![Wireless Set No](https://upload.wikimedia.org/wikipedia/commons/c/c7/Wireless_Set_No._10.jpg)
The
British Army
The British Army is the principal land warfare force of the United Kingdom, a part of the British Armed Forces along with the Royal Navy and the Royal Air Force. , the British Army comprises 79,380 regular full-time personnel, 4,090 Gurk ...
's Wireless Set, Number 10, was the world's first
microwave relay telephone
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
system. It transmitted eight full-duplex (two-way) telephone channels between two stations limited only by the
line-of-sight, often on the order of . The stations were mounted in highly mobile trailers and were set up simply by aiming the two
parabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or pa ...
s on the roof at the next station.
The basic concept became possible with the introduction of two key technologies in 1940: the
cavity magnetron, which produced
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
signals with reasonable efficiency; and
pulse-code modulation
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the ...
(PCM), which offered a simple way to encode the signals on a magnetron. As the available
bandwidth was high, eight channels were combined into a single link using
time-division multiplexing
Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fracti ...
.
Early experiments with single-duplex (one-way) systems were carried out in 1941 and 1942 which demonstrated the basic concept. By that point, improvements in electronics allowed for a full-duplex system. Testing of a long-range system began in 1942 and overwater tests followed. The system was ready for service in 1944, and military-quality sets were available for
D-Day
The Normandy landings were the landing operations and associated airborne operations on Tuesday, 6 June 1944 of the Allied invasion of Normandy in Operation Overlord during World War II. Codenamed Operation Neptune and often referred to as D ...
operations. The range was enough that it was used to provide secure communications from the D-Day beaches back to England across the
English Channel
The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" (Cotentinais) or ( Jèrriais), (Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Kana ...
. The system could be extended into a relay by connecting trailers together, or through existing landlines, and in doing so the range was eventually extended into Germany. Field Marshal
Bernard Montgomery
Field Marshal Bernard Law Montgomery, 1st Viscount Montgomery of Alamein, (; 17 November 1887 – 24 March 1976), nicknamed "Monty", was a senior British Army officer who served in the First World War, the Irish War of Independence and t ...
would later note:
Description
Radiotelephony
There had been many systems for transmitting
telephone
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
conversations over radio before
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, 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
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
to "
skip
Skip or Skips may refer to:
Acronyms
* SKIP (Skeletal muscle and kidney enriched inositol phosphatase), a human gene
* Simple Key-Management for Internet Protocol
* SKIP of New York (Sick Kids need Involved People), a non-profit agency aidin ...
" their signals. A
radio antenna
In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an ...
has to be within about an
order of magnitude
An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
of the
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
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
In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It is the ratio of the radiation intensity in a given direction from ...
'' of the antenna, its ability to form the signal into a beam. This is related to
optical resolution
Optical resolution describes the ability of an imaging system to resolve detail, in the object that is being imaged.
An imaging system may have many individual components, including one or more lenses, and/or recording and display components. ...
, which is improved with increasing antenna sizes, and decreased with increasing wavelength. The relatively long wavelengths of the signals being used made focussing difficult, and in many cases such signals were broadcast omni- or semi-directionally. 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 long-distance telephone conversation might make do with as little as 4 kHz of bandwidth, but at 150 kHz this represents a fairly large amount of the available signal. 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 tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
s that could operate in the
very-high frequency
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves ( radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VH ...
(VHF) band.
AT&T
AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile tel ...
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
Multiplex may refer to:
* Multiplex (automobile), a former American car make
* Multiplex (comics), a DC comic book supervillain
* Multiplex (company), a global contracting and development company
* Multiplex (assay), a biological assay which measur ...
calls into the existing landline network. Even at this early time,
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
noted that the system would be much more effective in centimetre wavelengths, and produced an illustration of a system using
horn antenna
A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are us ...
s that could carry hundreds of calls. Further experiments were curtailed by the start of the war.
Microwave development
![Original cavity magnetron, 1940 (9663811280)](https://upload.wikimedia.org/wikipedia/commons/7/78/Original_cavity_magnetron%2C_1940_%289663811280%29.jpg)
As part of the development of
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
, the early years of
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
produced rapid development of
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
-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
A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated ...
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
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to t ...
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.
PCM
In 1937, English engineer
Alec Reeves
Alec Harley Reeves (10 March 1902 – 13 October 1971) was a British scientist best known for his invention of pulse-code modulation (PCM). He was awarded 82 patents.
Early life
Alec Reeves was born in Redhill, Surrey in 1902 and was educated at ...
was working at the
Paris
Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...
laboratories of
IT&T
ITT Inc., formerly ITT Corporation, is an American worldwide manufacturing company based in Stamford, Connecticut. The company produces specialty components for the aerospace, transportation, energy and industrial markets. ITT's three businesses ...
when he came up with the idea of
pulse-code modulation
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the ...
(PCM). In this concept, the analogue waveform is reconstructed from a series of pulses whose width defines the amplitude at that instant. A series of such pulses are sent into a filter to recreate the original analogue signal. When the German forces invaded France in May 1940, Reeves returned to England, and the PCM concept was immediately picked up for a number of roles.
PCM was almost perfectly matched for transmission using a magnetron and the technique arrived in England just as the first production magnetrons were delivered. 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 PCM 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 PCM encoder and delaying its pulses slightly so that its signals were sent after the first. This solved the problem of
multiplexing
In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...
multiple signals into a single connection. Previously, telephone systems accomplished this with
frequency division multiplexing
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate ...
, 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 PCM, the signals were spread out in time instead of frequency, which was easily handled by the magnetrons. This makes the Number 10 the world's first
time-division multiplexing
Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fracti ...
(TDM) system.
The first conceptual design, introduced in 1941, was for a single channel
half-duplex system, 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.
Into service
The first experimental sets arrived in July 1942 and were used on a two-stage link between
Horsham
Horsham is a market town on the upper reaches of the River Arun on the fringe of the Weald in West Sussex, England. The town is south south-west of London, north-west of Brighton and north-east of the county town of Chichester. Nearby to ...
and Berkeley Court on
Baker Street
Baker Street is a street in the Marylebone district of the City of Westminster in London. It is named after builder William Baker, who laid out the street in the 18th century. The street is most famous for its connection to the fictional detec ...
, London. Overwater testing followed between
Ventnor
Ventnor () is a seaside resort and civil parish established in the Victorian era on the southeast coast of the Isle of Wight, England, from Newport. It is situated south of St Boniface Down, and built on steep slopes leading down to the sea. ...
on the
Isle of Wight
The Isle of Wight ( ) is a county in the English Channel, off the coast of Hampshire, from which it is separated by the Solent. It is the largest and second-most populous island of England. Referred to as 'The Island' by residents, the Isle of ...
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 moved to
Cherbourg
Cherbourg (; , , ), nrf, Chèrbourg, ) is a former commune and subprefecture located at the northern end of the Cotentin peninsula in the northwestern French department of Manche. It was merged into the commune of Cherbourg-Octeville on 28 Feb ...
. 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
Field Marshal Bernard Law Montgomery, 1st Viscount Montgomery of Alamein, (; 17 November 1887 – 24 March 1976), nicknamed "Monty", was a senior British Army officer who served in the First World War, the Irish War of Independence and t ...
'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 for use with
coaxial cable
Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
connections,
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
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.
In contrast, PCM was used mostly in military roles before being picked up for civilian uses. Among its most enthusiastic users was the
General Post Office
The General Post Office (GPO) was the state postal system and telecommunications carrier of the United Kingdom until 1969. Before the Acts of Union 1707, it was the postal system of the Kingdom of England, established by Charles II in 1660. ...
, which at that time ran the
long distance calling
In telecommunications, a long-distance call (U.S.) or trunk call (also known as a toll call in the U.K. ) is a telephone call made to a location outside a defined local calling area. Long-distance calls are typically charged a higher billing rate ...
services in the UK. They introduced a series of PCM-based systems that allowed long-distance links to be much further apart as they could cleanly rebuild the original series of pulses even with very low signal levels. In 1968 the company started the first all-PCM digital exchange, with no analog signals until they reached the customer's
local exchange.
Technical description
The complexity of the system was mostly related to the PCM encoding. The system was based on a master clock signal in the form of a
sine wave
A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...
at 9 kHz. The sine wave was processed in a series of tubes to produce a
sawtooth wave
The sawtooth wave (or saw wave) is a kind of non-sinusoidal waveform. It is so named based on its resemblance to the teeth of a plain-toothed saw with a zero rake angle. A single sawtooth, or an intermittently triggered sawtooth, is called a ...
, which was then
gated by the audio signal. When the voltage of the sawtooth was above the voltage of the audio signal, an output pulse was produced – the higher the voltage of the audio signal, the smaller the width of the sawtooth above that level, and the shorter the pulse. The final output was a train of pulses at the reference frequency, the width of each pulse being inversely proportional to the audio signal voltage.
The system as a whole had eight of these samplers, referred to at the time as ''separators''. Each output a pulse of up to 3.5 µs. A fixed delay was added in each sampler, such that channel 2's pulse was output about 5 µs after the one from channel 1. This resulted in a "chain" of pulses over one complete cycle of the master clock. A separate "sync pulse" of 20 µs was added in front of channel 1 to allow
clock recovery. The pulse chain from the encoders is amplified and sent to the transmission magnetron which produces pulses at the selected frequency.
Reception is much simpler; the clock is extracted from the sync signal and each channel is separated out by sampling at fixed times relative to the sync. The pulses are fed into
low-pass filters that directly produce the original audible signal.
The rest of the system is relatively simple. The magnetron output is sent to a dipole antenna in front of a diameter parabolic reflector. A second reflector antenna was positioned beside the first for reception. The original Mark I units sent signals upstream on 4550 or 4760 MHz, while the receiver was sensitive to the entire range between 4410 and 4888 MHz. The Mark II units shifted this to 4480 and 4840. The output of the transmitters was typically between 100 and 400 mW.
Range was normally on the order of , but sometimes "well over" . Normally, only seven of the eight telephone channels would be used, leaving one free for the operators at the station to use, or as a backup.
The system was mounted in a relatively small four-wheel trailer with the antennas on a
turntable on top. It could be powered by any mains supply from 100 to 250 volts. The Mark I units could also self-power using two Onan 3 kVA generators mounted on one end of the trailer, while the Mark II used a single PE 95 10 kVA generator with another Onan generator for backup.
Notes
References
Citations
Bibliography
*
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*
*
*
*
*
* {{cite book
, first=Bernard , last=Lovell
, url=https://books.google.com/books?id=tnGo5BywLj8C
, title= Echoes of War: The Story of H2S Radar
, publisher=CRC Press
, date=1991
, isbn=0852743173
External links
Wireless Set No.10
Microwave transmission
World War II British electronics
British military radio
1940 in technology