Radio Circuit Design

In 1864, James Maxwell (1831-1879), a Scottish physicist, produced his theory of the electromagnetic field which predicted that electromagnetic waves can propagate in free space at a velocity equal to that of light [9]. Experimental confirmation of this theory had to wait until 1887 when Heinrich Hertz (1857-1894) constructed the first high-frequency oscillator. When a voltage was induced in an induction coil connected across a spark gap, a discharge would occur across the gap setting up a damped sinusoidal high-frequency oscillation. The frequency of the oscillation could be changed by varying the capacitance of the gap by connecting metal plates to it. The detector that he used consisted of a second coil connected to a much shorter spark gap. The observation of sparks across the detector gap when the induction coil was excited showed that the electromagnetic energy from the first coil was reaching the second coil through space. These experiments were in many ways similar to those carried out in 1839 by Joseph Henry (1797-1878). Several scientists made valuable contributions to the subject, such as Edouard Branly (1844-1940) who invented the ''coherer'' for wave detection, Aleksandr Popov (1859-1906) and Oliver Lodge (1851-1940) who discovered the phenomenon of resonance.

In 1896, Guglielmo Marconi (1874-1937) left Italy for England where he worked in cooperation with the British Post Office on ''wireless telegraph''. A year later, he registered his ''Wireless Telegraphy and Signal Co. Ltd'' in London, England to exploit the new technology of radio. On the 12th of December 1901, Marconi received the letter '' S'' in Morse code at St, Johns, Newfoundland on his receiver whose antenna was held up by a kite, the antenna which he had constructed for the purpose having been destroyed by heavy winds. He had confounded the many skeptics who thought that the curvature of the earth would make radio transmission impossible [10].

Up to this point, no use had been made of ''electronics'' in telecommunication: high-frequency signals for radio were generated mechanically. The first electronic device, the diode, was invented by Sir John Ambrose Fleming (1849-1945) in 1904. He was investigating the ''Edison effect'' that is, the accumulation of dark deposits on the inside wall of the glass envelope of the electric light bulb. This phenomenon was evidently undesirable because it reduced the brightness of the lamp. He was convinced that the dark patches were formed by charge particles of carbon given off by the hot carbon filament. He inserted a probe into the bulb because he had the idea that he could prevent the charged particles from accumulating by applying a voltage to the probe. He soon realized that, when the probe was held at a positive potential with respect to the filament, there was a current in the probe but when it had a negative potential no current would flow: he had invented the diode. He was granted the first patent in electronics for his effort. Fleming went on to use his diode in the detection of radio signals - a practice which has survived to this day.

The next major contribution to the development of radio was made by Lee DeForest (1873-1961). He got into legal trouble with Marconi, the owner of the Fleming diode patent, when he obtained a patent of his own on a device very similar to Fleming's. He went on to introduce a piece of platinum formed into a zig-zag around the filament and soon realised that, by applying a voltage to what he called the ''grid'', he could control the current flowing through the diode. This was, of course, the triode - a vital element in the development of amplifiers and oscillators.

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Project Management Made Easy

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