The arc converter, sometimes called the arc transmitter or Poulsen arc after its inventor Danish engineer Valdemar Poulsen, is a device that used an electric arc to convert direct current electricity into radio frequency alternating current. It was used in radio transmitters for a short period around 1920 before it was replaced by vacuum tubes, and was one of the first technologies that was used to transmit sound (amplitude modulation) by radio. It is now on the list of IEEE Milestones as a historic achievement in electrical engineering.
Elihu Thomson discovered that a carbon arc shunted with a series tuned circuit would "sing". This "singing arc" was probably limited to audio frequencies. Bureau of Standards credits William Duddell with the shunt resonant circuit around 1900.
The English engineer William Duddell discovered how to make a resonant circuit using a carbon arc lamp. Duddell's "musical arc" operated at audio frequencies, and Duddell himself concluded that it was impossible to make the arc oscillate at radio frequencies.
Valdemar Poulsen, who had demonstrated the 'Telegraphone' (the world's first magnetic recording device) at the Paris Exhibition of 1900, turned his inventive genius to the problem and succeeded in raising the efficiency and frequency to the desired level; Poulsen's arc could generate frequencies of up to 200 kilohertz and was patented in 1903.
After a few years of development the arc technology was transferred to Germany and Great Britain in 1906 by Poulsen, his collaborator Peder Oluf Pedersen and their financial backers. In 1909 the American patents as well as a few arc converters were bought by Cyril F. Elwell. The subsequent development in Europe and the United States was rather different, since in Europe there were severe difficulties for many years implementing the Poulsen technology, whereas in the United States an extended commercial radiotelegraph system was soon established with the Federal Telegraph Company. Later the US Navy also adopted the Poulsen system. Only the arc converter with passive frequency conversion was suitable for portable and maritime use. This made it the most important mobile radio system for about a decade until it was superseded by vacuum tube systems.
In 1922, the Bureau of Standards stated, "the arc is the most widely used transmitting apparatus for high-power, long-distance work. It is estimated that the arc is now responsible for 80 per cent of all the energy actually radiated into space for radio purposes during a given time, leaving amateur stations out of consideration."
Unlike the spark-gap transmitter converter, the arc converter produces undamped or continuous waves (CW). This was an important feature as the use of damped waves resulted in lower transmitter efficiency and communications effectiveness, while covering the RF spectrum with interference. This more refined method for generating continuous-wave radio signals was initially developed by Danish inventor Valdemar Poulsen. The Poulsen arc converter can be likened to a continuous-duty-rated electric arc welder with a tuned circuit connected across the arc. The negative resistance characteristics of an electric arc permits the creation of a relaxation oscillator that converts direct current to radio frequency energy. The arc converter consisted of a water-cooled bronze chamber in which the arc burned in hydrogen gas between a carbon cathode and a water-cooled copper anode. Above and below this chamber there were two series field coils surrounding and energizing the two poles of the magnetic circuit. These poles projected into the chamber, one on each side of the arc to provide a magnetic field. This field helps to stabilize the arc and improve overall conversion efficiency. In today's world one can still find oscillators based on negative resistance devices; the tunnel diode is one of them.
It was most successful when made to operate in the frequency range of a few kilohertz to a few tens of kilohertz. The passive frequency multiplier was relied upon to bring the output frequency up to practical transmission frequencies. The frequency multiplier and antenna tuning had to be selective enough to suppress the high harmonic output of the arc converter.
Since the arc took some time to strike and operate in a stable fashion, normal on-off keying could not be used. Instead, a form of frequency shift keying was employed. In this compensation-wave method, the arc operated continuously, and the key altered the frequency of the arc by one to five percent. The signal at the unwanted frequency was called the compensation-wave. In arc transmitters up to 70 kW, the key typically shorted out a few turns in the antenna coil. For larger arcs, the arc output would be transformer coupled to the antenna inductor, and the key would short out a few bottom turns of the grounded secondary. Therefore, the "mark" (key closed) was sent at one frequency, and the "space" (key open) at another frequency. If these frequencies were far enough apart, and the receiving station's receiver had adequate selectivity, the receiving station would hear standard CW when tuned to the "mark" frequency.
The compensation wave method used a lot of spectrum bandwidth. It not only transmitted on the two intended frequencies, but also the harmonics of those frequencies. Arc converters are rich in harmonics. Sometime around 1921, the Preliminary International Communications Conference prohibited the compensation wave method because it caused too much interference.
The needed for the emission of signals at two differing frequencies was eliminated by the development of uniwave methods. In one uniwave method, called the ignition method, keying would start and stop the arc. The arc chamber would have a striker rod that shorted out the two electrodes through a resistor and extinguished the arc. The key would energize an electromagnet that would move the striker and reignite the arc. For this method to work, the arc chamber had to be hot. The method was feasible for arc converters up to about 5 kW.
The second uniwave method is the absorption method, and it involves two tuned circuits and a single-pole, double-throw, make-before-break key. When the key is down, the arc is connected to the tuned antenna coil and antenna. When the key is up, the arc is connected to a tuned dummy antenna called the back shunt. The back shunt was a second tuned circuit consisting of an inductor, a capacitor, and load resistor in series. This second circuit is tuned to roughly the same frequency as the transmitted frequency; it keeps the arc running, and it absorbs the transmitter power. The absorption method is apparently due to W. A. Eaton.
The design of switching circuit for the absorption method is significant. It is switching a high voltage arc, so the switch's contacts must have some form of arc suppression. Eaton had the telegraph key drive electromagnets that operated a relay. That relay used four sets of switch contacts in series for each of the two paths (one to the antenna and one to the back shunt). Each relay contact was bridged by a resistor. Consequently, the switch was never completely open, but there was a lot of attenuation.
 See also
- "Milestones:Poulsen-Arc Radio Transmitter, 1902". IEEE Global History Network. IEEE. Retrieved 29 July 2011.
- Little 1921, p. 125
- Bureau of Standards 1922, p. 404
- Bureau of Standards 1922, p. 400
- Bureau of Standards 1922, pp. 415–416
- Bureau of Standards 1922, figure 228. The series resonant tuned circuit would be the antenna coil in series with the antenna.
- Bureau of Standards 1922, figure 229
- Possibly the Preliminary International Conference on Electrical Communications, 1920; see http://www.archives.gov/research/guide-fed-records/groups/043.html at 43.2.11
- Little 1921, p. 125
- Bureau of Standards 1922, pp. 416–419
- Bureau of Standards 1922, figure 229-A
- Eaton 1921
- Little 1921, p. 125
- Eaton 1921, p. 115
- Bureau of Standards (1922), The Principles Underlying Radio Communication (Second ed.), U.S. Army Signal Corps, Radio Communications Pamphlet No. 40. Revised to April 24, 1921. http://www.forgottenbooks.org
- Eaton, W. A. (April 1921), "Description of a Uni-Wave Signaling System for Arc Transmitters", Electric Journal 18: 114–115
- Howeth, Linwood S. (1963), History of Communications-Electronics in the United States Navy, U.S. Govt. Printing Office
- Little, D. G. (April 1921), "Continuous Wave Radio Communication", Electric Journal 18: 124–129. Elihu Thomson made singing arc before Duddell, p. 125.
- Morecroft, J. H.; Curry (1921), Principles of Radio Communication, New York: John Wiley & Sons Inc
- http://oz6gh.byethost33.com/poulsenarc.htm, Modulation of the Poulsen arc, from the book Radio Telephony, 1918 by Alfred N. Goldsmith.
- http://www.stenomuseet.dk/person/hb.ukref.htm, English summary of the Danish Ph.D. dissertation, The Arc Transmitter - a Comparative Study of the Invention, Development and Innovation of the Poulsen System in Denmark, England and the United States, by Hans Buhl, 1995
- Morse, A. H. (1925), Radio: Beam and Broadcast, London: Ernest Benn Limited. History of radio in 1925. Page 25: "Professor Elihu Thomson, of America, applied for a patent on an arc method of producing high-frequency currents. His invention incorporated a magnetic blowout and other essential features of the arc of to-day, but the electrodes were of metal and not enclosed in a gas chamber." Cites to US Patent 500630. Pages 30-31 (1900): "William Du Bois Duddell, of London, applied for a patent on a static method of generating alternating currents from a direct-current supply, which method followed very closely upon the lines of that of Elihu Thomson of 1892. Duddell suggested electrodes of carbon, but he proposed no magnetic blow-out. He stated that his invention could be used for producing oscillations of high frequency and constant amplitude which could "be used with advantage in wireless telegraphy," especially where it was "required to tune the transmitter to syntony." Duddell's invention (Br. Pat. 21,629/00) became the basis for the Poulsen Arc, and also of an interesting transmitter evolved by Von Lepel." Page 31 (1903): "Valdemar Poulsen, of Copenhagen, successfully applied for a patent upon a generator, as disclosed by Duddell in 1900, plus magnetic blow-out proposed by Thomson in 1892, and a hydrogenous vapour in which to immerse the arc. (Br. Pate 15,599/03; U.S. Pat 789,449.)" Also Ch. IV, pp 75-77, "The Poulsen Arc". Refinements by C. F. Elwell.
- Elwell, C. F. (1923), The Poulsen Arc Generator, London: Ernest Benn Limited
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