Radio communication: non-eavesdroppable ship-to-ship communication (1954 – 1962)

Non-eavesdroppable 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 1952, after research into the usability of visible light, infrared and ultraviolet radiation, 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, with the aim of gaining some experience with a microwave connection over water, the Royal Netherlands Navy commissioned the start of developing a complete communication system suitable for testing on board 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 Fresnel lens. Two independent beams were created by polarisation separation, which made it possible to keep both stations automatically aligned. 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. In order 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-eavesdroppable 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 used in a toothpaste factory to make tubes for containing glue. These cylinders were formed with a mandrel into square pipes. These pipes were then glued together into a lens 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.

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 a number of uncertainties, among other things due to the occurrence of anomalous propagation over the sea, as is for instance almost always present in the Mediterranean.
Despite much appreciation for the achieved, the research was discontinued. The need for stabilisation and the size and weight of the antenna part were an objection. In fact, the technique was not sufficiently advanced to be able to realise the set goal. In this decision, the changed insights into sailing in the context of a nuclear threat also played a role.