Waveguides

The advantage of microwave links was that lots of channels could be transmitted without needing to lay cables in the ground. Does that mean that guiding structures (microwave cables are usually referred to as waveguides or, more generally, as guiding structures) were entirely unsuitable for microwave communications? No, they were not. At one time the prospects looked exciting.

Waveguides are made of hollow metal tubes which may have rectangular or circular cross-sections, as shown in Fig. 9.6. The electromagnetic waves carry the information inside the tubes very much like water. Information flows in at one end and comes out at the other end. An alternative view is to look at wave propagation as a result of a series of reflections by the walls, as shown in Fig. 9.7 for a rectangular waveguide.

Unfortunately waveguides, like any other guiding structures, have attenuation which depends on the size of the waveguide and on the electric field configuration within them. Attenuation usually increases with frequency, so that if we want to accommodate lots of channels and employ higher and higher frequencies, the waveguides become less and less suitable for information transmission. There is however an interesting exception, discovered theoretically in the 30s by Sally Mead of Bell Laboratories. For one particular electric field configuration (see Fig. 9.8) the attenuation of a circular waveguide declines with frequency. Taking a fairly large waveguide diameter of 7.5 cm and a frequency of 75 GHz (a wavelength of 4 mm), the attenuation is no more than 0.23 db/km.5 Other electric field configurations are also possible: they are called modes. Since they are important for guided wave propagation there is a little more about them in Box 9.1 at the end of this chapter.

Fig. 9.6 Schematic representation of waveguides of rectangular and circular cross section.

Fig. 9.6 Schematic representation of waveguides of rectangular and circular cross section.

Fig. 9.7 A wave may propagate by subsequent reflections from waveguide walls.

Waveguide wall

Waveguide wall

Fig. 9.7 A wave may propagate by subsequent reflections from waveguide walls.

Electric field lines

Fig. 9.8 Electric field lines in a circular waveguide. The only configuration that leads to low loss.

How many telephone channels are available? Using a range of frequencies between 30 GHz and 100 GHz the number of available channels with the most economic Single Sideband Modulation comes to 1.75 million, or to about 100000 if PCM (Pulse Code Modulation, see Chapter 11) is used. No wonder that communications engineers got excited about it. But why use waveguides which need to be buried in somebody else's land, and pay rents? Why abandon the chain of towers? The answer is that atmospheric absorption is unduly high for such high frequencies so free space propagation is out of the question. On the other hand, with the prospect of 100000 channels it will still be economic to pay rent and undertake all the expenses of manufacturing and laying the waveguides.

At this stage it may be worth making a little excursion into economics. Say the two cities connected by this waveguide are 100 km from each other and, say, 15 p is charged for each 3 minute call between them. The revenue would come to about a quarter of a billion pounds in a year, even if only ten per cent of the capacity is used. This is not a sum to be sniffed at.

Thus the motivation to go ahead was quite strong. Research started a few years after the end of the war, more or less simultaneously in the US and in Britain. The British effort was concentrated at Standard Telecommunications Laboratories (the principal research Laboratory of International Telephone and Telegraph), at University College, London and at the Post Office Research Station. The American research was mainly conducted at Bell Laboratories. By the middle 60s most of the technical problems had been solved. The only thing needed was customers. Unfortunately, there were no two cities anywhere in the world which would have cried out for 100 000 brand new telephone channels. And besides, the system would have been quite expensive to install. AT&T's experimental line cost $100000 per mile.

AT&T hoped to stimulate demand with its Picturephone which was the sensation of the 1964 New York World Fair. It was introduced in service in Pittsburgh in the summer of 1970. Had it caught on it would have gobbled up bandwidth, one picture channel being equal to one hundred telephone channels, but as it happened the public wanted to be heard but not seen.6 The British Post Office and AT&T had no other option but to wait patiently for demand to pick up but before this happened, suddenly, a competitor appeared in the form of the optical fibre (see Chapter 12). It was enormously bad luck. It often happens in technological history that the solution envisaged for a particular problem turns out to be deficient in one or another aspect and the designers have to go back to the drawing board. As far as I know long distance communications by microwave waveguide is the only case when a technically excellent solution never saw the light of the day because, before it could be put into practice, a series of new inventions made possible a cheaper, alternative solution. As someone who worked on the project for a couple of years I mourned its demise.

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