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DF BRD
Portable radio direction finder
DAG-1 1 was a portable radio direction finder,
manufactured from late 1942
onwards by Airplane & Marine Instruments, Inc.
in Clearfield (Pennsylvania, USA) for the US Navy.
The device was used during and after
World War II (WWII)
for finding and locating enemy (clandestine) radio stations.
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The device is housed in a
military green metal enclosure,
and is suitable for Medium Wave (MW) and Short Wave (SW) frequencies
in 3 bands from 1.6 to 18.2 MHz,
using AM or CW (morse code).
A bearing to the intercepted radio transmitter is obtained by using a
loop antenna in combination with a vertical sensing antenna
and a magnetic compass, all supplied with the kit.
It allows the direction to the transmitter to be plotted on a map.
This is done by determining the axis
of minimum signal strength 2 (null) from at least two different positions.
This method is known as triangulation.
The accuracy of the location can be improved by using more than two positions,
and by connecting the device to ground (earth).
In RDF operation, the sensing antenna
and the loop antenna are coupled
via an adjustable balance circuit, so that it becomes easier to find the
signal minimum (null).
The device can also be used as a general purpose
communications receiver,
or intercept receiver,
in which case the sense antenna is coupled directly
to one side of the loop, so that maximum signal is obtained.
The sense antenna can also be used on its own, or can be disabled
altogether when not required.
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At the front of the device is a hinged cover that consists of two halves,
held together in the middle by another hinge. For operation, only the top half
has to be opened, as shown in the image above.
For transport, both antennas and the ground pin are
stowed inside the cover.
The device shown here was used in the period after WWII by the
Bijzondere Radio Dienst (BRD) 3
– the Dutch Special Radio Service – for finding
clandestine spy radio stations, operated by Eastern Block spies and agents,
during the early days of the Cold War.
The BRD used it alongside other portable direction finders like the
GPO WT-11, the GPO WL-53400
and the Quante StSG-52 [1].
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Models DAG, DAG-1 and DAG-2 are believed to be identical [A].
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Generally speaking it is easier to determine the angle of minimum signal
strength. Obviously this gives a 90° error compared to the angle of
maximum signal strength.
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From 1945 to 1946 known as Bureau Nationale Veiligheid (BNV) – Bureau for
National Security – and from 1947 to 1952 as Bureau Bijzondere Diensten (BD)
– Bureau for Special Services. In 1975 dissolved into the Radio Controle
Dienst (RCD) of the PTT – the national Radio Monitoring Service
(now: AT). ➤ More
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The diagram below gives an overview of the controls and connections of the
device, all of which are located at the front of the receiver, in the top
half of the storage case. It is fully self contained and is powered by internal
batteries that are installed behind the large panel at the bottom left.
The only connections are for the optional headphones
and (also optional) for a ground pin.
At the top are two sockets for the antennas, protected by
screw caps. Once
installed, the loop antenna can be rotated with the Bearing knob at the top
left, whilst the scale is used to read the angle.
At the center is a large knob for adjusting the frequency in the currently
selected band. At the right is the internal speaker, which is disabled
automatically as soon as a pair of headphones is connected.
At the right edge is a screw terminal for connection of a wire
to the ground pin.
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ANT Communications receiver BAL Find the bearing with the lowest signal strength (null) OFF Loop antenna only SENSE Search for a signal (using both antennas)
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The actual receiver is housed in the top half of the storage case,
and can be removed by releasing eight tumb screws along the edges.
A 4-pin Jones socket, fitted to the chassis,
connects the device to the battery compartment.
A separately available service cable
can be used to operated the receiver outside the case.
The antennas can be inserted into the receptacles inside the receiver.
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Although the receiver has a built-in speaker, it was often used with
a pair of headphones, such as the military R-30-U shown in the image on
the right, as that would not reveal the operator's position when the
device was used in the field.
The headphones have a PL-55 plug that can be
inserted into the socket
at the right edge of the receiver. This disables the internal speaker.
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The loop antenna is used for determining the bearing to an intercepted
radio signal. As it is shielded (except for a small part at the top) it
has a narrow aperture (opening angle). Furthermore, it is only
sensitive to magnetic waves (i.e. the H-field of the RF signal).
Note that the loop antenna must always be installed, even when the device
is used as a communications receiver with the vertical telescopic antenna,
as otherwise the RF input circuit is misaligned.
When unused, the loop antenna is
stowed inside the front cover.
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The sensing antenna is used to pick up a signal before finding a bearing
to the transmitter. It is also used to cancel out the residual signal from
the loop antenna when trying to find the angle of minimum signal strength,
resulting in a much sharper null. Extend the telescopic
antenna to its maximum length when using the device as a regular
communications receiver.
When unused, the antenna is
stowed inside the front lid, close towards
the upper edge, where it is held in place by two metal clips.
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When using the device as a direction finder, the bearing accuracy can be
increased by grounding the device properly. This is done inserting the
ground pin shown in the image on the right into the ground (earth) and
connecting it to the screw terminal at the
right edge of the receiver.
When using the device as a communications receiver, the ground connection
is not necessary. When unused, the ground pin is
stowed inside the front cover,
close to the hinge.
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When using the ground pin above (i.e. when using the receiver as a
direction finder), it should be connected to the screw terminal at the
right edge of the receiver, using the short wire shown in the image on the
right.
The wire has a crocodile clip at one end, that can be attached to the ground
pin (or to another ground line). The other end has a U-shaped fork that
fits the screw terminal on the receiver.
When unused, it is stowed behind the hinged door
of the spare parts compartment.
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The radio was powered by three batteries: two identical LT/HT batteries
that provide +1.5V and +90V, connected in parallel, and one battery that
provides a -7.5V bias voltage for the grids.
These batteries are no longer in production and suitable replicas are
currently unavailable.
No image available at present.
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The device was supplied with a numbered copy of the instruction book,
which was applicable to the DAG-1 and DAG-2 radio direction finders.
It contains instruction for setting up and using the device, and general
information about direction finding. It also contains a full wiring diagram
and the complete circuit diagram.
➤ Download the manual
➤ Circuit diagram
➤ Wiring diagram
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When serviceing or repairing the device, it might be necessary to operate
the receiver whilst it is outside the case. For this purpose, the service
cable shown in the image on the right was used.
The cable is approx. 1.5 metres long, has a 4-pin male Jones connector
at one end, and a mating female connector at the other end. One side plugs into
the frame, whilst the other end plugs into the connector behind the front
panel of the receiver. When unused, the cable is
stowed in the accessories compartment.
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The DAG-1 and DAG-2 direction finders came with a double set of spares,
such as valves (tubes), light bulbs, etc., packed in the heavy metal container
shown in the image on the right.
The closed container measures 320 x 255 x 165 mm and has a bare weight of
5.3 kg (without the contents). If necessary, it can be locked with a padlock
at the front, as shown here.
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In the YouTube video below, collector Steve Ellington shows the DAG-1
radio direction finder in operation on all three bands.
As the batteries for this device
are no longer in production, he has taken it out of the case and has
connected it to a pair of laboratory power supply units (PSUs) [2].
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The operating principe of the DAG-1 receiver is illustrated in the block
diagram below. The circuit is built around 8 valves (tubes). From left to
right: an RF pre-amplifier (1T4) followed by a mixer (1R5), where the signal
from the variable frequency oscillator (VFO) (1T4) is subtracted.
This results in a 465 kHz Intermediate Frequency signal (IF)
that is amplified in two stages (2 × 1T4).
The 465 kHz is then passed through an AM-detector (1R5) and amplified
to speaker level in an AF-stage (1S4). In the output path of the AF-stage
is an impedance transformer that provides the signal for the internal 8Ω
speaker or a pair of 600Ω headphones (the latter disables the speaker).
For the reception of CW signals, the Beat Frequency
Ocillator (BFO) can be enabled with a switch.
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The main antenna is a magnetic loop, which has a metal
shield that is connected to ground and has an opening at the top. As a result
it is only sensitive to the magnetic field (H-field) of the RF signal from
the intercepted transmitter. The highest signal strength is obtained when
the antenna is in line with the transmitter. Likewise, the lowest signal
strength (null) is obtained when the loop antenna is perpendicular to the
transmitter. The latter is illustrated in the diagram below.
The diagram below shows the relation between the two antennas and the effect
of the MODE-selector.
The loop antenna provides a symmetric signal to the input
transformer (TR), of which the secondary side forms a tuned
circuit with the adjustable capacitor (Ct). This signal (rf1)
is passed to the input of the RF pre-amplifier. In position (4)
of the MODE-selector (SENSE), the signal of the sensing antenna (rf2)
is added to this. The extra components (Rs) and (Cs) ensure the correct
phase and magnitude, which can be further adjusted by altering the
antenna length.
In position (2) of the MODE-slector (BAL), the sensing antenna is connected to
a balancer circuit (Cb) at the ends of the loop antenna. By adjusting (Cb),
the signal from the sensing antenna can be made to counteract with the signal
in the loop antenna, resulting in a much sharper null.
In position (1) of the MODE-selector (ANT), the sensing antenna is connected
directly to one side of the loop antenna, so that maximum signal is
coupled into the circuit, allowing the device to be used as a
communications receiver.
In position (3) of the MODE-selector only the loop is used.
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The interior of the receiver can easily be accessed by loosening the 8
thumb screws along the edges of the front panel, after which the receiver
can be pulled out of the chassis. At the front edge of the chassis is a
4-pin Jones socket that mates with a 4-pin plug behind the front panel.
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This plug carries the four power lines from the batteries. It can be extended
by means of a 4-way service cable,
that allows the receiver to be used outside the chassis.
This is most usefull when serviceing or repairing the unit in the field.
The image on the right shows the interor of the receiver – seen from the
rear – after removing it from the chassis. It consists of an aluminium frame
that is bolted to the front panel. All large parts, such as valves, filters,
tuning capacitors and transformers, are mounted at the top. At the centre
is the cylindrical (yellow-ish) tuning scale.
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All passive components (resistors, capacitors, etc.) are mounted at the
bottom side of the chassis, and are easily accessible.
Also at the bottom is a 4-deck ceramic rotary switch, which is coupled to the
band selector at the front panel.
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When we obtained the DAG-1 featured here – in December 2020 – it was in near
mint condition, but had not been switched on for at least 30 years or more,
so we knew we had to be careful.
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After a thorough visual inspection, in which
nothing was found, we decided to create two test cables. The first one is
a reproduction of the original service cable which was available
for repairs at the time. It allows the receiver to be battery powered, whilst outside the enclosure.
The second cable is a short extension to the first one, but allows external
power supply units to be used instead of the internal battery pack. 1
The 4-way external power cable is shown in the image
on the right, and has plugs for 0V (black), +1.5V (red), -7.5V (yellow)
and +90V (green).
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The +1.5V for the filaments of the valves was connected first and the unit
was switched ON. In this state, the filaments draw a current of approx. 360 mA,
which appears to be normal. Next, the -7.5V and the +90V supplies were
connected and the speaker immediately started to produce noise, of which the
volume could be controlled with the Volume adjustment at the front panel.
A strong signal from a broadcast station near 17 MHz was received, and the
loop antenna was rotated to find the angle of minimum signal strength.
Apart from a few contact problems and a broken scale light, the device still
worked perfectly, nearly 80 years after it was manufactured.
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The original batteries are missing from this unit, but even if they had
survived, there would have been no chance to retrieve any power from them
after so many years. The original batteries were not rechargeable.
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The diagram below shows the configuration of the batteries. At the left is the LT/HT
battery that provides 1.5V (LT) and 90V (HT). According to the manual, two such battery
packs were used in parallel (identified as the A and B battery),
but in the device featured here, only one connector is
available. This might be an aftermarket modification for use with a different battery type.
At the right is the battery that provides the -7.5V bias voltage. It is connected with
its (+) pole to the 0V rail, which is a single black wire in the battery compartment.
The (-) pole of the battery should be connected to a screw terminal at the upper edge
of the battery compartment.
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The receiver is connected to the battery system by means of a 4-pin male Jones connector,
fitted behind the front panel at the bottom edge,
that mates with a female socket that is fitted to the front edge of the chassis.
Below is the pinout when looking into the female socket on the chassis.
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- -7.5V
- +90V
- 0V (GND)
- +1.5V
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Manufacturer Airplane & Marine Instruments, Inc. Year 1942 onwards Principle Superheterodyne Purpose (1) Radio Direction Finder, (2) Communications Receiver Frequency 1.6 - 18.2 MHz Bands 3 (see below) IF 465 kHz Modulation AM, CW Valves 8 (see below) Phones 600 Ω (not supplied) Output 6 mW Dimensions 415 x 350 x 180 mm Weight 14.5 kg (batteries included)
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- 1.6 - 3.6 MHz
- 3.6 - 8.1 MHz
- 8.1 - 18.2 MHz
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CBR Burgess Battery Company, Freeport Illinois CIA Airplane & Marine Instruments, Inc., Clearfield Pennsylvania CJD J.F.D. Manufacturing Company, Brooklyn New York CZD David White Company, Milwaukee, Wisconsin
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© Crypto Museum. Created: Monday 07 December 2020. Last changed: Sunday, 01 January 2023 - 14:20 CET.
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