A Doppler Direction Finder
for Mobile VHF Operation

INTRODUCTION

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.

EXPLANATION

All the information at this ( personal ) 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...

If this project seems interesting, but too "ambitious" for your technical skills, I can provide PC boards ( for a price, see OPTIONS ) either "bare boards" or "assembled / tested".

WEBSITE CONTENTS

A lot of enhancements have been 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, and a serial RS232 interface, to drive a computer display program. 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 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 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.

Somewhere in the middle of all this, 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 )

The compass can also be operated as a separate instrument, if you have no particular interest in DF equipment... a PIC program is provided which will allow the compass to drive the IBM display programs available on this site.

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.

Check the link called "OPTIONS" on the home page for terms of business, or for more information about these enhancements. Each of the enhancements is described ( in detail ) on this website, so you can ( if you are ambitious ) build them yourself. Source code for the 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....

LEGAL DISCLAIMER
AND
COPYRIGHT DECLARATION

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

All information at this site is....

COPYRIGHT 1999
Bob Simmons
1445 Harbor View Drive
Apt. 143
Santa Barbara, California
93103
(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 :

NO WARRANTY OF SUITABILITY
FOR ANY PURPOSE
IS EXPRESSED OR IMPLIED.

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... talk to me about buying one. ( or renting one )

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.]

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