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Radio Direction Finders

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). In addition, several techniques are available for automatic direction finding. Known methods are:

DF equipment on this website
Telefunken E-383-N (EP2a) LW radio direction finder
GPO Receiver (UK)
German Gürtelpeiler, used during WWII
DAB and DAB-3 radio direction finder (US)
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
Bendix ADF-T-12-C automatic direction finder for LF/MF
HRR-26 homing system used by the CIA
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
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 the near-field of the transmitter. They can be selective or non-selective, with the latter being insensitive to strong (unwanted) nearby radio signals. The image above shows a hand-held FS-meter that was used by the Dutch RCD.

Examples of field-strength meters
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, 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 to 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 bit.

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.

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:

As Adcock and Doppler are the most common ones, we will briefly describe them below, before going into the other methods. The descriptions below are largely based on a 1978 report by the US Department of Transport, in which three methods (Adcock, Doppler and Homer) are evaluated. For a more detailed description with mathematical backgrounds, please refer to that report [1].

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 [2]. Further information on Wikipedia [0].

Examples of Adcock direction finders

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.

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.

 Read the full report

The Watson-Watt principle is very similar to the Adcock principle, in that it is based on amplitude differences between the signals from its antennas. Depending on the implementation though, it often resembles the Doppler method, in which a (pseudo) rotating antenna is used. By using a directional antenna for this — such as a Yagi antenna, a Log-Periodic antenna or a Beverage antenna — a reference antenna is no longer needed as the 180° ambiguity does not exist.

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 away 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 a Watson-Watt direction finder
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.

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
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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 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

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

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 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

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

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

  1. 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.

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

  3. Wikipedia, Adcock antenna
    Retrieved December 2016.
Further information
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© Crypto Museum. Created: Saturday 17 December 2016. Last changed: Sunday, 12 April 2020 - 20:46 CET.
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