AM (Amplitude Modulation) Broadcasting the first eighty years of the 20th century

AM (Amplitude Modulation) Broadcasting

AM was the dominant method of broadcasting during the first eighty years of the 20th century and remains widely used into the 21st.

AM radio began with the first, experimental broadcast on Christmas Eve of 1906 by Canadian experimenter Reginald Fessenden, and was used for small-scale voice and music broadcasts up until World War I. San Francisco, California radio station KCBS claims to be the direct descendant of KQW, founded by radio experimenter Charles "Doc" Herrold, who made regular weekly broadcasts in San Jose, California as early as June 1909. On that basis KCBS has claimed to be the world's oldest broadcast station and celebrated its 100th anniversary in the summer of 2009. The great increase in the use of AM radio came late in the following decade as radio experimentation increased worldwide following World War I. The first licensed commercial radio services began on AM in the 1920s. XWA of Montreal, Quebec (later CFCF, now CINW) claims status as the first commercial broadcaster in the world, with regular broadcasts commencing on May 20, 1920. The first licensed American radio station was started by Frank Conrad, KDKA in Pittsburgh, Pennsylvania. Radio programming boomed during the "Golden Age of Radio" (1920s–1950s). Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music.

Scheme AM (Amplitude Modulation) Broadcasting

AM radio technology is simpler than Frequency Modulated (FM) radio, Digital Audio Broadcast (DAB), Satellite Radio or HD (digital) Radio. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification.

In North American broadcasting practice, transmitter power input to the antenna for commercial AM stations ranges from about 250 watts to 50,000 watts. Experimental licenses were issued for up to 500,000 watts radiated power, for stations intended for wide-area communication during disasters including Cincinnati station WLW, which used such power on occasion before World War II. WLW's superpower transmitter still exists at the station's suburban transmitter site, but it was decommissioned in the early 1940s and no current commercial broadcaster in the US or Canada is authorized for such power levels. Some other countries do authorize higher power operation (for example the Mexican station XERF formerly operated at 250,000 watts). Antenna design must consider the coverage desired and must direct the transmitted signal so as not to interfere with other stations operating on the same or adjacent frequencies.

AM radio is broadcast 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 the U.S., for example) subject to international agreements.

Long wave is 148.5 kHz–283.5 kHz, with 9 kHz channel spacing generally used. Long wave is used for radio broadcasting in Europe, Africa and parts of Asia (ITU region 1), and is not allocated in the Western Hemisphere. In the United States and Canada, Bermuda and U.S. territories this band is mainly reserved for aeronautics navigational aids, though a small section of the band could theoretically be used for microbroadcasting under the United States Part 15 rules. Due to the propagation characteristics of long wave signals, the frequencies are used most effectively in latitudes north of 50°.

Medium wave is 520 kHz–1,610 kHz. In the Americas (ITU region 2) 10 kHz spacing is used; elsewhere it is 9 kHz. ITU region 2 also authorizes the Extended AM broadcast band between 1610 kHz and 1710 kHz. Medium wave is by far the most heavily used band for commercial broadcasting. This is the "AM radio" that most people are familiar with.

Short wave is 1.711 MHz–30.0 MHz, divided into 15 broadcast bands. Shortwave broadcasts generally use a narrow 5 kHz channel spacing. Short wave is used by audio services intended to be heard at great distances from the transmitting station. The long range of short wave broadcasts comes at the expense of lower audio fidelity. The mode of propagation for short wave is different (see high frequency). AM is used mostly by broadcast services – other shortwave users may use a modified version of AM such as SSB or an AM-compatible version of SSB such as SSB with carrier reinserted.

Frequencies between the broadcast bands are used for other forms of radio communication, and are not broadcast services intended for reception by the general public.

Some microbroadcasters and pirate radio broadcasters, especially those in the United States under the FCC's Part 15 rules, broadcast on AM to achieve greater range than is possible on the FM band. On mediumwave (AM), such radio stations are often found between 1610 kHz and 1710 kHz. Hobbyists also use low-power AM transmitters to provide local programming for antique radio equipment in areas where AM programming is not widely available or is of questionable quality; in such cases the transmitter, which is designed to cover only the immediate property and perhaps nearby areas, is connected to a computer or music player.