A Doppler Direction Finder
for Mobile VHF Operation


This website describes the construction of an effective VHF Doppler - type radio direction finder, ( a Doppler "DF" ) suitable ( among other things ) for mobile use in amateur radio T-hunts. An external FM receiver is required, ( the user must provide this ) and a special DF antenna must be constructed.

Doppler DF s have become very popular in T-hunts because they are fairly simple to construct and use, and they are quite effective. They are named for the "Doppler shift" principle, which causes the frequency of a received signal to "shift" ( up or down ) from it s "normal" frequency value, due to relative motion between the transmitter and receiver.

All the information required for a skilled hobbyist to build it is available here. That includes artwork for the printed circuit boards, ( three in all ) schematics, a parts list, ( with various catalogue part numbers, about $100 ) a basic technical description of the device, and some "tips" on its installation and use.

The PicoDopp DF

In 2003, a more sophisticated version of Doppler DF was added to this website, using a microcomputer ( and software ) to achieve almost all the necessary tasks. The antenna design was also improved, for greater sensitivity and frequency range. It is only available as a set of assembled and tested PC boards, and the user must still perform a non-trivial amount of completion work, to obtain a working DF unit.

For those who want to build their own equipment "from scratch", and / or who want to understand the technical details of Doppler DF units, ( i.e. self-education ) the original website Doppler is probably best. For others, who simply want to buy and use a Doppler DF, the new Doppler design ( called "PicoDopp" ) is better. The old DF design ( on this website ) is no longer available from me, ( FEB 2005 ) so you must make your own boards if you want it. The SquawkBox, MiniAntenna and MultiDisplay boards are still available ( on the new PicoDopp website ) but they are only available assembled and tested. The PicoDopp performs much better than the oriinal DF design, it is simpler to assemble into a finished instrument, and it has some extra features, such as an infrared data link that allows the use of a PDA ( Palm ) to provide a cordless / handheld display, and also a GPS input.

Information about the PicoDopp DF is available on a second website, by clicking [HERE].


All the information at this ( personal / hobby ) website is provided "gratis". I built one of these DF's for my own use in ham radio "T - hunts", conducted by my local amateur radio club. To learn some new PCB CAD software I bought, I decided to create some "respectable / genuine" PC boards for this device, and offer it to other club members. Once this was done, ( and to justify learning HTML for web pages ) I decided to create this website and offer it to other folks, with similar interests.

Perhaps more than anything, this website serves as a "showcase" for my technical skills, to any potential employers, anywhere in the world. That is why everything is so completely and meticulously described... I believe these are worthwhile virtues to demonstrate to a potential employer, but everyone else who visits this website benefits from this philosophy as a "side effect".

By the way, EVERYTHING on this website has actually been built and tested by me, and demonstrated to work... ( a few photos are provided )

Anyway, I probably couldn't make any ( serious ) money on these things... I know hams...

Again, this DF design is no longer available from me... since the introduction of the new PicoDopp DF, sales of this design have dropped almost to zero, and I discontinued offering it in February 2005. There is enough technical information here to make the old design yourself, if you have the necessary skills. It is quite an undertaking, and the new PicoDopp performs much better and has more features, so I reccomend it as an alternative unless you just want to get the experience of building small projects like this.


A lot of enhancements were added to the basic DF design as options, since the unit was first completed. The options are designed so that the DF user can begin with a "basic DF" unit, and add the options later, as desired.

The options include a ( 3 - digit ) numeric bearing readout, a serial RS232 interface, ( to drive an IBM or Palm PDA computer display program) an electronic compass, and an improved ( fully integrated ) MINI-ANTENNA for operation to 1 GHz.

In October 2002, a sophisticated MULTIDISPLAY board was added, which offers a hi-resolution pelorus display ( 36 LEDs ) combined with a 3-digit numeric readout in a single board. It requires the RS232 option, since it is driven by RS232 messages. The message format is the ( popular ) "Agrello DF" message, so the display can be employed with DFs from ( some ) other manufacturers.

The numeric readout has been adapted ( by various technical folks ) to operate with other types of Doppler DF's, and this is also true for the RS232 interface... The signals required to drive them can generally be found in any type of Doppler DF.

The RS232 interface employs a PIC 16F84 microcomputer, ( very popular with hobbyists ) and all the source code for the PIC is provided on this website.

An IBM PC display program ( MsDOS ) for the RS232 interface was added in late July, 2000, and has proven to be very popular. It has a simulation mode that allows operation without a DF, for learning purposes, and the RS232 input data format is compatible with other ( popular ) DF's. ( Agrello DF message format )

Later, a second RS232 Palm display program was added, for operation on a Palm PDA, ( personal digital assistant, about US$ 120 in August 2001 ) which provides a very economical alternative to a laptop computer display. ( not as many features as the IBM, though )

Somewhere in the year 2000, an electronic compass was also added to the DF, to "cancel" any variations of DF bearing measurements that are ( accidentally ) caused by changes of vehicle direction... This is a VERY nice feature, and it allows the IBM PC ( and Palm PDA ) display program to "sum" multiple DF bearings, automatically. ( A new version of the IBM program was created, to use the "extra" compass information )

IMPORTANT : ( September 2002 ) Since the summer of 2002, the PNI V2X compass module was DISCONTINUED, and is no longer available... I have spoken with PNI Corporation, and they intend to replace it with a new, enhanced model that has a smaller footprint and similar price. I am told they will offer an "adapter" PC board that will allow the new model to be installed as a "drop in" replacement for the V2X. This should become available ( I am told ) in late 2002... wait and see...

The operation of the original DF antenna board suffered above 250 MHz due to design deficiencies.. I later created an integrated "mini - antenna" which consists of a circular PC board with 8 antennas and all the necessary switching circuits... it has been tested to 1 GHz with good results... It is small enough to be housed in a plastic "radome". ( Photos are provided )

In a similar DF "theme", A separate ( portable ) TDOA DF unit was added in April 2000, because hidden transmitters are not always located adjacent to roads or highways. It employs acoustic feedback with stereo headphones, using antennas mounted on the headset, to keep the hands and eyes of the DF hunter free for other tasks. ( not as popular as the Doppler DF, but not as difficult, either )

Finally, something not really related to DF was added in 2001... a single board computer using the Intel 8052 micro, which has a built-in 8K BASIC interpeter, RS232 port, onboard EPROM programmer, and room for a PIC micro. A download program was later added, so that programs can be written, edited, and saved on an IBM PC.

The PC board for this SBC was created with ExpressPCB CAD, which is available ( free ) at their website. This CAD program will enable you to submit the SBC CAD file ( available for download on my website ) directly to Express PCB, ( along with a credit card number ) and get high quality PC boards with plated - through holes, directly from Express PCB. It's a neat service, and I often use it for other ( work - related ) PC boards.

Source code for the Doppler IBM display program ( written in QuickBasic 4.0 ) is available on request by sending an e-mail to me.

At this time, ( August 2001 ) maybe 50 people ( besides myself ) have built this Doppler unit, including a few folks in Europe, Africa, Asia, North and South America. Some clubs and organizations have also built them, and a few students have built them for academic credit. A few folks have used them for remote DF "networks", ( using APRS telemetry links ) because the equipment was designed to be "APRS compatible".

There actually might be more of them out there, because there is enough information posted on this website to eliminate any need for correspondence with me... one fellow didn't contact me until he was completely finished.

And now for something completely different....


Just to encourage honesty, I decided to include the following COPYRIGHT DECLARATION :

All information at this site is....

Bob Simmons
1445 Harbor View Drive
Apt. 143
Santa Barbara, California
(805) 568 - 3697 ( home )
E-MAIL : pelican2@silcom.com.
All rights reserved.

That being said, ANYONE may copy and / or distribute the information from this site, WITHOUT prior permission from me, PROVIDED THAT :

( 1 ) DUE CREDIT is given to me for the original work. That includes printing my call sign letters ( WB6EYV ) on the printed circuit boards.

( 2 ) NO PROFITS are involved... you can recover your expenses, ( photocopying, making boards, etc. ) but NOTHING MORE. If you think you can make money with this, ( you poor, deluded soul ) contact me.

Finally, a DISCLAIMER :


If you have trouble building one of these things, I'll try to help you with it... to a point. If you don't know how to solder, don't know how to identify pin 1 on an IC package, don't know how to read a resistor color code, etc., this project is NOT for you...

If you've never used a Doppler before, they are very nice, but they are NOT a "panacea"... A good receiver with a signal strength indicator, and a small yagi are reccomended to complement the virtues of a Doppler, for serious T - hunts. For "accidental" hunts, ( stuck mike PTT buttons, "false alarm" ELT's / EPIRB's ) Dopplers can do most ( or sometimes all ) of the D/F work. Doppler work well with strong or moderate signals, but poorly with weak signals, and this one is no exception... see "CAVEATS".

Poorly designed "economy" receivers ( wide or assymetric I/F passband ) sometimes just don't work with Dopplers... it's very rare, but it does happen. Check the link called "CAVEATS" for more information, or click [HERE.]