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Doppler Systems LLC

Radio Direction Finders and Direction Finding Systems

The products on this page are no longer in production.  We do continue to support these products with customer assistance and repairs.  Contact the factory if you have need help with any of these products.

Series 5900 Direction Finding System

The 5900 series features wide frequency coverage using a magnetic mounted RF summing circuit. A wide range of antennas is offered to cover frequencies between 88 and 1000 MHz. This series is recommended for low cost homing from a vehicle.

A complete setup requires a processor, a remote display, an RF summer, and an antenna. A narrow band fm receiver is also required. Good quality scanners, transceivers, service monitors, and spectrum analyzers can be used; however, care must be taken to avoid transmitting into the direction finder if a transceiver is used.


The block diagram below shows the setup required for a mobile (homing) system.


When used with an appropriate narrow band FM receiver or scanner, the 5900 series direction finder meets the following specifications:

DDF5931 Processor

The DDF5931 processor generates the control voltages required to electronically rotate the antenna (using the DDF5980 RF Summer). It also processes the audio signal from the narrow band fm receiver to determine whether a signal is present, and if it is, the best estimate of the bearing of that signal. Digital processing allows the receiver to be operated with the squelch "open" for maximum sensitivity. If the receiver provides a Received Signal Strength (RSS) output, this is digitized and scaled to drive the S-meter on the remote display. The audio signal is filtered to remove the tone modulation impressed by the simulated Doppler rotation of the antenna, then amplified to drive a loudspeaker. The DDF5931 provides an RS232 output which is normally connected to the DDF5921 for remote display of the bearing and signal strength. The same output may alternatively be connected to a PC for changing internal parameters (normally not required). Front panel connections are provided for an external speaker and DC power to the receiver. Cables or connectors are included for connecting to the automobile power (cigar lighter), the receiver audio output, and the receiver RSS output.

The DDF5931 can be permanently mounted to the floor of the vehicle. It measures (HxWxD): 32x216x140 mm (1.25x8.5x5.5 inches) and weighs 0.635 kg (1.40 lbs).

DDF5921 Remote Display

Bearing angle is indicated by an array of 16 LEDs forming a compass rose and by a 3 digit 7-segment digital display. Signal strength is displayed using a 10 segment colored LED bar graph. All LED displays are high intensity for daylight viewing and may be dimmed by pressing a front panel switch. Other front panel controls include an averaging and attenuator switch, a bearing calibration switch, and volume up/down switches. The remote display has an internal loudspeaker for monitoring the received signal.

All signals are encoded in standard ASCII format, and the unit is connected to the DDF5931 processor with a 3.66 m (12 ft) RS232 cable which is included. Operating power is obtained over this cable also. The unit mounts to the windshield with a double pivoted arm that can be easily adjusted for best viewing.

The DDF5921 measures 60x114x32 mm (2.375x4.5x1.25 inches) and weighs 0.236 kg (0.52 lbs).

DDF5980 Mobile RF Summer

DDF5980 is a magnetically mounted remote RF summer with a cable length of 4.27 m (14 feet). In addition to high dynamic range preamplifiers, the RF summer contains a preamplifier bypass switch which is equivalent to 24 dB attenuation  which may be switched in or out. The unit has four TNC antenna inputs and a single TNC output. The unit comes with detachable control and RF cables. Dual shielded RG58 is used for the coax.

The DDF5980 measures  127x127x38 mm (5x5x1.5 in) and weighs 0.77 kg (1.7 lbs).

DDF5961 and 5962 Mobile Antennas

These consist of a set of four magnetically mounted bases with matched coaxial cables. Whips are supplied that may be cut to the frequency of interest using the cutting chart provided. The mounts are spaced in a square pattern 1/4 wavelength apart on the car's roof. Antenna leads are 1.8 m (6 ft.) of dual shielded RG58 style coax with TNC connectors to mate with the DDF5980.

DDF5962 includes 2.5 mm (0.100 in.) diameter, 0.48 m (19 in) whips that can be cut for 136 to 500 MHz.

DDF5961 includes 2.5 mm (0.100 in.) diameter, 0.84 m (33 in) whips that can be cut for 88 to 136 MHz.

These two antennas are identical except for the whips. Additional whips may be ordered so that the same magnetic mount bases can be used to cover multiple frequency bands.

Model Frequency Range (MHz) Stock Number of Whip
DDF5961 88 to 136 MHz ANT008
DDF5962 136 to 500 MHz ANT022

DDF5947 Mobile Antenna

DDF5947 consists of a 342 mm (13.5 in.) ground plane and four stub antennas having TNC bases which connect directly to the DDF5980 magnetic mounted RF summer. Frequency coverage is 700 to 1000 MHz. The antenna is shown above installed on a DDF5980. A heavy plastic cover provides a windscreen for the ground plane. The antenna measures 406 mm in diameter by 178 mm high (16 in diameter by 7 in high). It weighs 1.56 kg (3.45 lb.). A safety strap is also provided with this antenna, and it should always be used where the magnetic mount cannot be relied upon.