Series 5931 Radio Direction Finding System

The products for the series 5931 direction finders on this page are no longer in production.  However, we do continue to support these products with customer assistance and repairs. Contact the factory if you need help with any of these products.

The series 5931 direction finders feature wide frequency coverage using a magnetic mounted RF summing circuit. Doppler offers a wide range of antennas to cover frequencies between 88 and 1000 MHz. We recommend the series 5931 direction finders for low cost homing from a vehicle.

A complete setup requires a processor, a remote display, an RF summer, and an antenna. It also requires a narrow band fm receiver. You can use good quality scanners, transceivers, service monitors, and spectrum analyzers. However, be careful to avoid transmitting into the direction finder if you use a transceiver.

Features of the Series 5931 Direction Finders

  • Remote display of bearing angle and signal strength
  • Broadband – covers 88 to 1000 MHz
  • 5 deg bearing accuracy (1 sigma, no reflections)
  • Digital processing for high sensitivity and multipath reduction
  • Notch filter eliminates tone from built-in speaker output
  • Selectable moving average and RF attenuator

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 5931 series direction finder meets the following specifications:

  • Type: simulated Doppler, 4 antenna elements
  • Commutation frequency: 1228 Hz nominally
  • RF operating frequency: 88 to 1000 MHz
  • Accuracy: 5 degrees 1 sigma
  • DF sensitivity: -126 dBm
  • Sampling rate: Best estimate calculated twice per second
  • Averaging: Moving average of last 2 or 4 bearings selectable from remote display
  • Audio output: Notch filter to eliminate commutation tone, 0.5 W maximum output
  • Audio input: 0.01 to 0.6 VRMS
  • Display hold: 10 seconds
  • RF attenuator: 24 dB selectable from remote display
  • Power: 11 to 14 VDC, 0.7 amps
  • Operating temperature: 0 to 50 deg C (Display & Processor), -50 to 100 deg C (RF Summer & Antennas)
  • CE compliant

DDF5931 Processor

processor for series 5900 direction finders
DDF5931 Processor

The DDF5931 processor generates the control voltages necessary to rotate the antenna (using the DDF5980 RF Summer). It also processes the audio from the narrow band fm receiver to determine whether a signal is present. Digital processing lets the receiver operate with the squelch “open” for maximum sensitivity. If the receiver provides a Received Signal Strength (RSS) output, it 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 antenna rotation. Next it is amplified to drive a loudspeaker. The DDF5931 provides an RS232 output which is normally connected to the DDF5921 to display the bearing and signal strength. You may alternatively connect the same output to a PC for changing internal parameters.

Front panel connections are provided for an external speaker and DC power to the receiver. We include cables or connectors for connecting to the automobile power (cigar lighter), the receiver audio output, and the receiver RSS output.

Users can permanently mount the DDF5931 to the floor of the vehicle. It measures (HxWxD): 32x216x140 mm (1.25×8.5×5.5 inches) and weighs 0.635 kg (1.40 lbs).

DDF5921 Remote Display

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 easily adjusts for best viewing.

The DDF5921 measures 60x114x32 mm (2.375×4.5×1.25 inches) and weighs 0.236 kg (0.52 lbs).

DDF5980 Mobile RF Summer

DDF5980 Mobile RF Summer

The DDF5980 consists of 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. This is equivalent to 24 dB attenuation, and it can 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. The coax uses dual shielded RG58.

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

DDF5961 and 5962 Mobile Antennas

DDF5961 & DDF5962 Antennas

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

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

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

These two antennas are identical except for the whips. You may order additional whips so that the same magnetic mount bases can cover multiple frequency bands.

ModelFrequency Range (MHz)Stock Number of Whip
DDF596188 to 136 MHzANT008
DDF5962136 to 500 MHzANT022

DDF5947 Mobile Antenna

DDF5947 Mobile Antenna

The 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. See the antenna installed on a DDF5980 in the photo above. 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.). Doppler also provides a safety strap with this antenna. Always use the strap when you cannot rely on the magnetic mount.