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Origins of microwave

Post Office tower under construction, 1964

The need for bandwidth : the crowded airwaves

Communications channels are often likened to pipes that carry information. In plumbing, the fatter the pipe, the greater the volume of water you can pump through it. In telecommunications, wires and radio channels have similar constraints; there's an upper limit to the information you can force down them at once. A standard telephone wire is like a thin pipe - fine for voice communication, but you can't use it to connect supercomputers on the Internet.

It's the same on the airwaves; a single television station occupies the same amount of space in the radio spectrum as 667 separate simultaneous AM radio stations. In fact the whole of the medium wave broadcast band could not provide enough bandwidth for even one television channel.

The radio spectrum only becomes suitable for transmitting broadband (high speed, high volume) signals in the microwave region, starting at 2GHz. These microwave frequencies are shared by many different businesses from mobile telephones to local area networks and satellite communications. 

Goonhilly dish at night

Why use a dish? : high beam radiowaves

Why does a car headlamp have a reflector behind the bulb? It is to trap all the light energy from the bulb and focus it out forward where it is needed.

In the same way, radio aerials work most efficiently when arranged to direct or 'focus' the radio energy in the direction required.

The shape of the reflector depends on the frequency or wavelength of the radio signal in use - at VHF and UHF frequencies the reflector is usually made out of a metal rod or 'elements' pointed in a specific direction, but at the shorter microwave wavelengths (which behave more like light), a parabolic reflector, like a satellite dish, is more efficient.

The dishes are used to receive satellite television at home, as well as at satellite earth stations and at the Jodrell Bank radio telescope. The dish itself focuses the radio waves it receives onto the active antenna element at the focal point in the centre - just like a car headlamp.

GPO poster of Post Office microwave tower in Birmingham

The need for microwave : shifting the volume

As more lines were installed after the First World War, the volume of calls on the British telephone network started to increase at a dramatic rate.

Routing all these calls along the trunk network meant providing bigger and bigger cables to provide the capacity that was needed.

But extending the trunk network over wild countryside or across water posed a problem - how to do so quickly and without having to lay large cables, which were unwieldy and expensive.

Microwave provided a possible answer - a wireless link that would use focused high intensity point-to-point beams to carry high volumes of traffic simultaneously.

 

 

Sir Jagadis C Bose

Bose's microwave demonstration (1895) : Indian pioneer

Up to 1900 the focus of invention had been on sending and receiving communication signals. As the 20th century progressed scientists worked with longer radio wavelengths (lower frequency signals) to achieve ever-greater distances.

But some scientists were going the other way, looking at the properties of very short wavelengths. The theory was that by shortening the wave, you could pack more electromagnetic energy into the signal.

One of the pioneers was J.C. Bose in India. In 1895 he gave his first public demonstration of very short wavelengths, ranging from 2.5cm down to 5mm - equivalent to a frequency of 60 Gigahertz (GHz). He used these transmissions to ring a bell remotely and to explode a charge of gunpowder.

Artillery radar equipment demonstration in England, 1945

Early signs of radar (1935) : short waves detect far away objects

In 1932 Sir Robert Watson-Watt figured out a way to harness the power of very short waves to detect objects far away. He came up with the idea of pulsing energy out on very short wavelengths in order to 'bounce' it off a target and detect the reflected signals.

He wrote a paper (with A.F. Wilkins) describing this new technique in 1935 and the idea was taken up rapidly . By the autumn of 1938 his Radio Direction Finding (RDF) systems were in place along the south and east coasts of Britain.

During the Battle of Britain in 1940, the British were able to detect enemy aircraft at any time of day and night and in any weather conditions, proving the defensive value of RDF or, as it would soon come to be called, 'radar' - short for Radio Detection And Ranging.

Cross channel microwave demonstration

The first microwave links (1930) : beams across the Channels

The advantage of ultra short wave and microwave transmissions was that they could carry huge amounts of signal information using wider channels or 'bandwidth'. This meant they could carry large numbers of phone calls simultaneously - very useful for extending the trunk phone network, particularly where conventional cables were impossible such as over water. However, the range was relatively limited.

Microwave links first came into practical use during the 1930s. In 1931 Britain's Standard Telephone & Cable Ltd (STC) demonstrated its 'Micro-Ray' microwave communications link across the Channel between Dover and Calais. The following year, Britain's first ultra short wave radio telephone link was set up by The Post Office across the Bristol Channel, spanning a distance of 13 miles.

 

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The UK's first permanent gallery dedicated to the history of information and communication technologies opens in the new Information Age gallery at London’s Science Museum.

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