APRS-related software and its use with ARDF and moving-map
displays. I recently noted that my older APRS links
no longer worked: I would recommend doing a search for
APRS to find current information.
GPSS (GPSSoftware) - This software, written by Robin
Lovelock, was intended primarily for moving-map and vehicle
tracking applications using GPS. Several years ago, at the
request of the amateur radio and radio-collar tracking
community, Robin added the capability to use the bearings from a
RDF unit (in Agrelo format) in addition to NMEA GPS data,
combining the two to produce a moving-map display that displays
the bearings relative to the vehicle's position: Given
enough bearings, the likely position of the transmitter is
calculated and displayed. For more information, see the
pages specific to the use of GPSS in Amateur
Radio and RDF,
as well as the "More GPSS
info" page. This is the only one of the
two programs mentioned here that I have actually used and I have
been pleased with it. GPSS is "nagware" - that is, you can
use it non commercially for free as long as you tolerate a
"reminder" that you can pay a reasonable fee to get rid of that
Another suite of software that may be useful for the ARDF
enthusiast is "UI-View." I know relatively little
about this software and more info may be found via an internet
WB6EYB's "Google Hunt"
- This is free program, platform-insensitive, that allows "live" maps,
interfaced with Google Earth. Since it is generally compatible
with GPS NMEA and the Agrelo outputs from the Montreal Doppler receiver
(and others!) it should, in theory, work! (I have not used it personally, but I have seen reports of success on "PicoDopp" Yahoo group.)
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
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.
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.
Antenna Arrays - This page describes some
typical antenna arrays (for mobile and home use) that may be
used with RDF systems.
WB6EYB's Doppler D/F
Instruments page has a bunch of info on various DF
equipment - how they work, and how to built them. A
worthwhile description of how these devices work may be found here.
UARC RDF page - This page has a few links to
projects and other pages related to ARDF, including:
wide-dynamic range field strength meter ("Mark 1")
- This is a simple field strength meter, using more-or-less
standard components, that can provide over 50 dB of dynamic
range without the need for changing ranges. In
conjunction with a directional antenna, it may be used to
locate the source of a radio emission.
wide-dynamic range field strength meter ("Mark 2")
- This is an updated version of the above field strength
meter, using an AD8307 logarithmic amplifier chip. This
unit has in excess of 80 dB of dynamic range and the chip's
built-in temperature compensation provides repeatable accuracy
as good as 1 dB.
A highly recommended project is Joe's
Measure Beam Optimized for Radio Direction Finding
- a cheap and simple-to-build (and nearly indestructible)
3-element Yagi that works very nicely for direction-finding
purposes. This Yagi is especially useful with
field-strength meters and receivers with wide dynamic range
S-meters. I've used this antenna for years and can
A PIC-based TDOA unit with an
audible field-strength meter - This page
describes a TDOA (Time Difference of Arrival) unit that
uses a small PIC microprocessor to improve performance and add
some features such as indication using tone pitch and LEDs as
well as a field strength meter for when you are fairly close to
the transmitter. Go
here for more information on pre-programmed
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.