AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmissions, but also on the longwave and shortwave radio bands.
166-505: WPRD (1440 AM ) is a radio station broadcasting a Spanish-language variety format. Licensed to Winter Park, Florida , United States, WPRD is owned and operated by J & V Communications, Inc. WPRD serves as the main flagship station for J & V Communications, Inc. and is headquartered in Orlando, Florida , in the same building previously occupied by former stations WABR and WBJW (the latter now WOMX-FM ). Some programming on WPRD
332-442: A coupled oscillator , producing beats (see top graphs) . The oscillating radio frequency energy was passed rapidly back and forth between the primary and secondary resonant circuits as long as the spark continued. Each time the energy returned to the primary, some was lost as heat in the spark. In addition, unless the coupling was very loose the oscillations caused the transmitter to transmit on two separate frequencies. Since
498-437: A damped wave . The frequency f {\displaystyle f} of the oscillations, which is the frequency of the emitted radio waves, is equal to the resonant frequency of the resonant circuit, determined by the capacitance C {\displaystyle C} of the capacitor and the inductance L {\displaystyle L} of the coil: The transmitter repeats this cycle rapidly, so
664-472: A "chain". The Radio Corporation of America (RCA), General Electric , and Westinghouse organized a competing network around its own flagship station, RCA's WJZ (now WABC) in New York City, but were hampered by AT&T's refusal to lease connecting lines or allow them to sell airtime. In 1926 AT&T sold its radio operations to RCA, which used them to form the nucleus of the new NBC network. By
830-401: A "closed" resonant circuit containing the detector . A radio system with a "two circuit" (inductively coupled) transmitter and receiver was called a "four circuit" system. The first person to use resonant circuits in a radio application was Nikola Tesla , who invented the resonant transformer in 1891. At a March 1893 St. Louis lecture he had demonstrated a wireless system that, although it
996-846: A 10 shilling receiver license fee. Both highbrow and mass-appeal programmes were carried by the National and Regional networks. The period from the early 1920s through the 1940s is often called the "Golden Age of Radio". During this period AM radio was the main source of home entertainment, until it was replaced by television. For the first time entertainment was provided from outside the home, replacing traditional forms of entertainment such as oral storytelling and music from family members. New forms were created, including radio plays , mystery serials, soap operas , quiz shows , variety hours , situation comedies and children's shows . Radio news, including remote reporting, allowed listeners to be vicariously present at notable events. Radio greatly eased
1162-428: A 25 kW alternator (D) turned by a combustion engine. The first spark gap and resonant circuit (S1, C1, T2) generated the high voltage to charge the capacitor (C2) powering the second spark gap and resonant circuit (S2, C2, T3) , which generated the output. The spark rate was low, perhaps as low as 2 - 3 sparks per second. Fleming estimated the radiated power was around 10 - 12 kW. The transmitter
1328-595: A 400 ft. wire antenna suspended from a kite . Marconi announced the first transatlantic radio transmission took place on 12 December 1901, from Poldhu , Cornwall to Signal Hill, Newfoundland , a distance of 2100 miles (3400 km). Marconi's achievement received worldwide publicity, and was the final proof that radio was a practical communication technology. The scientific community at first doubted Marconi's report. Virtually all wireless experts besides Marconi believed that radio waves traveled in straight lines, so no one (including Marconi) understood how
1494-475: A Spanish-language variety format under "La Fantastica 1440". The name was later changed to "La Voz 1440" in late 2014, as it remains known to this day. This article about a radio station in Florida is a stub . You can help Misplaced Pages by expanding it . AM broadcasting The earliest experimental AM transmissions began in the early 1900s. However, widespread AM broadcasting was not established until
1660-570: A central station to all parts of the United States", he was unable to overcome the inherent distance limitations of this technology. The earliest public radiotelegraph broadcasts were provided as government services, beginning with daily time signals inaugurated on January 1, 1905, by a number of U.S. Navy stations. In Europe, signals transmitted from a station located on the Eiffel Tower were received throughout much of Europe. In both
1826-430: A circuit does not produce radio waves. A resonant circuit with an antenna radiating radio waves (an "open" tuned circuit) loses energy quickly, giving it high damping (low Q, wide bandwidth). There was a fundamental tradeoff between a circuit which produced persistent oscillations which had narrow bandwidth, and one which radiated high power. The solution found by a number of researchers was to use two resonant circuits in
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#17327880966651992-442: A combination of oscillating electric and magnetic fields could travel through space as an " electromagnetic wave ". Maxwell proposed that light consisted of electromagnetic waves of short wavelength, but no one knew how to confirm this, or generate or detect electromagnetic waves of other wavelengths. By 1883 it was theorized that accelerated electric charges could produce electromagnetic waves, and George Fitzgerald had calculated
2158-610: A concerted attempt to specify performance of AM receivers through the 1993 AMAX standard, a joint effort of the EIA and the NAB, with FCC backing... The FCC rapidly followed up on this with codification of the CQUAM AM stereo standard, also in 1993. At this point, the stage appeared to be set for rejuvenation of the AM band. Nevertheless, with the legacy of confusion and disappointment in the rollout of
2324-424: A conductor which suddenly change their velocity, thus accelerating. An electrically charged capacitance discharged through an electric spark across a spark gap between two conductors was the first device known which could generate radio waves. The spark itself doesn't produce the radio waves, it merely serves as a fast acting switch to excite resonant radio frequency oscillating electric currents in
2490-635: A consortium, the British Broadcasting Company (BBC), established on 18 October 1922, which was given a monopoly on broadcasting. This enterprise was supported by a tax on radio sets sales, plus an annual license fee on receivers, collected by the Post Office. Initially the eight stations were allowed regional autonomy. In 1927, the original broadcasting organization was replaced by a government chartered British Broadcasting Corporation . an independent nonprofit supported solely by
2656-564: A continuous-wave (CW) transmitter. Fessenden began his research on audio transmissions while doing developmental work for the United States Weather Service on Cobb Island, Maryland. Because he did not yet have a continuous-wave transmitter, initially he worked with an experimental "high-frequency spark" transmitter, taking advantage of the fact that the higher the spark rate, the closer a spark-gap transmission comes to producing continuous waves. He later reported that, in
2822-580: A few " telephone newspaper " systems, most of which were established in Europe, beginning with the Paris Théâtrophone . With this in mind, most early radiotelephone development envisioned that the device would be more profitably developed as a "wireless telephone" for personal communication, or for providing links where regular telephone lines could not be run, rather than for the uncertain finances of broadcasting. The person generally credited as
2988-463: A few hundred ( Hz ), to increase its rotational speed and so generate currents of tens-of-thousands Hz, thus producing a steady continuous-wave transmission when connected to an aerial. The next step, adopted from standard wire-telephone practice, was to insert a simple carbon microphone into the transmission line, to modulate the carrier wave signal to produce AM audio transmissions. However, it would take many years of expensive development before even
3154-408: A higher frequency, usually 500 Hz, resulting in a spark rate of 1000 Hz. The speed at which signals may be transmitted is naturally limited by the time taken for the spark to be extinguished. If, as described above, the conductive plasma does not, during the zero points of the alternating current, cool enough to extinguish the spark, a 'persistent spark' is maintained until the stored energy
3320-516: A home audio device prior to the introduction of the high-fidelity , long-playing record in the late 1940s. Listening habits changed in the 1960s due to the introduction of the revolutionary transistor radio (Regency TR-1, the first transistor radio released December 1954), which was made possible by the invention of the transistor in 1948. (The transistor was invented at Bell labs and released in June 1948.) Their compact size — small enough to fit in
3486-474: A listening site at Plymouth, Massachusetts. An American Telephone Journal account of the December 21 alternator-transmitter demonstration included the statement that "It is admirably adapted to the transmission of news, music, etc. as, owing to the fact that no wires are needed, simultaneous transmission to many subscribers can be effected as easily as to a few", echoing the words of a handout distributed to
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#17327880966653652-482: A long series of experiments to increase the transmission range of Hertz's spark oscillators and receivers. He was unable to communicate beyond a half-mile until 1895, when he discovered that the range of transmission could be increased greatly by replacing one side of the Hertzian dipole antenna in his transmitter and receiver with a connection to Earth and the other side with a long wire antenna suspended high above
3818-487: A majority of early broadcasting stations operated on mediumwave frequencies, whose limited range generally restricted them to local audiences. One method for overcoming this limitation, as well as a method for sharing program costs, was to create radio networks , linking stations together with telephone lines to provide a nationwide audience. In the U.S., the American Telephone and Telegraph Company (AT&T)
3984-506: A national scale. The introduction of nationwide talk shows, most prominently Rush Limbaugh 's beginning in 1988, was sometimes credited with "saving" AM radio. However, these stations tended to attract older listeners who were of lesser interest to advertisers, and AM radio's audience share continued to erode. In 1961, the FCC adopted a single standard for FM stereo transmissions, which was widely credited with enhancing FM's popularity. Developing
4150-421: A new type of spark gap was developed by German physicist Max Wien , called the series or quenched gap. A quenched gap consisted of a stack of wide cylindrical electrodes separated by thin insulating spacer rings to create many narrow spark gaps in series, of around 0.1–0.3 mm (0.004–0.01 in). The wide surface area of the electrodes terminated the ionization in the gap quickly by cooling it after
4316-430: A pair of flat spiral inductors with their conductors ending in spark gaps. A Leyden jar capacitor discharged through one spiral, would cause sparks in the gap of the other spiral. See circuit diagram. Hertz's transmitters consisted of a dipole antenna made of a pair of collinear metal rods of various lengths with a spark gap (S) between their inner ends and metal balls or plates for capacitance (C) attached to
4482-603: A particular transmitter by "tuning" its resonant frequency to the frequency of the desired transmitter, analogously to the way one musical instrument could be tuned to resonance with another. This is the system used in all modern radio. During the period 1897 to 1900 wireless researchers realized the advantages of "syntonic" or "tuned" systems, and added capacitors ( Leyden jars ) and inductors (coils of wire) to transmitters and receivers, to make resonant circuits (tuned circuits, or tank circuits). Oliver Lodge , who had been researching electrical resonance for years, patented
4648-536: A patent on his radio system 2 June 1896, often considered the first wireless patent. In May 1897 he transmitted 14 km (8.7 miles), on 27 March 1899 he transmitted across the English Channel , 46 km (28 miles), in fall 1899 he extended the range to 136 km (85 miles), and by January 1901 he had reached 315 km (196 miles). These demonstrations of wireless Morse code communication at increasingly long distances convinced
4814-399: A policy allowing AM stations to simulcast over FM translator stations. Translators had previously been available only to FM broadcasters, in order to increase coverage in fringe areas. Their assignment for use by AM stations was intended to approximate the station's daytime coverage, which in cases where the stations reduced power at night, often resulted in expanded nighttime coverage. Although
4980-593: A practical radio communication system. In addition to Tesla's system, inductively coupled radio systems were patented by Oliver Lodge in February 1898, Karl Ferdinand Braun , in November 1899, and John Stone Stone in February 1900. Braun made the crucial discovery that low damping required "loose coupling" (reduced mutual inductance ) between the primary and secondary coils. Marconi at first paid little attention to syntony, but by 1900 developed
5146-486: A prototype alternator-transmitter would be ready, and a few years beyond that for high-power versions to become available. Fessenden worked with General Electric 's (GE) Ernst F. W. Alexanderson , who in August 1906 delivered an improved model which operated at a transmitting frequency of approximately 50 kHz, although at low power. The alternator-transmitter achieved the goal of transmitting quality audio signals, but
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5312-424: A radio system incorporating features from these systems, with a two circuit transmitter and two circuit receiver, with all four circuits tuned to the same frequency, using a resonant transformer he called the "jigger". In spite of the above prior patents, Marconi in his 26 April 1900 "four circuit" or "master tuning" patent on his system claimed rights to the inductively coupled transmitter and receiver. This
5478-456: A receiver's earphones the signal sounds like a steady tone, whine, or buzz. In order to transmit information with this signal, the operator turns the transmitter on and off rapidly by tapping on a switch called a telegraph key in the primary circuit of the transformer, producing sequences of short (dot) and long (dash) strings of damped waves, to spell out messages in Morse code . As long as
5644-423: A result, AM radio tends to do best in areas where FM frequencies are in short supply, or in thinly populated or mountainous areas where FM coverage is poor. Great care must be taken to avoid mutual interference between stations operating on the same frequency. In general, an AM transmission needs to be about 20 times stronger than an interfering signal to avoid a reduction in quality, in contrast to FM signals, where
5810-566: A risky gamble for his company. Up to that time his small induction coil transmitters had an input power of 100 - 200 watts, and the maximum range achieved was around 150 miles. To build the first high power transmitter, Marconi hired an expert in electric power engineering, Prof. John Ambrose Fleming of University College, London, who applied power engineering principles. Fleming designed a complicated inductively-coupled transmitter (see circuit) with two cascaded spark gaps (S1, S2) firing at different rates, and three resonant circuits, powered by
5976-452: A shirt pocket — and lower power requirements, compared to vacuum tubes, meant that for the first time radio receivers were readily portable. The transistor radio became the most widely used communication device in history, with billions manufactured by the 1970s. Radio became a ubiquitous "companion medium" which people could take with them anywhere they went. The demarcation between what is considered "experimental" and "organized" broadcasting
6142-427: A short-range "wireless telephone" demonstration, that included simultaneously broadcasting speech and music to seven locations throughout Murray, Kentucky. However, this was transmitted using induction rather than radio signals, and although Stubblefield predicted that his system would be perfected so that "it will be possible to communicate with hundreds of homes at the same time", and "a single message can be sent from
6308-496: A single frequency , but a continuous band of frequencies. They were essentially radio noise sources radiating energy over a large part of the radio spectrum , which made it impossible for other transmitters to be heard. When multiple transmitters attempted to operate in the same area, their broad signals overlapped in frequency and interfered with each other. The radio receivers used also had no resonant circuits, so they had no way of selecting one signal from others besides
6474-470: A stationary electrode. The spark rate was equal to the rotations per second times the number of spark electrodes on the wheel. It could produce spark rates up to several thousand hertz, and the rate could be adjusted by changing the speed of the motor. The rotation of the wheel was usually synchronized to the AC sine wave so the moving electrode passed by the stationary one at the peak of the sine wave, initiating
6640-438: A storm 17 September 1901 and he hastily erected a temporary antenna consisting of 50 wires suspended in a fan shape from a cable between two 160 foot poles. The frequency used is not known precisely, as Marconi did not measure wavelength or frequency, but it was between 166 and 984 kHz, probably around 500 kHz. He received the signal on the coast of St. John's, Newfoundland using an untuned coherer receiver with
6806-406: A vibrating arm switch contact on the coil called an interrupter repeatedly breaks the circuit that provides current to the primary winding, causing the coil to generate pulses of high voltage. When the primary current to the coil is turned on, the primary winding creates a magnetic field in the iron core which pulls the springy interrupter arm away from its contact, opening the switch and cutting off
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6972-439: A wide bandwidth , creating radio frequency interference (RFI) that can disrupt other radio transmissions. This type of radio emission has been prohibited by international law since 1934. Electromagnetic waves are radiated by electric charges when they are accelerated . Radio waves , electromagnetic waves of radio frequency , can be generated by time-varying electric currents , consisting of electrons flowing through
7138-461: A widespread audience — dates back to the founding period of radio development, even though the earliest radio transmissions, originally known as "Hertzian radiation" and "wireless telegraphy", used spark-gap transmitters that could only transmit the dots-and-dashes of Morse code . In October 1898 a London publication, The Electrician , noted that "there are rare cases where, as Dr. [Oliver] Lodge once expressed it, it might be advantageous to 'shout'
7304-406: Is comparable to or better in audio quality than a standard analog broadcast". Despite the various actions, AM band audiences continued to contract, and the number of stations began to slowly decline. A 2009 FCC review reported that "The story of AM radio over the last 50 years has been a transition from being the dominant form of audio entertainment for all age groups to being almost non-existent to
7470-402: Is dissipated, permitting practical operation only up to around 60 signals per second. If active measures are taken to break the arc (either by blowing air through the spark or by lengthening the spark gap), a much shorter "quenched spark" may be obtained. A simple quenched spark system still permits several oscillations of the capacitor circuit in the time taken for the spark to be quenched. With
7636-502: Is largely arbitrary. Listed below are some of the early AM radio broadcasts, which, due to their irregular schedules and limited purposes, can be classified as "experimental": People who weren't around in the Twenties when radio exploded can't know what it meant, this milestone for mankind. Suddenly, with radio, there was instant human communication. No longer were our homes isolated and lonely and silent. The world came into our homes for
7802-750: Is repeated on WOTS 1220 kHz in Kissimmee , WSDO 1400 kHz in Sanford , WTJV 1490 kHz in DeLand , and WUNA 1480 kHz in Ocoee . WPRD also shares programming with Puerto Rican station WCMN . Following the demise of WAJL, WPRD started broadcasting on the same 1440 frequency as "Pride 1440", playing music from the 1940s through the 1960s. Kids' Choice Broadcasting Network, later Imagination Station Network , started broadcasting on March 31, 1990, on WPRD as its originating station. During this time
7968-645: Is simpler than later transmission systems. An AM receiver detects amplitude variations in the radio waves at a particular frequency, then amplifies changes in the signal voltage to operate a loudspeaker or earphone . However, the simplicity of AM transmission also makes it vulnerable to "static" ( radio noise , radio frequency interference ) created by both natural atmospheric electrical activity such as lightning, and electrical and electronic equipment, including fluorescent lights, motors and vehicle ignition systems. In large urban centers, AM radio signals can be severely disrupted by metal structures and tall buildings. As
8134-578: The Electronic Industries Association (EIA) and the National Association of Broadcasters (NAB) with the intention of helping AM stations, especially ones with musical formats, become more competitive with FM broadcasters by promoting better quality receivers. However, the stereo AM and AMAX initiatives had little impact, and a 2015 review of these events concluded that Initially the consumer manufacturers made
8300-625: The Emergency Alert System (EAS). Some automakers have been eliminating AM radio from their electric vehicles (EVs) due to interference from the electric motors , but the lawmakers argue that this is a safety risk and that car owners should have access to AM radio regardless of the type of vehicle they drive. The proposed legislation would require all new vehicles to include AM radio at no additional charge, and it would also require automakers that have already eliminated AM radio to inform customers of alternatives. AM radio technology
8466-493: The Federal Emergency Management Agency (FEMA) expressed concerns that this would reduce the effectiveness of emergency communications. In May 2023, a bipartisan group of lawmakers in the United States introduced legislation making it illegal for automakers to eliminate AM radio from their cars. The lawmakers argue that AM radio is an important tool for public safety due to being a component of
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#17327880966658632-483: The VHF , UHF , or microwave bands. In his various experiments, Hertz produced waves with frequencies from 50 to 450 MHz, roughly the frequencies used today by broadcast television transmitters . Hertz used them to perform historic experiments demonstrating standing waves , refraction , diffraction , polarization and interference of radio waves. He also measured the speed of radio waves, showing they traveled at
8798-431: The electrolytic detector and thermionic diode ( Fleming valve ) were invented by Reginald Fessenden and John Ambrose Fleming , respectively. Most important, in 1904–1906 the crystal detector , the simplest and cheapest AM detector, was developed by G. W. Pickard . Homemade crystal radios spread rapidly during the next 15 years, providing ready audiences for the first radio broadcasts. One limitation of crystals sets
8964-599: The " Kennelly–Heaviside layer " or "E-layer", for which he received the 1947 Nobel Prize in Physics . Knowledgeable sources today doubt whether Marconi actually received this transmission. Ionospheric conditions should not have allowed the signal to be received during the daytime at that range. Marconi knew the Morse code signal to be transmitted was the letter 'S' (three dots). He and his assistant could have mistaken atmospheric radio noise ("static") in their earphones for
9130-430: The " capture effect " means that the dominant signal needs to only be about twice as strong as the interfering one. To allow room for more stations on the mediumwave broadcast band in the United States, in June 1989 the FCC adopted a National Radio Systems Committee (NRSC) standard that limited maximum transmitted audio bandwidth to 10.2 kHz, limiting occupied bandwidth to 20.4 kHz. The former audio limitation
9296-677: The 1920s, following the development of vacuum tube receivers and transmitters. AM radio remained the dominant method of broadcasting for the next 30 years, a period called the " Golden Age of Radio ", until television broadcasting became widespread in the 1950s and received much of the programming previously carried by radio. Later, AM radio's audiences declined greatly due to competition from FM ( frequency modulation ) radio, Digital Audio Broadcasting (DAB), satellite radio , HD (digital) radio , Internet radio , music streaming services , and podcasting . Compared to FM or digital transmissions , AM transmissions are more expensive to transmit due to
9462-603: The 1930s, most of the major radio stations in the country were affiliated with networks owned by two companies, NBC and CBS . In 1934, a third national network, the Mutual Radio Network , was formed as a cooperative owned by its stations. A second country which quickly adopted network programming was the United Kingdom, and its national network quickly became a prototype for a state-managed monopoly of broadcasting. A rising interest in radio broadcasting by
9628-660: The 1940s two new broadcast media, FM radio and television , began to provide extensive competition with the established broadcasting services. The AM radio industry suffered a serious loss of audience and advertising revenue, and coped by developing new strategies. Network broadcasting gave way to format broadcasting: instead of broadcasting the same programs all over the country, stations individually adopted specialized formats which appealed to different audiences, such as regional and local news, sports, "talk" programs, and programs targeted at minorities. Instead of live music, most stations began playing less expensive recorded music. In
9794-532: The AM radio industry in the United States developed technology for broadcasting in stereo . Other nations adopted AM stereo, most commonly choosing Motorola's C-QUAM, and in 1993 the United States also made the C-QUAM system its standard, after a period allowing four different standards to compete. The selection of a single standard improved acceptance of AM stereo , however overall there was limited adoption of AM stereo worldwide, and interest declined after 1990. With
9960-531: The British public pressured the government to reintroduce the service, following its suspension in 1920. However, the government also wanted to avoid what it termed the "chaotic" U.S. experience of allowing large numbers of stations to operate with few restrictions. There were also concerns about broadcasting becoming dominated by the Marconi company. Arrangements were made for six large radio manufacturers to form
10126-411: The Twenties when radio exploded can't know what it meant, this milestone for mankind. Suddenly, with radio, there was instant human communication. No longer were our homes isolated and lonely and silent. The world came into our homes for the first time. Music came pouring in. Laughter came in. News came in. The world shrank, with radio. The idea of broadcasting — the unrestricted transmission of signals to
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#173278809666510292-417: The U.S., for example) subject to international agreements. Spark-gap transmitter A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark . Spark-gap transmitters were the first type of radio transmitter, and were the main type used during the wireless telegraphy or "spark" era, the first three decades of radio , from 1887 to
10458-694: The United States and France this led to a small market of receiver lines geared for jewelers who needed accurate time to set their clocks, including the Ondophone in France, and the De Forest RS-100 Jewelers Time Receiver in the United States The ability to pick up time signal broadcasts, in addition to Morse code weather reports and news summaries, also attracted the interest of amateur radio enthusiasts. It
10624-439: The antenna but by the resonant circuit, so it could easily be changed by adjustable taps on the coil. The antenna was brought into resonance with the tuned circuit using loading coils . The energy in each spark, and thus the power output, was no longer limited by the capacitance of the antenna but by the size of the capacitor in the resonant circuit. In order to increase the power very large capacitor banks were used. The form that
10790-523: The antenna wire. This meant that the full transmitter power flowed through the microphone, and even using water cooling, the power handling ability of the microphones severely limited the power of the transmissions. Ultimately only a small number of large and powerful Alexanderson alternators would be developed. However, they would be almost exclusively used for long-range radiotelegraph communication, and occasionally for radiotelephone experimentation, but were never used for general broadcasting. Almost all of
10956-506: The antenna, which was determined by its length; it acted as a half-wave dipole , which radiated waves roughly twice the length of the antenna (for example a dipole 1 meter long would generate 150 MHz radio waves). Hertz detected the waves by observing tiny sparks in micrometer spark gaps (M) in loops of wire which functioned as resonant receiving antennas. Oliver Lodge was also experimenting with spark oscillators at this time and came close to discovering radio waves before Hertz, but his focus
11122-421: The antenna. The transmitter repeats this cycle at a rapid rate, so the spark appeared continuous, and the radio signal sounded like a whine or buzz in a radio receiver . The cycle is very rapid, taking less than a millisecond. With each spark, this cycle produces a radio signal consisting of an oscillating sinusoidal wave that increases rapidly to a high amplitude and decreases exponentially to zero, called
11288-476: The atmosphere between two 600 foot wires held aloft by kites on mountaintops 14 miles apart. Thomas Edison had come close to discovering radio in 1875; he had generated and detected radio waves which he called "etheric currents" experimenting with high-voltage spark circuits, but due to lack of time did not pursue the matter. David Edward Hughes in 1879 had also stumbled on radio wave transmission which he received with his carbon microphone detector, however he
11454-482: The audience has continued to decline. In 1987, the elimination of the Fairness Doctrine requirement meant that talk shows, which were commonly carried by AM stations, could adopt a more focused presentation on controversial topics, without the distraction of having to provide airtime for any contrasting opinions. In addition, satellite distribution made it possible for programs to be economically carried on
11620-428: The bandwidth of transmitters and receivers. Using a resonant circuit (also called tuned circuit or tank circuit) in transmitters would narrow the bandwidth of the radiated signal, it would occupy a smaller range of frequencies around its center frequency, so that the signals of transmitters "tuned" to transmit on different frequencies would no longer overlap. A receiver which had its own resonant circuit could receive
11786-476: The broad resonance of the antenna, and responded to the transmissions of all transmitters in the vicinity. An example of this interference problem was an embarrassing public debacle in August 1901 when Marconi, Lee de Forest , and G. W. Pickard attempted to report the New York Yacht Race to newspapers from ships with their untuned spark transmitters. The Morse code transmissions interfered, and
11952-404: The characteristics of arc-transmitters . Fessenden attempted to sell this form of radiotelephone for point-to-point communication, but was unsuccessful. Fessenden's work with high-frequency spark transmissions was only a temporary measure. His ultimate plan for creating an audio-capable transmitter was to redesign an electrical alternator , which normally produced alternating current of at most
12118-449: The clicks of the transmitter. Marconi made many subsequent transatlantic transmissions which clearly establish his priority, but reliable transatlantic communication was not achieved until 1907 with more powerful transmitters. The inductively-coupled transmitter had a more complicated output waveform than the non-syntonic transmitter, due to the interaction of the two resonant circuits. The two magnetically coupled tuned circuits acted as
12284-403: The commission estimated that fewer than 250 AM stations were transmitting hybrid mode signals. On October 27, 2020, the FCC voted to allow AM stations to eliminate their analog transmissions and convert to all-digital operation, with the requirement that stations making the change had to continue to make programming available over "at least one free over-the-air digital programming stream that
12450-433: The conductors of the attached circuit. The conductors radiate the energy in this oscillating current as radio waves. Due to the inherent inductance of circuit conductors, the discharge of a capacitor through a low enough resistance (such as a spark) is oscillatory ; the charge flows rapidly back and forth through the spark gap for a brief period, charging the conductors on each side alternately positive and negative, until
12616-475: The continued migration of AM stations away from music to news, sports, and talk formats, receiver manufacturers saw little reason to adopt the more expensive stereo tuners, and thus radio stations have little incentive to upgrade to stereo transmission. In countries where the use of directional antennas is common, such as the United States, transmitter sites consisting of multiple towers often occupy large tracts of land that have significantly increased in value over
12782-437: The continuous wave AM transmissions made prior to 1915 were made by versions of the arc converter transmitter, which had been initially developed by Valdemar Poulsen in 1903. Arc transmitters worked by producing a pulsating electrical arc in an enclosed hydrogen atmosphere. They were much more compact than alternator transmitters, and could operate on somewhat higher transmitting frequencies. However, they suffered from some of
12948-428: The current stopped. In the inductively coupled transmitter, the narrow gaps extinguished ("quenched") the spark at the first nodal point ( Q ) when the primary current momentarily went to zero after all the energy had been transferred to the secondary winding (see lower graph) . Since without the spark no current could flow in the primary circuit, this effectively uncoupled the secondary from the primary circuit, allowing
13114-434: The decades, to the point that the value of land exceeds that of the station itself. This sometimes results in the sale of the transmitter site, with the station relocating to a more distant shared site using significantly less power, or completely shutting down operations. The ongoing development of alternative transmission systems, including Digital Audio Broadcasting (DAB), satellite radio, and HD (digital) radio, continued
13280-567: The decline of the popularity of the traditional broadcast technologies. These new options, including the introduction of Internet streaming, particularly resulted in the reduction of shortwave transmissions, as international broadcasters found ways to reach their audiences more easily. In 2022 it was reported that AM radio was being removed from a number of electric vehicle (EV) models, including from cars manufactured by Tesla, Audi, Porsche, BMW and Volvo, reportedly due to automakers concerns that an EV's higher electromagnetic interference can disrupt
13446-569: The demonstration witnesses, which stated "[Radio] Telephony is admirably adapted for transmitting news, stock quotations, music, race reports, etc. simultaneously over a city, on account of the fact that no wires are needed and a single apparatus can distribute to ten thousand subscribers as easily as to a few. It is proposed to erect stations for this purpose in the large cities here and abroad." However, other than two holiday transmissions reportedly made shortly after these demonstrations, Fessenden does not appear to have conducted any radio broadcasts for
13612-581: The end of World War I. German physicist Heinrich Hertz built the first experimental spark-gap transmitters in 1887, with which he proved the existence of radio waves and studied their properties. A fundamental limitation of spark-gap transmitters is that they generate a series of brief transient pulses of radio waves called damped waves ; they are unable to produce the continuous waves used to carry audio (sound) in modern AM or FM radio transmission. So spark-gap transmitters could not transmit audio, and instead transmitted information by radiotelegraphy ;
13778-418: The energy stored in the antenna was quickly radiated away as radio waves, so the oscillations decayed to zero quickly. The radio signal consisted of brief pulses of radio waves, repeating tens or at most a few hundreds of times per second, separated by comparatively long intervals of no output. The power radiated was dependent on how much electric charge could be stored in the antenna before each spark, which
13944-648: The expectation is that listeners will primarily be tuning into the FM signal rather than the nominally "primary" AM station. A 2020 review noted that "for many owners, keeping their AM stations on the air now is pretty much just about retaining their FM translator footprint rather than keeping the AM on the air on its own merits". In 2018 the FCC, led by then-Commission Chairman Ajit Pai , proposed greatly reducing signal protection for 50 kW Class A " clear channel " stations. This would allow co-channel secondary stations to operate with higher powers, especially at night. However,
14110-445: The fall of 1900, he successfully transmitted speech over a distance of about 1.6 kilometers (one mile), which appears to have been the first successful audio transmission using radio signals. However, at this time the sound was far too distorted to be commercially practical. For a time he continued working with more sophisticated high-frequency spark transmitters, including versions that used compressed air, which began to take on some of
14276-532: The first "syntonic" transmitter and receiver in May 1897 Lodge added an inductor (coil) between the sides of his dipole antennas, which resonated with the capacitance of the antenna to make a tuned circuit. Although his complicated circuit did not see much practical use, Lodge's "syntonic" patent was important because it was the first to propose a radio transmitter and receiver containing resonant circuits which were tuned to resonance with each other. In 1911 when
14442-401: The first time. Music came pouring in. Laughter came in. News came in. The world shrank, with radio. Following World War I, the number of stations providing a regular broadcasting service greatly increased, primarily due to advances in vacuum-tube technology. In response to ongoing activities, government regulators eventually codified standards for which stations could make broadcasts intended for
14608-624: The general public, for example, in the United States formal recognition of a "broadcasting service" came with the establishment of regulations effective December 1, 1921, and Canadian authorities created a separate category of "radio-telephone broadcasting stations" in April 1922. However, there were numerous cases of entertainment broadcasts being presented on a regular schedule before their formal recognition by government regulators. Some early examples include: Because most longwave radio frequencies were used for international radiotelegraph communication,
14774-417: The general public, or to have even given additional thought about the potential of a regular broadcast service, and in a 1908 article providing a comprehensive review of the potential uses for his radiotelephone invention, he made no references to broadcasting. Because there was no way to amplify electrical currents at this time, modulation was usually accomplished by a carbon microphone inserted directly in
14940-476: The ground. These antennas functioned as quarter-wave monopole antennas . The length of the antenna determined the wavelength of the waves produced and thus their frequency. Longer, lower frequency waves have less attenuation with distance. As Marconi tried longer antennas, which radiated lower frequency waves, probably in the MF band around 2 MHz, he found that he could transmit further. Another advantage
15106-421: The horizon. The dipole resonators also had low capacitance and couldn't store much charge , limiting their power output. Therefore, these devices were not capable of long distance transmission; their reception range with the primitive receivers employed was typically limited to roughly 100 yards (100 meters). I could scarcely conceive it possible that [radio's] application to useful purposes could have escaped
15272-414: The influence of Maxwell's theory, their thinking was dominated by the similarity between radio waves and light waves; they thought of radio waves as an invisible form of light. By analogy with light, they assumed that radio waves only traveled in straight lines, so they thought radio transmission was limited by the visual horizon like existing optical signalling methods such as semaphore , and therefore
15438-554: The isolation of rural life. Political officials could now speak directly to millions of citizens. One of the first to take advantage of this was American president Franklin Roosevelt , who became famous for his fireside chats during the Great Depression . However, broadcasting also provided the means to use propaganda as a powerful government tool, and contributed to the rise of fascist and communist ideologies. In
15604-410: The key is pressed the spark gap fires repetitively, creating a string of pulses of radio waves, so in a receiver the keypress sounds like a buzz; the entire Morse code message sounds like a sequence of buzzes separated by pauses. In low-power transmitters the key directly breaks the primary circuit of the supply transformer, while in high-power transmitters the key operates a heavy duty relay that breaks
15770-411: The lack of any way to amplify the signals meant they were somewhat weak. On December 21, 1906, Fessenden made an extensive demonstration of the new alternator-transmitter at Brant Rock, Massachusetts, showing its utility for point-to-point wireless telephony, including interconnecting his stations to the wire telephone network. As part of the demonstration, speech was transmitted 18 kilometers (11 miles) to
15936-419: The late 1960s and 1970s, top 40 rock and roll stations in the U.S. and Canada such as WABC and CHUM transmitted highly processed and extended audio to 11 kHz, successfully attracting huge audiences. For young people, listening to AM broadcasts and participating in their music surveys and contests was the social media of the time. In the late 1970s, spurred by the exodus of musical programming to FM stations,
16102-399: The marketplace decide" which was best. The lack of a common standard resulted in consumer confusion and increased the complexity and cost of producing AM stereo receivers. In 1993, the FCC again revised its policy, by selecting C-QUAM as the sole AM stereo implementation. In 1993, the FCC also endorsed, although it did not make mandatory, AMAX broadcasting standards that were developed by
16268-432: The message, spreading it broadcast to receivers in all directions". However, it was recognized that this would involve significant financial issues, as that same year The Electrician also commented "did not Prof. Lodge forget that no one wants to pay for shouting to the world on a system by which it would be impossible to prevent non-subscribers from benefiting gratuitously?" On January 1, 1902, Nathan Stubblefield gave
16434-448: The multiple incompatible AM stereo systems, and failure of the manufacturers (including the auto makers) to effectively promote AMAX radios, coupled with the ever-increasing background of noise in the band, the general public soon lost interest and moved on to other media. On June 8, 1988, an International Telecommunication Union (ITU)-sponsored conference held at Rio de Janeiro, Brazil adopted provisions, effective July 1, 1990, to extend
16600-507: The narrow passband of the receiver's resonant circuit could only be tuned to one of these frequencies, the power radiated at the other frequency was wasted. This troublesome backflow of energy to the primary circuit could be prevented by extinguishing (quenching) the spark at the right instant, after all the energy from the capacitors was transferred to the antenna circuit. Inventors tried various methods to accomplish this, such as air blasts and Elihu Thomson 's magnetic blowout . In 1906,
16766-537: The necessity of having to transmit a high power carrier wave to overcome ground losses, and the large antenna radiators required at the low broadcast frequencies, but can be sent over long distances via the ionosphere at night; however, they are much more susceptible to interference, and often have lower audio fidelity. Thus, AM broadcasters tend to specialize in spoken-word formats, such as talk radio , all-news radio and sports radio , with music formats primarily for FM and digital stations. People who weren't around in
16932-405: The new frequencies. It was now estimated that the expanded band could accommodate around 300 U.S. stations. However, it turned out that the number of possible station reassignments was much lower, with a 2006 accounting reporting that, out of 4,758 licensed U.S. AM stations, only 56 were now operating on the expanded band. Moreover, despite an initial requirement that by the end of five years either
17098-428: The new policy, by 2011 there were approximately 500 in operation, and as of 2020 approximately 2,800 of the 4,570 licensed AM stations were rebroadcasting on one or more FM translators. In 2009 the FCC stated that "We do not intend to allow these cross-service translators to be used as surrogates for FM stations". However, based on station slogans, especially in the case of recently adopted musical formats, in most cases
17264-491: The notice of such eminent scientists. Italian radio pioneer Guglielmo Marconi was one of the first people to believe that radio waves could be used for long distance communication, and singlehandedly developed the first practical radiotelegraphy transmitters and receivers , mainly by combining and tinkering with the inventions of others. Starting at age 21 on his family's estate in Italy, between 1894 and 1901 he conducted
17430-413: The operator switched the transmitter on and off with a telegraph key , creating pulses of radio waves to spell out text messages in Morse code . The first practical spark gap transmitters and receivers for radiotelegraphy communication were developed by Guglielmo Marconi around 1896. One of the first uses for spark-gap transmitters was on ships, to communicate with shore and broadcast a distress call if
17596-411: The original station or its expanded band counterpart had to cease broadcasting, as of 2015 there were 25 cases where the original standard band station was still on the air, despite also operating as an expanded band station. HD Radio is a digital audio broadcasting method developed by iBiquity . In 2002 its "hybrid mode", which simultaneously transmits a standard analog signal as well as a digital one,
17762-438: The oscillations die away. A practical spark gap transmitter consists of these parts: The transmitter works in a rapid repeating cycle in which the capacitor is charged to a high voltage by the transformer and discharged through the coil by a spark across the spark gap. The impulsive spark excites the resonant circuit to "ring" like a bell, producing a brief oscillating current which is radiated as electromagnetic waves by
17928-489: The oscillations, while the secondary winding ( L2 ) was connected to the wire antenna ( A ) and ground, forming an "open" resonant circuit with the capacitance of the antenna ( C2 ). Both circuits were tuned to the same resonant frequency . The advantage of the inductively coupled circuit was that the "loosely coupled" transformer transferred the oscillating energy of the tank circuit to the radiating antenna circuit gradually, creating long "ringing" waves. A second advantage
18094-404: The outer ends. The two sides of the antenna were connected to an induction coil (Ruhmkorff coil) (T) a common lab power source which produced pulses of high voltage, 5 to 30 kV. In addition to radiating the waves, the antenna also acted as a harmonic oscillator ( resonator ) which generated the oscillating currents. High-voltage pulses from the induction coil (T) were applied between
18260-422: The output is a repeating string of damped waves. This is equivalent to a radio signal amplitude modulated with a steady frequency, so it could be demodulated in a radio receiver by a rectifying AM detector , such as the crystal detector or Fleming valve used during the wireless telegraphy era. The frequency of repetition (spark rate) is in the audio range, typically 50 to 1000 sparks per second, so in
18426-406: The output power of a loop antenna . Fitzgerald in a brief note published in 1883 suggested that electromagnetic waves could be generated practically by discharging a capacitor rapidly; the method used in spark transmitters, however there is no indication that this inspired other inventors. The division of the history of spark transmitters into the different types below follows the organization of
18592-433: The overheating issues of needing to insert microphones directly in the transmission antenna circuit. Vacuum tube transmitters also provided high-quality AM signals, and could operate on higher transmitting frequencies than alternator and arc transmitters. Non-governmental radio transmissions were prohibited in many countries during World War I, but AM radiotelephony technology advanced greatly due to wartime research, and after
18758-476: The patent was renewed the Marconi Company was forced to buy it to protect its own syntonic system against infringement suits. The resonant circuit functioned analogously to a tuning fork , storing oscillating electrical energy, increasing the Q factor of the circuit so the oscillations were less damped. Another advantage was the frequency of the transmitter was no longer determined by the length of
18924-450: The primary circuit. The circuit which charges the capacitors, along with the spark gap itself, determines the spark rate of the transmitter, the number of sparks and resulting damped wave pulses it produces per second, which determines the tone of the signal heard in the receiver. The spark rate should not be confused with the frequency of the transmitter, which is the number of sinusoidal oscillations per second in each damped wave. Since
19090-407: The primary current. Then the magnetic field collapses, creating a pulse of high voltage in the secondary winding, and the interrupter arm springs back to close the contact again, and the cycle repeats. Each pulse of high voltage charged up the capacitor until the spark gap fired, resulting in one spark per pulse. Interrupters were limited to low spark rates of 20–100 Hz, sounding like a low buzz in
19256-441: The primary early developer of AM technology is Canadian-born inventor Reginald Fessenden . The original spark-gap radio transmitters were impractical for transmitting audio, since they produced discontinuous pulses known as " damped waves ". Fessenden realized that what was needed was a new type of radio transmitter that produced steady "undamped" (better known as " continuous wave ") signals, which could then be "modulated" to reflect
19422-402: The receiver. In powerful induction coil transmitters, instead of a vibrating interrupter, a mercury turbine interrupter was used. This could break the current at rates up to several thousand hertz, and the rate could be adjusted to produce the best tone. In higher power transmitters powered by AC, a transformer steps the input voltage up to the high voltage needed. The sinusoidal voltage from
19588-650: The reception of AM transmissions and hurt the listening experience, among other reasons. However the United States Congress has introduced a bill to require all vehicles sold in the US to have an AM receiver to receive emergency broadcasts. The FM broadcast band was established in 1941 in the United States, and at the time some suggested that the AM band would soon be eliminated. In 1948 wide-band FM's inventor, Edwin H. Armstrong , predicted that "The broadcasters will set up FM stations which will parallel, carry
19754-436: The reporters on shore failed to receive any information from the garbled signals. It became clear that for multiple transmitters to operate, some system of "selective signaling" had to be devised to allow a receiver to select which transmitter's signal to receive, and reject the others. In 1892 William Crookes had given an influential lecture on radio in which he suggested using resonance (then called syntony ) to reduce
19920-406: The resonant circuit took in practical transmitters was the inductively-coupled circuit described in the next section. In developing these syntonic transmitters, researchers found it impossible to achieve low damping with a single resonant circuit. A resonant circuit can only have low damping (high Q, narrow bandwidth) if it is a "closed" circuit, with no energy dissipating components. But such
20086-799: The same deficiencies. The lack of any means to amplify electrical currents meant that, like the alternator transmitters, modulation was usually accomplished by a microphone inserted directly in the antenna wire, which again resulted in overheating issues, even with the use of water-cooled microphones. Thus, transmitter powers tended to be limited. The arc was also somewhat unstable, which reduced audio quality. Experimenters who used arc transmitters for their radiotelephone research included Ernst Ruhmer , Quirino Majorana , Charles "Doc" Herrold , and Lee de Forest . Advances in vacuum tube technology (called "valves" in British usage), especially after around 1915, revolutionized radio technology. Vacuum tube devices could be used to amplify electrical currents, which overcame
20252-400: The same program, as over their AM stations... eventually the day will come, of course, when we will no longer have to build receivers capable of receiving both types of transmission, and then the AM transmitters will disappear." However, FM stations actually struggled for many decades, and it was not until 1978 that FM listenership surpassed that of AM stations. Since then the AM band's share of
20418-409: The same speed as light. These experiments established that light and radio waves were both forms of Maxwell's electromagnetic waves , differing only in frequency. Augusto Righi and Jagadish Chandra Bose around 1894 generated microwaves of 12 and 60 GHz respectively, using small metal balls as resonator-antennas. The high frequencies produced by Hertzian oscillators could not travel beyond
20584-459: The secondary resonant circuit and antenna to oscillate completely free of the primary circuit after that (until the next spark). This produced output power centered on a single frequency instead of two frequencies. It also eliminated most of the energy loss in the spark, producing very lightly damped, long "ringing" waves, with decrements of only 0.08 to 0.25 (a Q of 12-38) and consequently a very "pure", narrow bandwidth radio signal. Another advantage
20750-446: The ship was sinking. They played a crucial role in maritime rescues such as the 1912 RMS Titanic disaster. After World War I, vacuum tube transmitters were developed, which were less expensive and produced continuous waves which had a greater range, produced less interference, and could also carry audio, making spark transmitters obsolete by 1920. The radio signals produced by spark-gap transmitters are electrically "noisy"; they have
20916-410: The sounds being transmitted. Fessenden's basic approach was disclosed in U.S. Patent 706,737, which he applied for on May 29, 1901, and was issued the next year. It called for the use of a high-speed alternator (referred to as "an alternating-current dynamo") that generated "pure sine waves" and produced "a continuous train of radiant waves of substantially uniform strength", or, in modern terminology,
21082-420: The spark circuit broken, the transmission frequency is solely determined by the antenna resonant circuit, which permits simpler tuning. In a transmitter with a "rotary" spark gap (below) , the capacitor was charged by AC from a high-voltage transformer as above, and discharged by a spark gap consisting of electrodes spaced around a wheel which was spun by an electric motor, which produced sparks as they passed by
21248-747: The spark era. Inspired by Marconi, in the late 1890s other researchers also began developing competing spark radio communication systems; Alexander Popov in Russia, Eugène Ducretet in France, Reginald Fessenden and Lee de Forest in America, and Karl Ferdinand Braun , Adolf Slaby , and Georg von Arco in Germany who in 1903 formed the Telefunken Co., Marconi's chief rival. The primitive transmitters prior to 1897 had no resonant circuits (also called LC circuits, tank circuits, or tuned circuits),
21414-399: The spark gap was in the antenna, which functioned as the resonator to determine the frequency of the radio waves. These were called "unsyntonized" or "plain antenna" transmitters. The average power output of these transmitters was low, because due to its low capacitance the antenna was a highly damped oscillator (in modern terminology, it had very low Q factor ). During each spark
21580-460: The spark rate was equal to twice the frequency of the AC power (often multiple sparks occurred during the peak of each half cycle). The spark rate of transmitters powered by 50 or 60 Hz mains power was thus 100 or 120 Hz. However higher audio frequencies cut through interference better, so in many transmitters the transformer was powered by a motor–alternator set, an electric motor with its shaft turning an alternator , that produced AC at
21746-408: The spark when the capacitor was fully charged, which produced a musical tone in the receiver. When tuned correctly in this manner, the need for external cooling or quenching airflow was eliminated, as was the loss of power directly from the charging circuit (parallel to the capacitor) through the spark. The invention of the radio transmitter resulted from the convergence of two lines of research. One
21912-482: The station used the moniker "The Imagination Station". Station owner, Metroplex Communications, invested in the network. In September, the station sponsored a concert for the one year anniversary of non-profit Arnold Palmer Hospital for Children and Women featuring Peter Yarrow , a network vice president and investor. On February 27, 1991, the Imagination Station Network stopped broadcasting on
22078-476: The station. At 2 p.m. that day, the station changed over to all-news with the CNN Headline News service. About 100 calls were received by the station regarding the discontinuation of the network on the station. For a brief period in 1993, the call sign were changed to WWZN before reverting to WPRD. Since the mid-1990s, WPRD has been owned by J & V Communications, Inc. and started broadcasting in
22244-451: The subject used in many wireless textbooks. German physicist Heinrich Hertz in 1887 built the first experimental spark gap transmitters during his historic experiments to demonstrate the existence of electromagnetic waves predicted by James Clerk Maxwell in 1864, in which he discovered radio waves , which were called "Hertzian waves" until about 1910. Hertz was inspired to try spark excited circuits by experiments with "Reiss spirals",
22410-411: The technology for AM broadcasting in stereo was challenging due to the need to limit the transmissions to a 20 kHz bandwidth, while also making the transmissions backward compatible with existing non-stereo receivers. In 1990, the FCC authorized an AM stereo standard developed by Magnavox, but two years later revised its decision to instead approve four competing implementations, saying it would "let
22576-400: The transformer is applied directly to the capacitor, so the voltage on the capacitor varies from a high positive voltage, to zero, to a high negative voltage. The spark gap is adjusted so sparks only occur near the maximum voltage, at peaks of the AC sine wave , when the capacitor was fully charged. Since the AC sine wave has two peaks per cycle, ideally two sparks occurred during each cycle, so
22742-401: The translator stations are not permitted to originate programming when the "primary" AM station is broadcasting, they are permitted to do so during nighttime hours for AM stations licensed for daytime-only operation. Prior to the adoption of the new policy, as of March 18, 2009, the FCC had issued 215 Special Temporary Authority grants for FM translators relaying AM stations. After creation of
22908-433: The transmitter produces one pulse of radio waves per spark, the output power of the transmitter was proportional to the spark rate, so higher rates were favored. Spark transmitters generally used one of three types of power circuits: An induction coil (Ruhmkorff coil) was used in low-power transmitters, usually less than 500 watts, often battery-powered. An induction coil is a type of transformer powered by DC, in which
23074-399: The transmitter's frequency, which lighted a Geissler tube . This system, patented by Tesla 2 September 1897, 4 months after Lodge's "syntonic" patent, was in effect an inductively coupled radio transmitter and receiver, the first use of the "four circuit" system claimed by Marconi in his 1900 patent (below) . However, Tesla was mainly interested in wireless power and never developed
23240-425: The transmitter, with their coils inductively (magnetically) coupled , making a resonant transformer (called an oscillation transformer ); this was called an " inductively coupled ", " coupled circuit " or " two circuit " transmitter. See circuit diagram. The primary winding of the oscillation transformer ( L1 ) with the capacitor ( C1 ) and spark gap ( S ) formed a "closed" resonant circuit which generated
23406-416: The two sides of the antenna. Each pulse stored electric charge in the capacitance of the antenna, which was immediately discharged by a spark across the spark gap. The spark excited brief oscillating standing waves of current between the sides of the antenna. The antenna radiated the energy as a momentary pulse of radio waves; a damped wave . The frequency of the waves was equal to the resonant frequency of
23572-551: The upper atmosphere, later called skywave propagation. Marconi did not understand any of this at the time; he simply found empirically that the higher his vertical antenna was suspended, the further it would transmit. After failing to interest the Italian government, in 1896 Marconi moved to England, where William Preece of the British General Post Office funded his experiments. Marconi applied for
23738-496: The upper end of the Region 2 AM broadcast band, by adding ten frequencies which spanned from 1610 kHz to 1700 kHz. At this time it was suggested that as many as 500 U.S. stations could be assigned to the new frequencies. On April 12, 1990, the FCC voted to begin the process of populating the expanded band, with the main priority being the reduction of interference on the existing AM band, by transferring selected stations to
23904-407: The voltage that could be used was limited to about 100 kV by corona discharge which caused charge to leak off the antenna, particularly in wet weather, and also energy lost as heat in the longer spark. A more significant drawback of the large damping was that the radio transmissions were electrically "noisy"; they had a very large bandwidth . These transmitters did not produce waves of
24070-548: The war the availability of tubes sparked a great increase in the number of amateur radio stations experimenting with AM transmission of news or music. Vacuum tubes remained the central technology of radio for 40 years, until transistors began to dominate in the late 1950s, and are still used in the highest power broadcast transmitters. Unlike telegraph and telephone systems, which used completely different types of equipment, most radio receivers were equally suitable for both radiotelegraph and radiotelephone reception. In 1903 and 1904
24236-461: The waves had managed to propagate around the 300 mile high curve of the Earth between Britain and Newfoundland. In 1902 Arthur Kennelly and Oliver Heaviside independently theorized that radio waves were reflected by a layer of ionized atoms in the upper atmosphere, enabling them to return to Earth beyond the horizon. In 1924 Edward V. Appleton demonstrated the existence of this layer, now called
24402-613: The world that radio, or "wireless telegraphy" as it was called, was not just a scientific curiosity but a commercially useful communication technology. In 1897 Marconi started a company to produce his radio systems, which became the Marconi Wireless Telegraph Company . and radio communication began to be used commercially around 1900. His first large contract in 1901 was with the insurance firm Lloyd's of London to equip their ships with wireless stations. Marconi's company dominated marine radio throughout
24568-404: The youngest demographic groups. Among persons aged 12–24, AM accounts for only 4% of listening, while FM accounts for 96%. Among persons aged 25–34, AM accounts for only 9% of listening, while FM accounts for 91%. The median age of listeners to the AM band is 57 years old, a full generation older than the median age of FM listeners." In 2009, the FCC made a major regulatory change, when it adopted
24734-572: Was 15 kHz resulting in bandwidth of 30 kHz. Another common limitation on AM fidelity is the result of receiver design, although some efforts have been made to improve this, notably through the AMAX standards adopted in the United States. AM broadcasts are used on several frequency bands. The allocation of these bands is governed by the ITU 's Radio Regulations and, on the national level, by each country's telecommunications administration (the FCC in
24900-474: Was approved by the FCC for use by AM stations, initially only during daytime hours, due to concerns that during the night its wider bandwidth would cause unacceptable interference to stations on adjacent frequencies. In 2007 nighttime operation was also authorized. The number of hybrid mode AM stations is not exactly known, because the FCC does not keep track of the stations employing the system, and some authorized stations have later turned it off. But as of 2020
25066-477: Was built in secrecy on the coast at Poldhu , Cornwall , UK. Marconi was pressed for time because Nikola Tesla was building his own transatlantic radiotelegraphy transmitter on Long Island, New York , in a bid to be first (this was the Wardenclyffe Tower , which lost funding and was abandoned unfinished after Marconi's success). Marconi's original round 400-wire transmitting antenna collapsed in
25232-647: Was done by adjusting the length of the antenna. This patent gave Marconi a near monopoly of syntonic wireless telegraphy in England and America. Tesla sued Marconi's company for patent infringement but didn't have the resources to pursue the action. In 1943 the US Supreme Court invalidated the inductive coupling claims of Marconi's patent due to the prior patents of Lodge, Tesla, and Stone, but this came long after spark transmitters had become obsolete. The inductively coupled or "syntonic" spark transmitter
25398-468: Was efforts by inventors to devise a system to transmit telegraph signals without wires. Experiments by a number of inventors had shown that electrical disturbances could be transmitted short distances through the air. However most of these systems worked not by radio waves but by electrostatic induction or electromagnetic induction , which had too short a range to be practical. In 1866 Mahlon Loomis claimed to have transmitted an electrical signal through
25564-473: Was granted a British patent, but the US patent office twice rejected his patent as lacking originality. Then in a 1904 appeal a new patent commissioner reversed the decision and granted the patent, on the narrow grounds that Marconi's patent by including an antenna loading coil (J in circuit above) provided the means for tuning the four circuits to the same frequency, whereas in the Tesla and Stone patents this
25730-400: Was immediately recognized that, much like the telegraph had preceded the invention of the telephone, the ability to make audio radio transmissions would be a significant technical advance. Despite this knowledge, it still took two decades to perfect the technology needed to make quality audio transmissions. In addition, the telephone had rarely been used for distributing entertainment, outside of
25896-423: Was intended for wireless power transmission , had many of the elements of later radio communication systems. A grounded capacitance-loaded spark-excited resonant transformer (his Tesla coil ) attached to an elevated wire monopole antenna transmitted radio waves, which were received across the room by a similar wire antenna attached to a receiver consisting of a second grounded resonant transformer tuned to
26062-596: Was not capable of longer distance communication. As late as 1894 Oliver Lodge speculated that the maximum distance Hertzian waves could be transmitted was a half mile. To investigate the similarity between radio waves and light waves , these researchers concentrated on producing short wavelength high-frequency waves with which they could duplicate classic optics experiments with radio waves, using quasioptical components such as prisms and lenses made of paraffin wax , sulfur , and pitch and wire diffraction gratings . Their short antennas generated radio waves in
26228-421: Was on waves on wires, not in free space. Hertz and the first generation of physicists who built these "Hertzian oscillators", such as Jagadish Chandra Bose , Lord Rayleigh , George Fitzgerald , Frederick Trouton , Augusto Righi and Oliver Lodge , were mainly interested in radio waves as a scientific phenomenon , and largely failed to foresee its possibilities as a communication technology. Due to
26394-460: Was persuaded that what he observed was induction . Neither of these individuals are usually credited with the discovery of radio, because they did not understand the significance of their observations and did not publish their work before Hertz. The other was research by physicists to confirm the theory of electromagnetism proposed in 1864 by Scottish physicist James Clerk Maxwell , now called Maxwell's equations . Maxwell's theory predicted that
26560-399: Was proportional to the capacitance of the antenna. To increase their capacitance to ground, antennas were made with multiple parallel wires, often with capacitive toploads, in the "harp", "cage", " umbrella ", "inverted-L", and " T " antennas characteristic of the "spark" era. The only other way to increase the energy stored in the antenna was to charge it up to very high voltages. However
26726-399: Was remedied by the quenched-spark and rotary gap transmitters (below) . In recognition of their achievements in radio, Marconi and Braun shared the 1909 Nobel Prize in physics . Marconi decided in 1900 to attempt transatlantic communication, which would allow him to dominate Atlantic shipping and compete with submarine telegraph cables . This would require a major scale-up in power,
26892-423: Was that it allowed a large primary capacitance (C1) to be used which could store a lot of energy, increasing the power output enormously. Powerful transoceanic transmitters often had huge Leyden jar capacitor banks filling rooms (see pictures above) . The receiver in most systems also used two inductively coupled circuits, with the antenna an "open" resonant circuit coupled through an oscillation transformer to
27058-439: Was that these vertical antennas radiated vertically polarized waves, instead of the horizontally polarized waves produced by Hertz's horizontal antennas. These longer vertically polarized waves could travel beyond the horizon, because they propagated as a ground wave that followed the contour of the Earth. Under certain conditions they could also reach beyond the horizon by reflecting off layers of charged particles ( ions ) in
27224-473: Was the first organization to create a radio network, and also to promote commercial advertising, which it called "toll" broadcasting. Its flagship station, WEAF (now WFAN) in New York City, sold blocks of airtime to commercial sponsors that developed entertainment shows containing commercial messages . AT&T held a monopoly on quality telephone lines, and by 1924 had linked 12 stations in Eastern cities into
27390-431: Was the first type that could communicate at intercontinental distances, and also the first that had sufficiently narrow bandwidth that interference between transmitters was reduced to a tolerable level. It became the dominant type used during the "spark" era. A drawback of the plain inductively coupled transmitter was that unless the primary and secondary coils were very loosely coupled it radiated on two frequencies. This
27556-406: Was the lack of amplifying the signals, so listeners had to use earphones , and it required the development of vacuum-tube receivers before loudspeakers could be used. The dynamic cone loudspeaker , invented in 1924, greatly improved audio frequency response over the previous horn speakers, allowing music to be reproduced with good fidelity. AM radio offered the highest sound quality available in
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