, Nikola Tesla "''Alternating Electric Current Generator''" (March 10, 1891) (1891, 15 kHz), Salomons and Pyke (1891, 9 kHz), Parsons and Ewing (1892, 14 kHz.), Siemens (5 kHz), B. G. Lamme (1902, 10 kHz), but none was able to reach the frequencies required for radio transmission, above 20 kHz.
Construction
In 1904, Reginald Fessenden
Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father. During his life he received hundre ...
contracted with General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
for an alternator that generated a frequency of 100,000 hertz for continuous wave radio. The alternator was designed by Ernst Alexanderson
Ernst Frederick Werner Alexanderson (January 25, 1878 – May 14, 1975) was a Swedish-American electrical engineer, who was a pioneer in radio and television development. He invented the Alexanderson alternator, an early radio transmitter used ...
. The Alexanderson alternator was extensively used for long-wave radio communication
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
s by shore stations, but was too large and heavy to be installed on most ships. In 1906 the first 50-kilowatt alternators were delivered. One was to Reginald Fessenden at Brant Rock, Massachusetts
Ocean Bluff-Brant Rock is a census-designated place (CDP) in Plymouth County, Massachusetts, United States, composed of the neighborhoods of Ocean Bluff, Brant Rock, Fieldston, and Rexhame in the town of Marshfield. The population of the CDP ...
, another to John Hays Hammond, Jr. in
Gloucester, Massachusetts
Gloucester () is a city in Essex County, Massachusetts, in the United States. It sits on Cape Ann and is a part of Massachusetts's North Shore. The population was 29,729 at the 2020 U.S. Census. An important center of the fishing industry and a ...
and another to the
American Marconi Company in
New Brunswick, New Jersey
New Brunswick is a city (New Jersey), city in and the county seat, seat of government of Middlesex County, New Jersey, Middlesex County, in the U.S. state of New Jersey.[modulated
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...]
to carry the human voice. Until the invention of
vacuum-tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as a ...
(valve)
oscillator
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
s in 1913 such as the
Armstrong oscillator, the Alexanderson alternator was an important high-power
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
, and allowed
amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
radio transmission of the human voice. The last remaining operable Alexanderson alternator is at the
VLF transmitter Grimeton
Grimeton Radio Station () in southern Sweden, close to Varberg in Halland, is an early longwave transatlantic wireless telegraphy station built in 1922–1924, that has been preserved as a historical site. From the 1920s through the 1940s it wa ...
in Sweden and was in regular service until 1996. It continues to be operated for a few minutes on
Alexanderson Day
Alexanderson Day, named after the Swedish radio engineer Ernst Fredrik Werner Alexanderson and held either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2 July, is the day of the open house at the Swedish go ...
, which is either the last Sunday in June or first Sunday in July every year.
World War I and the formation of RCA
The outbreak of
World War I
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
forced European nations to temporarily abandon development of international radio communications networks, while the United States increased efforts to develop transoceanic radio. By the end of the war the Alexanderson alternator was operating to reliably provide transoceanic radio service.
British Marconi offered
General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
$5,000 in business in exchange for exclusive rights to use the alternator, but just as the deal was about to go through, the American president
Woodrow Wilson
Thomas Woodrow Wilson (December 28, 1856February 3, 1924) was an American politician and academic who served as the 28th president of the United States from 1913 to 1921. A member of the Democratic Party, Wilson served as the president of ...
requested that GE decline the offer, which would have given the British (who were the leader in
submarine communications cables
A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean and sea. The first submarine communications cables laid beginning in the 1850s carried tel ...
) dominance over worldwide radio communications. GE complied with the request and joined with
American Telephone and Telegraph
AT&T Corporation, originally the American Telephone and Telegraph Company, is the subsidiary of AT&T Inc. that provides voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agen ...
(AT&T), the
United Fruit Company
The United Fruit Company (now Chiquita) was an American multinational corporation that traded in tropical fruit (primarily bananas) grown on Latin American plantations and sold in the United States and Europe. The company was formed in 1899 fro ...
, the
Western Electric Company
The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...
and the
Westinghouse Electric and Manufacturing Company
The Westinghouse Electric Corporation was an American manufacturing company founded in 1886 by George Westinghouse. It was originally named "Westinghouse Electric & Manufacturing Company" and was renamed "Westinghouse Electric Corporation" in ...
to form the
Radio Corporation of America
The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
(RCA), giving American companies control of American radio for the first time.
Stations
Thorn L. Mayes identified the production of ten pairs of 200 KW Alexanderson alternators, totaling 20 transmitters, in the period up to 1924:
U.S. military use during and after World War II
Beginning in 1941, seven of the twenty original 200 KW alternators were put into service by the U.S. Navy and Air Force:
During World War II the U.S. Navy recognized the need for reliable distant longwave (VLF) transmissions to the Pacific fleet. A new facility was constructed at Haiku in Hawaii, where two 200 KW Alexanderson alternators transferred from the mainland were installed. The Navy also operated an existing transmitter at Bolinas, California, again for Pacific ocean communication. Both Haiku alternators were sold for salvage in 1969, possibly to Kreger Company of California.
In the late 1940s the Air Force assumed control of the Haiku and Marion, Massachusetts facilities. The Air Force found that longwave transmissions were more reliable than shortwave when sending weather information to Arctic researchers as well as bases in Greenland, Labrador, and Iceland. The two Marion transmitters were used until 1957. One was scrapped in 1961 and the other was reportedly handed over to the U.S. Bureau of Standards and stored in a Smithsonian Institution warehouse.
[Mayes (1989), quoting July 15, 1976 correspondence from "the Commanding Officer of the USCG Station Hawaii", page 180.]
Design
The Alexanderson alternator works similarly to an AC electric generator, but generates higher-frequency current, in the very low frequency (VLF) radio frequency range. The rotor has no conductive windings or electrical connections; it consists of a solid disc of high tensile strength magnetic steel, with narrow slots cut in its circumference to create a series of narrow "teeth" that function as magnetic poles. The space between the teeth is filled with nonmagnetic material, to give the rotor a smooth surface to decrease aerodynamic drag. The rotor is turned at a high speed by an electric motor through a speed–increaser gearbox.
The machine operates by variable
reluctance
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the ge ...
(similar to an electric
guitar pickup
A pickup is a transducer that captures or senses mechanical vibrations produced by musical instruments, particularly stringed instruments such as the electric guitar, and converts these to an electrical signal that is amplified using an instru ...
), changing the
magnetic flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ( ...
linking two coils. The periphery of the rotor is embraced by a circular iron stator with a C-shaped cross-section, divided into narrow poles, the same number as the rotor has, carrying two sets of coils. One set of coils is energized with
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
and produces a magnetic field in the air gap in the stator, which passes axially (sideways) through the rotor.
As the rotor turns, alternately either an iron section of the disk is in the gap between each pair of stator poles, allowing a high magnetic flux to cross the gap, or else a non-magnetic slot is in the stator gap, allowing less magnetic flux to pass. Thus the magnetic flux through the stator varies sinusoidally at a rapid rate. These changes in flux induce a
radio-frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
voltage in a second set of coils on the stator.
The RF collector coils are all interconnected by an output
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
, whose secondary winding is connected to the antenna circuit.
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
or
telegraph keying of the radio frequency energy was done by a
magnetic amplifier
The magnetic amplifier (colloquially known as a "mag amp") is an electromagnetic device for amplifying electrical signals. The magnetic amplifier was invented early in the 20th century, and was used as an alternative to vacuum tube amplifiers wh ...
, which was also used for
amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
and voice transmissions.
The
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of the current generated by an Alexanderson alternator in
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...
is the product of the number of rotor poles and the revolutions per second. Higher radio frequencies thus require more poles, a higher rotational speed, or both. Alexanderson alternators were used to produce radio waves in the
very low frequency
Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave a ...
(VLF) range, for transcontinental wireless communication. A typical alternator with an output frequency of 100 kHz had 300 poles and rotated at 20,000 revolutions per minute (RPM) (333 revolutions per second). To produce high power, the clearance between the rotor and stator had to be kept to only 1 mm. The manufacture of precision machines rotating at such high speeds presented many new problems, and Alexanderson transmitters were bulky and very expensive.
Frequency control
The output frequency of the transmitter is proportional to the speed of the rotor. To keep the frequency constant, the speed of the electric motor turning it was controlled with a feedback loop. In one method, a sample of the output signal is applied to a high-Q
tuned circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
, whose
resonant frequency
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
is slightly above the output frequency. The generator's frequency falls on the "skirt" of the LC circuit's impedance curve, where the impedance increases rapidly with frequency. The output of the LC circuit is rectified, and the resulting voltage is compared with a constant reference voltage to produce a feedback signal to control the motor speed. If the output frequency gets too high, the impedance presented by the LC circuit increases, and the amplitude of the RF signal getting through the LC circuit drops. The feedback signal to the motor drops, and the motor slows down. Thus the alternator output frequency is "locked" to the tuned circuit resonant frequency.
The sets were built to operate at wavelengths of 10,500 to 24,000 meters (28.57 to 12.5 KHz). This was accomplished by three design variables. The alternators were built with 1220, 976 or 772 poles. Three gearboxes were available with ratios of 2.675, 2.973 and 3.324 and the 900 RPM driving motor was operated at slips of 4% to 20%, giving speeds of 864 to 720 RPM. Transmitters installed in Europe, operating on 50-cycle power, had a wavelength range of 12,500 to 28,800 meters due to the lower speed of the driving motor.
Performance advantages
A large Alexanderson alternator might produce 500 kW of output radio-frequency energy and would be water- or oil-cooled. One such machine had 600 pole pairs in the stator winding, and the rotor was driven at 2170 RPM, for an output frequency near 21.7 kHz. To obtain higher frequencies, higher rotor speeds were required, up to 20,000 RPM.
Along with the
arc converter
The arc converter, sometimes called the arc transmitter, or Poulsen arc after Danish engineer Valdemar Poulsen who invented it in 1903, was a variety of spark transmitter used in early wireless telegraphy. The arc converter used an electric arc ...
invented in 1903, the Alexanderson alternator was one of the first radio transmitters that generated
continuous wave
A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particle ...
s. In contrast, the earlier
spark-gap transmitter
A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the main type use ...
s generated a string of
damped wave
Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples inc ...
s. These were electrically "noisy"; the energy of the transmitter was spread over a wide frequency range, so they interfered with other transmissions and operated inefficiently. With a continuous-wave transmitter, all of the energy was concentrated within a narrow
frequency band
A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spectrum.
The frequency range of a system is the range over which i ...
, so for a given output power they could communicate over longer distances. In addition, continuous waves could be
modulated
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
with an
audio signal
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...
to carry sound. The Alexanderson alternator was one of the first transmitters to be used for
AM transmission.
The Alexanderson alternator produced "purer" continuous waves than the arc converter, whose nonsinusoidal output generated significant
harmonics
A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
, so the alternator was preferred for long-distance telegraphy.
Disadvantages
Because of the extremely high rotational speed compared to a conventional alternator, the Alexanderson alternator required continuous maintenance by skilled personnel. Efficient lubrication and oil or water cooling was essential for reliability which was difficult to achieve with the lubricants available at the time. In fact, early editions of the
Royal Navy
The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against F ...
's "Admiralty Handbook of Wireless Telegraphy" cover this in considerable detail, mostly as an explanation as to why the navy did not use that particular technology. However, the US Navy did.
Other major problems were that changing the operating frequency was a lengthy and complicated process, and unlike a spark transmitter, the carrier signal could not be switched on and off at will. The latter problem greatly complicated "listening through" (that is, stopping the transmission to listen for any answer). There was also the risk that it would allow enemy vessels to detect the presence of the ship.
Because of the limits of the number of poles and rotational speed of a machine, the Alexanderson alternator is capable of generating transmission frequencies up to around 600kHz in the lower
Medium wave
Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the daytime ...
band, with
shortwave
Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
and higher frequencies being physically impossible.
See also
*
Alexanderson Day
Alexanderson Day, named after the Swedish radio engineer Ernst Fredrik Werner Alexanderson and held either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2 July, is the day of the open house at the Swedish go ...
*
Tonewheel
A tonewheel or tone wheel is a simple electromechanical apparatus used for generating electric musical notes in electromechanical organ instruments such as the Hammond Organ and in telephony to generate audible signals such as Ringing tone. It ...
*
Resolver (electrical)
A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analog device, and has digital counterparts such as the digital resolver, rotary (or pulse) encoder.
Description
The most commo ...
Notes
References
Further reading
Antique Wireless Association - column edited by Frank Lotito * David E. Fisher and Marshall J. Fisher, ''Tube, the Invention of Television'' Counterpoint, Washington D.C. USA, (1996)
* Hammond, John Winthrop
''Men and Volts, the Story of General Electric'' Philadelphia & New York: J. B. Lippincott (1941), pp. 349–352, 372.
* Notes from the Navy Institute proceedings 1952 from M.G. Abernathy files.
* Letter to M.G. Abernathy from G. Warren Clark Captain USNR (Ret)
* Letter to Mr. Mayes from Lt. Francis J. Kishima Commanding Officer USCG Omega Station Hawaii
Milestones:Yosami Radio Transmitting Station, 1929* E. F. W. Alexanderson, ''High Frequency Alternator''
* N. Tesla,
{{refend
External links
"The World's Greatest Wireless Station"(Radio Central), ''The Book of Radio'' by Charles Taussig, pages 312-327.
"American Marconi Station, Marion, MA"by Henry Brown
Alexanderson Alternators at Haiku Valley, Oahuby David Jessup
"Kahuku Marconi Wireless Station, O'ahu, Hawaii"by Jonathan H, June 20, 2007
"Marion Mass. Alternators"(picture gallery) by David Jessup, 2012
(gallery)
"The Radio Station SAQ Grimeton"Alexanderson alternator preserved at
UNESCO
The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture. It ...
World Heritage Site
A World Heritage Site is a landmark or area with legal protection by an international convention administered by the United Nations Educational, Scientific and Cultural Organization (UNESCO). World Heritage Sites are designated by UNESCO for h ...
in Grimeton, Sweden
History of radio
Radio electronics
Alternators
Swedish inventions