Radiotelephony 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. The first was that in order to gain long-range transmission, these systems had to work at relatively low frequencies in the kilohertz range or somewhat higher
longwave frequencies that could take advantage of the
ionosphere to "
skip" their signals. A
radio antenna has to be within about an
order of magnitude of the
wavelength in order to be efficient, and in practice, is often sized to exactly the wavelength to form a
half-wave dipole. Thus, these systems used very large antennas. Another related radio physics effect is the
directivity of the antenna, its ability to form the signal into a beam. This is related to
optical resolution, which is improved with increasing antenna sizes, and decreased with increasing wavelength. The relatively long wavelengths of the signals made focussing difficult without resorting to enormous antenna arrays, and in many cases such signals were broadcast omni- or semi-directionally instead. This meant the signals could be received by ground stations other than the intended one, sometimes thousands of miles away, leading to interference. For secure military communications, such a system had obvious drawbacks. Finally, the amount of information that can be carried by a radio signal is a function of its
bandwidth. A telephone conversation might make do with a bandwidth as small as 4 kHz, but at 150 kHz this represents a fairly large fractional bandwidth. Depending on the antenna and receiver design, the spread of frequencies that can be efficiently received may limit the link to one or two conversations. All of these problems are reduced by moving to shorter wavelengths. There was considerable experimentation in the immediate pre-war era with newer
vacuum tubes that could operate in the
very-high frequency (VHF) band.
AT&T led a number of these efforts, including a system operating at 150 MHz. This allowed the signal to be more tightly focused, and the increased bandwidth allowed a dozen lines to be carried on the signal using the same equipment used to
multiplex calls into the existing landline network. Even at this early time,
Bell Labs noted that the system would be much more effective in centimetre wavelengths, and produced an illustration of a system using
horn antennas that could carry hundreds of calls. Further experiments were curtailed by the start of the war. Higher frequency systems had been experimented with, but were significantly limited by the low power of the microwave tubes of the era. An experimental 1931 system across the
English Channel produced only 0.5 W of output, and was not used commercially. A commercial system followed in 1935, but while the 300 MHz frequency were considered microwaves at the time, today these would be known as
UHF. 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 , using single-channel links and very tall antennas.
Microwave development As part of the development of
radar, the early years of
World War II produced rapid development of
microwave-frequency electronics and techniques. One of the key advances was the introduction of the
cavity magnetron in 1940. One of the reasons for the intense interest in microwaves was the issue of antenna size; in the VHF region, radar antennas were on the order of metres long, which made them difficult to use on
night fighters. In contrast, the magnetron produced wavelengths of 9 cm, with antennas half that length. This meant they could easily fit within the nose area of a
night fighter. 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. This made the system dramatically more useful; not only was the radio energy focussed into a small area and thus produced far stronger reflections, but those reflections could also be accurately located in space by moving the reflector to point at the target. The magnetron's potential in communications was understood from the start, but in this role, it had a significant problem. In most radio systems of the era, the audio signal and the radio frequency carrier signal are generated separately and then mixed to produce an
amplitude modulated signal that is then amplified for transmission. This requires an amplifier that can produce a range of output frequencies, at least as great as the bandwidth of the audio signal. 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. There is no way to modulate the output using a separate signal.
PWM General Electric Company (GEC) delivered the first production magnetrons in 1940. PWM was almost perfectly matched for transmission using a magnetron. 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. The result was a series of microwave pulses representing the audio signal. On reception, the chain of pulses is sent into a circuit that averages the total energy received, reproducing the audio for output. As the pulses were quite short compared to the 9 kHz sampling time, much of the signal was empty. This could be easily taken advantage of by using another PWM encoder and delaying its pulses slightly so that its signals were sent after the first. This solved the problem of
multiplexing multiple signals into a single connection. Previously, telephone systems accomplished this with
frequency division multiplexing, shifting each of the channels by a different carrier frequency so they could all be broadcast at the same time in the same way that many radio stations can share the airwaves on different "channels". As the magnetron could not change its frequency, which is based on its physical construction, this technique would not work. With PWM, the signals were spread out in time instead of frequency. This makes the Number 10 the world's first
time-division multiplexing (TDM) system. The first conceptual design, introduced in 1941, was for a single channel
half-duplex system. 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. As development continued, accurate filters able to cleanly separate two closely spaced microwave frequencies were developed. 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. This was not a difficult change; the recently introduced
GL Mk. III radar also used separate dishes for transmission and reception and was easily adapted to the new role.
Into service The first experimental sets arrived in July 1942 and were used on a two-stage link between
Horsham and
64 Baker Street in London. Overwater testing followed between
Ventnor on the
Isle of Wight and
Beachy Head on the south coast. A production order was sent in early 1944. The first operational use occurred shortly after D-Day when the transceiver at Beachy Head was moved to
Cherbourg. As the Allies advanced into Europe, repeaters were created up by connecting two No. 10 trailers back-to-back with conventional telephone wiring, allowing messages to be relayed over longer distances. Where long-distance landlines were available, these were used to extend the connections between stations. The result was a network of landlines and No. 10 sets that eventually stretched from Germany back to London. In April and May 1945, a network of seven repeaters linked 21st Army Group with its various field headquarters. The sets were extremely successful. In the entirety of the war, Field Marshal
Bernard Montgomery's headquarters lost a direct line to London for a total of one hour. In post-war debriefings, German radio engineers boasted that they were able to gather British signals with ease. Careful examination of these claims revealed that No. 10 communications had not only never been intercepted, but that the Germans were entirely unaware of its existence.
Post-war During the late-war period, the
klystron tube had also improved to become a useful system. 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. This allowed communications systems to be constructed using frequency division multiplexing. 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. ==Technical description==