Radio Communication: Non-eaves-droppable ship-to-ship communication (1954 – 1962)


Non-eaves-droppable ship-to-ship communication (1954 – 1962)

Non-eavesdropping communication between navy ships has always been found to be extremely important, for example, the commonly used signal lamp. Fear of the Navy is being detected at a large distance and being localised by electromagnetic emissions within a fleet context. This has led to concepts of radio and radar silence.

In 1948 and 1949, the Meetgebouw worked on the development of telephony over light (“optiphone”). The idea was to modulate the light beam of a 10-inch (a.p. 3860A) signal searchlight. However, the Royal Netherlands Navy required the signal searchlight’s Morse function to be usable at all times. The transmitter and receiver had to be of small size. In its research assignment, The Navy stated that a combination of visible light with infrared light would increase the utility value of the connector.

At the end of 1948, two signal searchlights of the HNLMS Karel Doorman and a 600 V/1-2 A plate voltage device were temporarily lent to the Meetgebouw for tests.  In 1949, the Karel Doorman was fitted with new signal searchlights and the Meetgebouw could continue to use the signal searchlights. Research continued until 1953. Despite the initially positive research results, no operational system resulted as the state of the atmosphere at sea was decisive for the reliability of the communication.

In 1952, after the aforementioned research into the usability of visible light, infrared and even ultraviolet, it was proposed to investigate the possibilities of microwaves for the realisation of a ‘bilge link’ between naval vessels.
After a simple setup in 1954-55 with 3.2 cm transceivers, to gain some experience with a microwave connection over water, the Royal Netherlands Navy commissioned the start of developing a complete communication system suitable for testing onboard ships in 1956. The aim was to evaluate the operational value of microwave connections within a fleet context with problems such as making a connection with not exactly known positions of vessels and maintaining that connection.
This led to a rather complicated rotating microwave antenna system, based on a microwave Fresnel lens, and a gyroscope-stabilised platform. Two independent beams were created by polarisation separation, which made it possible to keep both stations automatically aligned. The size and weight of the antenna made the choice of a smaller wavelength desirable. Ideally would have been a wavelength of 6 mm, but because of the availability of 1.8 cm transmission tubes that wavelength was chosen. To make the emission detection area as small as possible, the power of each transmitter was automatically adjusted by the counter station to the minimum required for the connection.

Non-eavesdroppable ship-to-ship communication unit
Non-eaves-droppable ship-to-ship communication unit; rotatable polarisation frame at the front

Building this equipment turned out to be time-consuming and lasted until 1959, mainly because the specific character meant that almost all microwave components had to be manufactured ourselves. The Fresnel lens is made of aluminium cylinders that were acquired from the Velpon factory which used the cylinders to create glue tubes. The cylinders were formed with a mandrel into square pipes. These pipes were then glued together and subsequently filled with liquid wax. This gave the possibility to model the lens on the lathe. Then the wax was melted out and the whole lens part was cleaned. The lens concentrated the normally spherical microwave wavefront into a narrowly focused bundle.  

Fresnel grid from toothpaste tubes
Fresnel grid from aluminium glue tubes

Due to the small beam width in elevation, it was necessary to have a stabilised platform for onboard testing. In 1962, the French Navy offered us access to a floating facility at the Île d’Oléron. The counter station was placed on land near Toulon. The ordeal was successful, but it did show that there were still many uncertainties, among other things due to the occurrence of anomalous propagation over the sea, as is for instance almost always present in the Mediterranean.

Toulon: the barrel-shape of the device is visible
Toulon: The barrel shape of the device is visible

Despite much appreciation for the achievement, the research was discontinued. The need for stabilisation and the size and weight of the antenna part was an objection. The technique was not sufficiently advanced to be able to realise the set goal. In this decision, newer insights about fleet operations in the context of a nuclear threat also played a role.