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DSP-RDF Doppler Radio Direction Finder Kit

Price $255.00 U.S + S&H

This kit is based upon the N0GSG DSP Doppler featured in the November 2002 QST Magazine. Doppler RDF units determine the bearing to a signal's source by electronically rotating four antennas, arranged in a circular pattern, at a high rate of speed. The signals received at each antenna are combined and passed on to your FM receiver which is tuned to the frequency of interest. The FM receiver detects the incoming signal and produces an audio tone which is then passed back to the Doppler unit. Inside the Doppler unit are two microprocessors which control the antenna switching and process the audio tone, from the receiver, using DSP techniques. The result of this processing determines the relative direction to the signal source. The direction is displayed continuously in real-time on a circular array of 16 LEDs, each representing 22-1/2 degrees of angle. In the case of intermittent signals the microprocessors save and display the last directional information (shown as a flashing bearing indicator) until the signal is again active. Active signals are displayed as a steady directional indication. The DSP-RDF Doppler kit being offered by W6DFW incorporates another feature not in the original design. This feature allows direction finding of a jamming signal even when the jammer is broadcasting a tone intended to interfere with Doppler units.

Notice: Notice: Most current serial port GPS's offer the $GPRMC NMEA output sentence. If your GPS does not furnish the $GPRMC NMEA sentence. the true bearing features will not operate. All other features will operate including both visual display and RS-232 outputs as well as GPS data streaming but all bearings will be relative rather than true. (IF YOUR GPS USES A USB PORT, TO CONNECT TO A COMPUTER, IT WILL NOT WORK WITH THE DSP DOPPLER.
Follow the link in the sidebar for a suitable GPS.
Version 2 Enhancements:
  • RS-232 output
    • The RS-232 interface consists of 5 surface mount components. Because of the skill levels needed, these components are pre mounted.
    • 4800 baud, 8 bit, no parity, 1 stop bit.
    • Standard Agrelo serial data format for RS-232 Doppler Data.
    • Supported by: WinAPRS, SAPlus, GPSS to name a few programs.
    • Bin size for RS-232 bearings is reduced to 3.6 degrees, rounded up to the nearest integer value for RS-232 output. This means that the circle is divided into 100 parts Visual display bin size remains 22-1/2 degrees, (always relative).
    • Only 1 computer serial port is needed as both GPS and Doppler information are combined before sending to the computer.
  • Separate GPS serial data connector.
    • All NMEA GPS data formats are streamed and appear on the RS-232 output interleaved with Doppler Bearings when Available.
    • Uses $GPRMC NMEA GPS sentence, when available, to convert RS-232 output bearings to true bearings for map displays.
(NOTE: VERSION 1 is no longer supported.)
The DSP-RDF Doppler kit consists of all necessary components including pre-programmed Microprocessors, and a set of six professional quality printed circuit boards:
  • The MAIN SIGNAL PROCESSOR board which processes the receiver output audio and displays the directional information.
  • The ANTENNA CONTROL board which performs the electrical switching operation for the antennas and combines the four resulting signals into a single signal to be connected to the FM receiver's antenna input.
  • Four ANTENNA INTERFACE boards, one for each antenna in the array. These boards contain the switching circuitry for each individual antenna.
The ANTENNA CONTROL and ANTENNA INTERFACE board kits come with components for both 2 Meter and 440 MHz operation. An extra set of Control and Interface Boards may be ordered separately if the user wishes to construct both 2 meter and 440 MHz antenna arrays.

  1. Small hand tools, low-wattage soldering iron, 63/37 rosin cored electronic solder, Anti-static wrist strap, Volt-Ohm Meter.
  2. Enclosures and antennas. The kit does not include enclosures or antennas.
  3. A standard DB-9 male to female serial cable. This cable connects the ANTENNA CONTROL to the MAIN SIGNAL PROCESSOR. This cable is wired pin for pin and is available from any computer store as a modem connection cable
  4. A standard DB9 Male to DB-25 Female Serial cable is used for the connection to the computer port. DO NOT USE A NULL MODEM CONFIGURED CABLE.

Four whip antennas can easily be constructed by soldering a 1/4 wave length section of brass or copper plated steel 3/32 inch diameter gas welding / brazing rod to the center conductor of a PL-259 Male UHF connector.
Each vertical whip should be one-quarter wavelength, this length is calculated using the formula: Length in inches = 2808 divided by the frequency in Megahertz, ( 2808/F ) The length should also include the length of the PL-259 connector pin which plugs into the SO-239 socket.
The four whip antennas connect directly to the SO-239 connector on each of the four ANTENNA Interface boards. The antenna array can be constructed several ways. A sheet of plywood covered with aluminum or galvanized steel can be used as a ground plane.
Antennas should be placed in a square pattern spaced about 0.22 wavelength, Spacing = 2471 divided by the frequency in Megahertz ). In the case of 2 meters this is about 17 inches.
with the four whips mounted in a square pattern, the ANTENNA CONTROL board will be mounted in a metal shielded box in the center of this square. Antenna # 1 will be the left front, #2 the right front, #3 the right rear and #4 the left rear.
An alternative method, which I use, is to mount each of the ANTENNA INTERFACE boards in a small die cast aluminum box to which is attached to a ring magnet. The whip magnets are connected to the ground side of the whip. The ANTENNA CONTROL is likewise mounted in standard 4-1/2 inch die cast outdoors electrical junction box, also with a ring magnet attached. In use, the control box is positioned on the center of the vehicle roof and the antennas are positioned in a square pattern as described above. The roof of the vehicle serves as the ground plane. Proper shielding of the ANTENNA CONTROL BOX and ANTENNA INTERFACES is critical, as is the insulation of the four antenna magnets from the vehicle. All RF connectors are Female UHF SO-239 connectors and mate with MALE PL-259 connectors.
The following PDF files are available:
Assembly Manual V2.01 (2.4MB)
Operation Manual V2.01 (127KB)

Main Board Schematic
Antenna Control Schematic
Antenna Interface Schematic
PACTEC CM6-300 Enclosure DRILL Templates

ORDERING: US Only. For International, see below.

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