Click for homepage
Bendix ADF-T-12-C
Automatic direction finder

ADF-T-12-C is an airborne radio direction finder, manufactured by The Bendix Corporation in Fort Lauderdale (Florida, USA). The device covers the LW/MW frequency bands (190-1750 kHz) and is intended for the aviation industry, where it is used to accurately measure the bearing to a radio beacon, using a servo-controlled variant of the Bellini-Tosi goniometer direction finder.

The set consists of an analogue 3-band receiver with Variable Frequency Oscillator (VFO), a visual indicator that shows the angle of incidence of the intercepted signal, a loop antenna, a sense antenna (either a rod or a wire) and (optionally) an external audio amplifier, so that it can also be used as a regular MW/LW broadcast receiver.

In the Netherlands, the device was also used in the 1970s and 1980s, by the Radio Monitoring Service (RCD) of the PTT, for locating clandestine stations (pirates) in the AM broadcast band. The device appeared to be highly effective for this.
  
Bendix ADF-T12C

The Bendix ADF-T12B and C models, are basically transistorized versions of the old valve-based Motorola ADF-12. The device was succeeded by the digital ADF-T12D, which had thumbwheels to set the frequency, but was mechanically far less reliable than its analogue predecessors.

Receiver and indicator
Bendix ADF-T12C
ADF-T-12C complete set
Receiver
Indicator (rose) seen from the front
Receiver front view
Receiver front panel
Rear view of indicator connected to receiver and loop antenna
A
×
A
1 / 8
Receiver and indicator
A
2 / 8
Bendix ADF-T12C
A
3 / 8
ADF-T-12C complete set
A
4 / 8
Receiver
A
5 / 8
Indicator (rose) seen from the front
A
6 / 8
Receiver front view
A
7 / 8
Receiver front panel
A
8 / 8
Rear view of indicator connected to receiver and loop antenna

Features
The diagram below provides an overview of the controls at the front panel of the receiver. At the bottom left is the MODE selector, which also acts as the power switch. At the right is a 3-position band selector. At the centre is the tuning scale, with the tuning knob immediately below it. At the top left is the signal strength meter (S-meter), which can be used as an aid for optimally tuning to the desired radio beacon (or clandestine radio station, when used as a law enforcement tool).

Click to see more

All connections are via a 16-way Amphenol connector at the rear, that mates with a receptacle inside the metal cradle that holds the receiver. The cradle is wired to an external audio amplifier – here in the form of a self-built Velleman kit – and to a 9-pin connector – shown here at the left – that is fitted to the rear of the visual indicator. The fixed loop antenna is also connected to the rear of the indicator via a 4-pin Cannon connector. The sense antenna is connected directly to the PL-259 socket at the rear of the cradle.

Setup
The diagram below shows the various components of the system and illustrates how they are connected. At the heart of the system is the Receiver Model 210 (RX), which also acts as the central power hub. It has connections for a sense antenna, an external audio amplifier and the Model 551RL indicator/servo unit. The fixed double-loop antenna is connected to the indicator.



Radio Monitoring Service   RCD
In the 1970s and 80s, the Netherlands was flooded with clandestine radio stations – also known as pirates – that operated in the regular broadcast radio bands, in between legal radio stations. In particular the FM broadcast band (87-108 MHz) and the MW band AM were popular at the time.

As clandestine transmitters cause interference with legal stations and do no pay for the use of the frequency spectrum, it was the duty of the Radio Controle Dienst (RCD) — at the time part of the state-owned telecom provider PTT — to locate them and enforce the telecom regulations.

To do this effectively, the RCD had a series of unobtrusive cars — initially Ford Granadas — in which a 19" equipment rack was installed on the co-seat, as shown in the image on the right. The indicator and receiver of the ADF-T12 are visible at the top left. The speaker is at the right edge.
  
Interior of Ford Granada

Apart from the ADF-T12, which was mainly used for locating pirates in the MW broadcast band, the rack contains an OAR ADF-940 direction finder for the 27 MHz band, a Handic 0016 scanner, a tape recorder, a mobile radio and a pager. A separate receiver – not visible here – was used for monitoring the VHF FM broadcast band (87-108 MHz) on which pirates were also highly active.

 Full description of the Ford Granada with equipment

Receiver with cradle and indicator
Receiver front panel
Connections at the rear
16-pin Amphenol receptacle inside the cradle
Rear view of indicator connected to receiver and loop antenna
Loop antenna plug
Connections at the rear
Wiring
B
×
B
1 / 8
Receiver with cradle and indicator
B
2 / 8
Receiver front panel
B
3 / 8
Connections at the rear
B
4 / 8
16-pin Amphenol receptacle inside the cradle
B
5 / 8
Rear view of indicator connected to receiver and loop antenna
B
6 / 8
Loop antenna plug
B
7 / 8
Connections at the rear
B
8 / 8
Wiring




Click to see more

Parts
ADF Receiver 201D
RX
Servo Amplifier Indicator 551RL
Fixed loop antenna
Sense antenna (optional)
Audio amplifier
Poudspeaker
Instruction and maintenance manuals
Receiver   201D
The Model 201D is the heart of the system. It's an analogue 3-band receiver, which measures 212 x 159 x 71.5 mm and weights 771 grams. It has a 16-pin Amphenol connector at the rear, that mates with a receptacle inside the cradle.

The receptacle is wired to the (12V) power source, the indicator, the audio amplifier and – via the indicator – to the loop antenna. A separate sense antenna must be connected to the PL-259 socket at the rear.

  
Receiver

Indicator   551RL
The angle of incidence of the intercepted radio signal, is measured by applying the signals of the two perpendicular loop antennas to a servo-controlled RF-resolver inside the indicator, to find the angle of minimum signal strength (null).

It is basically a Bellini-Tosi goniometer driven by a servo-motor. The motor is controlled by an error signal provided by the receiver, and stops when the null is reached. As there are two nulls (displaced by 180°), the sense antenna is used to find the correct one, by controlling the rotational direction of the servo-motor.

  
Indicator (rose)

Fixed loop antenna   2321E
One of the unique features of the ADF-T12 is the so-called fixed loop antenna, that allows the angle of incidence to be measured automatically.

It consists of two perpendicular loop antennas, wound around a flat ferrite body, potted in a strong weather resistant compound. The loop antenna is connected to the indicator via two cables (one for each loop). Inside the indicator is an RF-resolver (goniometer) of which the rotor coil is connected to the input of the receiver.

  
Loop antenna with wiring

Sense antenna
A sense antenna should be connected directly to the receiver, to resolve the 180° ambiguity of the loop antenna (and the servo system). By looking at the phase difference between the signal from the sense antenna and the (delayed) signal from the loop antenna (via the RF resolver), the servo is driven in the correct direction.

The sense antenna is also used when the device is used for the reception of regular broadcast transmissions in the MW radio band.

  
Sense antenna

Audio amplifier
In order to get an acoustic feedback of the intercepted signal, an (optional) external amplifier was available, such as the original Model 102A/B amplifier. It amplifies the line signal from the receiver to speaker level.

In the Netherlands however, the RCD used a low-cost amplifier kit from Velleman, and mounted it on top of the cradle.

  
Audio amplifier (Velleman kit)

Speaker
In the Netherlands, the RCD used a standard cylindrical speaker from the German peripheral manufacturer Peiker, the same one as was commonly supplied in the 1970s and 80s with the mobile radios of the emergency services, and with the first generation of mobile (car) phones.

The speaker has a diameter of 58 mm and was usually embedded in the front panel of the equipment rack inside the intercept vehicle.

  
Speaker

Manuals
Two manuals are available for the ADF-T12-C: (1) an installation manual and (2) a maintenance manual. Both are available for download below.

The manuals describe the operating principle of the direction finder and shows how it should be installed in an airplaine. It also shows how the loop antenna should be installed and calibrated.

 Installation manual
 Maintenance manual
  
Manuals

ADF-T-12C complete set
Receiver
Indicator (rose) seen from the front
Indicator (rose)
Connections at the rear of the indicator
Indicator, receptacles for receiver and loop antenna
Loop antenna top
Loop antenna bottom (receptacle)
Loop antenna wiring
Loop antenna with wiring
Sense antenna
Audio amplifier (Velleman kit)
Speaker
C
×
C
1 / 14
ADF-T-12C complete set
C
2 / 14
Receiver
C
3 / 14
Indicator (rose) seen from the front
C
4 / 14
Indicator (rose)
C
5 / 14
5 / 14
C
6 / 14
Connections at the rear of the indicator
C
7 / 14
Indicator, receptacles for receiver and loop antenna
C
8 / 14
Loop antenna top
C
9 / 14
Loop antenna bottom (receptacle)
C
10 / 14
Loop antenna wiring
C
11 / 14
Loop antenna with wiring
C
12 / 14
Sense antenna
C
13 / 14
Audio amplifier (Velleman kit)
C
14 / 14
Speaker

Block diagram
Below is the simplified block diagram of the system, in which only part of the receiver is shown at the top right. For clarity, the tuning sections and the IF stages have been omitted. At the bottom left is the indicator with the servo-controlled RF resolver. The two loops of the loop antenna, are connected to the two static windings of the RF resolver. The rotor coil of the resolver is fed to the receiver, where it is amplified, delayed by 90° and then mixed with a 47 Hz reference signal that is provided by the indicator. The resulting signal is added to the signal from the sense antenna.

Block diagram of the servo loop

Once the receiver is tuned to the desired frequency, its audio output is fed back to the indicator, where the 47 Hz component is extracted from the audio signal and fed to a balance amplifier that compares the phase of the 47 Hz signal from the receiver with the phase of the 47 Hz signal from the reference oscillator. The result is used to drive the servo-motor. As soon as the needle points to the angle of incidence, the error signal from the RF resolver is null and the servo-motor stops.

Receiver
The block diagram of the bare receiver is shown below. At the left is the sense antenna, of which the signal is amplified in an RF amplifier (RF). It is then mixed with the signal from a Variable Frequency Oscillator (VFO) and amplified in three stages (IF1, IF2, IF3). This signal is then used to feed the Automatic Gain Control (AGC), which avoids overloading on strong RF signals. It is also fed to an AM detector, after which the audio signal is amplified in three stages (AF1, AF2, PA). On receiver 201 models B, B1 and D, a Beat Frequency Oscillator (BFO) is present for CW reception.

Block diagram of the receiver

The section at the left, where the error signal from the loop antenna is injected, has been omitted here for clarity. The 47 Hz signal, injected by the reference oscillator of the servo system, is taken from the first audio stage (AF1) and fed back to the indicator, where it is part of the servo loop.

Servo amplifier-indicator
The drawing below was taken from the installation manual [A], and explains how the servo system works. For clarity, the various parts have been coloured. The signals from the two loops inside the loop antenna, are connected to the stator coils of the RF resolver (red, green), which is in fact a Bellini-Tosi goniometer. The rotating coil of the RF resolver is connected to the receiver.

Construction of the servo amplifier-indicator

A separate 47 Hz signal from a reference oscillator inside the servo/indicator, is also fed to the receiver (not shown here). The audio output from the receiver is fed back to the servo/indicator, where its phase is compared with the original signal from the reference oscillator, and used to drive the servo-motor. The latter drives the RF resolver (and the indicator) via a gear box.

RF Resolver   goniometer
The principle of the Bellini-Tosi Goniometer (RF resolver), is illustrated below. North (N) represents the forward direction of the vehicle. The loop antenna consists of a rectanglular flat ferrite body, with two separate windings at 90° angles, shown in red and green respectively. One winding (red) is responsible for the north/south (N/S) magnetic field of the intercepted signal (H). The other winding (green) picks up the west/east (W/E) component of the magnetic field.


The two antenna windings are connected to the corresponding stator fields of the RF resolver (goniometer). The rotating part of the goniometer, i.e. the rotor coil, picks up the fields from the N/S and W/E stator coils, and is connected to the input of the receiver. It is rotated by the servo-motor (under control of the receiver output) until a signal minimum (null) is found.


Interior
Getting access to the interior of the 201 Receiver is straightforward. When it is installed in the cradle, 1 which is usually the case, it can be removed by releasing the locking bolt – accessible through a hole in the front panel – after which the receiver can be pulled from the metal cradle.

As the receiver is connected to the outside world via a single 16-way connector at the rear, there is no need to disconnect any of the wires before opening it. The image on the right shows the bare receiver after it has been taken out of the cradle and the front panel has been removed.

The device consists of a molded aluminium frame with a large printed circuit board (PCB) at the bottom. All parts are mounted on the PCB. At the centre is a large 3-way tuning capacitor that controls the Local Oscillator (VFO) as well as the tuned circuits of the RF input stages in tandem.
  
Receiver interior seen from the front left

The right part of the frame is taken by the tuned circuits of the Local Oscillator, the antenna input circuits and the mixer, with separate adjustments for each of the 3 frequency bands. Adjustment of the receiver is possible by temporarily using a service cable to connect it to the cradle. 1

  1. In the Bendix documentation, the cradle is referred to as the wiring harness.

Unlocking the cradle
Receiver with cradle and indicator
Receiver
Receiver interior seen from the front left
Receiver interior seen from the rear right
Receiver interior seen from the rear left
Receiver adjustments
Receiver, tuning capacitor
D
×
D
1 / 8
Unlocking the cradle
D
2 / 8
Receiver with cradle and indicator
D
3 / 8
Receiver
D
4 / 8
Receiver interior seen from the front left
D
5 / 8
Receiver interior seen from the rear right
D
6 / 8
Receiver interior seen from the rear left
D
7 / 8
Receiver adjustments
D
8 / 8
Receiver, tuning capacitor

Restoration
When we acquired our ADF-T-12C in December 2020, it had not been used for at least 30 years. Nevertheless it worked first time and only required minor – prodominantly cosmetic – work on the exterior. On the inside, the tuning scale – which was corroded – was carefully cleaned , and the glass of the S-meter had to be refitted, for which the instrument had to be partly disassembled.

The following restoration work has been carried out:

  • Exterior cleaned
  • Meter glass repaired
  • Tuning scale repaired (was heavily corroded)
  • Front panel and knobs cleaned
  • Knobs refitted
  • Power connection restored
Connections
Amphenol Blue-Ribbon connector
Below is the pin-out of the 16-way Amphenol connector at the rear of the receiver, when looking onto the contacts. This is the same as looking at the solder side of the receptacle. Note that only the receiver has to be powered by an external 12V DC source. Any connected peripheral, gets its DC power from the receiver. Full wiring diagram on the last page of the installation manual [A].

  1. Sense antenna
  2. Loop shield
  3. Loop input R1 (tin)
  4. Loop input R2 (copper)
  5. Optional Audio Amplifier A
  6. Dial lamps
  7. Tuning meter (ext)
  8. 47 kHz switching phase 2

  9. Ground
  10. Servo DC output
  11. Spare
  12. 47 kHz switching phase 1
  13. Servo DC output
  14. Headset (500Ω)
  15. DC in (+)
  16. ADF Servo signal output
Fixed loop antenna
The loop antenna has a 7-pin male connector at the bottom, that mates with a rather special female cable part that came with the antenna. It has two fixed 2-wire shielded cables – one for each of the loops – that should be connected to the indicator (rose) by means of a Cannon WK 4-21C-5/8 connector. This 4-pin connector is also know by its Bendix part number S-240611-11. Below is the pinout when looking into the receptacle at the rear side of the indicator.

  1. A2
  2. A1
  3. B1
  4. B2
Specifications
  • Frequency
    190-1750 kHz
  • Bands
    3 (see below)
  • Modulation
    AM, MCW, CW
  • IF
    140 kHz
  • Sensitivity
    (ADF) 75µV, (REC) 10µV
  • Selectivity
    4 kHz (-6dB), 12 kHz (-60dB)
  • Output
    70 mW, 500 Ω
  • Response
    200-2000 Hz (10dB)
  • Settling
    7 seconds (max.)
  • Accuracy
    ±3°
  • Power
    12 or (28V) (selectable on the rear panel)
  • Current
    Receiver: 100 mA (170 mA), Indicator: 120 mA (60 mA)
  • Dimensions
    212 x 159 x 71.5 mm
  • Weight
    Receiver 1588 g, Indicator 771 g
Frequency range
The device covers a frequency range from 190 to 1750 kHz, spread over three bands:

  1. 190-440 kHz
  2. 420-900 kHz
  3. 850-1750 kHz
Parts
  • 210D Receiver
  • 2321E loop antenna
  • 551-series servo indicator
Documentation
  1. ADF-T-12C/D, Installation Manual
    I.B. 2012-1. Bendix, Avianics Division, March 1971

  2. ADF-T-12B/C, Maintenance Manual
    I.B. 2012A. Bendix, Avionix Division, October 1969.
    Reprint October 1970.
References
  1. Cor Moerman, Personal correspondence
    December 2020.
Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Saturday 17 December 2016. Last changed: Wednesday, 23 December 2020 - 08:46 CET.
Click for homepage