BUILD YOUR OWN DF UNIT.

These pages are made by Dutch Hamradio-amateurs.
Home-Page Last modified June 15 2001.
All these pages best viewed at 640x480 and with Netscape 3.0 and Explorer 4.0 or higher. LEFT MAIN UNIT and RIGHT KLIK ON REMOTEBOX PICTURE TO SEE RDF ANIMATION.

HOW TO BUILD A RDF-UNIT

ON THIS HOMEPAGE YOU WILL FIND INFORMATION
AND DIAGRAMS TO BUILD YOUR OWN RDF AND DISPLAYUNIT.
PLUS THE SPECIAL ANTENNA COMBINER WITH BUILD-IN RX-PREAMPS.
AND ALL THIS INFORMATION IS FREE FOR DOWNLOAD.
IMPORTANT NOTICE;
We do NOT have any plans to manufacter or getting involved in selling complete build units !
We do NOT give any permission to reproduce, or use this information for others than HamRadio use.
We are NOT a company, only some Dutch HamRadio Amateurs and this project is just a big HOBBY of ours.

techtalk front/back
view antenna switch main diagram computer interface

THIS RDF-UNIT CONVERTS ANY NBFM TRANSCEIVER INTO A RADIO DIRECTION FINDER.
BUILD-IN R.F. ACTIVATED ANTENNA RELAY DIVERTS TRANSCEIVER OUTPUT TO NORMAL ANTENNA DURING TRANSMIT OR WHEN D.F. ATTACHMENT IS SWITCHED 0FF.
WITHIN THE LATEST VERSION, LOW NOISE RX PREAMPS ARE BYING ADDED INTO THE ANTENNA COMBINER UNIT !!!
( for better sensitivity for low level signals ).

FEATURES

• This unit works with any existing narrow-band FM receiver or transceiver. No modifications are needed. The only connections required are to the external speaker and antenna jacks.
• Gives a clear directional readout on a circular array of sixteen ultra bright red LEDs.
• Display holds last reading when signal drops out.
• Very easy to use and install.
• Only a single coaxial cable needed between display unit and antenna combiner.
• Antenna combiner unit connects to four omnidirectional antennas to simulate a spinning antenna. Depending on the choice of antennas, tested operation is possible from 20 to 500MHz. (20 to 1000 MHz. is possible but not fully tested)
• Professional quality at low cost. Display unit is based on two P.T.H. circuit boards. Gasket sealed combiner unit houses two conventional double sided PCBs.

SYSTEM DESCRIPTION

DFunit is a Radio Direction Finding (RDF) system which is designed as an add-on accessory for any existing narrow band FM communications receiver or transceiver.
The only connections required to the receiver are to the antenna input and the external loudspeaker jack.
The system comprises two separate units. One contains control and display electronics and is located at the receiver; the other is a special antenna combining unit containing its own drive electronics and requiring only a single coaxial cable to connect to the control unit.
Directional read out is via sixteen ultra bright red LEDs arranged in a circle at 22,5 degrees intervals on the main- or the remote unit.
When a signal is received its bearing relative to the antenna is indicated by whichever of the sixteen LEDs illuminates. In mobile applications this permits "homing" onto the signal, and at fixed stations when the antenna has been correctly aligned the compass bearing of the signal is directly indicated.
When used with transceivers an RF activated relay built into the control unit allows 'talk through' by diverting the transmitter signal to the normal antenna.
This RDFunit will work with FM receivers ranging from pocket scanners to mobile or marine radio telephones and including VHF amateur radio and CB transceivers.
In addition to the display unit and the antenna combiner unit, a complete system needs four omnidirectional antennas (e.g. conventional quarter wave whips or half wave dipoles) mounted in a square array (see later sections).

APPLICATIONS

This RDFunit costs between ten and a hundred times less than conventional RDF systems, and therefore opens up new application areas for both professional and hobby users.
Applications include:
VHF amateur radio, Citizen's Band radio, aircraft spotting, tracking gliders and light aircraft, locating lost model aircraft, private mobile radio systems, coastal and marine radio, tracking and locating anti-social radio operators, locating 'tagged' animals in the wild, helping to identify or trace unknown transmissions, law enforcement.

Diagram for all of the buttons and other components.

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HOW IT WORKS

The RDFunit uses the so called Doppler principle in which a single antenna moves rapidly in a horizontal circular path.
The cyclic motion towards and then away from the transmitter adds phase modulation to the received signal and after demodulation in the detector of an FM receiver followed by filtering, a sinusoidal signal is obtained. The frequency of this signal is equal to the rotation rate of the antenna, and its phase relative to the antenna rotation is related to the direction of arrival of the radio signal.
To avoid the obvious drawbacks of mechanical rotation, the RDF simulates a rotating antenna electronically. Four quarter wave whip or half wave dipole antennas are mounted at the corners of a square, the separation being between about 0.05 and 0.3 of a wavelength. A special electronically controlled attenuator using PIN diodes smoothly transfers the receiver connection from one antenna to another. The resulting signal is then similar to that obtained from an antenna actually moving in a cirde yet without any mechanical complications.

TECHNICAL DETAILS

The RDF system has been designed using the latest technology to give excellent performance at remarkably low cost.
Installation is greatly simplified by the fact that only a single coaxial cable is needed to connect the display unit to the antenna combining unit. The normal control cables associated with Doppler RDF systems have been eliminated by placing a phase locked loop system plus drive electronics at the antenna. This reduces installation costs, especially where the antenna is remotely located, and also means that existing coaxial cable runs can be used if desired.
It also means that alternative special purpose antenna systems can be easily used with the system as and when they become available.
A major technical feature of the RDF is the smooth switching technique used in the head unit. This avoids one of the problems associated with conventional non- mechanical Doppler RDF systems. Typically these use abrupt switching techniques to step from one antenna to the next in the circular array of antennas. With this technique the phase modulation waveform is stepped instead of smooth, and therefore all incoming signals have sidebands added corresponding to harmonics of the switching frequency. High order sidebands from a strong adjacent channel signal can often be sufficiently strong to cause serious interference to the desired signal. The PIN diode smooth-switching technique used in the RDF system was optimised using computer simulation methods and reduces such effects to a very low level.
Two commutated capacitor filters are used in the signal processing circuits. One of them extracts the modulation signal from speech and noise prior to the digital phase comparison circuits. The bandwidth of this filter is controlled by the knob labelled "RESPONSE" and determines the speed of response of the display to a changing bearing.
The second filter notches out the superimposed modulation signal so that it does not disturb normal reception. After notching, the signal passes via the "VOLUME" control to a 2 watt audio power amplifier which then drives an external loudspeaker.

INSTALLATION AND OPERATION

A complete RDF system requires the following items of equipment.

1. A narrow band FM receiver for the desired frequencies.
2. RDF control and display unit.
3. RDF antenna combiner unit with a magnetic base for car roof installations and with a four metre output lead - see below.
4. Four identical antennas with coaxial cables to connect to item 3.
5. An external loudspeaker to plug into the display unit.

In addition a coaxial cable jumper lead will be required to connect between the receiver and the control unit.
The control unit requires an external power source of 12 volts DC
(from the car electrical system in mobile applications or from a mains power supply).
The nature of the antennas mentioned in item 4 will depend on the application.
For mobile use four magnetically mounted quarter wave whip antennas are ideal,
since the antenna combiner unit can be conveniently mounted between the antennas on the car roof.
For fixed station use, four ground plane antennas or four vertical dipoles should be mounted symmetically on a vertical mast together with RDF antenna combiner unit.
In either case the distance between adjacent antennas should be between about 0.05 and 0.3 wavelengths.
The connectors fitted to the Display Unit are as follows:

Audio input (Phono),
Loudspeaker output (Phono),
R.F. output to receiver (BNC coaxial),
R.F. input from RDF antenna (BNC coaxial),
R.F. input from normal antenna (BNC coaxial),
RS232 output to external PC or interface (** optional DIN connector),
Power supply (3.5mm jack, and concentric type DC connectorwired in parallel).
Connection to the antenna combiner unit is via five BNC FEMALE connectors.

THE ANTENNA ARRAY

KLIK ON ABOVE PICTURE
FOR DOWNLOAD THE DIAGRAMS FOR ANTENNA-SWITCH (with RX preamps).

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MAIN DIAGRAM

KLIK ON THE ABOVE PICTURE
FOR DOWNLOAD THE DIAGRAM OF THE MAIN BOARD.
For those who like to build the interface,
Above you are able to download the diagrams of the main and antennaswitch units.
Its a easy to build and 'no tune' based unit just 'plug and play'.
This is the only information at this moment I do not have other usefull information like boards layout availeble.
The boards we made are used on both sides of the PCB.
The only thing i can help you with, are PCB layouts on 'film'.
These 'films' I'm not able to copy and put on these pages.
When you like and perhaps at a later stage, I will try to put some files containing these PCB boards layout on this page aswell.
There are also some wiring diagrams availeble.
Remember,
I'am not a company and this is just a hobby project so its freeware and I do not ask you any money for this information!!!
Only can tell you, we build several interfaces and all working great and without any big problems.

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COMPUTER INTERFACE HISTORY

KLIK on picture to enlarge.
Somewhere around they end of 1997,
We start thinking if it is possible to connect the DF to a PC.

In November 1998,
we start to examine how to build a RS-232 interface for our DFunit to interface with PC serial.

In Januari 1999,
We are getting in contact with Klaus, and he is willing to give us a hand with designing a RS-232 interface for connecting our DF to PC serial ports.

On the end of March 1999,
We have finish some final test of RS-232 interface together with the RDF unit.
And finally, and many thanks to Klaus, its working !!!!
The microprocessor within the interface is reading the bearings from the RDF-unit and transmit $DFBRG,xxx,*xx data true the RS-232 interface to the PC com-poorts of the PC
( where xxx = degrees and *xx = [CR] [LF] ).
Within Windows terminal at 4800 baud,
we can see the bearings actualy being display on the computer screen.
Within the interface there is also a function that give full support to a remote GPS unit.
You must only put two wires from the RS-232 interface to the GPS-unit NMEA data output,
and that part is working also.
Now you must find yourself a nice pice of GPS and mapping software that is also supporting the $DFBRG,xxx,*xx data, transmitted by the RDF RS-232 interface,
and you are on your way to total perfection.
The diagram of the RS-232 interface is free to download or you may ask it by email.
So this is the end of this chapter and we are going further to the nex step,

Find us a pice of GPS mapping software that support
DFBRG,xxx,*xx or the %xxx/x DF format.
( see for the latest developments about this, our GPS software chapter )

On the end of June 1999,
We are ready with things and working on other projects.
So we close it for time being.

On April 10-2000,
We re-start this ARDF project and are going to find out if it is possible to use this ARDF unit together with APRS.
We ask Roger Baker, the author of UIVIEW, if he is willing to import ARDF functions into his wonderfull APRS program UIVIEW.

On Februari 10-2001,
Roger Baker is not replying or intrested to make DF availeble in UIVIEW.
A local Dutch amateur made a simple pice of DF software.
This software runs in background of UIVIEW-32 as uses %xxx/xx DF data format to plot DF bearings on APRS maps.

On March 1-2001,
A ADD-ON for Roger Baker his UIVIEW is availeble on UIVIEW pages.
This software only runs in a registrated copie of UIVIEW-32
The DF ADD-ON plot DF bearings on APRS UIVIEW maps.
ADD-ON COULD BE DOWNLOAD ON THIS LINK

On february 1-2002,
A SET OF BRAND NEW DESIGN PCB BOARDS ARE MADE AND AVAILEBLE TO PURCHASE!
The set contain ALL 4 PCB boards,
all diagrams, componentlists who are needed to start.
There are NO KITS availeble now or in near future.
Price indication somewhere near 150,00 EURO's.
Please contact: Henk PE0SSB by email
KLIK HERE FOR PCB BOARDS AND OTHER RDF STUFF

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GPS AND RDF MAPPING SOFTWARE

For more information about our special and wonderfull DF software made by BUTEL-SOFTWARE, and to see realtime simulation! Please visit our software page.

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FEETBACK OR CONTACT INFORMATION

When you need more information here you find our email addres.

OUR EMAIL

BACK TO THE INDEX PAGE

On our RDF index pages you will find more links to RDF/MAPPING software,
other DF units, test reports and also some GPS/DF software test drives!

INDEX PAGE