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RDF
Radio Direction Finding

Over the years, many techniques have been developed for locating the position of a transmitter. The most simple but nevertheless effective method, is to use a receiver or a field strength meter in combination with an adjustable attenuator. A slightly better method is to use a directional antenna to (manually) find the direction in which the signal is the strongest (or the weakest).

DF equipment on this website
Telefunken E-383-N (EP2a) LW radio direction finder
GPO interference investigation receiver WT No. 11
GPO Receiver (UK)
GPO
German Gürtelpeiler, used during WWII
Portable radio direction finder (US, 1942)
Mobile radio direction finder DBF-1
DAB and DAB-3 radio direction finder (US)
FH-4 HF/DF (huff-duff) high frequency direction finder
Wilhelm Quante St.Sg.52 (Germany)
Telefunken PE-484 Kleinstpeilempfänger
RZ-301 Pospisil (CZ)
Russian R11-PA valve-based body-wearable direction finder
WWI portable direction finder in suitcase
Belgian post-war version of the UK Type 36/1 (MCR-1), made by MBLE (Philips)
Soyka (USSR)
Filin (USSR)
Sinitsa (USSR)
Kopchik aperiodic surveillance detection receiver
Czechoslovak MRP-4 (Barabara) radar locator
Radio Direction Finder for 121.5 MHz distress beacons
EM-038-B French binaural aperiodic intercept receiver in suitcase
Nahfeldausforschungsgerät (near-field direction finder) used by the Austrian authorities
ADF-940 automatic direction finder for 27 MHz band
HRR-26 homing system used by the CIA
Bendix ADF-T-12-C automatic direction finder for LF/MF
TAIYO TD-L1706 direction finder
General coverage panoramic intercept receiver (1 GHz)
General coverage panoramic intercept receiver (2 GHz)
Rohde & Schwarz EB-200 Monitoring Receiver 10 kHz - 3 GHz
Sadelco FS-3 field strength meter
NRP field strength indicator with built-in frequency counter
ZAP Checker - field strength indicator
ZAP
PR-100 Portable Digital Receiver
 Detailed descriptions


Methods
The following basic direction finding techniques are described below:

  1. Field strength measurement
  2. Manual direction finding
  3. Automatic direction finding

Triangulation
The most common method for locating the position of a transmitter during World War II (WWII) and in the early years of the Cold War, was by using two or more mobile reception stations with directional antennas. As soon as the transmission started, each mobile station would try to find the angle at which the signal strength was the highest, 1 and draw it as a line on the city map.

Locating the position of a transmitter by means of triangulation

As a directional antenna is not perfect, and because of reflections of the radio waves against buildings etc. the angle has a certain tolerance. The drawing above illustrates how this works, when two mobile Radio Direction Finding (RDF) stations (A and B) have determined a bearing on a rogue transmitter (T). Because of the triangle A-B-T, this method is known as triangulation.

In the example above, there is still an uncertainty of several houses. Furthermore, the method does not work when the transmitter is located exactly in between the two reception stations. This could be solved by using three or more RDF stations at strategic positions around the city. Once the position of the transmitter had roughly been determined, the interceptors would move closer towards the target, in order to narrow down the angle of tolerance. Once the average position was known, the interceptors would generally use field-strength measurement for the final stage.

From the above example it will be evident how dangerous it is to operate a rogue transmitter at wartime. For this reason, agents often changed the location of their transmitter and tried to keep their transmissions as short as possible. This was done be minimising the length of a message, and by sending the message as a high-speed burst, by means of a so-called burst transmitter.

  1. In practice it is often better to determine the angle of the lowest signal strength, as it is easier to find and has a smaller tolerance. The direction to the transmitter in then 90° higher or lower.


1. Field strength mesasurement
One of the simplest and most effective ways to find a rogue transmitter, is to use a field-strengh meter with an adjustable attenuator. An interceptor with a mobile Radio Direction Finding (RDF) station would first drive down a main road to see whether the signal becomes stronger or weaker.

He would then adjust his plan. When getting closer to the transmitter, the attenuation would be increased, so that the field-strength indicator still produces a usable reading on its scale. In practice, an experienced interceptor can usually determine the position in less than 30 minutes.

Field-strength measurement is still widely used today, often used in combination with other RDF methods – such as Triangulation and Automatic Direction Finding (ADF) – especially during the final stage of the job, to find the exact building from which the rogue transmitter is operated.
  
Example of a field-strength meter that was used by the Dutch Radio Monitoring Service

Field-strength (FS) meters are relatively simple instruments that can be made with just a couple of components. Yet they are very effective, especially in close proximity of the transmitter. They can be selective or non-selective, with the former being insensitive to strong (unwanted) nearby radio signals. The image above shows a non-selective handheld FS-meter with built-in frequency counter, that was used by the Dutch Radio Monitoring Service (RCD) during the 1980s and 90s.

Examples of field-strength meters
Sadelco FS-3 field strength meter
NRP field strength indicator with built-in frequency counter
ZAP Checker - field strength indicator
ZAP
2. Manual direction finding
The basic form of true direction finding, is by using a (somewhat) directional antenna, such as a dipole, open loop, window, Adcock, Yagi or Log Periodic. By connecting the antenna to a receiver and turning it around, the angle of maximum or minimum signal strength can be determined.

In case of a portable receiver and antenna, the interceptor could walk towards the transmitter until it is found. A good example of a modern manual direction finder, is the Rohde & Schwarz PR-100 in combination with the HE-300 portable antenna, as shown in the image on the right.

Older systems include the DAG-1, which is of WWII vintage. It was designed for use by the US Navy, probably for navigation purposes, but was also used after the war to find clandestine radio stations (pirates) and even illegal radio stations operated by East European (Cold War) agents.
  
PR-100 portable monitoring receiver and HE-300 anenna

The DAG-1 uses a magnetic loop antenna, which is sensitive to the magenetic field of the radio wave only. Like other simple directional antennas (dipole, Adcock), it is symmetric and suffers from an 180° ambiguity. This means that it can not tell the difference between front and back.

Bellini-Tosi Goniometer
A significant improvement was the Goniometer, also known as Bellini-Tosi or B-T Goniometer, invented in 1907 by Ettore Bellini and Alessandro Tosi, and described in US Patent 943,960. It uses two loop antennas, typically long wires wound around a wooden frame, at perpendicular angles (i.e. 90° apart). The wires of these two loops are connected to the static windings of the goniometer (stators), whilst a single rotor winding is connected to the input of the receiver.

Bellini-Tosi Goniometer
Bellini-Tosi Goniometer · 1907

This invention allowed the user to hunt for signals by rotating the goniometer, without actually moving the (large) antenna. The drawback of this method is the 180° ambiguity of the antenna plus the sensitivity to ground and ionospheric reflections, especially on higher frequencies.

Adcock antenna
The latter was solved by using four vertical monopole Adcock antennas, invented in 1918 by Frank Adcock and described in UK Patent 130,490. It consists of two pairs of cross-connected vertical dipoles. Like the Bellini-Tosi window antennas, it suffers from the 180° ambiguity. Both the B-T Goniometer and the Adcock antenna were later used with Automatic Direction Finders.

Circuit diagram of two Adcock antenna-pairs
Adcock antenna · 1918


Example of manual direction finders
Telefunken E-383-N (EP2a) LW radio direction finder
GPO interference investigation receiver WT No. 11
GPO Receiver (UK)
GPO
German Gürtelpeiler, used during WWII
Portable radio direction finder (US, 1942)
DAB and DAB-3 radio direction finder (US)
PR-100 Portable Digital Receiver
Wilhelm Quante St.Sg.52 (Germany)
Telefunken PE-484 Kleinstpeilempfänger
RZ-301 Pospisil (CZ)
Russian R11-PA valve-based body-wearable direction finder
WWI portable direction finder in suitcase
Belgian post-war version of the UK Type 36/1 (MCR-1), made by MBLE (Philips)
Soyka (USSR)
Filin (USSR)
Sinitsa (USSR)
Kopchik aperiodic surveillance detection receiver
Czechoslovak MRP-4 (Barabara) radar locator
Radio Direction Finder for 121.5 MHz distress beacons
3. Automatic direction finders
In order to understand the principle behind the various techniques that are used for automatic direction finders (ADF), it may be useful to examine the differences between them. Generally speaking, the following techniques are available for automatic radio direction finding:

  1. Watson-Watt
  2. Adcock
  3. Homer
  4. Bellini-Tosi
  5. Doppler
  6. Angle of Arrival (AoA)
  7. Interferometer
  8. Time of Arrival (ToA)
3a. Watson-Watt
High Frequency Direction Finding, commonly abbreviated as HF/DF or referred to by its nickname huff-duff, was one of the first methods of automatic direction finding. It is based on a system developed in 1926 by Robert Watson-Watt for locating lightning strikes, and displays the signal from two loop antennas on an oscilloscope, so that the angle of incidence can be read instantly.


The system was widely used during WWII, for example during the Battle of Britain for guiding British fighers to their destination, and during the Battle of the Atlantic against German U-boats. The signals from the two loop antennas are processed by two highly identical receivers, and fed to the X and Y inputs of an oscilloscope. A line will then be displayed at the angle of incidence.


Watson-Watt also solved the 180° ambiguity problem, by adding a so-called sense antenna to one side of a loop, to that the opposite side would be less visible on the oscilloscope. This was later improved by adding a third (highly identical) receiver to the setup, and using the signal from the sense antenna to drive the Z-axis (i.e. the beam luminance) of the oscilloscope. The concept is also known as Cathode-Ray Direction Finding (CRDF), Twin-Path DF and Watson-Watt DF.

 More about HF/DF
 Read Arthur Bauer's paper about HF/DF

Examples of Watson-Watt direction finders
FH4 High Frequency Direction Finder (huff-duff) - Watson-Watt
3b.  Adcock   Watson-Watt
Adcock is not a direction finding principle as such, but merely an antenna design, invented in 1918 by Lt. Frank Adcock, that was used in combination with the Watson-Watt principle. It is officially known as Adcock/Watson-Watt, but is commonly referred to simply as Adcock DF.

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 incidence (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 [4]. Further information on Wikipedia [5].

Examples of Adcock direction finders
Mobile radio direction finder DBF-1
ADF-940 automatic direction finder for 27 MHz band
HRR-26 homing system used by the CIA

3c.  Homer
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).


3d.  Bellini-Tosi
Automatic Bellini-Tosi is an improved variant of the Bellini-Tosi Goniometer (B-T Goniometer) of 1907, in which the rotor coil of the goniometer, or RF Resolver, is not operated by hand but by a servomotor in combination with a feedback loop from the receiver. Furthermore, the signal from a sense antenna is injected somewhere in the signal path, to resolve the inherent 180° ambiguity.


The diagram above illustrates the principle. By injecting a low-frequency component into the RF signal from the goniometer, and phase-comparing it with the same component in the output of the receiver, the servo-motor is driven until a signal minimum (null) is found. This concept was used for many years in the aviation industry, and has the advantage over the HF/DF and Adcock based systems, that it does not require three highly identical receivers, but just a single one.

Example
Bendix ADF-T-12-C automatic direction finder for LF/MF
3e.  Doppler
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 evenly 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.


3f.  Angle of Arrival   AoA
The Angle of Arrival (AoA) principle, also known as Power of Arrival (PoA), directly measures the angle of incidence of the intercepted signal, by observing the amplitude changes of a rotating directional antenna. By using four or more discrete directional antennas in a circular arrangement, and switching between them in quick succession, the effect of a rotating antenna is simulated. As a directional antenna is used — such as a Yagi, Log-Periodic or Beverage antenna — a reference antenna is no longer required as the 180° ambiguity does not exist. This simplifies the design.

Watson-Watt antenna arrangement with ideal directional antenna element

Imagine an ideal directional antenna, which rotates at constant speed in a horizontal plane as illustrated in the diagram above. The antenna produces a signal with amplitude (A), which is a function of the angle of rotation (α). When the antenna is facing the transmitter, the maximum amplitude is obtained (A0). In the opposite direction (180°), the minimum amplitude is obtained. For an ideal directional antenna, the amplitue as a function of the angle, is calculated as:

Amplitude as a function of the angle

With a non-ideal directional antenna, the pattern will be similar, although the sinewave will be further above from the X-axis. The variations in signal strength of the intercepted signal, will result in a similar fluctuation of the strength of the IF signal of a receiver (commonly at 10.7 MHz). This can be regarded as a form of Amplitude Modulation (AM), which looks like this:

Variations in amplitude of the IF signal

By applying this signal to an AM detector and removing the DC component, the phase of the resulting sinewave can be determined. By comparing the phase to the forward direction (N), the angle of incidence can be calculated. This is usually done by means of a microcontroller.

AM-envelope after detection and filtering

The above diagram shows the position of the AM envelope when the transmitter is placed at 45° east from the north. In this case, the maximum amplitude will occur at 45° from the start. The same 45° shift can be observed at the zero-crossing points, which are generally easier to find. By observing the direction of the tangent at the zero-crossings, the 180° ambiguity can be avoided.

Example of an AoA direction finder
TAIYO TD-L1706 direction finder

3g. Correlative interferometer
A correlative interferometer uses two or (preferably) three highly identical antennas and receivers, spaced at known distances, plus the signal from a (4th) reference antenna, and analyse them in a computer system, using advanced mathematical techniques such as Fast Fourier Transform (FFT). The phase of the signals is then used to calculate the bearing to the transmitter. When used from an aircraft with three antennas, it is possible to calculate both the azimuth and the elevation [8].


The illustration above shows how this works for a system with 3 antennas placed in a triangle. The antennas are connected to three highly identical receivers that are tuned in parallel, by means of a common local oscillator (LO) that is controlled by a computer system. The signals of the three receivers a converted to the digital domain (i.e. digitised) and then fed to the computer.

Simplified block diagram of a correlative interferometer

The principles behind the correlative interferometer were first described in 1961 by C.W. McLeish and N. Burtnyk [8]. One of the first companies to manufacture a commercially available correlative interferometer in the mid-1980s, was Watkins-Johnson in Gaithersburg (Maryland, USA) [9].

In 1987 it was improved by David Tong, and sold by his company Datong Electronics Ltd. in Leeds (UK), as an alternative to Doppler [10]. The Datong solutions were used by various European law-enforcement and intelligence agencies until at least the early 2000s, for fixed as wel as vehicle-based DF, in particular for following people, vehicles and money by means of tracking beacons.

Example of a correlative interferometer direction finder
Datong DF-5 direction finder
DF-5

3h. Time of Arrival   ToA
A Time-of-Arrival (ToA) direction finder is similar to the correlative interferometer shown above, but uses differences in time between the arrival of the incident wave front at each of the three antennas. By analysing the signals in a computer, it is possible to calculate the bearing to the transmitter (e.g. a tracking beacon) with an accuracy of 1° or better



Direction finders on this website
The following direction finders are described in more detail on this website. Read the short introductions below and click on More information, or click the image on the right for further details. Alternatively, click any of the thumbnails at the top of this page.

GPO
This small receiver was used during WWII to locate clandestine transmitters, mainly operated by German agents in and around London. The unit is housed in a Bakelite enclosure and its lid acts as the frequency range 'plug-in' as well as the direction-sensitive antenna.

The receiver is commonly known as the GPO-receiver, but its official name was Tester WL-53400. It was only built in small quantities.

 More information

  
GPO Receiver (Tester WL.53400)

BC-792-A   SCR-504-A
BC-792 – also known as SCR-504 – was a portable covert direction finder, developed in 1943 and used by the OSS (the predecessor of the CIA) for finding clandestine transmitters operated by spies during World War II.

The device is concealed as a regular leather travel suitcase, and was also used during the early part of the Cold War in various European countries.

 More information

  
BC-792-A direction finder inside leather suitcase

Gürtelpeiler   wanted item

The Gürtelpeiler was probably the first body-worn intercept receiver. It was used during WWII by the German secret services to locate clandestine transmitters operating in countries occupied by the Germans.

The valve-based receiver can be concealed under the operator's clothing with a loop antenna around the neck.

 More information

  
Original Gürtelpeiler

USSR   Soviet Union
During the Cold War, the USSR (Russia) developed a series of highly portable intercept receivers that were deployed in most Warsaw Pact countries. Such receivers where generally carried around the operator's waist, hidden under his clothing.

They also developed stationary and mobile intercept radios and other direction finding equipment.

 More information

  
Soyka intercept receiver

Wilhelm Quante StSG-52
This German-built portable direction finding receiver was used in the Netherlands in the early 60s to track down clandestine radio stations and foreign secret agents. The receiver is housed in a wooden case, so that the internal window-antenna can be used. It is operated by a trigger-switch hidden under the carrying handle.

 More information
  
Click to see more

Telefunken PE-484
The PE-484 was a body-wearable miniature direction finder (Kleinstpeilemfänger) introduced around 1958 by Telefunken in Germany. It could be hidden inconspicuously under the operator's clothing and was intended for tracking down clandestine radio stations. In some countries, the PE-484 was used until the early 1980s.

 More information
  
PE-484/3 receiver

RZ-301   POSPÍŠIL
RZ-301 is a rare short-wave direction finder built in Chechoslowakia around 1948. It comes with four plug-in modules that can be inserted at the bottom of the unit. Each plug-in unit covers a specific frequency range.

It was used by the Czech Secret Police (StB) to track down clandestine (spy) radio stations during the Cold War.

 More information
  
RM-301 with three additional plug-in units

FBA Peiler   Nahfeldausforschungsgerät
This small hand-held direction finder that covers 3 to 145 MHz, was built by the Austrian Radio Monitoring Service of the ÖPT in 1963, especially for unobtrusively searching for clandestine radio stations, such as Cold War spy transmitters.

The receiver is fully transistorized and comes with 15 frequency plug-in units, nicely packed in a sturdy metal carrying case.

 More information
  
FU-303 ready for use

ADF-9xx
The American company OAR built a wide range of radio direction finders that were intended as a navigational aid aboard ships. Special versions, the so-called 9xx-range, were made for locating clandestine radio stations (pirates).

The image on the right shows the ADF-940 which has a built-in 40-channel scanner for the 27 MHz citizens band (CB).

 More information
  
ADF-904

TAIYO direction finder   TD-L1706
TD-L1706 was a direction finder made by the Japanese company TAIYO Musen. Although it was intended for the maritime industry, it appeared to be very adequate for finding clandestine transmitters (priates) on land.

The system consists of a main unit, a compass display (shown on the right) and an flat antenna that could be disguised as the sunroof of a car.

 More information
  
TAIYO compass display

PAN-1000
PAN-1000 was a high-end general coverage panoramic receiver, developed by the Dutch Radar Laboratory (NRP) for the Dutch Radio Monitoring Service (RCD) in the early 1980s.

The receiver covers a frequency range from 0.1 to 1000 MHz and could be fitted inside a car. It was intended for locating clandestine radio stations (pirates).

 More information
  
Complete PAN-1000 set

PAN-2000
In the mid-1990s, the PAN-1000 was succeeded by the PAN-2000, developed by the German company ELCOM GmbH. It was based on a (modified) ICOM IC-R9000 communications receiver, with an external FFT Processor.

The PAN-2000 was capable of intercepting signals up to 2000 MHz (2 GHz). The TAIYO DF unit was reused with this set.

 More information

  
Complete PAN-2000 intercept system

EB-200
The EB-200 is the successor to the EB-100. It is a portable receiver that covers all frequencies between 10 kHz and 3 GHz, with a wide variety of modulation types: AM, FM, CW, LSB, USB, Pulse and I/Q. It is one of the first receivers that has a fully digital IF-stage with DSP technology.

The radio was intended for monitoring of the frequency spectrum and for locating sources of transmission, including covert listening devices.

 More information

  
EB-200 receiver in operation


References
  1. Wikipedia, Radio direction finder
    Retrieved December 2020.

  2. Wikipedia, Automatic direction finder
    Retrieved December 2020.

  3. 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.

  4. Frank Adcock, British Patent 130,490
    Filed 20 August 1918, Issued 7 August 1919.

  5. Wikipedia, Adcock antenna
    Retrieved December 2016.

  6. Ettore Bellini and Alessandro Tosi, US patent 943,960
    Filed 1 October 1907. Issued 21 December 1909.

  7. Arthur O. Bauer, HF/DF An Allied Weapon against German U-Boats 1939-1945
    27 December 2004.

  8. McLeish & Burtnyk, The Application of the Interferometer to HF Direction Finding
    IEE, Vol. 108, Issue 41, September 1961, pp. 495-499, 1961.

  9. Moell & Curlee, Transmitter Hunting, Radio Direction Finding Simplified
    ISBN 0-8306-2701-4. 1987. pp. 147-150, 264-265.

  10. US Patent 4,809,012, Direction Finding Equipment
    David A. Tong, filed 27 May 1987.
Further information
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