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DAG-1
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.

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.
  
DAG-1 ready for use with front cover half open

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].

  1. Models DAG, DAG-1 and DAG-2 are believed to be identical [A].
  2. 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.
  3. 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

DAG-1 closed
DAG-1 with front cover half open
DAG-1 with front cover fully open
DAG-1 radio direction finder ready for use
Front view, with cover fully open
DAG-1 ready for use with front cover half open
Loop antenna installed
Telescopic sense antenna installed
Compass installed on the base of the loop antenna
A
×
A
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DAG-1 closed
A
2 / 9
DAG-1 with front cover half open
A
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DAG-1 with front cover fully open
A
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DAG-1 radio direction finder ready for use
A
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Front view, with cover fully open
A
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DAG-1 ready for use with front cover half open
A
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Loop antenna installed
A
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Telescopic sense antenna installed
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Compass installed on the base of the loop antenna

Features
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.

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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.

Mode selector
  1. ANT
    Communications receiver
  2. BAL
    Find the bearing with the lowest signal strength (null)
  3. OFF
    Loop antenna only
  4. SENSE
    Search for a signal (using both antennas)
Socket for loop antenna
Rotating the loop antenna
Adjusting the frequency
Antennas and ground pin stowed inside front cover
Storage compartment for compass, cables and headphones
Battery compartment
Ground wire connected to receiver
B
×
B
1 / 8
1 / 8
B
2 / 8
Socket for loop antenna
B
3 / 8
Rotating the loop antenna
B
4 / 8
Adjusting the frequency
B
5 / 8
Antennas and ground pin stowed inside front cover
B
6 / 8
Storage compartment for compass, cables and headphones
B
7 / 8
Battery compartment
B
8 / 8
Ground wire connected to receiver




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Parts
Storage case
Receiver CIA-46174-A
RX
Headphones (optional)
Magenetic compass in leather wallet
Directional magenetic  loop antenna
Telescopic sense antenna
Ground wire
Ground wire
LT and HT batteries
Operating and technicalmanual
Service power cable
Storage case
The device is housed in an aluminium alloy case that measures 415 x 350 x 180 mm and weights 14.5 kg, batteries included. The case is actually a frame that consists of two halfs. The top half contains the actual receiver, whilst the lower half holds the battery compartment and a small compartment for the accessories and spares.

The case has a hinged lid at the front, on the inside of which the antennas and the ground pin are stowed. It can be opened in two parts, so that it is not in the way during operation.

  
DAG-1 closed

Receiver   CIA-46174-A
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.
  
Receiver

Headphones   R-30-U
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.

  
Headphones R-30-U

Compass   CZD-10210
A magnetic compass – manufactured by David White Company in Milwaukee (Wisconsis, USA) – was provided to accurately measure the angle between the device and the magnetic northpole. The compass could be installed on a special bracket at the base of the loop antenna.

As it is a delicate instrument, it was packed in a leather wallet and stowed in the spares compartment when unused.
  
Leather wallet with compass

Loop antenna
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.

  
Magnetic loop antenna

Sensing antenna
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.

  
Vertical telescopic sense antenna

Ground pin
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.

  
Ground pin

Ground wire
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.

  
Ground wire

Batteries
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.
  

Manual
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

  
Click to see more

Service cable
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.

  
Service cable

Leather pocket
Leather wallet with compass
Compass
Magnetic loop antenna
Loop antenna - base
Compass and fitting on loop antenna base
Compass installed on the base of the loop antenna
Headphones R-30-U
Military headphones R-30-U
Headset connected to receiver
DAG-1 and military headphones
Vertical telescopic sense antenna
Ground pin
Ground wire
Service cable
C
×
C
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Leather pocket
C
2 / 16
Leather wallet with compass
C
3 / 16
Compass
C
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Magnetic loop antenna
C
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Loop antenna - base
C
6 / 16
Compass and fitting on loop antenna base
C
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Compass installed on the base of the loop antenna
C
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Headphones R-30-U
C
9 / 16
Military headphones R-30-U
C
10 / 16
Headset connected to receiver
C
11 / 16
DAG-1 and military headphones
C
12 / 16
Vertical telescopic sense antenna
C
13 / 16
Ground pin
C
14 / 16
Ground wire
C
15 / 16
15 / 16
C
16 / 16
Service cable

Video
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].




Block diagram
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.

Antennas
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.

Top view of the loop antenna and the axis to the intercepted signal

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.


Interior
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.

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.
  
Receiver interior

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.

Empty chassis after removing the receiver
Receiver
Receiver interior
Receiver - bottom view
IF-stages, detector, BFO and AF-stage
Tuning section and filters
RF pro-amplifier, VFO and mixer
Adjustable capacitors
Band selector
IF and AF stages
4-pin Jones socket at the edge of the chassis
D
×
D
1 / 11
Empty chassis after removing the receiver
D
2 / 11
Receiver
D
3 / 11
Receiver interior
D
4 / 11
Receiver - bottom view
D
5 / 11
IF-stages, detector, BFO and AF-stage
D
6 / 11
Tuning section and filters
D
7 / 11
RF pro-amplifier, VFO and mixer
D
8 / 11
Adjustable capacitors
D
9 / 11
Band selector
D
10 / 11
IF and AF stages
D
11 / 11
4-pin Jones socket at the edge of the chassis

Restoration
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 or more years, so we knew we had to be careful.

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 test cable which was available to service personnel 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).
  
External power cable

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.

The following restoration work has been carried out:

  1. 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.

Service cable
External power cable
Leather pocket
Headphones R-30-U
E
×
E
1 / 4
Service cable
E
2 / 4
External power cable
E
3 / 4
Leather pocket
E
4 / 4
Headphones R-30-U

Connections
Battery
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.

Power
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.

  1. -7.5V
  2. +90V
  3. 0V (GND)
  4. +1.5V
Specifications
  • 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)
Frequency bands
  1. 1.6 - 3.6 MHz
  2. 3.6 - 8.1 MHz
  3. 8.1 - 18.2 MHz
Valves   Tubes
  • 1T4 (5x)
  • 1R5 (2x)
  • 1S4
Part numbers
  CIA-46174 Receiver in carrying case
  CJD-66054 Sense antenna
  CIA-69077 Loop antenna
  CZD-10210 Magnetic compass
  CBR-19045 (2x) Battery pack A & B, +90V & +1.5V
  CBR-19011 Pattery pack C, 7,5V
Manufacturer codes
  • 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
Documentation
  1. Product specification, DAG, DAG-1 and DAG-2
    Catalog of Naval Electronic Equipment, April 1946.

  2. Instruction Book for Navy models DAG-1 & DAG-2 portable radio direction finder
    Airplane & Marine Instruments, Inc., Clearfield (Pennsylvania, USA).
    14 June 1943. First released 21 December 1942.

  3. DAG-1 wiring diagram
    Extracted from original instruction book [B].

  4. DAG-1 circuit diagram
    Extracted from original instruction book [B].
References
  1. Cor Moerman, DAG-1 portable radio direction finder - THANKS !
    Received December 2020.

  2. Steve Ellington (N4LQ), DF Receiver WW2 DAG-1 Directional Finder
    Retrieved December 2020.
Further information
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Crypto Museum. Created: Monday 07 December 2020. Last changed: Sunday, 13 December 2020 - 15:01 CET.
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