Milestones in Radio Technology

An overview of the key milestones in the history of radio technology and its development

Radio Coherer History Includes:

Radio history / timeline     Radio receiver history     Superhet radio history    

Early beginnings - It is impossible to say exactly where the story of radio starts. The early scientists who saw and investigated the effects of electricity and magnetism were crucial to the story. But a number of people also noticed effects that were probably caused by radio waves. Galvani is thought to have witnessed the effects of electromagnetic waves as he was investigating the conduction of electricity. Others too including Henry, Edison and others may also have witnessed effects as well.

Maxwell proves the existence of e/m waves - It was James Clerk Maxwell who first proved the existence of an electromagnetic (e/m) wave. He proved it mathematically, and published his findings in a number of papers. Much of this early work was performed whilst he was at Kings College London. His work was summarised in a book entitled "Treatise on Electricity and Magnetism." In 1871 Maxwell moved to Cambridge where he became the first director of the Cavendish Research Laboratory.

1887 Hertz discovers radio waves - In a series of experiments started in 1887 Heinrich Hertz proved the physical existence of radio waves that Maxwell had shown to exist mathematically. His transmitter consisted of a simple spark gap across an induction coil with a loop of wire to act as an antenna. The receiver consisted of a smaller gap in a loop the same size as that in the transmitter. In his experiments Hertz also discovered many of their properties. The results of his experiments performed in 1887-8 were published between 1888-90. Whilst other people had seen the effects before, nobody had realised what they were, and Hertz is rightly credited with having discovered radio or Hertzian waves as they were first called.

1894 The coherer is popularised - The coherer, an item used to detect radio waves took many years to develop with the earliest observations dating back to 1850. The first person to use the idea of the coherer was a Frenchman named Edouard Branly. He discovered that the resistance of a glass tube filled with metal filings fell to a few hundred ohms when an electrical discharge occurred nearby. The filings could then be "de-cohered" by a sharp tap on the tube. These devices were effective in detecting the transmissions of a spark transmitter. The idea was popularised by Oliver Lodge, especially as a result of a lecture he gave in 1894.

1894 Marconi's first experiments - In the autumn of 1894 Marconi performed his first experiments with radio waves in the attic of his parents house in Bologna. Initially he was only able to achieve distances of a few metres, but he made significant progress, steadily increasing the distance over which he send the signals. He managed to send signals over a distance of about 2 kilometres, and realising the possibilities of the system for maritime communications he gave a demonstration to the Italian authorities. Unfortunately they were not impressed, and as a result Marconi moved to England.

1897 Marconi demonstrates radio waves travel over water - Marconi gave his first public demonstrations in December 1896. One major use for radio could be in crossing stretches of water. Cables were expensive and very vulnerable. Accordingly in the summer of 1897 Marconi set up a link spanning the 14 kilometres of the Bristol Channel. After this Marconi put on many other demonstrations and gave lectures: many were to the press and in this way he was able to gain the maximum amount of publicity. It also stimulated the interest of other experimenters.

1899 First cross channel link - Marconi steadily increased the range of his wireless system. In the spring of 1899 a first link was set up to cross the English Channel between an existing station at South Foreland in England and a station set up at Wimereux near Bologne in France. This was the first international wireless transmission. Another demonstration was organised later in the year and it was found that the signals from Wimereux could be heard back at Marconi's factory in Chelmsford over 130 kilometres away.

1901 First transatlantic transmission - With the successes in using radio waves to cross the English channel Marconi turned his eyes towards greater distances and being able to send messages across the Atlantic. If he could succeed in this he would be able to use his system to send messages across the Atlantic more cheaply than using a cable, and also keep in contact with ships over vast distances. Marconi started this venture by setting up stations at Polhu in Cornwall England and Cape Cod in Massachussetts, USA. Storms destroyed the huge antennas at both sites, and Marconi rebuilt the antenna at Poldhu, but relocated the station from Cape Cod to a site in Newfoundland. However the letter "S" being transmitted by the station in England was just received although with great difficulty in Newfoundland on 12th December 1901.

1904 Fleming invents the diode valve - Professor Ambrose Fleming of University College London acted as a consultant to Marconi and he was very aware that detectors used in the receivers were the weak link in the receiving equipment of the day. Having seen the Edison effect - an effect Edison discovered when he was trying to extend the life of electric light bulbs, Fleming wondered whether this could be sued to detect radio signals. He gave details to his assistant who set up an experiment that worked. The diode valve had been discovered. It consisted of a heated element in an evacuated glass bulb. A second element was also placed in the bulb but not heated. It was found that an electric current only flowed in one direction with electrons leaving the heated cathode and flowing towards the second element called the anode, and not in the other direction.

1906 De Forest invents the triode - In the USA de Forest, replicated Fleming's diode and went a stage further by adding an additional element to give a device he called and Audion. Although de Forest applied for several patents in the years between 1905 and 1907, the invention of the triode is normally taken to be 1906. Initially the triode was only used as a detector. Its operation was not understood, and this prevented its full potential from being utilised.

1912 The triode used as an amplifier - It took some time before the full potential of the triode was realised. Eventually it was de Forest who succeeded in using it as an amplifier and in 1912 he built an amplifier using two devices. This was demonstrated to AT&T who understood its potential for use as a repeater in long distance telephone circuits.

1918 Armstrong invents the superhet - Although thermionic valves (tubes) enabled far greater performance to be gained in radio receivers, the performance of the devices was still very poor and receivers of the day suffered from insensitivity and poor selectivity. During the First World War a considerable amount of effort was devoted into resolving these problems. An intermediate solution was developed by a Frenchman named Lucien Levy, but in 1918, Edwin Armstrong developed a receiver where the incoming signal was converted down to a fixed intermediate frequency. Here it could be satisfactorily amplified and filtered. Unfortunately the idea did not gain much acceptance at first because the war ended, and superhet receivers were very expensive because of the numbers of valves they used. It took until the late 1920s before the number of transmitting stations rose to a level that the performance of the superhet was required and further developments meant they could be made more cheaply.

1920 Early broadcasts by the Marconi Company - The possibilities of broadcasting entertainment and news using radio soon arose. In Britain initial transmissions were made by the Marconi Company from their Chelmsford works for experimental purposes. These broadcasts started in February 1920. Although only two daily programmes were broadcast they had an enormous impact. The famous international singer Dame Nellie Melba took part in one on 20th June 1920.

1921-23 Short wave transatlantic success - Long distance communications had until now been concentrated on the long wavelengths. Accordingly radio amateurs had been only allowed to use the short wave bands which were thought to be of little commercial value. However in the USA a number of amateur stations had made contacts over considerable distances. This made people wonder whether it would be possible to make contact across the Atlantic. After several sets of tests commencing in 1921 contact was finally made between the American stations 1MO and 1XAM and the French station 8AB in November 1923. Then in October of 1924 a station at Mill Hill School in the North of London made contact with one in Dunedin New Zealand on the opposite side of the globe. Meanwhile commercial interests were also exploring the capabilities of the short wave bands, and a number of links were set up.

1922 2LO and the BBC - In May 1922 the Marconi Company was allowed to set up a broadcast station in London. Later in the year the British Broadcasting Company was formed and this took over the station that had the famous call 2LO. Initially it only had a power of 100 watts, which gave it a maximum coverage of about 40 miles. Later with a power of 1.5 kW its coverage was greatly increased.

1925 International Broadcasting - The Westinghouse Company set up a short wave broadcasting station (KDKA) in East Pittsburgh which transmitted on a wavelength of 62.7 metres. This was successfully received and rebroadcast in Britain. In Britain the amateur station 2NM run by Gerald Marcuse started broadcasts in 1927. These gained great popularity and shortly after their start the BBC commenced broadcasts from an experimental station. However it took until 1932 before the Empire service (the forerunner of the BBC World Service) was officially opened, broadcasting from Daventry in the Midlands.

1934 Frequency Modulation - Although wideband FM is widely used today it took many years for its advantages to be discovered, previously everyone had tried to improve its performance by reducing the bandwidth. It was Edwin Armstrong who made the breakthrough. However it took many years for him to convince people about its superiority. In 1934 he brought his idea to the attention of RCA, and a year later he set up a demonstration. Then in 1939 he used his own money to finance a station to prove that the system worked. Shortly after this the idea began to take off and by January 1940 150 applications for FM broadcast stations had been submitted in the USA. In 1954 the BBC launched its wideband FM service.

1945 Arthur C Clarke proposes geostationary satellites - Up until this time all international communications relied on either short wave radio transmissions or cable links. Short wave radio was unreliable and subject to high levels of interference, and international telephone cables were exceedigly expensive. In 1945 the author Arthur C Clarke wrote a historic article in Wireless World describing a system that used satellites in geostationary orbit. Signals would be transmitted up to the satellite that would rebroadcast them back to the earth. In view of their altitude above the earth the signals would be able to be received many thousands of miles away from the original transmitting station. Clarke calculated that only three satellites would be required to cover around the globe. His idea was revolutionary, and it took many years before the technology was available for it to be implemented.

1957 Sputnik 1 launched - On 4th October 1957 the USSR (Russia) launched the first satellite into orbit. The satellite was in a very low orbit which took 96 minutes to circle the earth. It only transmitted a bleep, but it was sufficient to prove that satellites could be successfully put into orbit.

1962 Telstar launched - Several other satellites followed Sputnik. Some were launched by the Soviet Union, and others by the Americans. However the launch of Telstar proved to eb a major milestone in satellite development. On 23rd July 1962 it was used to make the first live transatlantic television transmissions. Signals from the USA were seen live in many homes around Europe, making communication shistory. Another satellite named Relay was used to beam the pictures of the funeral of the Late President Kennedy to people all over the world. Since then the number of satellites has considerably increased, along with improvements in technology. Now most international communications are routed via satellites. Apart from this they provide many other useful functions including navigation, geological surveys, weather information, direct television broadcast and so forth.

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