Editing Wireless telegraphy (section)

==Methods==
The primitive [[spark-gap transmitter]]s used until 1920 transmitted by a modulation method called [[damped wave]].  As long as the telegraph key was pressed, the transmitter would produce a string of transient pulses of radio waves which repeated at an audio rate, usually between 50 and several thousand [[hertz]].<ref>{{Cite web |title=Spark Transmitter Basics |url=http://home.freeuk.net/dunckx/wireless/sparktx/sparktx.html |access-date=2024-05-21 |website=home.freeuk.net}}</ref> In a receiver's earphone, this sounded like a musical tone, rasp or buzz.  Thus the Morse code "dots" and "dashes" sounded like beeps.  Damped wave had a large frequency [[bandwidth (signal processing)|bandwidth]],  meaning that the radio signal was not a single frequency but occupied a wide band of frequencies. Damped wave transmitters had a limited range and interfered with the transmissions of other transmitters on adjacent frequencies.<ref>{{Cite book |last1=Siwiak |first1=Kazimierz |url=https://books.google.com/books?id=SW1hJpjZbDwC&pg=PA15 |title=Ultra-wideband Radio Technology |last2=McKeown |first2=Debra |date=2004-06-07 |publisher=Wiley |isbn=978-0-470-85931-5 |pages=1–20 |language=en}}</ref>

After 1905 new types of radiotelegraph transmitters were invented which transmitted code using a new modulation method: [[continuous wave]] (CW)<ref name=":1">{{Cite web |title=continuous wave |url=https://encyclopedia2.thefreedictionary.com/continuous+wave |access-date=2024-05-21 |website=TheFreeDictionary.com |language=en}}</ref> (designated by the [[International Telecommunication Union]] as emission type A1A).<ref name=":2">{{Cite web |last=ID |first=FCC |title=Emissions Designator A1A |url=https://fccid.io/Emissions-Designator/A1A |access-date=2024-05-21 |website=FCCID.io |language=en}}</ref> As long as the telegraph key was pressed, the transmitter produced a continuous [[sine wave|sinusoidal wave]] of constant amplitude.<ref name=":1" />  Since all the radio wave's energy was concentrated at a single frequency, CW transmitters could transmit further with a given power, and also caused virtually no interference to transmissions on adjacent frequencies. The first transmitters able to produce continuous wave were the [[arc converter]] (Poulsen arc) transmitter, invented by Danish engineer [[Valdemar Poulsen]] in 1903,<ref>{{Cite book |last=Poulsen |first=Valdemar |chapter-url=https://books.google.com/books?id=JHgSAAAAYAAJ&pg=PA963 |title=Transactions of the International Electrical Congress, St. Louis, 1904 |publisher=J.B. Lyon Company |year=1905 |volume=2 |pages=963–971 |language=en |chapter=System for producing continuous electric oscillations}}</ref> and the [[Alexanderson alternator]], invented 1906–1912  by [[Reginald Fessenden]] and [[Ernst Alexanderson]].<ref>{{Cite web |date=2015-12-31 |title=Milestones:Alexanderson Radio Alternator, 1904 |url=https://ethw.org/Milestones:Alexanderson_Radio_Alternator,_1904 |access-date=2024-05-23 |website=IEEE Global History Network |language=en}}</ref> These slowly replaced the spark transmitters in high power radiotelegraphy stations.

However, the radio receivers used for damped wave could not receive continuous wave.  Because the CW signal produced while the key was pressed was just an unmodulated [[carrier wave]], it made no sound in a receiver's earphones.<ref>{{cite web|url=http://www.utexas.edu/research/cemd/nim/Agif/CarrWave.html|title=Carrier wave with no modulation transports no information.|publisher=[[University of Texas]]|archive-url=https://web.archive.org/web/20080414012934/http://www.utexas.edu/research/cemd/nim/Agif/CarrWave.html|archive-date=2008-04-14|url-status=dead}}</ref> To receive a CW signal, some way had to be found to make the Morse code carrier wave pulses audible in a receiver.

This problem was solved by Reginald Fessenden in 1901. In his "heterodyne" receiver, the incoming radiotelegraph signal is mixed in the receiver's [[detector (radio)|detector]] crystal or vacuum tube with a constant sine wave generated by an [[electronic oscillator]] in the receiver called a [[beat frequency oscillator]] (BFO).  The frequency of the oscillator <math>f_\text{BFO}</math> is offset from the radio transmitter's frequency <math>f_\text{IN}</math>.  In the detector the two frequencies subtract, and a [[beat frequency]] ([[heterodyne]]) at the difference between the two frequencies is produced: <math>f_\text{BEAT} = |f_\text{IN} - f_\text{BFO}|</math>.<ref name=":3">{{Cite journal |date=2009 |title=Heterodyne receiver |url=https://www.antiquewireless.org/wp-content/uploads/Vol.-22.pdf |journal=The AWA Review |publisher=The Antique Wireless Association, Inc. |volume=22 |pages=287–289 |isbn=978-0-9741994-1-2}}</ref> If the BFO frequency is near enough to the radio station's frequency, the beat frequency is in the [[audio frequency]] range and can be heard in the receiver's earphones.<ref name=":3" /> During the "dots" and "dashes" of the signal, the beat tone is produced, while between them there is no carrier so no tone is produced. Thus the Morse code is audible as musical "beeps" in the earphones.

The BFO was rare until the invention in 1913 of the first practical electronic oscillator, the vacuum tube feedback [[electronic oscillator|oscillator]] by [[Edwin Armstrong]].  After this time BFOs were a standard part of radiotelegraphy receivers. Each time the radio was tuned to a different station frequency, the BFO frequency had to be changed also, so the BFO oscillator had to be tunable. In later [[superheterodyne]] receivers from the 1930s on, the BFO signal was mixed with the constant [[intermediate frequency]] (IF) produced by the superheterodyne's detector. Therefore, the BFO could be a fixed frequency.<ref>{{Cite web |title=Superheterodyne reception {{!}} Radio waves, Frequency, Amplification {{!}} Britannica |url=https://www.britannica.com/technology/superheterodyne-reception |access-date=2024-05-23 |website=www.britannica.com |language=en}}</ref>

Continuous-wave vacuum tube transmitters replaced the other types of transmitter with the availability of power tubes after [[World War I]] because they were cheap.  CW became the standard method of transmitting radiotelegraphy by the 20s, damped wave spark transmitters were banned by 1930<ref name="Schroeder" /> and CW continues to be used today. Even today most [[communications receiver]]s produced for use in shortwave communication stations have BFOs.<ref>{{Cite web |last=Lu |first=Emma |date=2022-02-25 |title=Beat Frequency Oscillator- Principle and Applications |url=https://www.wellpcb.com/beat-frequency-oscillator.html |access-date=2024-05-21 |website=Circuit Board Fabrication and PCB Assembly Turnkey Services - WellPCB |language=en-US}}</ref>