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FBA Peiler   Nahfeldausforschungsgerät
Near-field direction finder

The FBA-Peiler 1 , or Nahfeldausforschungsgerät, was a portable near-field direction finder, developed in the early 1960s by the Fernmelde Betriebsamt (FBA) in Vienna, at the time part of the Austrian state-owned telecom operator ÖPT. The device was intended for finding radio and television interference (QRM) and clandestine transmitters, such as spy radios and radio bugs.
 
The device has a frequency span of 3 - 145 MHz and consists of a small hand-held receiver (the direction finder) and 15 frequency plug-in units.

The actual receiver measures 9 x 17 x 3 cm and is housed in a white plastic case, similar to a 1960s portable radio. It is fully self-contained in that it is battery powered, has a wire antenna and a leather carrying strap with a wired crystal earphone that delivers the audio to the operator.

The entire kit is nicely packed in a robust metal carrying case with a leather grip at the front.
  
Complete FU-303 direction finder in metal storage case

The case of the FBA-Peiler is constructed in such a way that the device can easily be hidden under the operator's clothing [1]. All one has to do is select the appropriate coil unit (i.e. frequency plug-in) and adjust the variable capacitor on the receiver to the desired frequency. The device is suitable for direct reception of AM signals and FM signals that have a sufficient AM-component.

In addition, a 1000 Hz tone can be injected directly at the receiver's input, so that the receiver can be used for the detection of transmitters with AM or FM modulation, or no modulation at all. After detection, this produces a 1000 Hz tone of which the strength varies with the strength of the intercepted signal. This feature is extremely useful when in close proximity of the transmitter.

It is currently unknown how many FBA direction finders were built, but given the quality of the cases, the building style of the electronics, the presentation of the manual and the known serial numbers (199, 303), it seems reasonable to assume that it was built in small quantities, probably a few hundreds. As they were only used by special services and agencies, not many units have survived once the receivers were decomissioned. At present, only three surviving units are known.
 
  1. Peiler is the German word for Direction Finder.

Closed case Complete FU-303 direction finder in metal storage case Taking the receiver from the case Receiver removed from storage case FU-303 ready for use Close-up of the controls Attachment of the leather carrying strap Unplugged frequency plug-in coil

 
Controls
Controlling the receiver is pretty straightforward. All controls are on the left side of the device. After installing the selected frequency plug-in coil on top of the receiver, the unit is turned on by turning the sensitivity adjustment away from its null-position (a click indicates that the receiver is switched on). The earpiece should now produce noise and the tuning knob is used to select the desired frequency. A slide switch allows the 1000 Hz generator to be activated (slide upwards).


In close proximity of a transmitter, the plug-in coils act as antennas. When further away, an external wire antenna can be connected to the side of the coil unit. When homing in on the transmitter, the optional 1000 Hz tone can be used for guidance. By reducing the sensitivity of the receiver each time you get closer to the transmitter, locating the transmitter will be very easy.

The receiver is designed to be worn on the left side of the body, with the leather strap over the right shoulder. This way the controls are facing towards the front and can easily be manipulated with the right hand. A simple wire is used as antenna, so that it can be hidden inside the clothing.
 
Frequency plug-ins
  1. 100-145 MHz
  2. 75-100 MHz
  3. 50-75MHz
  4. 36-50 MHz
  5. 28-36 MHz
  6. 24-28 MHz
  7. 19-24 MHz
  8. 15-19 MHz
  9. 12-15 MHz
  10. 9.5-12 MHz
  11. 8-9.5 MHz
  12. 6-8 MHz
  13. 5-6 MHz
  14. 4-5 MHz
  15. 3-4 MHz
Interior
The interior of the receiver can be accessed by removing one screw from the top of the unit. This allows the front cover to be removed. The device consists of three sections: the actual receiver at the top, the 1000 Hz AM-generator at the bottom, and the battery compartment at the centre.
 
A standard 4.5 flat-pack battery is used to power the device. This is roughly the size of three 1.5V AA-size cells. The image on the right shows the actual receiver, which resides in the upper section of the case. At the top are the two terminals for the frequency plug-in unit. They are placed assymetrically, so that the coil plug-in can not be inserted the wrong way around.

At the left side are the sensitivity potentiometer and the tuning capacitor. The three OC75 germanium transistors of the 3-stage amplifier are 'floating' side-by-side at the centre.
  
Receiver with 3-stage amplifier

The circuit diagram of the receiver is surprisingly simple. At the top left is the tuned circuit that is responsible for tuning to the desired frequency. It consists of a removable coil and an adjustable capacitor. The optional 1000 Hz homing signal (T1) is injected via a diode (D1) directly to the tuned circuit, thereby modulating the incoming signal in AM. After detection (D2), a 3-stage amplifier (T2, T3 and T4) delivers just enough signal to drive a crystal earphone. The circuit is powered by a standard 4.5V flat battery and takes just 8mA even with the 1000 Hz generator on.
 
Opening the case FU-303 interior FU-303 interior: left the 1000 Hz generator, in the middle the battery compartment and at the right the receiver. Receiver with 3-stage amplifier 1000 Hz AM-injector Battery terminals FU-303 interior, top view Interior of one of the plug-in coils


FBA-Peiler (Nahfeldforschungsgerät) circuit diagram [1]



 
Storage case
The FBP-Peiler is supplied in a professional sturdy green hammerite metal carrying case with two locks and a leather carrying grip at the front. Inside the top lid is the circuit diagram with some directions on how to use the receiver, plus a list of the frequencies covered by each plug-in unit.
 
The bottom part of the case contains a pre-shaped polystyrene (PS) block with velvet lining. A large bay is reserved for the actual direction finder and has a storage area for the leather carrying strap with the earphone underneath it.

The 15 frequency plug-in coils are each labelled with a letter of the alphabet (A-O). They are nicely arranged around the receiver, each with their two contact pins mating with two holes in the PS bottom. The image on the right shows the interior of the carrying case after the receiver has been removed. The earphone is still visible.
  
Receiver removed from storage case

Also visible in the storage area is a short piece of wire with a banana-plug at the end. It is used as a concealed antenna and should be connected at the side of the frequency plug-in. The length of the antenna is relatively short compared to the wave length, which reduces the sensitivity of the receiver, especially below 6 MHz [2]. In practice this was not considered a problem however.
 
Transmitters operating on such low frequencies generally use a lot of transmission power. Besides, the device was intended for use in the near-field (at close proximity of the transmitter) in which case the antenna length doesn't matter.

Because of the small quantities in which the receiver was built, it doesn't have an official model name or number. Instead it was called FBA-Peiler or Nahfeldausforschungsgerät. Each device does have a serial number however, plus the abbreviation of the department (Stelle) that used it. The image on the right shows the tag.
  
Serial number tag

According to the tag, it was used by the 'FU' department of the Fernmeldebetriebsamt (FBA) in Klagenfurt, and has serial number 303. Another FBA-Peiler, from the collection of Günter Hütter in Austria [4] carries a slightly different tag. It has serial number 199 and was apparently used by the 'FÜV' department of the Fernmeldebetriebsamt (FBA) in Innsbruck (Tirol, Austria).
 
Closed case Complete FU-303 set in metal storage case Circuit diagram in top lid Complete FU-303 direction finder in metal storage case Receiver removed from storage case Serial number tag Serial number tag Wire antenna

 
Glossary
FBA   Fermelde Betriebsamt
Former Austrian Communications Service, reponsible for the deployment, operation, exploitation and enforcement of telephone and telegraph lines. Part of the Austrian state-owned Post and Telegraph service ÖPT.

ÖPT   Österreichische Post- und Telegraphenverwaltung
Former Austrian state-owned Post and Telegraph service [3]. Known as PTV and ÖPT. (Wikipedia)

References
  1. FPA Wien (Vienna), Bescheibung zum Nahfeldausforschungsgerät
    Description of the near-field detection device (German).
    18 April 1963.

  2. FPA Wien (Vienna), Zur Beachtung...
    Additional remarks on using the FBA-Peiler (German). Date unknown.

  3. Wikipedia (Germany), Post- und Telegraphenverwaltung
    Retrieved July 2013.

  4. Günter Hütter, FBA-Peiler with serial number 199
    Crypto Museum, Austria, June 2009.

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