History of Radio and Electronics
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- Navigate this page:
- 21st Century
- History of the Coherer
- Early coherer history
- Edouard Branly
- Input from Oliver Lodge
- Marconi the coherer
- End of the coherer
- Crystal / Cats Whisker Radio Detector
- Cats whisker crystal detector
Transistors introducedIn the late 1940s the transistor was discovered. Initially the devices were not widely used because of their expense, and the fact that valves were being made smaller, and performed better. However by the early 1960s portable transistor broadcast radios were hitting the market place. These radios were ideal for broadcast reception on the long and medium wave bands. They were much smaller than their valve equivalents, they were portable and could be powered from batteries. Although some valve portable receivers were available, batteries for these were expensive and did not last for long. The power requirements for transistor radios were very much less, resulting in batteries lasting for much longer and being considerably cheaper. Although transistors gained a wide level of acceptance for broadcast sets, their introduction was a little slower in the professional market. The performance of the early transistors was much lower than that of valves, and this meant that high performance sets were not so easy to design. Nevertheless as transistor performance improved and field effect transistors were introduced, semiconductor technology soon started to overtake that of valves. This was particularly true of the size and weight characteristics. Further developments in semiconductor technology lead to the introduction of the integrated circuit. This enabled radio receiver technology to move forwards even further. The fact that integrated circuits enabled high performance circuits to be built for less cost, and significant amounts of space could be saved both gave advantages. As a result of these developments new techniques could be introduced. One of these was the frequency synthesizer that was used to generate the local oscillator signal for the receiver. By using a synthesizer it was possible to generate a very accurate and stable local oscillator signal. Also the ability of synthesizers to be controlled by microprocessors meant that many new facilities could be introduced apart fromt he significant performance improvements offered by synthesizers. 21st CenturyReceiver technology is till moving forward. Digital signal processing where many of the functions performed by an analogue intermediate frequency stage can be performed digitally by converting the signal to a digital stream that is manipulated mathematically is now widespread. Indeed the new digital audio broadcasting standard being introduced can only be used when the receiver can manipulate the signal digitally. Whilst today's radios are miracles of modern technology, filled with low power high performance integrated circuits crammed into the smallest spaces, the basic principle of the radio is usually the superhet, the same idea which was developed by Edwin Armstrong back in 1918. History of the Coherer- history of the coherer, detailing how it was discovered and where it was used including some landmark radio communications transmissionsThe coherer was a very early radio wave detector. It was used for detecting the signals for many early wire-less transmissions. The coherer was used with the spark transmitters of the day: these transmitters used a high voltage, often created by an induction coil arrangement, and this was then used to create a spark, and the resultant signal was radiated by an antenna connected to the system. Although not very sensitive, the coherer was often the best way of detecting these wireless signals and being able to read the radio communications transmission being sent. As such the coherer history forms an essential element within the whole development of radio communications technology and was a key enabler within many major firsts. Early coherer historyThere are a number of early observations that have been recorded that set the foundations for the developemtn of the coherer. In 1850 Pierre Guitard found that when dusty air was electrified, the particles of dust would tend to link together in strings. In another observation, but this time with an application two British inventors, C and S A Varley took out a patent in 1866 for a lightning arrestor. In this patent application, the arrestor had two copper points that were almost touching as in a normal arrestor. However the box that contained the arrestor had carbon power. The carbon would then provide a conduction path in the event of discharge that was not sufficient to jump across the spark gap, thereby removing any residual charge. This was an important discovery in the overall history of the coherer as it set out the fundamental principle of the coherer. The next major step in the development of the coherer was the discovery in 1878 by a researcher named Hughes that a glass tube containing loose zinc and silver filings was sensitive to electric sparks at a distance. The filings cohered in the presence of a nearby spark. In a further development another researcher named Onesti is believed to have experimented with a predecessor to the coherer in 1884. However the person to whom the credit for the invention of the coherer is generally given is a Frenchman called Edouard Branly.
He observed a change in the resistance of thin metallic films that were exposed to electric sparks. First he used platinum deposited on glass and he later discovered that the variations in the resistance of metals in a finely divided state were even more marked. He then went on to show that these changes were due to the presence of an electrical spark, and as a result of the newly discovered Hertzian waves (i.e. radio waves). He then conducted some investigations into the use of different types of filings and powered conductors to discover which produced the optimum effect.
However, it appears that he saw the findings of Branly at some stage. He improved the design taking the ideas a stage further forward. Lodge also called this improved detector a "coherer" and thus it was he who named the device. Lodge presented some of his findings to the Institution of Electrical Engineers, IEE, in London in 1890. Then in 1894 he devised and exhibited an adjustable point device. This had an end with a fine spiral spring that gently touched an aluminium plate.
He had been introduced to Oliver Lodge and most likely as a result he started to use the coherer for his radio systems. Marconi was a great experimenter but not a theorist. Marconi tried many different methods, perpetually trying new ideas to see if they made any improvements. He was then very careful not to allow his competition to gain the secrets of his developments. There were stories that his engineers took home the mixtures of filings over the weekend in case there was a break in at the offices by any of the competition. Marconi's favored coherer format was to have a sealed glass tube. This had two silver plugs which were attached to platinum wires. The inner ends of the plugs were cut at an angle, and brought within two millimetres of each other. The space between was generally filled with nickel and silver filings in the correct proportions. Occasionally other mixtures were used according to the experiments that had been undertaken. 
Coherer as used by Marconi Marconi used his coherers in many of his commercial systems. These were generally installed on ships where they enabled messages to be sent back to land based stations. End of the cohererThe coherer was never a very satisfactory form of radio detector. In the early days of radio, the detector was the main factor limiting the performance and achievable range of a system. In addition to this the coherer was expensive to produce, especially as significant amounts of silver were often used in their manufacture. As a result, much effort was placed into finding a better replacement for the coherer. In 1904 Ambrose Fleming invented the diode valve (vacuum tube). These devices were expensive to make and run. As a result thermionic technology was not widely used until later. Around 1907 the first viable alternative was found in the form of the cat's whisker diode rectifier. Although a little temperamental, their performance was far superior and the cohere soon fell out of use.
- details of the cat's whisker radio detector or crystal detector - how this detector worked, the different types and the history of its development.The cat's whisker detector is one of the most iconic components used in early radio sets. Also called the crystal detector it was an iconic component used in many early radio equipments. Today, many cat's whisker or crystal detectors sets can be seen on the antique and collectors market. In their day, the radios using the cats' whisker detector or the crystal detector performed well and for many years they were the main type of detector used in sets used for the reception of broadcast transmissions. The development of the crystal detector or cat's whisker itself resulted from the observations made by people researching into radio technology, although they had little understanding of the principles because they are a form of semiconductor diode, and the basic theory behind this technology was yet to be discovered. Cat's whisker crystal detectorThe crystal detector, or as it later became known, the cat's whisker detector provided a much superior form of detection and allowed a direct audible indication of the incoming signal rather than coherers that cohered to give an indication that could then be fed to headphones. However one of the first reported uses of a crystal detector was by an Indian professor of Physics at Presidency College in Calcutta named Jagadis Chandra Bose. He demonstrated the use of a diode using galena (lead sulphide) crystals and a metal point contact. He filed a U.S patent for a point-contact semiconductor rectifier for detecting radio signals in 1901. Then in 1906 a number of other patents were filed. One of the first fpr the year was that from Ferdinand Braun in Germany who patented his detector on 18 February 1906. This crystal detector was based on psilomelan - a hydrated oxide of manganese. 
Typical crystal detector / cat's whisker detector Then on 21 February 1906, L W Austen filed a patent for a tellurium-silicon detector. Then a month later in March 1906, General H H C Dunwoody in the USA patented a carborundum detector. This was followed by Greenleaf W Pickard filing a patent in August of that year for a silicon metal rectifier. This was the culmination of around four years work for Pickard who said he had been working on crystal detectors since 1902. Download 0.59 Mb. Share with your friends:
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