Automatic Friend or Foe system Royal Netherlands Navy (1938 – 1940)
As early as 1934, the Royal Netherlands Navy asked the Meetgebouw to develop an automatic reconnaissance signal (‘friend or foe’). First, a challenge signal is transmitted to an unknown ship. If it is a ship of the Royal Netherlands Navy, the system at that ship will immediately answer the reconnaissance signal with the (correct) answer associated to the challenge.
The automatic electronic reconnaissance system should replace the old manual system with Navy signal flags or signal lamps. It took a while to develop the requirements and initial design. In the final design, a ship ‘queries’ another, yet unknown, ship by sending a code. The code is set with two six-position switches (1 out of 30 codes). An electronic circuit with six tone generators and relays generates the selected code, amplifies it (radio tube technology), and sends the signal to the transmitter antenna. At the receiving side – the challenged ship – a bell (bell) rings as an indication of a challenge. The received signal is decoded using six filters and a number of relays.
If the challenge signal contains the correct code, the reconnaissance signal system automatically sends an encoded response specific to that ship. The system on the first, interrogating vessel then decrypts the ‘response’ of the interrogated vessel. The answer indicates whether the other ship is a “friend or foe”.
In the period 1938 – 1940, a prototype system is developed. In 1940, the Germans occupation stopped further development.
Only about thirty photos of the system remain as well as 35 mechanical drawings from February 1938 and three final electrical diagrams dated November 1939. The material lists for the modular cabinets take the salty sea environment into account: celoron (insulation material), aluminum and cadmium-plated steel.
The transmitter operates at a wavelength of 60 cm (500 MHz) and consists of a Kolster ring with a so-called apple tube TB04/8 as an oscillator. This type of tube is a laboratory favorite for the high wavelengths: apple tubes can also be found in the ‘electric listening device‘ and the ultrashortwave field phone. Our archives show that this frequency is claimed early by the Navy: in 1936, a letter by the Ministry of the Interior established that the frequency band 175-250 MHz (1.714-1.200 m) * is reserved for ‘the UKG communication for the automatic aircraft control and for the Automatic reconnaissance signal (Navy)”. [*number of wavelengths in a meter]
The reconnaissance signal system has two antennas: a directional antenna and a non-directional antenna.
The system has a few built-in tests; among other things, the system could be tested with a deactivated transmitter and antenna. In addition, an extensive failure analysis table with linked actions is provided (see photos below).