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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:
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DF equipment on this website
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Field strength mesasurement
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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.
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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.
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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.
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Examples of field-strength meters
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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.
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.
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Automatic direction finders
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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:
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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].
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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:
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.
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].
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Examples of Adcock direction finders
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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.
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.
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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).
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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
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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.
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:
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:
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.
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.
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Example of a Watson-Watt direction finder
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Direction finders on this website
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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.
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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
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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
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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
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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
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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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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
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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
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FBA Peiler
Nahfeldausforschungsgerät
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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
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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
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TAIYO direction finder
TD-L1706
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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
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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
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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
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© Crypto Museum. Created: Saturday 17 December 2016. Last changed: Sunday, 12 April 2020 - 20:46 CET.
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