Thearc converter, sometimes called thearc transmitter, orPoulsen arc after Danish engineerValdemar Poulsen who invented it in 1903,[1][2] was a variety ofspark transmitter used in earlywireless telegraphy. The arc converter used anelectric arc to convertdirect current electricity intoradio frequencyalternating current. It was used as aradio transmitter from 1903 until the 1920s when it was replaced byvacuum tube transmitters. One of the first transmitters that could generatecontinuous sinusoidal waves, it was one of the first technologies used to transmit sound (amplitude modulation) by radio. It is on thelist of IEEE Milestones as a historic achievement inelectrical engineering.[3]
Elihu Thomson discovered that a carbon arc shunted with a series tuned circuit would "sing". This "singing arc" was probably limited to audio frequencies.[4] Bureau of Standards credits William Duddell with the shunt resonant circuit around 1900.[5]
The English engineerWilliam Duddell discovered how to make aresonant circuit using acarbon arc lamp. Duddell's "musical arc" operated ataudio frequencies, and Duddell himself concluded that it was impossible to make the arc oscillate atradio frequencies.
Valdemar Poulsen succeeded in raising the efficiency andfrequency 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 arctechnology was transferred toGermany andGreat Britain in 1906 by Poulsen, his collaboratorPeder Oluf Pedersen and their financial backers. In 1909 the American patents as well as a few arc converters were bought byCyril Frank Elwell. The subsequent development in Europe and theUnited States was rather different, since inEurope there were severe difficulties for many years implementing the Poulsen technology, whereas in the United States an extended commercialradiotelegraph system was soon established with theFederal Telegraph Company. Later theUS 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 This made it the most importantmobile radio system for about a decade until it was superseded byvacuum tube systems.
In 1922, theBureau 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."[6]
This new, more-refined method for generating continuous-wave radio signals was initially developed by Danish inventorValdemar Poulsen. Thespark-gap transmitters in use at that time produceddamped wave which wasted a large portion of their radiated power transmitting strong harmonics on multiple frequencies that filled the RF spectrum with interference.Poulsen's arc converter produced undamped orcontinuous waves (CW) on a single frequency.
There are three types for an arc oscillator:[7]
Continuous or ‘undamped’ waves (CW) were an important feature, since the use ofdamped waves fromspark-gap transmitters resulted in lower transmitter efficiency and communications effectiveness, while polluting the RF spectrum with interference.
The Poulsen arc converter had atuned circuit connected across the arc. The arc converter consisted of a chamber in which the arc burned inhydrogen gas between acarboncathode and a water-cooledcopperanode. Above and below this chamber there were two seriesfield 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 amagnetic field.
It was most successful when operated in the frequency range of a fewkilohertz to a few tens of kilohertz. Theantenna tuning had to be selective enough to suppress the arc converter'sharmonics.
Since the arc took some time to strike and operate in a stable fashion, normalon-off keying could not be used. Instead, a form offrequency-shift keying was employed.[8] In thiscompensation-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 thecompensation-wave. In arc transmitters up to 70 kW, the key typically shorted out a few turns in the antenna coil.[9] 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.[10] 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'sreceiver had adequateselectivity, 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[11] prohibited the compensation wave method because it caused too much interference.[4]
The need for the emission of signals at two different frequencies was eliminated by the development ofuniwave methods.[12] In one uniwave method, called theignition method, keying would start and stop the arc. The arc chamber would have astriker 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 theabsorption method, and it involves two tuned circuits and asingle-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 tuneddummy antenna called theback shunt. The back shunt was a second tuned circuit consisting of an inductor, a capacitor, and load resistor in series.[13][14] 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.[4]
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.[15]
{{citation}}:ISBN / Date incompatibility (help). Revised to April 24, 1921.http://www.forgottenbooks.orgA really satisfactory theory of the operation of the Poulsen arc does not exist at present, a satisfactory theory being one which will enable the calculation of the results, the necessary data being given.