Wireless Set Number 10
The British Army's Wireless Set, Number 10, was the world's first multi-channel microwave relay telephone system.
[1] It transmitted eight full-duplex (two-way) telephone channels between two stations limited only by the line-of-sight, often on the order of 25 to 50 miles (40 to 80 km).
The range was enough that it was used to provide secure communications from the D-Day beaches back to England across the English Channel, and the network was eventually extended into Germany.
[2]There had been many systems for transmitting telephone conversations over radio before World War II, but they all suffered from a series of similar problems.
There was considerable experimentation in the immediate pre-war era with newer vacuum tubes that could operate in the very-high frequency (VHF) band.
[3] Higher frequency systems had been experimented with, but were significantly limited by the low power of the microwave tubes of the era.
Several similar experiments were also carried out in Germany, primarily by Telefunken, but they were stymied by low power levels and their multi-channel system was never successfully developed.
By the end of World War II they had built out a network, eventually reaching 70,000 kilometres (43,000 mi), using single-channel links and very tall antennas.
A simple half-wave dipole has little directivity, but once again the short wavelengths helped as a suitable focusing arrangement using a parabolic dish about a metre wide reduced the beam width to about 5 degrees.
The magnetron does not allow this; it produces a single frequency that is dependent on its physical construction, defined by the number and size of holes drilled into it.
While the magnetron could not be smoothly modulated in amplitude or frequency, it could be turned on and off very rapidly; it is this quality that makes it useful for radar where short pulses are desirable.
To carry communications, the original audio signal was sent into a PWM encoder whose pulsed output was then amplified and used as the power supply to the magnetron.
On reception, the chain of pulses is sent into a circuit that averages the total energy received, reproducing the audio for output.
This would operate like a conventional radio set, where users at either end of a connection have to take turns speaking as they share a single channel.
This led to a new version that used separate frequencies for the upstream and downstream directions, allowing full-duplex operation, albeit with the small downside that two magnetrons and antennas were required.
[2] The first experimental sets arrived in July 1942 and were used on a two-stage link between Horsham and 64 Baker Street in London.
10 trailers back-to-back with conventional telephone wiring, allowing messages to be relayed over longer distances.
In the entirety of the war, Field Marshal Bernard Montgomery's headquarters lost a direct line to London for a total of one hour.
[12] In post-war debriefings, German radio engineers boasted that they were able to gather British signals with ease.
In contrast to the magnetron, the klystron is a true amplifier, accepting a low-power input signal across a range of frequencies and then outputting it at much higher power.
As this was already widely used in telephony with coaxial cable connections, Bell Labs selected this solution for their TD-2 network that was built across the United States during the early 1950s and in many other countries during the later 1950s.
A separate sync pulse of 20 μs was added in front of channel 1 to allow clock recovery.
