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Tuning at the Dawn of Radio

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Tuning at the Dawn of Radio Feature Tuning at the Dawn of Radio chieving precise selective tuning was a key factor in Athe development of radio communication. To many late 19th century scientists, the discovery of the existence of a vast electromagnetic spectrum, with wavelengths extending from picometres to megametres, was strange and exciting. But in 1888, Heinrich Hertz had seen no practical utility for the radio waves that he had transmitted, received and studied in his laboratory. What use could be made of radiation with these wavelengths longer than heat and light, that could only be detected by instruments? The British scientist William Crookes (of cathode ray and radiometer fame) was an outstanding visionary who saw the future with astonishing clarity. Less than four years after Hertz published his discovery, he envisaged person-to-person wireless communication in remarkable detail, touching on many aspects of the radio art from the ability of the waves to pass through buildings and fog, to coding the messages for secrecy, and using reflecting and dielectric antennas to beam the signals in a desired direction. Marconi’s first tuned transmitter, made at Poole in 1899. The single turn secondary winding couples Remarkably, Crookes also foresaw the the circuit to the antenna. (Image credit: Museum of the History of Science, Oxford). concept of transmitter selection by receiver tuning. In the Fortnightly Review for 1 February 1892, he wrote: “I assume here in Britain using standing waves on open wire narrower bandwidth, and exhibited sharper that the progress of discovery would give transmission lines. Although an eminent resonance effects. He demonstrated these instruments capable of adjustment by turning researcher, Lodge was an amiable gentleman effects during a famous ‘syntonic Leyden jar’ a screw or altering the length of a wire, so as of relaxed character, and when he received lecture to the Royal Institution as early as to become receptive of wavelengths of any news of Hertz’s progress he famously March 1889. preconcerted length. Thus, when adjusted to continued his Austrian vacation, rather than Lodge also went further. In August 50 yards, the transmitter might emit, and the rushing back to England to publish before 1894, during a presentation to the British receiver respond to, rays varying between 45 the young German. Instead, he added a Association for the Advancement of Science and 55 yards, and be silent to all others.” [1] postscript to his own paper complimenting in Oxford, he made what some consider to By contrast, none of the 19 claims made Hertz on his work. Just as with Hertz, Lodge have been the first public demonstration in Marconi’s fundamental 1897 wireless initially had no thought of applying his of practical wireless telegraphy. According telegraphy patent made any mention of discoveries to signalling. But Hertz tragically to his own account, an assistant used a tuning. Nor did the publications of any died in 1894 at the age of only 36, whereas Morse key to send some dots and dashes of the other experimenters in England, Lodge lived until 89, and so he was able to from a spark transmitter in one building to Germany, Italy and Russia at that time. All contribute to the practical development of a coherer receiver in the lecture theatre of the inventors had practical systems of radio wireless for many years. the Clarendon laboratory 55m away, and communication, only so long as there were It was because Lodge experimented with this was later accepted by the US courts very few systems of radio communication. transmission lines rather than radiating as evidence of a demonstration of radio But that was soon to change. antennas that he observed the phenomenon communication at a date prior to Marconi’s of resonance more strikingly. Because of experiments near Bologna in 1895. the radiated energy, a spark transmitter Depending on the interpretation of ‘a Oliver Lodge connected to an antenna generated highly demonstration of wireless telegraphy’, this While Hertz was making his studies of damped waves that had a broad spectrum, priority claim remains a contentious issue. electromagnetic waves radiated in free whereas the less damped waves that it In a lecture to the Royal Society of Arts in space, Oliver Lodge was doing similar work excited in closed tuned circuits had a 1937, Ambrose Fleming stated that it was 26 September 2018 Feature unquestionable that Lodge had pre-empted when Marconi and others started claiming Today, this idea is so familiar to everyone Marconi by demonstrating electric-wave property rights in discoveries to which he had who has used a radio receiver that it may seem telegraphy at that time. But in her touching contributed. On 10 May 1897 he applied surprising that it was not at all obvious at the and imaginative biography of her father, for what was to become recognised as the time. But the patent had major implications Degna Marconi changed ‘unquestionable’ to basic radio patent on tuning. This described a for the design of future transmitters, receivers ‘questionable’ [2]. complete system of wireless telegraphy, but so and antennas, and it became increasingly had Marconi’s patent of 1896. The difference valuable as the radio spectrum became was that Lodge specifically described a system more and more crowded. In Lodge’s original Basic tuning patent in which transmitter and receiver could be description only the antenna circuits were Initially Lodge was reluctant to file patents, tuned to a particular frequency, and so could resonant, so the selectivity was limited by as he considered his scientific discoveries operate in the vicinity of other transmitters the fact that they were open tuned circuits public property. But he felt that he had to act operating on different frequencies. coupled to free space. Resonance was achieved by adjustable antenna tuning coils at the feed point, and the receiving antenna was coupled to the detector by a high General principle of the Marconi multiple tuner, omitting the fixed frequency transformer, also a very important capacitors, range and standby switches, and antenna protection. (Image innovation in electronics design. credit: Museum of the History of Science, Oxford). Surprisingly, despite Lodge’s earlier observations about their narrower bandwidth, there were no other closed tuned circuits in the transmitter or receiver configurations he described. (The secondary circuit of the receiver transformer wasn’t tuned to resonance). Equally strangely, he didn’t show a high frequency transformer for coupling the transmitter to its antenna. Since only the antennas were resonant in his system, Lodge discouraged the use of antenna designs that worked against ground, such as the quarter- wave vertical. Except in a few special cases, such as the ground of a ship’s hull in sea water, he considered that the properties of the ground connection were too uncertain for accurate tuning. Perfect interference The growing need for selective tuning became very apparent during the evaluation tests of radio equipment carried out by the US Navy in 1899 [3]. Having noted the very successful operation of Marconi’s system during the reporting of the America’s Cup Fleming’s cymometer absorption wavemeter used a 1m long coil and tubular variable capacitor with a yacht races in September and October, naval common slider. observers invited him to allow his equipment to be tried out between the cruiser New York, the battleship Massachusetts, the torpedo boat Porter, and a shore station at the Highlands of Navesink, overlooking the entrance to New York Bay. Marconi was initially very reluctant to agree to these tests, on the grounds that he had brought only short-range equipment to the US for the races, and that it didn’t incorporate the latest improvements. Nevertheless the tests were very satisfactory, except for the crucial fact that communication proved impossible if the shore station transmitted while one ship was The standard 1907 Charles Franklin multiple tuner covers 80 to 2600m in 4 ranges. The large Dr Bruce Taylor, HB9ANY terminal on the left is a protective micrometer spark gap. (Image credit: Alayne Alvis, Museums [email protected] Victoria, Melbourne). September 2018 27 Feature and manufactured radio equipment for its own use, some of which involved definite infringements. When the Marconi Company attempted to secure compensation for patent infringement from the US Government, the Supreme Court declared its tuning and valve patents invalid. Four 7s patent The master tuning patent No 7777 granted to Marconi on 26 April 1900 is one of the most famous in radio history. This ‘Four 7s’ patent was the subject of more litigation than any other, bar Lodge’s basic tuning patent itself and the thermionic valve patents of Lee de Forest. The essential difference from Lodge’s The ‘intensifier’ control on the right side of the multiple tuner rotates the coils in the ganged disclosure was that after much trial and error, variocouplers. (Image credit: Museo Nazionale della Scienza e della Tecnologia, Milan). rather than by theoretical analysis, Marconi had obtained better results with two tuned circuits at the transmitter and two at the receiving from the other. In the words of the was also somewhat secretive about how receiver. In addition to the tuned antennas US Navy historian, Linwood Howeth, “The the system actually worked. Despite this, described by Lodge, he used a closed tuned results of the interference tests were perfect. the Navy offered to purchase twenty sets exciter circuit in the transmitter and a closed That is, the interference was perfect.” of wireless equipment, but Marconi refused tuned detector circuit in the receiver, each According to the files in the Marconi to sell outright and offered to lease the coupled to its antenna by a high frequency archives, the spark gaps in the transmitters equipment instead.
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