Wireless Set Number 10

Lore & Background

The Wireless Set Number 10 was developed from experiments beginning in 1941 with single-duplex (one-way) systems, followed by full-duplex testing in 1942. Long-range and overwater tests were conducted that same year. The system became ready for service in 1944, with military-quality sets available for D-Day operations.

Its range allowed secure communications from the D-Day beaches back to England across the English Channel, and the network was later extended into Germany. The stations were mounted in trailers and set up by aiming two parabolic antennas on the roof at the next station. The system could be extended into a relay by connecting trailers together or using existing landlines to connect separated trailers.

The basic concept became possible with two key technologies introduced in 1940: the cavity magnetron, which produced microwave signals with reasonable efficiency, and pulse-width modulation (PWM), which offered a simple way to encode signals on a magnetron. Because the available bandwidth was high, eight channels were combined into a single link using time-division multiplexing. The magnetron could not be smoothly modulated in amplitude or frequency, but it could be turned on and off very rapidly.

The original audio signal was sent into a PWM encoder whose pulsed output was amplified and used as the power supply to the magnetron, producing a series of microwave pulses representing the audio signal. On reception, the chain of pulses was averaged to reproduce the audio. By delaying pulses from different channels, multiple signals were multiplexed in time rather than frequency, making the Number 10 the world's first time-division multiplexing system.

Reader's Guide

The Wireless Set Number 10 addressed several problems that had plagued earlier radiotelephony systems. Low-frequency systems required very large antennas and suffered from poor directivity, making signals receivable by unintended stations thousands of miles away. Bandwidth limitations restricted such links to one or two conversations.

Moving to shorter wavelengths reduced these issues, but pre-war microwave tubes produced only 0.5 watts of output, limiting their usefulness. The cavity magnetron, developed for radar, provided both higher power and the short wavelengths needed for compact, directional antennas. The magnetron's inability to be amplitude-modulated was overcome by using pulse-width modulation, which also enabled time-division multiplexing—a novel approach at the time.

The system's use on D-Day to provide secure communications across the English Channel demonstrated its practical value in military operations. Field Marshal Bernard Montgomery later noted the system's importance. The Number 10 represented a significant step in military communications, combining mobility, security, and multi-channel capacity in a single trailer-mounted unit.