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Automatic Direction Finder - under construction

ADF-940 was a Radio Direction Finder (RDF), also known as an Automatic Direction Finder (ADF), developed as part of a range of direction finders by Ocean Applied Research (OAR) in San Diego (Californa, USA). The device is based on low-cost civil ADF units like the ADF-320, and was made especially for governmental and law enforcement agencies. It is based on the Adcock principle.

A complete ADF system consisted of a receiver with built-in display, an antenna unit, special cables and a set of five antennas. The image on the right shows a typical ADF-940-400 unit that covers a frequency range of 26.7 to 28.5 MHz.

The left half of the front panel holds the display, which is common for all ADF models of the era. It consists of a circular green/blue cathode ray tube (CRT) with several adjustments to control the position, intensity, and gain of the image on the CRT. The right half holds the receiver which is implemented as a removable panel or insert.
ADF-940 direction finder for 27 MHz

In the early 1960s, OAR had become successful in the tuna fishing industry around San Diego. A homing transmitter, also produced by OAR, was attached to a tuna that was then put back in the water. Being a social animal, the tuna would try to join the nearest school of tunas, after which the direction finder was used to find the school. Law enforcement agencies soon discovered that the same principle could be applied to the car of a suspect, and started ordering OAR equipment. In some countries, such as the Netherlands, ADF units were also used to find illegal transmitters.

ADF-904 ADF-904 Rear view
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Rear view

Known models
The following models of the OAR ADF direction finder are known. Note that the 900-range was probably reserved for specials and that the last two digits indicate the number of channels. No information about these variants is publicly available at the moment.

  • ADF-210
    Early version for maritime users
  • ADF-320
    Later customizable version
  • ADF-335
  • ADF-420
  • ADF-928
    Covers the original 28 (AM) channels of the 27 MHz CB band 1
  • ADF-940
    Later version that covers 40 channels (AM/FM) of the CB band
  • HRR-26
    Special version for the CIA
  1. At the time, the CB-band consisted of 23 AM-modulated channels, plus five so-called A-channels or alpha-channels that were used for radio controlled models [7].  Wikipedia

All controls are at the front panel of the ADF-940. At the left is the typical cathode ray tube (CRT), or scope display, that was common to all ADF models at the time. It is basically a common oscilloscope of which the X and Y deflections are used to create a rotating vector display. It is surrounded by the usual knobs to adjust its brightness, position and gain. At the far right are three fixed knobs for the audio volume, frequency scanning and dimming of the scale light.

The right half holds a removable aluminium panel that is affixed to the front panel by means of four screws in the corners. This removable panel offered great flexibility, as it enabled OAR to create new variants of the device, and to make customized specials that were adapted to the customer's requirements. As an example: some units have a speaker fitted on this panel, whilst on the model shown above this place is used to accomodate a 40-channel 27 MHz scanner.

All connections are at the rear of the device. At the far right are (from top to bottom) a 6.3 mm jack socket for a pair of headphones, a 3A fuse, and the power input socket. In this case the device is wired for connection of a 12V DC source, such as the battery from the intercept vehicle.

At the far left are three BNC sockets to which the antennas of the Adcock system are connected. The upper socket (YEL) is for connection of the reference antenna that provides the signal that is heared through the speaker. It also provides the reference for the other two channels (X/Y). The lower two sockets are the X (BRN) and Y (RED) inputs that provide the signals for the deflection of the CRT display, resulting in a vector that points in the relative direction 1 of the transmitter.

  1. This is the direction relative to the driving direction of the vehicle.

Front panel Rear side Display Receiver Field strength meter Controls Connections at the rear Behind the insert
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Front panel
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Rear side
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Field strength meter
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Connections at the rear
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Behind the insert

The ADF-940 can be tuned manually between 26.7 and 28.5 MHz, using the large coarse/fine tuning knob. Alternatively, the receiver can be restricted to the 40 channels of the 27 MHz CB band by pressing the bottom right yellow button at the center of the insert. This enables the 2-digit red LED display, on which the currently selected channel number is shown (e.g. 14). The upper two yellow buttons are used to step UP/DOWN through the available channels (1-40).

The scanning function is enabled by pressing the left/middle yellow button, and makes the receiver step through all 40 channels, wrapping around at the end, until a signal is detected. The threshold at which scanning is stopped can be adjusted with the upper right knob on the front panel. Scanning can be stopped at any time, by pressing the right/middle yellow button. Manual tuning is resumed by pressing the bottom/left yellow button. This turns the display OFF again.

In use
OAR direction finders were not only popular in the maritime field, but also with law enforcement agencies around the world. They were used in particular by the FBI and the police to track the car of a suspected criminal, and by the radio monitoring service for finding clandestine transmitters.

When the police discovered that radio direction finders were successfully used to track down a school of fish, it was decided to do the same to the car of a suspected criminal. A small homing transmitter or beacon was invisibly attached to the suspect's car and allowed the police to follow it unobtrusively. In many cases, the suspect unwittingly exposed his accomplices this way.

In the Netherlands, ADF units were also used to find illegal radio stations, commonly known as pirates, operating in the FM broadcast band around 104 MHz. For this application, OAR built special receivers that covered a particular segment of the desired VHF or UHF frequency band.

In the mid-1970s, when the use of the 27 MHz Citizen's Band (CB) was still illegal, the country was flooded with affordable AM CB-radios that caused a lot of radio and television interference. The Radio Monitoring Service, that was tasked with enforcement of the radio regulations, used early ADF units to locate the illegal CB stations.

The image on the right shows the interior of an intercept vehicle of the mid-1970s, that was used for this task. The ADF unit is clearly visible in the middle. It is accompanied by several other receivers plus a Sadelco field strength meter.

According to a former member of the Dutch Radio Monitoring Service, the RCD, the use of ADF units on VHF and UHF in urban areas was not very successful, as most of the music stations used horizontal polarization, whilst the ADF was only suitable for vertically polarized radio signals. 1

Furthermore, the reflections on surrounding buildings caused scatter on the CRT display [3]. This was mainly due to the fact that OAR used the Adcock principle for its ADF devices, rather than the Doppler Principle which would have been more appropriate [4]. Doppler was successfully applied by OAR's competitor Rohde & Schwarz in several products at the time. And they still use it today.

 More about the RCD

  1. In July 1977, the Dutch Radar Laboratory (NRP) developed a new antenna for the CIA, that made Adcock direction finders suitable for horizontally polarized signals, but this was unknown to the RCD.

External antenna unit Power and antenna cables Homing or beacon transmitter
Antenna unit
Not yet available   

Not yet available   

Homing transmitter
Not yet available   

The OAR ADF-940 is housed in a sturdy metal 'sleeve' that holds a metal frame to which the front panel is attached. After removing just four screws from the edges of the rear panel, the complete assembly, consisting of front panel and metal frame, can be pulled from the front of the device.

The image on the right shows the interior of the device, after the frame has been removed from the outer case shell, as seen from the front left. Note that the speaker is mounted at the left side, but that some models had it fitted at the front.

Most of the left half is taken by the display with its circular CRT that extends nearly to the rear side, a HT voltage unit mounted in a small metal enclosure, and supporting electronic circuits. The right half of the case contains the actual receiver, which has been shielded carefully from the display electronics to avoid interference.

The controls of the receiver are mounted on a separate aluminium panel that is placed at the right half of the front panel. It is removable, so that the receiver can easily be adapted to the customer's requirements. When removing the screws from the panel, it can be collapsed forward.

When doing this, be careful not to damage the wiring as it is rather fragile and all front panel parts are mounted very close to the electronics on the boards behind it. When re-mounting the panel, make sure not to cause any short-circuits.

One of the most interesting parts of the ADF-940 is the metal can mounted at the far right. It contains the three nearly identical front-ends that are used for the reference receiver and for the receivers that cause the X and Y deflections on the CRT display. These sections are carefully calibrated to match the supplied antennas.
Interior - right side (metal panel removed)

The metal can to the left of the front-end contains the controller. It is carefully shielded from the other circuit in order to avoid interference. On this particular device, the controller handles the display, the channel selection (1-40) and the scanning of the fixed channels in the 27 MHz band.

Behind the insert Inside the insert Interior CRT HT unit Receiver Interior - top view Interior - bottom view
Interior - right side (metal panel removed) Front-ends Antenna inputs and mixers Wiring of the controls at the right side of the front panel Power input seen from the inside CRT wiring
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Behind the insert
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Inside the insert
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HT unit
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Interior - top view
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Interior - bottom view
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Interior - right side (metal panel removed)
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Antenna inputs and mixers
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Wiring of the controls at the right side of the front panel
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Power input seen from the inside
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CRT wiring

Adcock vs Doppler
In order to understand the basics of the Adcock antenna principle that is used by the OAR ADF direction finders, it may be useful to consider the differences between existing systems. Generally speaking, the following techniques are available for automatic radio direction finding:

As Adcock and Doppler are the most common ones, we will briefly describe them below, before drawing conclusions about their applicability. The descriptions and conclusions are largely based on a 1978 report by the US Department of Transport, in which the three methods are compared. For a more detailed description with mathematical backgrounds, please refer to this report [4].

An Adcock antenna consists of spaced vertical open antennas, which in principle respond only to the vertical component of polarization of an incident wavefront. A two-element Adcock antenna is equivalent to a frame antenna of which the top horizontal arm is removed. The two remaining vertical elements are spaced no more than λ/4. In a crossed Adcock antenna, two vertical element pairs are used in a mutually ortogonal arrangement, in such a way that the first pair produces the sin φ and the second one the cos φ of the angle of incident (φ). OAR uses this arrangement:

Adcock antenna arrangement

One pair is used to determine the west-east component (W/E) and another one to determine the north-south component (N/S). The signals from these antennas are used to control the X and Y deflection of the CRT. A central whip (here shown in red) acts as the reference antenna (R). It is used for resolving the 180° ambiguity. The latter is performed automatically by special circuits.

Circuit diagram of two Adcock antenna-pairs

The circuit diagram above shows how the two antenna-pairs are connected. Each element (X1) is cross-connected to the diagonally opposite one (X2). The resulting pair is connected to the receiver via a transformer (X). The second pair (Y1 and Y2) is placed at 90° and is also connected to the receiver via a transformer (Y). This antenna arrangement was invented in 1918 by Frank Adcock, and is described in British Patent 130,490 [8]. Further information on Wikipedia [(9].

In a Doppler-based direction finding system, the antenna can be imagined as rotating at constant speed in a horizontal plane as shown in the drawing below. The circular motion of the antenna (A) induces a sinewave frequency modulation (fm) in the received signal (fc). This effect is known as a doppler shift. The phase (φ) of the sinewave modulation (fm) will be determined by the angle of incidence (α) of the received signal (fc). By comparing the phase of the motor driving source and the demodulated signal, the original angle of incidence of the received signal can be determined.

Doppler antenna arrangement

In reality, the antenna rotation is simulated by electronically commutating (switching) between a number of discrete antennas that are evently spread around the circumference of the circle. In practice, most arrangements consist of four or eight discrete antennas. In addition, a reference antenna (R) is often placed at the center of the circle, for reception in the conventional manner. In that case, the reference signal is used to cancel out voice-induced frequency modulation, so that the phase of the doppler-induced frequency modulation (fm) can be determined more accurately.

The Homer principle is commonly used in aviation and consists of two forward facing directional antennas that are pointing slightly off-center. By using the antenna radiation patterns and quickly switching between the two antennas, jumps in signal strength can be measured, except when the transmitter is dead ahead, in which case the signal strength is equal on both antennas (point P).

The diagram above shows how this works. The two antennas (A) and (B) have identical radiation patterns, but are facing slightly different directions of, say, +15° and -15° from the front of the vehicle. When the transmitter is dead ahead, the signal from both antennas will be equal (A=B) and no jumps in signal strength are measured. This is the case at point P in the above diagram.

When the signal strength between the two antennas is different (e.g. when B>A), the jumps in signal level are a measure for the angle of incidence. This is the case at point Q in the example. The diagram above shows how the signal strength from both receivers is used to affect the reading of the indicator in both directions. In rest (A=B) the meter is pointing upwards (0V).

In urban areas, Doppler-based systems clearly provide better results, as they are less sensitive to the effects of multi-path propagation caused by reflections on surrounding objects, such as trees and buildings, especially on VHF and UHF frequencies. Furthermore, Doppler systems can be used for both horizontally and vertically polarized waves, whereas Adcock systems are only suitable for vertically polarized signals. In the open field (e.g. at sea), performance is nearly identical, but only when the transmitter is vertically polarized. In practice, the OAR was suitable for direction finding of CB signals (27 MHz), but far less so for the horizontally polarized FM radio stations (104 MHz).

 Read the full report

The device is powered by a 12V DC source, such as the battery of a car, which should be applied to the military 6-pin male socket at the rear. Only two pins of this socket are used (B and D). The diagram below shows the pinout when looking into the socket from the rear of the device.

  1. n.c.
  2. 12V DC
  3. n.c.
  4. 0V (GND)
  5. n.c.
  6. n.c.
Antenna control
  1. Wiring currently unknown
  2. ?
  3. ?
  4. ?
  5. ?
  6. ?
  1. Anonymous, ADF-940-400 automatic direction finder - THANKS !
    Received December 2016. #CM-302429-DF.

  2. IEEE, Model ADFS-320 VHF band Automatic Direction Finder
    IEEE Communications Magazine. September 1981. Page 66.

  3. Cor Moerman, Former member of the Dutch Radio Monitoring Service
    Interview at Crypto Museum, December 2016.

  4. Charles J. Murphy, An evaluation of shore-based Radio Direction Finding
    US Department of Transport, United States Coast Guard, Office of R&D.
    CG-D-28-78. Final Report. September 1978.

  5. Elizabeth Tucker & Caryle Murphy, US Contractor Pleads Guilty in Tax Case
    The Washington Post (newspaper). 3 December 1988

  6. The Free Library, Cubic Communications acquires direction-finding product line
    Website. 12 October 1995.

  7. Wikipedia, Citizens band radio
    Retrieved December 2016.

  8. Frank Adcock, British Patent 130,490
    20 August 1918.

  9. Wikipedia, Adcock antenna
    Retrieved December 2016.
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Crypto Museum. Created: Tuesday 06 December 2016. Last changed: Friday, 01 September 2017 - 05:47 CET.
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