A Short History of Radio Part 1

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Earliest Times
It all started about ten thousand years ago. As tribes evolved into city states and tribal skirmishes evolved into organised battles, the shouts of the tribal chiefs were no longer sufficient to direct his warriors. Writing eventually evolved and, although this was less prone to error than the spoken word and could be taken over much greater distances, the means of conveying it were only that of the runner or his horse. Speed was often of the essence, as in the case where reinforcements were needed, a siege was needed to be broken, or news of a threatening defeat or a great victory had to be sent.

One of the earliest recorded events of this sort was in the year 490 BC when a runner ran the 25 miles from Marathon to Athens to bring news of the Greek victory over the Persians. That’s where we get the modern Olympic event name from. We owe a couple more thing to the ancient Greeks; firstly, the idea that all matter was composed of simpler substances which they called “elements” each of which was the same throughout, and secondly that in each case there must be a tiniest amount of each element which could not be divided up into even tinier particles. These, they called atoms. They had no real evidence for either of these. They just thought it must be so. They only imagined four elements, Air, Earth, Fire and Water. In this they were wrong. We now know that there are 92 naturally occurring elements, (and a few unnatural or synthetic ones), but they were right about atoms although they had no conception of how small they would turn out to be.

The Romans, 2000 years ago used a combination of horsemen and bonfire beacons, to send messages and these persisted right through the middle ages until the coming of the railways at the beginning of the nineteenth century. In fact our local Ivinghoe Beacon was the site of a beacon ready to be lit in 1588 if the Spanish Armada succeed in landing and invading. The beacon is really only good for saying “The event we had told you might happen, has happened”, but it can be fast.

Evolving Requirements
Meanwhile, both on land and at sea, various flag signals and semaphore systems were developed. In fact, in 1857 the British Board of Trade published a list of 70,000 signals which could be sent using only 18 different flags, and it became known as “The International Code of Signals”. With the increasing understanding of science during the nineteenth century, it was realised that electricity probably held the secret of fast and accurate communication, at least on land. Communication between ships was a different matter and of course aeroplanes hadn’t been invented. Amongst electrical systems, various analogue systems were first tried in which currents of various strengths were sent along wires to a galvanometer at the receiving end to make its needle point to various different letters in turn marked on its dial. The best known of these was “The Wheatstone Telegraph” and it was used extensively by the early railways. The technical problem, (apart from it being slow), was that, due to leakage, the current received was sometimes less than the current sent which led to errors. A better system, based on the binary system, where the current only had to be “on” or “off” was invented by Samuel Morse in the USA in 1837 and within a few years Britain, the USA and much of Western Europe were criss-crossed with telegraph systems using copper wires supported on poles. There were even submarine cables to France and across the Atlantic, although these were not very long lived or successful due to their designers not understanding transmission line theory and the capacitance problem. The original London to Edinburgh wires were about a quarter of an inch thick for there were no amplifiers. Later, relay based amplifiers were used but reliability was always a problem. The telegraph was used extensively in the American Civil War between 1861 and 1865 and by the British during their colonisation of South Africa and India. It is interesting to note that Morse never actually envisaged his code being heard: the current was intended to operate an “inker” at the far end for reading later and as a permanent record. In fact, when the telephone was invented later by Alexander Graham Bell in 1876 and demonstrated to the chairman of the Western Union Telegraph Company, he said “This telephone has too many shortcomings to be seriously considered as a means of communication. The device is inherently of no value to us”. In 1897, the English physicist, Professor J. J. Thomson had discovered the electron and invented the cathode ray tube and got the Nobel Prize for it, although it didn’t have a great impact on communication for another decade. People were quite happy to use electricity without quite knowing what it was, but Thomson did destroy the notion that the atom of an element was the smallest possible particle. The electron was 2000 times smaller than the smallest atoms and, in a conductor, could easily move between them. (It is interesting to note that his son, Professor G. P. Thomson also got the Nobel prize for physics in the 1920s for proving that, under some circumstances, the electron behaves like a wave and not a particle. This “duality” between the particle theory and the wave theory is still a hot subject today. I was privileged to study atomic physics under G.P. when he was head of the physics department at Imperial College).

After about 1870 things started to move more rapidly. In 1873, James Clerk Maxwell the mathematician wrote his great “Treatise on Electricity and Magnetism” showing that a current produced a magnetic field, a changing magnetic field produced an electric field in space or a voltage in a wire, and this in turn could produce a current. Induction, dynamos and transformers quickly followed. In 1883 Edison observed a tiny flow of current in a modified lamp bulb and the thermionic diode was born, although no one had a use for it at that time. In 1888 Heinrich Hertz’s experiments with spark gaps showed that electro-magnetic waves could be transmitted across his laboratory from one “microwave” dipole to another, although his detector was incredibly insensitive by modern standards. It consisted of a tiny spark gap observed with the aid of a microscope. His receiver is depicted in Figure 1.

However, with this, he established that these waves moved with the velocity of light, he measured their wavelength, he proved that they were reflected by metal plates and refracted by prisms made of paraffin wax, and thus he experimentally completely verified Maxwell’s theory. The possibility of communication over a distance without wires was predicted and the search was on for much more sensitive detectors and much more efficient transmitters of these new waves.

At about the same time as Hertz was conducting his rather academic experiments, the Anglo-American inventor David Edward Hughes discovered that a loose contact between a steel point and a carbon block would not conduct current, but that if electromagnetic waves were passed through the junction point, it conducted well. In 1879 Hughes demonstrated the reception of radio signals from a spark transmitter located some hundreds of meters away, but although this formed the basis of the “crystal and cat’s whisker” detector which came into fashion some 30 years later it was not pursued at that time because an even more sensitive detector was making its appearance. This was the “Coherer”.

The coherer, was really the result of the work of many men, including Hughes, Lodge, Branley and Popoff among others. It consisted essentially of a small quantity of metal filings lying loosely between metallic electrodes. The first practical form of the device for telegraphic purposes was demonstrated by Marconi, although he didn’t invent it. In fact Marconi didn’t actually invent very much. His talent was recognising a good idea when he saw it, improving it, and putting it into use. There were several forms of the device, all known as coherers but all distinctly different, and with different sensitivities. I shall say more about these in the next part.
John, G0NVZ (Nov. 2000)

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